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[Tim_M]

Hi Jerry,
You have my interest! I don't really know how to calculate scale speed for a model. I worked on the premise that the model should cover 1/8 of the distance the real boat covered in the same time. Of course, that isn't correct in terms of the hydrodynamic forces. I remember doing dimensional analysis at tech college but it was so long ago that the example we used was Noah's ark. I would really appreciate it if you could explain where your figure comes from. I'm here to learn! (I'm sure others will be equally interested).

[Jerry Hill]

The scale speed is derived by multiplying the actual speed by the square root of the scale factor.

Now I did the calcs in my head before but here it is properly

I.E. at 1/8 scale
Square root of 8 = 2.828427124
At an actual speed of 37.5 mph × 2.828427124 = 106.0660171 mph scale speed


I'm not knowledgeable enough to be able to explain the meat behind the formula, all I can say is that I believe it comes from models used by development engineers in wind tunnel and tank tests. My attention came to it when I started thinking about the margins of some speeds, where some of the numbers just don't look right using the simple scale approach.

For example, if a little tenth scale speedboat wanted to do a scale speed of 100 mph then it would only be doing an actual 10 mph, and in my mind watching a boat at a trotting speed is not representative of 100 mph in scale. So that was why I looked into it really, and when you apply the formula the perception of a models speed as a small rendition of reality when watching models it all fits.

So, if you need to know a target model speed to reproduce a scale speed the formula changes to scale speed divided by the square root of scale factor.

I.E. at 1/8 scale,
Square root of 8 = 2.828427124
A target scale speed of 300 mph ÷ 2.82847124 = 106.0660171 mph actual speed

So it turns out that the scale speed of your estimated figure is the actual speed needed to get the target scale speed of 300. That pans out in all examples.

[Tim_M]

Glue, clamp, post.........

What's holding me back today is that I have run out of clamps! I have built up the sponsons; one of them has the outer skin fitted (clamped!) and the other is waiting. I built them as a pair; it's more important that they are the same as opposed to dead accurate. I've used polyethylene foam as reserve buoyancy (having seem first hand what happens if you get water in the sponsons and they are the most vulnerable part of the model  - when I'm driving, that is). It's nice because it cuts very easily and doesn't crumble like polystyrene. Plus it's what I had to hand.

I had thought of using the bandaged-ply method to make the top deck of the sponsons but I chickened out. It's a very tight radius with little margin for error (I need lots of margin for error!). Plus it's a slow, difficult technique. Oh, and I've run out of ply!! I opted to use block balsa for the curvy bits. Looks like I have added a lot of weight but in reality most of it gets carved away later.

I have also attached the cowl to the hull. I got the join within a couple of mm all round. I was going to go further for my own satisfaction but stopped myself as there was a good chance I would ruin a pretty good job. Photos to follow when the duct tape comes off.

Thanks for the info on scaling speeds, Jerry. What you've described is dynamic scaling and that I do understand (or did!). That gets the hydrodynamic (or aerodynamic - same thing, different fluid) forces right, which is why you rightly say it's used for wind tunnel and test tank models. I spent many happy (and scary) hours running serious model hovercraft and ships down test tanks as an apprentice. Sitting on the carriage at about 30 mph indoors and wondering if the automatic brakes were going to work today called for the right coloured trousers. I guess the best scale for a model is what looks right! To me, most models look a bit too fast and twitchy (because time is not scaled). Just my opinion. Tell you what though - my K7 will NOT be doing 100 mph! On my local lake I reckon that would give me about a  five second run (OK, maybe ten by the time it gets up to speed and, hopefully, slows down again). What I would like to figure out is how to get it to kick up some impressive rooster tails from the prop and sponsons. I'm even contemplating adding some tiny 'scoops'. Anybody got any ideas?

Answers Please.............
Rudders. On Mk 1 I put the rudder right behind the prop. It seemed to work well (too well??) and looked OK. I know many people fit the rudder alongside the prop. I could do that too with more or less the same hardware. Given that K7 is intended to go in a straight line only (apart from lining up), is there a clear best choice? (I'm expecting two different schools of thought here).

Decals. I've made my own decals in the past for craft projects. I looked online (Crafty Computer Papers) and I now have three choices. Inkjet decals, that have to be waterproofed after they are fitted, laserjet decals that are inherently waterproof, or 'rub down' transfer decals. I think the inkjet option is out because I don't want patches of varnish. Laserjet would be OK as we have the right printers at work. I rather fancy the rub-down option. Anybody had experience of those?

Finally, Paint. I was kindly informed of the correct colour (Mercedes Himmelblau). Now, I want to use an acrylic rather than a solvent based paint (fumes, clean up, you name it....) and I'm going to attempt to use an airbrush. Anybody recommend a product/colour that is similar enough and readily available?

[Jerry Hill]

Scoops work well to create rooster tails. Racing airboats have them to stop flipping when the engines are throttled off quickly, the high thrust line of the aero engines forces the bow down normally, remove that and troubles can ensue. We fitted an active brake to my lad's airboat, here it is deployed:



It slows the boat a little and undermines the steering, but you'd only need little ones to create a nice effect I would think. I'll dig up images of the device itself if its of use to you.


I agree with your approach to speed for this project entirely. It does make you appreciate the speed of these original machines I must say, it must have been an amazing spectacle to witness.

[Jerry Hill]

Just remembered this video, within it is the airboat above with brake on and off showing:

https://youtu.be/IhgnAs8RnKw


Rudders immediately behind surface piercing props give lots of effect at high speeds and less at lower speeds simply because of the thrust directing/cavitation thing. So a surface prop needs to start being on song before the rudder has must water pressure to direct, when suddenlybit has a lot. So either low throw with limited low speed ability, which is firmly what one of my boats has but is fine to live with, exponential settings on the radio gear, or mixing between throttle and steering. Different throttle settings also create changing steering trim. Frankly I find sound control overcomes the disadvantages for sports use, racers would have a very different perspective.

For smoother, more speed proportional, and for less drag an offset rudder is the way to go. Less affected by  planing height, clear of propwash and placed on the stbd side is the best match for prop torsion issues meaning right turns are best, in most cases. As the Bluebird K7 was about locked in straightline ability I think this is the option I'd be looking at. In essence this boat is a 'Rigger', so what those guys are doing may help with ideas perhaps.

[Tim_M]

That's the fella, Jerry! I'm thinking perhaps some tiny 'winglets' on the front fins. Something to think about. You get just the 'foamy' effect I'm looking for (even though your purpose is quite different).

Regards the rudder, I intend to offset the prop about 15-20 mm stb (as per Mk1). That would put the rudder way off centre but as the bracket will attach to the strut, it's possible. I tried dual rate on Mk1, which tamed the over steer. Throttle rudder mixing? Mmm...........well, I have that new fancy radio now. Perhaps I will have a go at that! I knew there was a reason for buying it.

[Tim_M]

Just for a change, I'm not waiting for anything to dry (except, perhaps, the weather; it's grim!). I have just come to a natural pause.


Yesterday, I attached the cowl to the hull. Today, I detached it. No, I don't mean I messed it up; I cut along the split line and lifted the top half away. What d'you know? It worked! Just look at all that lovely space I have to work in! Later, after I have tidied up and arranged the running gear, I will add a couple of frames to stiffen the open section and then the inner deck and hatch. (I have a cunning plan for that hatch - I'll tell you if it works    ) (Oh good, got the smileys back. they don't work in Microsoft 'Edge')


The sponsons have been rough sanded to shape. I'm very pleased with the way they turned out.


I weighed everything today:
Bluebird K7 Weights 14-08-2015 (g)
Hull and Cowl                                       835
Sponsons (2)                                      345
Motor                                                   475
Battery                                                 380
Speed Controller                                  135
Booms                                                  125
Rudder                                                 50 - 110 (depending on what bracketry I use)
Servo                                                    45
Prop and flex                                        50
'All up' (everything piled on the scales)      2361
That motor is a real lump! OK, there are still a few bits to add, some wood to take off and the finish to apply but the current weight at 2.4 kg is huge reduction from Mk1 at over 3.5 kg. Bodes well for easy running.
The past couple of weeks have been intensive on the build, to put it mildly. Now I need to slow down - way down. The build so far has gone better than I could have hoped for and I  have the foundations of a half-decent model - or a heap of junk. Now it's down to attention to detail and careful finishing (never my strong point). I'm not going true 'scale' - though the basic shape is pretty accurate - just aiming for a model that 'captures' the real thing. My interest is firmly in design and build rather than running so now it's time to get my money's worth out of the materials. I think it will keep me entertained for quite some time. I will still post the progress but it won't at the same pace. I'm sure you will forgive me!

[ooyah/2]

The scale speed is derived by multiplying the actual speed by the square root of the scale factor.

Now I did the calcs in my head before but here it is properly

I.E. at 1/8 scale
Square root of 8 = 2.828427124
At an actual speed of 37.5 mph × 2.828427124 = 106.0660171 mph scale speed


I'm not knowledgeable enough to be able to explain the meat behind the formula, all I can say is that I believe it comes from models used by development engineers in wind tunnel and tank tests. My attention came to it when I started thinking about the margins of some speeds, where some of the numbers just don't look right using the simple scale approach.

For example, if a little tenth scale speedboat wanted to do a scale speed of 100 mph then it would only be doing an actual 10 mph, and in my mind watching a boat at a trotting speed is not representative of 100 mph in scale. So that was why I looked into it really, and when you apply the formula the perception of a models speed as a small rendition of reality when watching models it all fits.

So, if you need to know a target model speed to reproduce a scale speed the formula changes to scale speed divided by the square root of scale factor.

I.E. at 1/8 scale,
Square root of 8 = 2.828427124
A target scale speed of 300 mph ÷ 2.82847124 = 106.0660171 mph actual speed

So it turns out that the scale speed of your estimated figure is the actual speed needed to get the target scale speed of 300. That pans out in all examples.

Firstly to Tim,
What a great job of building your K7, top marks.

Jerry,
Thanks for to the speed conversion formula.

My K7 at 1/12 scale does approx 40 m.p.h. but to get a true scale speed it would need to be 86.7 m.p.h. which is mind blowing.

Earlier in Tims thread I said that I get about a 7 sec run from one end of my club lake to the other at 40 m.p.h..

Last year in the summer on a very flat calm day I had my K7 at Loch Lomond  and I was able to send it 100 yds to my right and  100 yards to my left.
I had Mrs T  with the camera but unfortunately she finds it hard to shut one eye and focus thro' the lens of the camera with the other, hence the pic enclosed doesn't show K7 flat out.

What actualy happened was that as I openned full throttle K7 took off ( literaly ) as it wound up speed with the amount of run that it had, just as it was level with me it took off just like D.C.'s K7, as it's a prop drive the torque of the prop started to turn it over to starboard and I was quick enough to snap the throttle shut and it flopped down on the starboard sponson which saved me a swim in a very cold loch.

I have a book by Neil Sheppard ( DONALD CAMPBELL AND THE FINAL RECORD ATTEMPT ) which does a very technical study by him and a Dr Kieth Mitchell with so many titles and P.H.D's to his name I will leave out and although a medical man his pastimes are the areodynamics and design of large R.C models and Hydroplanes in particular D.C.s Bluebird K7.

After much study of the last attempt he concludes that at 320 m.p.h. K7 was actually flying and shows some pics of K7 riding on it's stabilising fins and the hull is above the water with only the rear wedge touching and with the long held thought that as the engine cut out and  there was no thrust to keep it level the stern dropped down , wind underneath and K7 took of with the resulting disaster and at the speed it was travelling at was way out of it's design criteria.

I seam to have let myself get carried away a bit hear but  if you look at the video link with my K7 you can see as it comes at you she is clearly riding on the edges of the sponsons and the prop C/Line and well out of the water and is producing quite a Rooster tail.

I don't think that we should be aiming for a scale speed as you rightly say that Tims K7 would need to acheive 106 + m.p.h. to get up to full size speed.and the spray from the prop will create a rooster tale not unlike the Jet powered K7 of D.C. and Ernie Lazenby's Turbine powered K7 ,  can't say that I approve of a brake to create a spray as it will only take more power to get it up on the plane.

Hope that this is of interest and the book is well worth the cost to any Bluebird K7 fan and what ever you do Tim don't try to turn your K7 at speed, it just will not do it, I have the TEE shirt to proove it.

George.

Pic at Loch Lomond and link to you-tube.

https://www.youtube.com/watch?v=fGPMBKTA1Wo

[Tim_M]

Hello George! I was concerned by your silence. All is well, it seems 




HELP!
I decided to take a break from building and check out the new radio (just in case it was faulty).
After a LOT of searching and direct contact with Hobby King I managed to get a sort of manual, but it only goes so far. With a lot of bleeping (literally and metaphorically) I have managed to enter the model name and set it up as a basic aircraft (sorry George). I could not get any response from the motor but eventually realised that the throttle is at zero in the mid position. Surely that's not right? Aircraft don't go back'ards, do they (except the ones I tried to fly)? I then managed to stop that 'model' working completely again by messing about with the throttle end points, so I'm using another memory.


Long and short of it is I could REALLY use some guidance on setting up the radio. It's a Turnigy 9x. (OK, I should have known better and bought an expensive one). Anybody got any experience of this model? I guess any programmable radio would work in the same way....


What makes me a bit cross is that anything that carries the CE mark has to be provided with instructions in the language of the country it's sold in by law.  Seems Hobby King know different.

[ooyah/2]

Hello George! I was concerned by your silence. All is well, it seems 




HELP!
I decided to take a break from building and check out the new radio (just in case it was faulty).
After a LOT of searching and direct contact with Hobby King I managed to get a sort of manual, but it only goes so far. With a lot of bleeping (literally and metaphorically) I have managed to enter the model name and set it up as a basic aircraft (sorry George). I could not get any response from the motor but eventually realised that the throttle is at zero in the mid position. Surely that's not right? Aircraft don't go back'ards, do they (except the ones I tried to fly)? I then managed to stop that 'model' working completely again by messing about with the throttle end points, so I'm using another memory.


Long and short of it is I could REALLY use some guidance on setting up the radio. It's a Turnigy 9x. (OK, I should have known better and bought an expensive one). Anybody got any experience of this model? I guess any programmable radio would work in the same way....


What makes me a bit cross is that anything that carries the CE mark has to be provided with instructions in the language of the country it's sold in by law.  Seems Hobby King know different.

Hi Tim,
Yes alls well at present with the exception of the arthritise in my neck which gives me quite a bit of greif with shoulder pains and head aches, this is all a back lash from my early years as a design draughtsman , in those days it was a flat board on wedges and a small stool to let you get to the top.

I have been following your build and I do admire your enthusism in the wood work and the Computer drawings of K7, I have found no reason to comment other than well done , I hope that when you get her out on the water I may be of more use then.

George.

[black magic racing]

Hello George! I was concerned by your silence. All is well, it seems 




HELP!
I decided to take a break from building and check out the new radio (just in case it was faulty).
After a LOT of searching and direct contact with Hobby King I managed to get a sort of manual, but it only goes so far. With a lot of bleeping (literally and metaphorically) I have managed to enter the model name and set it up as a basic aircraft (sorry George). I could not get any response from the motor but eventually realised that the throttle is at zero in the mid position. Surely that's not right? Aircraft don't go back'ards, do they (except the ones I tried to fly)? I then managed to stop that 'model' working completely again by messing about with the throttle end points, so I'm using another memory.


Long and short of it is I could REALLY use some guidance on setting up the radio. It's a Turnigy 9x. (OK, I should have known better and bought an expensive one). Anybody got any experience of this model? I guess any programmable radio would work in the same way....


What makes me a bit cross is that anything that carries the CE mark has to be provided with instructions in the language of the country it's sold in by law.  Seems Hobby King know different.

http://static1.squarespace.com/static/51e8d916e4b0aabd69a1d776/t/545baa9ae4b0a69b3b66e12d/1415293621723/TH-9X+Manual.pdf
hope this helps?

[Tim_M]

Thanks for that Mr Magic! I've seen that one and HK sent me a link to another, slightly more understandable: http://www.hobbyking.com/hobbyking/store/uploads/529892926x11510x2.pdf

The issue I have is that I have always assumed that the throttle stick goes all the way down to stop, all the way up for flat out. My radio at the moment is only using half the travel, which means I have to put the stick in the middle to get neutral before the ESC will arm. It wasn't like that on my JR set. I have set the radio to Mode 2 so it should be right. It just isn't :(

[Tim_M]

I finally managed to get the throttle working as I want it to. Whilst the solution was simple, getting to it was not and I had to go to HK online chat to get the answers. I thought I would log the solution here for my benefit and  because others may have the same problem. Advice seems to spread virally on the net.
This applies the the Turnigy 9X v2 Tx and Rx ,, mode 2 and the Turnigy Aquastar 160.
There are two important things to understand first:
1) The Turnigy 9X 'Mode 2' transmitter is physically constructed as mode 2 (throttle stick on the left) but the firmware defaults to mode 1. When you start to set up a model, you need to go to the 'Stick Set' menu and select 'Mode 2'. Be careful; to select the setting you scroll down and then give a SHORT push on the 'menu' button. A long push (for good measure) does not save the setting! (One nice feature of the radio is that you can go to a 'display' screen and check which stick is doing what).
2) The Aquastar 160 cannot be recalibrated. It automatically adjusts to maximum throttle but the neutral point cannot be adjusted.
If you want forward and reverse, then you are good to go but I want forward only, full stick travel.
After a lot of playing around, it turns out the set up is simple.  Set the ESC to 'Forward Only' Make sure the Tx really is set up in Mode 2. Adjust the throttle stick trim to full up. Adjust the throttle sub trim to maximum positive. Leave the end point settings alone. Done.  The ESC will arm with the stick fully down and the motor will start to spin just about as soon as you move the stick up (i.e.  you have the full stick movement available)
Sorry if this is **** obvious. It wasn't to me and, judging by the number of internet posts on the subject, it isn't obvious to others either. The procedure is different, by the way, if you have an ESC that does get calibrated for a new set-up. You just need to be sure the Tx is really in mode 2 then follow the instructions for your ESC.

[Tim_M]

Lots of interesting things have turned up this week including a new servo, more glass cloth, resin and fittings and new collets for my Dremel (hint: do not put a broken off needle file in a Dremel to open up small holes  ). But nothing quite as exciting as the beauty in the pictures below.


Those who have been following the thread will know that one of my pet obsessions (I have many) is thrust washers/bearings. The only way I know to ensure a thrust washer is actually working is to allow longitudinal float in the drive train; a sliding coupling or a sliding motor mount. Back around Christmas I posted the design of a sliding motor mount as an improvement on my previous efforts. A fellow Mayhemer, who wishes to be known as Mr Nonny Mouse, on account of he would be inundated with requests, volunteered to make the mount for me.


The result is quite something. You can see the quality of the machining, sure, but what you can't see is the level of craftsmanship that went into making this. The mount slides backwards and forwards as smooth as silk and there is absolutely so sideways play. That's going above and beyond just working to the drawing.


Needless to say, I am extremely grateful to my distant friend. You meet the nicest people on here. 

[Tim_M]

There has been quite a lot of progress on the build this week, little by little. I'm still sanding to shape but things are getting close to the point where I start sanding for finish.


Most importantly, I've done a dry fit of the hardware and I would welcome any input before I start making brackets and stuff.


I've got two choices; offset rudder or in-line (right behind the prop). As the prop is offset about 15 mm starboard (I need to check the number from Mk 1), I'm planning to offset the rudder to port. It fits quite well there. I can't offset to stb (conventional) because there isn't enough room. Bearing in mind K7 is only intended to go in a straight line (apart from low speed lining up) I'm going to go with this unless somebody tells me it will be a disaster.


Yes, the motor mount goes this way up! I want to get the motor close to the floor so I can have a straight driveline  with a bit of 'down' on the prop. If I put the mount the other way up, the angle on the shaft is too much. The ESC and receiver will also be mounted high up just in case of any leaks (what leaks??). Plan A is that the ESC and Rx will be on a hinged panel so they swing up out of the way to get at the battery, which is way up front. Hard to tell at the moment but I may be able to put the ESC alongside the motor instead - it depends how the wires behave when the motor is bolted in - then I won't need the hinged panel.


My thoughts:
1) OK, I agree the prop is a tad high 
2) That rudder is HUGE. I've already cut it down as much as I can below the water pick ups. I think I will probably cut it off a bit more  re-drill the pick up holes from the front and plug as required (The water channels cross, of course, just to make it more difficult. I think if I drill at a slight angle I can get the pick ups to break out a bit further back from the edge too then make the rudder narrower.
3) The rudder isn't really in line with the prop. Will that matter for straight line running? If it needs to go further back, I will abandon part of the 2-piece bracket and make a new one. (At this rate there won't be much left of the rudder apart from the nipples and the pivot - but that's what I bought it for!)
4) I'm using a 3 mm carbon fibre push rod, only because it was cheaper to buy a set that to buy a steel rod and the fittings separately. I was quite shocked when I saw the size of the servo but it is higher torque (4 kg.cm - stupid unit!!) and faster than the old JR one I used in Mk 1. Metal gears and digital too.


OK chaps, put me right before I start hacking out holes.  


(Sorry about the quality of the photos - they're only for technical. Sorry about the feet too. They're mine, as is, unfortunately, the belly.)

[Jerry Hill]

Forgive me if I've missed things but I'll continue from an assumed position. A lot of this sounds dramatic, but I'm cutting to the chase here.


Your radio gear should ideally be protected enough to permit complete submersion of the boat. Placing it high feels right but not if the boat ends up upside down. I guess it's normal practice for displacement type models to have open gear but in my view that should be limited to displacement hulls, anything fully planing can, and probably will at some point, end up inverted. Generally a waterproof radio box will be less than half the price of a basic receiver, as much as a budget servo, and a quarter of a sound quality ESC, no contest for me. For interest the last two models I've made, and continuing forward on the results I'm getting, used waterproof servos, so the the radio box is tiny, and it's easier to seal a servo cable than a pushrod through the box.



If you're aiming for surface piercing prop drive the usual starting point is the bottom of the strut level with the bottom of the hull. If that's the intended prop it's a 'Cleaver' style which will have lots of lift, you could have an unstable back end if you run it submerged, and it'll cavitate for a pastime if running at decent RPM, if not it'll be because of the motor being pulled down, which will kill it in time if it's the right size for the motor.
If you plan to used a rigid shaft then a 'Stinger' drive may be better.


Leave your rudder as it is, it may be long but the chord on race rudders is small. Fitting the rudder to Stbd is all very well, but on a three point hydro it's less important than a mono, the boat should turn flat so the thing about keeping the rudder in the water is academic. With it to Port the wash off will compress the water a little on that side of the prop, but again immaterial as this boat will only be turning at low speed to setup for a straight run right? So it's not an issue in my view. The primary reason rudders are run to Stbd is that the boats race on right hand turn circuits, a product of the direction of rotation of the engines, which being derived from aero engines back in the day means that they are backwards facing and run contrary to the real world where most courses need left turns, for the same reason of dealing with prop torque reaction trying to rotate the boat.


Your carbon rod should be OK, it's an unbalanced rudder as far as I can tell so turning forces will be higher than a balanced or partially balanced one, but so long as your rod ends are secure to the CF it will be fine, I use them on boats with 10cc engines no problem, at least with partially balanced rudders. Here's one on one of my home made rudders.The CF rod is epoxied into a threaded ferrule for connecting to the rod end, I put heatshrink over it to colour it.

[Jerry Hill]

By the way, again this may have been covered, but are you aware that flexible shafts shorten under load? Your motor mount is lovely, a good idea, but it would be wise to bias the relaxed motor position forward to ensure there's room for the shortening. A very rough guide for a 1/4" shaft in a boat of your size is that the shaft will shorten 1/4" under full load.


Just in case 

[Tim_M]

Hi Jerry,
thank you very much for taking the time with your detailed reply. First thing that's not clear from yesterday's post is that all the hardware will, in fact, be enclosed under a waterproof hatch. There are some more frames and decking to go in once the hardware is positioned so that all the mechanics will be sealed off. The upper hull (cowling) is purely decorative on this model. I will be adding another waterproof box for the receiver too. Been there, done that, got wet legs. 


Prop height is something that has given me concern but what I ended up doing on my previous build, under guidance, was to set it about 35 mm from the bottom of the boat (same height as the forward planing shoe edges). The original K7 had a planing wedge on the stern. Under advice, I took that off and the stern lifted nicely. The back end of the boat is completely out of the water once it gets going. By the way, the CG on this model is a long way forward, perhaps more than conventional. It seemed to work on my Mk 1 so that is my starting point for this model. (I'm not after high speed, strange though it might sound. I want the model to get up onto three points and look right at a moderate speed - something I can control and get more than a couple of seconds run on a 200 m lake (with ducks. Lots of ducks.))


I looked at the photos of the rudder again last night. I'm reassured that you agree it can stay on the port side and that it really isn't much too big. Yes it's a single sided horn; that's how it came. And yes, it will increase the load on the servo (but not the torque). Have to make sure I have a decent bracket! I have a cunning plan for sealing the push rod - keep watching.


The main reason for the sliding motor mount is to cater for shaft wind-up. I've got about 10 mm free float in there. The thrust washer will touch  - in other words it will BE a thrust washer - but the shaft will never go into tension. Only wish I could make something like that myself! (By the way, that's a pretty serious boat in your photo! Awesome!)


All your comments have been taken on board. It's reassuring to know that most of them are already built in to the build plan! Many thanks again.

[Jerry Hill]

Ideal, you are clearly on top of things.

I guess you shaft will see a some tension every time you throttle off, recently I adopted a method many folk use to protect their props which has revealed to me how much tension there can be. A piece of silicone tubing is placed between the prop and the prop nut, the idea being that in an impact the prop can compress the tubing and disengage itself from the drive dogs, hence stop rotating during contact. Whilst first trying to set this up I found it surprising how much compression is needed on the tubing to stop unwanted disengagement, on throttle off the prop would release from the drive leaving me essentially in neutral, as the prop would not re-engage reliably. By the time the tubing was compressed enough to ensure this didn't happen I came to the conclusion that the prop would still be damaged so abandoned the idea. But, my point is that it revealed that the tension put on the shaft must be significant. The concept is used by racers anyway, where the odd climbing over someone elses boat occurs. I don't race, not least because of that reason,  so its not needed so much. I'm also not trying to protect £50 props 

If your original boat and this one rides on the prop alone then all the more reason to leave the rudder length, prop walk becomes a bigger issue on a flying hull. On the K7 though I'm guessing that's not such an issue, it'll just show itself as neutral trim variation.  It's common for a race boat to show Port trim when trolling or running on idle, then to straighten up under full throttle (assuming the boats was trimmed at full speed) 'driving' the boat compensates for that, but a deep rudder naturally will assist.

You come across as an astute person, and if there's anything that will allow you to really open up opportunities in model making it's having access to a lathe and milling machine. I'm a model builder, some are model assemblers, I measure modelling achievement by the rewards folk get from the activity so the approach is not the issue for me, but I like to make as much as I can and I imagine a chap such as yourself would see great benefits from some small machines. Take a look in the Sports R&D section for my build of a dummy outboard that doesnt exist commercially, it may interest you.

Loving the desert camo shorts by the way  .

[Tim_M]

Hi Jerry,
Tension on throttle down. I'll confess I hadn't thought of that! Don't think it will cause a problem though. The motor mount has a limit to its travel and that should stop the shaft pulling out of the bearing. Might jump when I start going forward again though. Mmm, you learn something every day.

Machine tools. Ah yes, one day! I still remember my days as a boiler-suited apprentice, milling, turning, welding, filing. Ooh I hated filing! I've been following your outboard thread with interest. Lovely workmanship, good idea. Inspired by that I decided to renew my knackered buffing mop (I use it to polish the burners on the cooker after they've been cleaned). I was also inspired by Toolstation prices! For £4 you get 3 small wheels, an arbor and two polishing soaps! Not pro quality but they do the job. I've also decommissioned my bench grinder. Last time I turned it on it was clear that the wheels were well out of balance - it's been in my damp garage for years. Nothing like a burst grinding wheel to spoil your day. For another £3 I got a 6" buffing wheel. It fits the old grinder a treat so I will be getting another one and use them for coarse and fine soaps.

K7 is indeed a 'prop rider'. I've given some thought to the prop position after your comments. I won't try to explain what I'm doing here but it will all become clear when I get it built and take some photos.

BTW, the flip flops are also desert camo. Nicely coordinated........

[Tim_M]

Slow but steady...............


The hull is pretty much sanded to shape (not finish) and the last couple of weeks have been spent sorting out the 'mechanicals' of the model. There wasn't a lot of point in posting every little cut and fit along the way.


Drive
Rather than a fixed tube and skeg, I'm using a centreboard. I'm sure other people have done it before. I have two pins that fit the tube; one fits the motor collet and the other fits the strut. They're both a snug fit ('turned' up in the electric drill ). The lateral position of the motor and strut are fixed (15 mm off centre). When the time comes to put the drive in place (after painting, which is a huge benefit) I set the prop depth and motor height. The prop tube finds its own line to set the a angle and the motor bearers (already fitted to the motor mount) are glued it wherever they end up. The prop tube is sealed to the centreboard box with a formed-in-place silicone grommet (aka a blob of sealer) and the skeg gets locked into place with a single pinch bolt - not glued. I haven't fitted the top, back or front to the centreboard box yet so it's easier to see how it works. It's not intended for adjustment 'on the fly' but it makes setting up a doddle and if my predictions for prop position turn out wrong I can change it without major surgery. Moving the prop on Mk1 wrecked it![size=78%]


Steering
The push rod passes through an oval tube. I'm going to fit the bellows seal on the inside of the boat because I don't want to see it. Plan A was to use a finger from a rubber glove but I have ordered a 'large' bellows 'cos it looks better. If it doesn't fit then it's back to plan A. (Not quite as daft as it sounds; you get a really soft gaiter - in both directions - using a rubber glove finger. Just seems a bit crass). Yes, the servo is mounted inverted. That's just the way everything lines up.


The second oval tube is where the water pipes will run. They will be sealed in with silicone.


I took a look at the rudder bracket. It was totally out of proportion to the strut and very heavy. I set to with the drill and file and did some pretty drastic weight reduction; there was a lot of redundant material there. I got it down from 68 g to 35 g. Not much in the great scheme of things but worthwhile as the bracket is so for from the CG. There's still ample meat left on it. If it breaks I will be a) very surprised, b) hugely impressed with the strength of my transom  . I think I will probably finish the rudder and strut in matt black. I love the sight of all the brightly coloured shiny hardware hung on the back of a racer but it isn't supposed to be there on K7. I'd like it to be as discreet as possible.


Layout
Everything has a home now. The ESC will be hung from a hinged bracket so it swings out of the way to get the battery in and out. The bracket isn't shown in the photos but it is made. This gives a nice easy run to all the cables. The receiver will be getting its own waterproof box.


Remember, all of this lot will be protected by a waterproof hatch as well, which is the next item in the build. I think the reasons for what I've done will be a lot clearer once you see how that hatch fits




 [/size]

[Tim_M]

Plodding along.


Steady progress this week but nothing very exciting to report I'm afraid. The problem is I use slow glue  . I have two types of cyano (Gorilla and Zap a Gap) but I really don't like either of them. Useful for fixing small parts or tacking but I find the joints are just too brittle (probably because they are not very good joints). Resin W for me.


The nature of the build has changed tack a little. The early part of the build was rather like assembling a kit. OK, the parts weren't pre-cut but basically all I had to do was cut all the bits to the drawings and stick them together. I'm now very much into scratch building; sorting out all the details that aren't in the CAD model and working out how to do things as I go. Fun but slow.


This week I have concentrated on building the coaming and hatch (yawn....) - a vital part of this design. I made the coaming up around some tree wood formers and then wrapped the hatch sides around them so it all fits together pretty well. I'll share something that didn't work. There is a lip around the coaming and the hatch where they meet. My cunning plan was to stretch an elastic band around that lip so it would both hold the hatch on and form a good seal. Mmm............ It worked really well on the corners but not on the straight bits. I should have seen that one coming but foresight is never quite as clear as hindsight. I still think it would work very well on a circular hatch if you ever have the need.


After some thought, I decided to substitute some silicone tube for the elastic band, split along its length (fiddly to do). The tube is stuck to the underside (only) of the lip on the coaming - one time when cyano works really well. Plan B (given that plan A didn't work) is to put the hatch in place and then pull the tube round the lip using a little hook tool. I tried it on the bench and it worked well. Not sure how it will work in situ as there is not a lot of room round the coaming. If it's not practical then Plan C is to just put the hatch down on top of the tube and clip it in place. The slit tube gives a nice compliant seal. If all else fails, it's Plan D - tape it up like everybody else does    The coaming is now fitted to the hull but I want to tidy up a bit before I photograph it - and the glues not dry yet [size=78%].[/size]

I've made the hinged bracket for the ESC and the rubber bellows has arrived. That just about fits the oval tube but it's a bit of a stretch. I may try to get some smaller conduit for the sake of neatness. All in all, I should be able to get the mechanics dry-fitted completely in the week.

[/size]I had a go with the airbrush today. Ooh, that's fun! I think even I can get a half decent paint finish with that; very controllable. I have a question though. I want to use water-based paint (partly because I don't want to use gallons of thinners cleaning out the airbrush). Hobbycraft do 'Crafters' brand acrylics; they are cheap and they also have just the right colour (I may need to mix two blues together) but the finish is matt. Anybody know of a water-based gloss that would work? Otherwise I guess I need to paint and then apply a water-based varnish afterwards. (I thought about using the Aerokote two-part resin I'm using for the glass cloth but I think that is pretty much a recipe for a ruined airbrush).

[/size]Don't worry, it will get more exciting.

[Ramon]

Hi Tim - Haven't looked in for a while but it's all looking very good indeed 


A couple of things that may be of help. I have been using TiteBond 111 (as opposed to TiteBond and TiteBond 11) for some time now. Apart from the fact that it's totally waterproof the thing I like most about it is it's extremely quick grab time 20 -30 mins in a warmish environment - can't recommend it enough and no, no connection 


Airbrushing - as someone who does a lot of it virtually on a daily basis at present a constant need to strip and clean the brush is a major part of use. Using any two pack or one pack varnish through it will not be a problem providing it's thin enough but be aware - when using two pack you really do not want to breath the fumes.


There are many acrylic paints suitable for airbrushing that you can use though may be the colour match might be a problem. Tamiya do a gloss aerosol spray with a reasonably close blue but that may not be near enough - check out Tamiya TS spray paint - but personally I would go the matt route then finish with a clear coat. There are some 'Floor Finishes' that make superb topcoats. The last I used on model aircraft was a two pack from a national paint company called Morrelli. If the airbrush is cleaned immediately after use with cellulose thinner you should have no problems - shooting cellulose thinner through the brush is not the same as cleaning it BTW 


One thing to be aware of with using Acrylic colour coat is that if applied over a heavy-ish cellulose base coat eg sanding sealer there's a very good chance it will eventually craze like an old pottery glaze. I believe this is due to the cellulose always slowly moving whilst the acrylic being very hard and inflexible doesn't - definitely got the tee shirt on that disaster 


Hope that might be of use


Regards - Ramon

[Tim_M]

My Goodness! (family forum rule  ). That hatch gave me some jip. In the end I wasn't happy with the external seal so I ripped it all off. I now have an internal ledge that may use to support a pipe 'O' ring type seal. On the other hand, I will probably tape it shut  - like everybody else.


Time got the better of me this weekend but I did manage to do a complete dry fit of the hardware. The hinged ESC mount works well; you might notice that I had intended to mount it at right angles to where it ended up (from the slots). Turned out the water pipes needed a lot more room. The Velcro strap will be a tie-wrap in the finished version. I also found the best space for the Rx was further back. I'll need to extend the lead from the ESC but it's been damaged anyway so I'll do a proper job on that later. I looked around for something interesting to put the Rx in but in the end I've settled for a simple wooden waterproof box (under construction). I shall do something to protect the servo too (not sure what yet). You can see the tube for the pushrod with its internal bellows and the second tube for the water pipes. The entire rear compartment gets sheeted over once all the rear-end hardware is fitted. The top is fitted to the centreboard box now too.


Next work is to tidy up the cowl joint and work out the latching and then it's sanding ready for glass and paint. Lots of sanding!


By the way, I have been using two-part filler where I need a strong material. I wanted to dispense small amounts onto edges and into holes and I found you can do it easily with a disposable syringe. Best bit is, it doesn't stick to the syringe so you can clean it out easily and use it again. I expect you already knew that but maybe someone didn't

[Tim_M]

Thought for the day......


Straying slightly off-topic, you have often heard me banging on about universal joints -  Hooke's Joint - and how they cause vibration. I was reading yet another post complaining about vibration from a UJ and blaming the quality of the component. It doesn't matter how much you spend on a UJ, it WILL cause vibration if there is the slightest angle on it - it's a geometric thing and you cannot get away from it. (2 works better but you still have the shaft between them speeding up and slowing down on every rev so you still get vibration).


What you need is a Constant Velocity (CV) joint, like in your car. I was thinking 'if only somebody made them for models'. Like a car - model. MODEL CARS! They have CV joints. (I even checked on the chassis I have hung on the wall as a source of interesting bits).

Me, I prefer straight lines but I'll be interested to see if any of the bent shaft boys take this one up....................

I'm sure if this is in the wrong place one of the mods will move it (please).