Topic: Bob's H.M.S. Polyphemus (Read 263610 times)

Subculture · « **Reply #50 on:** July 09, 2012, 05:33:02 pm »

If you're running the tanks completely full, or completely empty, you don't need to worry about baffles- water can't slosh unless there is air inside the tanks.

Bob K · « **Reply #51 on:** July 09, 2012, 07:22:35 pm »

Thanks Gentlemen. You are both correct, providing my calculations are reasonably accurate, and fill / empty times are as brief as I hope. To establish this my next simulation should be to adapt a 2 L plastic bottle of water so that I can pump air in then vent it using a Blagdon air valve. I guess it would do no harm to add anti slosh baffles, plus a couple of 6 mm brass pipes between tanks, as retro fitting them may be difficult afterwards.

Subculture · « **Reply #52 on:** July 09, 2012, 08:02:52 pm »

Big holes at the bottom is the key to swift flood times when you're dealing with only a few millibars of pressure differential. Slots will give a lot of area, and be less obvious underneath the hull. When you test with your milk bottle, don't forget to add a bit of weight to the bottom, otherwise it will take quite a while to flood. A spanner held on with a couple elastic bands will do the job.

Bob K · « **Reply #53 on:** July 09, 2012, 08:16:04 pm »

H.M.S. Polyphemus (1881), Waterlines

Excellent advice Andy. I was intending to use several 10 mm holes underneath, but slots make better sense. Maybe this is why WWII subs had a profusion of slots in their casings. Good idea about the spanner, easier to attach than suspended weights

Reason for Ballasting

Sketch showing ballasted and non ballasted waterlines. Blue line is max practical buoyancy for ‘safe sailing’ under moderate model boating lake conditions. Freeboard is approx 45 mm with stern and rudder just under water. Red line is more ‘true scale’ for the ship in attack mode, but is only 22 mm freeboard. A portion of the bulges are still above water which encourages waves to wash across the deck as per the real ship.

Trimmed down mode is for use in millpond sailing conditions only.

Subculture · « **Reply #54 on:** July 10, 2012, 09:38:46 am »

Quote from: Bob K on July 09, 2012, 08:16:04 pm

Maybe this is why WWII subs had a profusion of slots in their casings.

Yep, fast dive times were essential to evade air attack. Do you know how long the 1:1 version took to flood tanks?

Bob K · « **Reply #55 on:** July 10, 2012, 08:39:38 pm »

Quote from: Subculture on July 10, 2012, 09:38:46 am

Yep, fast dive times were essential to evade air attack. Do you know how long the 1:1 version took to flood tanks?

Nope. Online info on trimming down is rather vague, only that she was "partially submersible" and that the "bulging ballast tanks" were one of her most distinctive visual features. The original plans are very detailed, although without much in the way of sectional views. Here I am making some assumptions based on contemporary photos and illustrations. She would have needed sea going freeboard to reach Malta, but the picture showing her breaking harbour boom defenses (see reply #18) looks almost awash like a US Kalamazoo class monitor. Quite an amazing design feature some twenty years before the RN's first Holland Class subs.

I would guess something similar to the steam powered K class dive time average of around 5 minutes (record 3 mins 25 seconds).
However, the nice big vacuum pump with 1/4" tubing should shift 2.7 Litres quickly. As soon as I have the bulkheads cut out I will build the 2 L simulation bottle (with spanner !)

Bob K · « **Reply #56 on:** July 11, 2012, 12:05:50 pm »

H.M.S. Polyphemus (1881), bulkheads

The jig worked nicely. Card template for side bulkheads shown below. Now all I have to do is cut this shape from hard 3 mm plastic. Two off, handed.
Horizontal top sections for the tanks will form the plane for the underdeck.

From my drawing office days I used to have a Flexicurve, unfortunately long gone, but a strip of lead sheet works well to copy the shape of the lower hull profile for the bases of the transverse bulkheads.

Sectional diagram showing sloping armour backed by coal bunkers. I am guessing the actual tanks may have been on similar lines to Holland submarines, at the bottom of the bulges and maybe across the keel area too.

Note the drop keel which could be released in an emergency, and the float off life rafts.

Subculture · « **Reply #57 on:** July 11, 2012, 01:15:19 pm »

The Holland boats had a large main tank in the keel, and a smaller trim tank housed within the main tank to presumably to allow for different water densities.

They also featured kingston valves (valves on the bottom, in addition to valves on the top), which continued in use for subsequent RN submarines. I believe these were eventually abandoned, and open slots at the bottom and valves at the top was adopted. This was to allow speedier diving, which became increasingly improtant as ASW evolved.

I think Russian submarines are one of the last to have these valves fitted to their boats.

Bob K · « **Reply #58 on:** July 13, 2012, 03:28:23 pm »

Darn, after carefully copying the template profiles onto the clear hard 3 mm sheet I then carefully scribed the basic rectangles about a dozen times on both sides with a Stanley Knife, slid a block of 10 mm wood under the joint, gently pressed down - and it shattered like glass

I am going to have to think again, and maybe invest in a proper power jig saw with appropriate blades before trying again. This stuff is very hard and very inflexible, unlike regular polystyrene. Sorry. I was hoping to show an assembled bullheads set in a few days.

Subculture: Thanks for the insight into Kingston valves. I have been searching the online resources on these. Fascinating.

Norseman · « **Reply #59 on:** July 13, 2012, 08:18:08 pm »

Any use Bob? I was looking for a proper technical drawing really.

http://books.google.co.uk/books?id=goomAQAAIAAJ&pg=PA1188&dq=kingston+valves+technical+drawing&hl=en&sa=X&ei=pnMAULy_FsOb1AW1572oBw&ved=0CDoQ6AEwAA#v=onepage&q=kingston%20valves%20technical%20drawing&f=false

Dave

Subculture · « **Reply #60 on:** July 13, 2012, 09:03:50 pm »

Bob, just get some polycarbonate sheet, that won't shatter, I promise you.

Bob K · « **Reply #61 on:** July 13, 2012, 09:34:41 pm »

Dave: I just mentioned Kingston Valves after reading up on them after Subcultures interesting post about them.

Subculture: OK, just feeling a bit down right now. Never had problems cutting polystyrene before. Usually not this hard and brittle. Model boats R&D can often feel like three steps forwards and two steps back. You know what they say about omelettes and broken eggs.

Subculture · « **Reply #62 on:** July 14, 2012, 11:16:55 am »

The qualities of plastic can vary enormously. I see from an earlier posting you purchased your plastic from B&Q, and that's a bit of a red flag, as the stuff DIY stores sell does tend to be rather brittle.

If you want certainty stick to brands, ICI perspex for PMMA/acrylic, Lexan or makrolon for polycarbonate etc.

Bob K · « **Reply #63 on:** July 14, 2012, 03:29:13 pm »

Quote from: Subculture on July 14, 2012, 11:16:55 am

The qualities of plastic can vary enormously. I see from an earlier posting you purchased your plastic from B&Q, and that's a bit of a red flag, as the stuff DIY stores sell does tend to be rather brittle.

If you want certainty stick to brands, ICI perspex for PMMA/acrylic, Lexan or makrolon for polycarbonate etc.

It seemed like a good idea at the time as I could see it and buy it locally, rather than ordering enormous sheets online and the hassle of vague delivery dates. All part of the learning curve. Whatever did we do before internet and Google? I will do some searches based on those brand names, and sizes in between A4 and "garage roof". At least I have now invested in a Bosch jigsaw as fret saws etc not much use on big sheets, although I may try using a fret for smaller pieces and under-deck cutouts etc..

Subculture · « **Reply #64 on:** July 14, 2012, 06:54:45 pm »

Here you go- http://www.ebay.co.uk/itm/LEXAN-POLYCARBONATE-SHEET-3MM-CLEAR-210MM-X-297MM-A4-/120745535546?pt=LH_DefaultDomain_3&hash=item1c1cfea83a

Wouldn't bother going beyond 3mm thick- the sheet doesn't need to withstand water pressure, just needs to be watertight. 2mm would probably be okay actually.

Bob K · « **Reply #65 on:** July 14, 2012, 08:04:11 pm »

Thank you Andy, I was just searching on those brand names you quoted. I have opted for 1000 x 500 sheets from the same supplier, one in 3 mm for the side bulkheads which are 820 long, plus two sheets in 2 mm for the under deck and hatches. You are correct that 2 mm would probably be OK but I have gone with 3 mm to allow for any hull flexing. It has to stay fairly airtight.

Once the transverse bullheads (cellular 5 mm Marlon Polycarbonate) are secured the internal boxed structure should be be very solid. Using clear material will allow me to see what is going on for testing and commissioning.

RAAArtyGunner · « **Reply #66 on:** July 14, 2012, 10:50:25 pm »

Bob,

It has been mentioned by some members that "sheeting" can be obtained 'locally' if you have plastic sign makers nearby.

Plastic and illuminated sign makers us the brands mentioned.

You can generally get off cuts and sizes which are too small for signs but OK for modelling use and doesn't cost the earth.

Bob K · « **Reply #67 on:** July 14, 2012, 11:37:19 pm »

Good idea for a source of low cost offcuts. Usually rigid signs tend to be a lot thicker, and rarely clear, but usefull information which I am sure will come in useful.

RAAArtyGunner · « **Reply #68 on:** July 15, 2012, 12:03:19 am »

Bob,

My experience here in OZ is, have been able to get clear perspex in thin sizes 3mm, such as used in built up signs, lettering etc, which are colour back sprayed, which I used to build several disply cases for models.

Sizes varied from 1200mm x 300mm width and in between.

Was able to select the "bits" I wanted from a large heap of clear offcuts at a bargain price.

It also still has the protective film intact which aids in marking out and protects the surface when handling and re cutting to size.

Way to go.

Bob K · « **Reply #69 on:** July 15, 2012, 12:38:04 pm »

H.M.S. Polyphemus (1881), build notes

All comments and suggestions welcome.

Although I could just post pics and notes on each completed stage I am attempting to record a more complete process (including occasional setbacks) to show the development of an unusual non-kit model. Hopefully this R&D effort may be of some use to others. Previously I have only done bulkheads in ply or thinner ABS.

For anyone interested in building this ship it would of course be so much easier to have a higher fixed freeboard and just go ahead mounting the motors shafts and electrics as normal. However, I am game for a good challenge. It does mean though I tend to spend more time in research, trialling and jigs etc, than construction.
Figuring out how to create the next bit is half the fun.

So please bear with me. I estimate this one could take around six months, bearing in mind it is 1/60 so just about all fittings etc will have to be scratch made.

Thinking ahead

Dry dock view of the ram with torpedo tube bow cap, also showing the twin forward rudders for close manoeuvring inside hostile harbours.
I understand they were not very effective, but that was generations before bow thrusters were invented.

joppyuk1 · « **Reply #70 on:** July 16, 2012, 04:29:28 pm »

I'm enjoying reading of your tribulations (the read, that is, not the tribulations) in building this unique vessel. One of the reasons I never got started on mine. I was browsing the other day, and in John Beeler's BIRTH OF THE BATTLESHIP there is a photo of her at speed and this shows the very low freeboard to good effect. Apologies if you've already spotted it. This book has various pictures of Polyphemus scatered throughout, most of which are the usual ones seen before, and a good bit of text.
IJ

Bob K · « **Reply #71 on:** July 16, 2012, 08:10:49 pm »

Thank you Ian: Builds like this keeps the ‘little grey cells’ rust-free. I probably will not find many of them at club events, and it will be fun to sail with the smoke unit and multi cylinder steam engine sound module.

That book sounds like a “must have” book for this project. I located one on Amazon and ordered it. Thanks. Is this the illustration you referred to?

Almost awash, more like a surfaced submarine at speed.

joppyuk1 · « **Reply #72 on:** July 16, 2012, 08:36:05 pm »

Not that one, which looks like a 'breaking the boom' illustration. The one I mentioned is a photograph, fully broadside, and the bow is hidden by spray nearly to the superstructure.
IJ

Bob K · « **Reply #73 on:** July 16, 2012, 09:55:33 pm »

The above illustration depicts the Berhaven harbour raid exercise of 1885, so an actual photograph of her broadside on at speed would be invaluable. I eagerly await the book to arrive. The shape of the bows would appear to encourage water to build up towards the f'o'c'sle at speed, and that the top of the side bulges being above the waterline even when flooded down would cause water to ramp up over the deck at 17 kts especially in a quartering sea. Under these conditions I doubt if either f'o'c'sle or main deck would be tenable on an attack run, the ship having to be operated from flying deck level and the armoured conning tower.

Hopefully, if i get everything right on the model, lake trials should replicate this.

Bob K · « **Reply #74 on:** July 19, 2012, 02:06:29 pm »

H.M.S. Polyphemus (1881), ballast rig bath test

Before installing the ballast system controls it was necessary to build a test rig to check them out. An empty 2 litre diet drink bottle (I am following medical advice) fitted with a Blagdon pond style air valve and a pipe to the air pump. Square slots cut into the bottom plus a ½ inch spanner held on with elastic bands to keep it upright and provide some mass. Although pump is 12V test carried out using 6V.

With air valve closed no water ingress for ½ hour. Passed.
With air valve open it sank in 75 secs.
Close air valve, turn on pump. It fully blew in 45 secs.

With one air valve on each fitted ballast chamber operated by a common servo, and the pump connected to both chambers, this should fill the full 2.7 litres in 51 seconds and blow it in 61 seconds. Maybe I should opt for the full 12V. If my calculations on centre of mass for the ballast are not spot on then trimming will be interesting. If only I still had access to 3D software.

Broadside on at speed.
Was this the picture Ian ? Shows challenges of low freeboard operation well.

“Polyphemus under way. Her hull was designed so that water would flow over it when steaming at speed”

Clear Lexan sheets have arrived.
Now I can get on creating the bulkheads from the card templates. More soon.

News:

Author Topic: Bob's H.M.S. Polyphemus (Read 263610 times)