Print

Please login or register.
1 Hour 1 Day 1 Week 1 Month Forever
Login with username, password and session length.

[ayrshire andy]

Hello folks,

As so many of us have been so deeply into the subject of making the 1/12 Severn go like stink, I thought that a few figures and a gentle equation might be illuminating...the 1:1 model is powered by 2 mighty Caterpillar diesels each rated at 1250SHP at 2300rpm, which, at a conversion factor of 745.7 Watts per SHP, equals 932125 Watts going through each of the two prop shafts..but in our 1:12 scale models, assuming the drivetrain and propeller efficiencies to be as high as in the fullsize vessel-of which more later!-the theoretical power needed at each prop shaft is equal to 932125 Watts divided by 12 cubed which equals 539.42 Watts...however due to a number of complex hydrodynamical factors relating to the unshrinkability of water and other scale effects, we need to add a generous margin to that figure to ensure that the correct dynamic scale speed and wave pattern is achieved by our dinky little models, perhaps something in the region of 30-55% as an educated guess (any advances on that please anyone? Naval architects and other prop experts your Forum needs you!).

Having just worked out that we need something like 700 to 840 Watts per shaft in order to achieve the desired scale level of go-like-stinkiness, and knowing that the full size beast reaches 25 knots (28.79 MPH) flat out, we can work out the model's correct dynamic scale speed as being equal to the full size speed divided by the square root of the scale factor, ie 25 divided by 3.464 which equals an impressive 7.217 knots, or in landlubber money, 8.31 MPH !
Of course if all that sounds a little bit safe then we can apply yet more power however the author accepts no responsibility whatsoever for RNLI lifeboats taking off in UK airspace...PS did you know that you can now buy a brushless motor and suitably matching ESC which will give a continuous power output of around 1000 Watts for under £80 ? or 5000 Watts for under £180 ? we have the technology...and the car battery for cheap long run times if you don't deep discharge...adieu mon speed freaks![ /i]   

[johno 52-11]

Having been involved in getting a Severn to perform http://www.lifeboatmodels.co.uk/severnspeedtesting.htm there are a few things I would add to what you have said. The scale props on a model Severn are 75mm and to get the boat to perform at scale speed these need to run at about 6000 RPM. You cannot fit bigger props because of the hull shape so you need to run the model props faster to get the boat to perform. Running on 18V each of the motors we are using draws about 15-18 amps about 300 watts.  We are using 12amp SLA batteries and get around 20 mins of decent running. If as you say we need 700 to 840 watts per shaft then of SLA batteries you would have a running time of less than 10 mins assuming the batteries were able to supply the amps? The other thing that you need to take into account is weight a 1/12 Severn needs to weigh less than 25Kg to float to its lines. We are running with about 6 Kg of batteries which puts us down to its lines so there is not the weight capacity for the  "car battery for cheap long run times" if you want to increase your running times then you need to look at high capacity NiMh D or F cells or even lipo's.

Regards

John

[ayrshire andy]

Hi Johno,

Thanks for the detailed info and correction on the "cheap long run time" hypothesis , though I suppose that even 10 mins at full power would be long-ish for the fast-electric folks! Perhaps our experiences and calculations will spur debate and further development in the areas of structural weight reduction and increased motor efficiency, because if, taking your excellent 1/12 Severn figures as an example, we could reduce the model weight by 21% from 19kg down to 15kg, we could then raise the battery weight up by a whopping 67% from your current 6kg up to 10kg, which is in the region of the lighter car or leisure batteries and would, props allowing of course, allow you to raise your maximum shaft power into the 500 Watt region while preserving your excellent "20 mins of decent running" without recourse to the more costly cells which we often need. The brushless option is worth exploration due to their generally higher electrical efficencies and power/weight ratios though we would have to carefully match the motor kV to the prop to obtain what might be the ideal at least in some regards, two relatively small, light, efficient and powerful motors each driving its shaft directly...reducing motor weight, heat loss and drivetrain inefficiency...and reducing strain on our poor batteries and pockets!

Hearty congratulations on the excellent Severns and for showing what can be done!

Ayrshire Andy

[fatcat123]

Hi,

As for the weight issue, myself and Mike P have had the same issue of excess weight on the trent he is building me. He ended up building a superstructure from thin wood which is a lot lighter than the original grp superstructure..

Dan

[ayrshire andy]

Hi Dan,

Good luck with the Trent build, I sympathise with the weight issue as an occasionally slightly overweight boat builder (probably need to lay off the chocolate...), can I recommend the use where possible of a sandwich material? As the rigidity of a panel is proportional to the cube of its thickness, (all other things being equal) if you could have a lightweight relatively thick core sandwiched between 2 thin strong dense outer skins, you could have have both rigidity and light weight... checkout a cracking material called Selkie Board at www.rearo.co.uk, the foam-cored version is light and rigid though a bit pricey...may help all you builders who need to save a few kilos at the expense of a few pounds?!

Best wishes,

Andy