Newbies questions

Hello All,
as a total Newbie to the world of model subs who is about to start a scratchbuilt submarine project because I love challenges, I'd like to ask "Those Who Know" or indeed anyone who just has an opinion ,a couple of questions that so far I've been unable to find any answers to in this Forum.

1/ In general terms is there a formula to calculate the power requirement for a realistic speed of a 4-6 ft model ? Obviously I understand that much depends on the hydrodynamic 'cleaness' of any hull, it's length and displacement, or what size or wattage of motor[s] do you guys/galls use on say boats of say 4ft or 6ft length?

2/ I have seen somewhere that the ballast tank should have approx 15% extra capacity to be able to cope with water density variation due to temperature/waterborne crud/ hull mods or repairs etc. Does this approximation hold true for all systems ie cylinders/bags/ air/water?

Many Thanks,
Trevor

Hi Trevor,

I think that there is a danger with models of trying to get too mathematical and precise. Scale effects are so large (just things like surface tension for example) that using design principles for full size boats does not really work.

With a model you are not trying to optimise a propulsion system, you are having to choose an electric motor that is available (unless you build your own!)

That said, wetted surface area and fiddly bits of superstructure have to be taken into account when choosing a motor. Also what speed "looks right" for the model. (Don't get me started on scale speeds - which one would you like to choose?)

For me the the rough right answer for this model size, fairly clean hull would be a 555 motor, geared 3:1, probably drawing less than a couple of amps on 12v. Others please comment!!

Again temperature variations tend to get swamped by everything else. Ballast tank size is determined by how much superstructure you want to show!

Have you had a look at Norbert Bruggens book - an excellent and fairly mathematical approach to model design?

David

Hi David ,
thanks for your response, quite helpful. I certainly don't mean to get bgged down on theory and calculations, my [aero]modelling over the past 60 years when not building from someone's design has been of the " ILAR " approach and as a former aircraft engineer understand about scaleing of speeds and the unscaleability of air and water, so what I really get to is to allow for a good representation of the real thing. I have seen some videos of subs travelling at ridiculous speeds, more like a torpedo!
Your suggestion of motor and gearing has reinforced my 'guestimation' and that a motor that is a bit big is beter than one a bit small, and there are a heck of a lot to choose from. 12volt at 2amps running gives me a good starting point, for battery size etc.
As I'm building a fully submersible boat I wondered about reserve tank/bladder capacity and the environmental factors that might effect that.
Thanks for the suggestion of the book which I shall see if I can find,we are avid readers as our house has been an Idiot Lantern free house for the past 40 years

Trevor

Hi Trevor,

Just to give you some "proof of the pudding" the drive configuration is the one I used for my R class which is 5 foot 3 inches long but has a very "clean" hull.

You can see a video of her in action so that you can gauge the speed: (She is the one with the tall conning tower coming on after the K class)
https://www.theassociationofmodelsubmariners.com/t293-k-class-r-class-and-type-ix-at-barrow?highlight=R+class

Clearly my memory is failing me because I gave you the wrong motor (Actually a 777) and the current was a little more. The following is an extract from the AMS magazine article on the R class:

Drive Motor
This larger model needed more oomph than the otherwise beautiful Buhler motor could provide so I used a 3 to 1 geared (Graupner gears) 777 motor (From Model Motors Direct, Somerset) to a 1.75” prop. This draws about 2-3 amps in the water. The motor mount was the one featured recently in “In Depth” and I found that a geared arrangement with the shaft running alongside the motor produced a more compact arrangement than the direct coupling I had used previously.

David

PS Norberts book "Model Submarine Technology" is out of print but worth obtaining second hand.

Good Morning David,
Thank you for posting the link of one of your boats in action, just about what I had in mind ,certainly no faster, although the ability to have reserve power is of course always useful.
I've trawled around Amazon and ordered a copy of Bruggen's book which should be arriving in a day or two.
Trevor