Motors and props (again…)

[project_starterParticipant @project_starter]

Hello,

although this question has been posed many, many times before (and I've spent many hours reading all the answers here – thanks to all who have taken the time to reply in the past), I'd be very grateful if the collective wisdom of this forum could be brought to bear on my problem.

I was recently taken by a magazine article detailing the building of a much-simplified dynamic-diving submarine. I'm in the process of designing my own version based on the principles from the magazine (essentially a squared-up hull, small, sealed equipment pod and all ply and balsa construction) and am now considering motor options.

My model will be 1m long with a beam of about 60mm, based on a type-ix u-boat. As the full-size has twin motors, I'm designing mine to have the same, despite the complications and potential over-powering this implies. At 1m scale, the largest propeller possible is 20mm diameter and, as a nod towards authenticity, I'll be using 3-blade props, probably from Raboesch.

My current plan is to use 2845 in-runner brushless motors on separate 30A ESCs. It's tight inside the hull, but there's just room for a 2S 2200mAh battery – running time will be about 4 minutes at 60% duty-cycle with these motors. I'm a bit undecided, though, on whether I should go for higher or lower-speed motors. Even though the props are tiny, to my mind I don't want to spin them like mad and just thrash the water up behind them, but maybe I'm wrong?

Something like a BM400 from Overlander would give 2100KV, so about 15500rpm unloaded at full power. The Turnigy XK range could give as low as 1980KV for about 14600rpm or a 3650KV version for about 27000rpm. I prefer brushless motors in any setup, but would brushed be a better choice here?

Any thoughts and opinions would be very gratefully received. Apologies if that was all rather long and meandering – I suppose a tl;dr would be:

What's best for 20mm props, slow or fast?

What's best for props on a submarine, slow or fast?

Thanks very much

[project_starterParticipant @project_starter]

What’s the best motor config for a given prop size..?

[Brian Sweeting 2Participant @briansweeting2]

I am by no means an expert on these things but could a single motor running two shafts be an option.

It might give you more room inside and also allow some additional gearing if needed.

Just thinking out loud…

[ashley needhamParticipant @ashleyneedham69188]

I don’t think you want the props to go that fast! Once the sub is decks awash it won’t have to go very fast to dive.

A pair of speed 280 or 6v capable 280 can motors probably all you will need.

My Oberon dynamic dive sub, 3 foot long has one 35 (or might be a 40mm) prop and a “cooking” can 500 low power motor and on a 7.2v Nimh I barely use 1/2 throttle if that.

Ashley

[project_starterParticipant @project_starter]

Hello both,

thanks very much for your thoughts.

Your suggestion, Brian, of using a single motor and gearing, is a good one – I'll have to explore that further, I think.

The behaviour of your sub, Ashley, is helpful to hear about – I've never ventured below the surface before, so quite how a sub performs is all new territory to me. It does sound like a much lower power solution is required, here…

Thanks very much for your time and thoughts,

Paul

[Richard SimpsonParticipant @richardsimpson88330]

With this type of model you are very much caught in a compromise. If you want it to look realistic on the water and sit at a fairly normal surfaced water line you will need a lot more motor power than you will if you are happy to accept a less than scale look and go for only a very small amount of positive buoyancy. This would require a lot less power and, can actually be argued that the forward speed is far closer to scale to get it to dive than the water line model would be, which has to go a good bit faster than scale speed to get it to dive.

I think most people, as Ashley suggests, go for a decks awash, only slightly positive buoyancy, to enable smaller motors to be used to get the model to dive and look that little bit more realistic in the process.

I once bought a Robbe U-47 second hand, which sat lower than normal but higher than decks awash. The thing went at a ridiculous speed to get it to dive but when it did the bow suddenly shot downwards. I remember once ending up with the bow stuck in the mud at the bottom of the pond with the stern sticking out of the water! Very slowly it broke free!

[ashley needhamParticipant @ashleyneedham69188]

Balance and lowness of immersion are key. Been there with Richards scenario.

Its also not appreciated how much depends on the temperature of the water in affecting buoyancy.

If I play with my sub, I probably spend 20 mins fiddling with very small weights before it behave itself.

Always, in the centre of the pond,it dives very easily due to it being colder.

Ashley

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