Triple Screw Boat

In my view increasing from one prop and motor only has generally a negligible effect on speed. Reason being that the speed is governed by the revs. It might increase slightly as the load on each motor will be reduced and the motors are able to rev higher but it won't be significant.

The advantage for more than one motor is acceleration and more power which will increase the speed of a big heavy displacement hull pushing lots of water out of the way, to a point.

I think that when some folks, particularly with planing hulls, ask about twin motor set ups they think that the motors only need to be about half the size but that isn't the case. Maybe slightly smaller than a single motor set up but that's about it.

So I think when the third motor kicks in it initially takes load off the other two and they accelerate, hence the lunge, but then the top speed is then restricted by the max revs being achieved. And of course the third prop when not running is acting as a big brake which will then contribute to the lunge!

Increasing the voltage might help a bit but sounds as they it might be over-propped as well. Any reason why you used 5 bladed props and why you're considering 4 blade? Would have thought that 3 blade and maybe 50mm would be the best bet? I bet there is a fair amount of weight in that boat as well with three motors, prop shafts and rudders etc? What battery are you using?

Chris

Edited By Chris Fellows on 15/09/2023 13:03:38

Thank you for your insightful comment. Much appreciated. Indeed, this is a heavy boat. Big and heavy. I went with 5-bladed because some of the real triple-screw boats I studied all had 5-blade props. Several years ago, I did an experiment on another boat: 4-blade vs 5-blade, both same diameter. When I switched to 5-blade, the boat gained acceleration, but lost a bit of top-speed. So, I wonder if reducing the number of blades might increase the top-speed of my triple-screw boat. I reckon with three propellers, I already have plenty enough acceleration, so reducing the number of blades might be acceptable.

The battery is a 12-volt NiMH, which I had custom manufactured so it can handle a high current-draw.

You've answered my question as regards the battery as it was the current draw I was thinking about, as when operating at full throttle even a slight drop off will affect performance particularly when marginal.

Can you get plastic 3 blade props in the size you want. That would be the best for testing purposes before investing in the nice shiny ones, especially when you need 3!

I don't know much about brushed motors but looking/listening to the video reinforces my view that it's a lack of revs which is the problem. It could be argued that a coarser pitch could provide more speed but if the revs drop as a consequence no advantage is obtained.

Chris