Triple Screw Boat

[ashley needhamParticipant @ashleyneedham69188]

That`s really nice Q, really nice, well done…and even better as it is a `head` build.

. Just the right amount of detail, great colour scheme, very neat.

A couple of pictures of it at speed would be good.

Ashley

[Chris FellowsParticipant @chrisfellows72943]

As Ashley says very nice. You certainly had room for triple screws!

How does it go at speed?

Chris

[Queequeg QuintParticipant @queequegquint38270]

I will get some more action photos next week.

As for how it handles, with all three motors running, the turn radius is miserable. It makes an extremely wide turn because the thrust of the three propellers nearly overpowers the three rudders. So, we have set it to run the center-motor independently of the two port- and starboard-motors. If I deactivate the center-motor on turns, it turns very nicely.

More pictures:

The three 5-blade propellers on the Dolphin look like a treasure of gold just beneath the stern!

Engine room or Torpedo bay!

[Chris FellowsParticipant @chrisfellows72943]

You've sorted it anyway, but the rudders do look as though they'd benefit from being a bit longer. As you say nice props!

Meant to say before, impressive workshop!

Chris

[Queequeg QuintParticipant @queequegquint38270]

[Queequeg QuintParticipant @queequegquint38270]

Here is a video of my first run of the boat. Boat is very stable. Steering is good, even at full throttle. However, the boat is not as fast as I had hoped. The battery might have needed a better charge, but what i discovered is this: on two motors (port & starboard), the boat runs plenty fast. Activating the center motor adds only a little bit more speed. I was hoping for a significant increase in speed upon activating the third center motor. The bow does lift a bit when you activate the third motor, and the boat makes a lunge forward, but it is not significant. I will try again with a fresh charge of the battery soon.

I'm using three 5-blade propellers with 55mm diameter, being turned by three MFA Como 800 motors on 12-volts. I wonder if switching to 4-blade will increase my top-speed a bit.

[Chris FellowsParticipant @chrisfellows72943]

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

[Queequeg QuintParticipant @queequegquint38270]

Posted by Chris Fellows on 15/09/2023 12:47:13:

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.

[Chris FellowsParticipant @chrisfellows72943]

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

[foamfoilsParticipant @foamfoils]

Hi Chris: My name is Steve from Hamilton Ontario Canada and have been into flying for over 40 years but due to my age now 69 and getting tired of crashing it’s time for me to come down earth or should I say water. I have a 1 meter sail boat and a Billings police boat with 2 screws that works well enough for me to have fun with but I recently purchased a Graupner Weser fire boat kit 43″ long with 3 screws on 3 550 motors. I read your article on your 3 screw 3 rudders and found it very informative, my boat has 2 rudders with the 3 screws , rudders only on the 2 outboard units no rudder for the center screw. I would like to try and do the same thing you did with being able to shut down the 2 outboard motors and run the middle motor only. If it’s not too much trouble to ask could you please draw up a schematic drawing of how you managed to do this, and what ESC’s you used for a boat, I am only familiar with aircraft equipment and have to learn all the equipment for boats. You can post it in the forums or email me direct to [email protected]   I will be using my Spektrum DX6 radio for this as I need all 6 channels to make all the bits work but if the setup requires more channels then I will be shopping for a new radio. Thanks for letting me ramble on and for all the help you can offer.  Steve ( formally from Manchester UK )

[ashley needhamParticipant @ashleyneedham69188]

Q. As a cheap easy try out I would get three x50 props to try. You may have to search for handed ones, but they are inexpensive and although two blade, may actually give more power than the larger 5 blade ones as the motor will rev more.

looks good though, and stable for such a large boat. The small speed increase when adding essentially 1/3 more power is undoubtably due to the fact that to go twice as fast you need 8 times more power, not just twice the power due to the resistance of the water and air increasing non linearly to the speed (without looking it up, but I think it’s right), especially  more so with a heavy boat like yours.

Ashley

[foamfoils]

On foamfoils Said:

Hi Chris: My name is Steve from Hamilton Ontario Canada and have been into flying for over 40 years but due to my age now 69 and getting tired of crashing it’s time for me to come down earth or should I say water. I have a 1 meter sail boat and a Billings police boat with 2 screws that works well enough for me to have fun with but I recently purchased a Graupner Weser fire boat kit 43″ long with 3 screws on 3 550 motors. I read your article on your 3 screw 3 rudders and found it very informative, my boat has 2 rudders with the 3 screws , rudders only on the 2 outboard units no rudder for the center screw. I would like to try and do the same thing you did with being able to shut down the 2 outboard motors and run the middle motor only. If it’s not too much trouble to ask could you please draw up a schematic drawing of how you managed to do this, and what ESC’s you used for a boat, I am only familiar with aircraft equipment and have to learn all the equipment for boats. You can post it in the forums or email me direct to [email protected]   I will be using my Spektrum DX6 radio for this as I need all 6 channels to make all the bits work but if the setup requires more channels then I will be shopping for a new radio. Thanks for letting me ramble on and for all the help you can offer.  Steve ( formally from Manchester UK )

 

Steve, it is interesting to note that there are actually very few dedicated model boat orientated radio sets available.  In fact the only ones I can think of are the Robbe F-14 and F-16 units.  99% of us use a generic radio, most of which are designed for model aircraft use.  Your Spektrum is a perfect example, with all the functionality required to operate either fixed or rotating wing aircraft.  The plus side for an ex aircraft modeller is that you are probably far more familiar with the mixing and programming capabilities of the radio than most model boaters will ever be!  I have a Dx6i and a Dx6e but still think mixing is something my wife does when she is baking.

Formally from Manchester but now in Ontario!  You lucky person.  I used to live in Manchester, Sale to be exact, and spent my early years on cargo ships across the North Atlantic between Manchester and Montreal.  I have always thought one of the best places to live would be Canada.  An absolutely beautiful place.  I bought quite a few christmas tree decorations in those days, real ceramic and wood, so I get a reminder every year when we decorate the tree.

[Colin BishopModerator @colinbishop34627]

Steve,

A lot of us use products from Component Shop who sell the ACTion Electronics range of speed controllers, mixers and many other electronics items.

https://www.componentshop.co.uk/

Useful examples of wiring applications using their gear can be found on this archived site:

http://vp-airsoft.co.uk/ms.html

Colin

[Author]

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