Ideal prop angle through the hull

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22 May 2020 at 12:33 #88238
Joe Beckett
Participant
@joebeckett71895
Afternoon All,

Huntsman build is going well, skins on, superstructure started. I will post a build log when I work out how!

I am starting to think about powertrain setup. I have done this very rough mock-up for starters.

Top blob being the motor, one of these;

Bottom blob being the propeller.

This will all be duplicated as I as I'm going for a twin motor set up. Question is, what is the ideal angle for the prop to go through the hull? I've been told somewhere around 11-12deg? Looking at this picture

of the actual boat, it looks to be more than that? My thinking is the greater the angle, the more the bow will be pushed down so I need to keep it minimal?

I would like to fit the motors as close to the stern as possible, ideally just under the raised rear section of the rear cockpit floor, this will keep it as close to the full size as possible. In my mock-up, the distance between the motor & prop is 24cm, giving an angle of approx 12deg whicj puts the motors where I want them.

I'm also going for Radio Active HD couplings and there should be enough free play to fit these in.

Can anyone see any flaws with this idea before I drill holes in the hull?!

Thanks , I really appreciate the feedback.

Joe
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22 May 2020 at 12:33 #2920
Joe Beckett
Participant
@joebeckett71895
SLEC Fairey Huntsman 47″
22 May 2020 at 15:18 #88245
ashley needham
Participant
@ashleyneedham69188
Joe. Everyone but me has built a huntsman, or some sort of Fairey. I have no doubt someone will tell you EXCTLY what to fit, but in the meanwhile, that water cooled motor is no good as it has much too high a Kv rating. In theory, a motor running at a comfortable load does not need cooling.

Propshafts ideally should have a shallow angle, but again, someone who really knows will advise. Hold off until the Fairey guru, he who wears the loincloth of balsa (DM) advises.

What design/plan is the boat using?

Ashley
22 May 2020 at 17:48 #88254
Joe Beckett
Participant
@joebeckett71895
Hi Ashley,

That's good advice, thanks. The boat is the normal SLEC kit, designed for 1 motor. DM says it was originally designed to run on a large glo engine but I want to try to replicate the real thing and fit twin engines. I am admittedly a rank amateur, especially at the electronics side of things but from the research I have done, the kv rating is the maximum revs per volt for that motor. A Ferrari has lots of theoretical power but is not driven on the red line everywhere I chose this one because it is an outrunner (therefore more torque than an inrunner) and has a smallish can size that should help the prop angle. By having 2 motors, in theory, you halve the amount of effort required by each motor., hence they should run cooler but the cooling is an added bonus really as I also want to have cooling water coming out of the exhausts, so I may as well use it! I'm more interested in this thread about prop angles and the effect on a model boat of changing that angle and what the maximum you can go to (like the real thing about 15-20degs?)

All info is useful. Thanks again.
22 May 2020 at 18:10 #88256
Dave Milbourn
Participant
@davemilbourn48782
I'm sorry but I'm afraid I can be of little help on this project. I've never built a twin-prop H31 and I don't suppose I ever will. Even 'gurus' know the limitations of their ambitions!

As regards the prop shaft angle there are two very different camps – one says it doesn't matter that much and the other maintains the purity of zero degrees. Like the designer of the real thing, I'm in the former group. 15° works on the 1:1 scale version and Alan Burnard was far more qualified than I'll ever be.

For what it's worth I think that 2800kv is daft without some serious reduction gearing – like a Ferrari – so why complicate matters? Also I don't think that those couplings willt stand that sort of load for more than a few minutes.

Balsa loincloth?? What is he on???

DM

Edited By Dave Milbourn on 22/05/2020 18:42:14
22 May 2020 at 19:31 #88264
Paul T
Participant
@pault84577
We have been down this well trodden path many times before as the discussion of the angle of dangle pops up every twelve months and the 'gurus' have firmly entrenched ideas on which is correct.

Of course only one side can be correct and that is, obviously, the purity of zero degrees, propellers are designed to operate at maximum efficiency on a perfectly horizontal shaft, every degree away from zero equals a two percent loss of efficiency.

2800kv is an impressive amount of power and coupled with a 15deg shaft should let the boat perform cartwheels down the lake.
23 May 2020 at 10:09 #88278
Malcolm Frary
Participant
@malcolmfrary95515
A 2800KV motor under load will probably need the water cooling as it will be producing a lot more heat than thrust at the revs that the prop will let it achieve. Probably a more valid efficiency consideration than any very minor shaft angle change. Depending on the voltage intended, probably something in the range of 1000-1500KV. The actual power rating of the motor(s) is another consideration and set of numbers altogether.

On a hull designed to plane, lifting the back end by putting power through an angled prop does not automatically shove the front end down – the resulting forward motion of the boat, due to the shape of the hull, causes the front end to lift in compensation. That way, the boat rides level.

When doing a scale model, it is usually a good idea to follow the original design where possible. If it was a successful boat, the designer probably knew what he was doing. Horizontal shafts are great on tugs and trawlers, where there is enough hull under the water to give the room needed. Horizontal also works great if using a surface piercing prop. The Huntsman is neither.
23 May 2020 at 10:33 #88280
Charles Oates
Participant
@charlesoates31738
Hi Joe, part of my learning curve with brush less motors included a high kV motor, just because it was a bargain.. Don't go there, I had a ridiculous amount of revs, the props kept cavitating, or whatever the correct term is, and the model was useless in the water. 12 to 1400 kV is about right based on my models. If you install them now, you'll just end up taking them out again with all the problems that entails.

If you want water coming from the exhausts, just fit a scoop and restrictor, they would look a bit silly with the amount of flow from a water cooled motor.

As to the prop angle, your choice, but leave room for playing with a larger prop so you can experiment. I recommend you settle on prop clearance after a rethink of the motor, because that might affect the prop size.

Never fit a coupling that might just do, follow the advice on here and get a butch one, the shock load on a drivetrain can be huge.

People on here mostly offer advice based on mistakes we've all made to try and help you avoid the same ones, 'nuff said.

Charles
23 May 2020 at 11:05 #88282
Joe Beckett
Participant
@joebeckett71895
Good morning All,

Thanks Dave, Paul, Malcolm & Charles for the very comprehensive (and really useful) replies, this is exactly the kind of information I am hoping to find.

Hull angle is now sorted in my mind, which is a relief! My other post was on motors but as we're here…

Would anyone care to give advice on an actual set up? The model, as you probably know is 47" long and will be about 6-7kg when finished (I weighed the skinned bare hull and it is already 2.25kg!)

If I was fitting a single motor,it would be a ….. with Kv of…. and Watts of….using Lipo ?S.

I am fitting twin motors, so is it as easy as halving everything to give me the sum total of the single motor?

What are your opinions on the Turnigy 3720? (Outrunner,1500Kv, 890W, 60A). Would 2 of these be too powerful? I'm not stuck on Turnigy, so any other recommendations gratefully received.

Once again, thanks for all the input, this forum really is a goldmine of information!

Joe
23 May 2020 at 15:07 #88300
Chris Fellows
Participant
@chrisfellows72943
Joe

I'm not being funny but we advised you before and you ignored our advice and are planning on using totally unsuitable motors.

Best advice I will give you now is to ask UKMike on his build that I pointed to you before what motors he is using. Same hull, twin motors and Mike selects materials and components based on sound reasoning and research. His motors fall within the parameters we were suggesting.

Chris
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