Prop angle on a hydroplane

Hi.
I am new to this forum

I am designing rc model boats (speed hydroplanes).
I allready build two of them. The third one is the largest soo far:



The plan is to scale up my models to the real 1:1 boat.

On the topic...
Before I build my own boat is gona take me a awhile.
But I like to discus the project before it hits the drawing board.

I am planing to use a Subaru 2.0Turbo gas engine (or something with 250HP) in a homebuild hydroplane.
The question is:
How to find a right angle for the propeller?
The model boats have Flexshafts that are adjustable with hight and angle:


On the model the angle is between 0 and 4 degrees.
But on the real hydroplane it seams to be much larger angle:


Is any possiblitity to have a ajdustable angle or not?
The problem is if (in the future) I want tho change the angle I must reposition the whole motor and the strut

Thaks for brain storming with me.

Matej from Slovenia

 

The big problem you will find in moving from RC boats to full scale is the power to weight ratio. I recently built a fast electric monohull along what I thought were reasonable hull-lines. The big problem we had was that over about 75% throttle (6-cells, fully charged) the boat was flipping over (in roll), due to the prop torque. This was not a failing of the hull, as much as the submerged drive lifting the entire hull out of the water, so there was nothing to react the torque.

Deeper V boats with surface drives were not quite as quick (when we were upright), so the next challenge is to react the torque without significantly adding to the weight or resistance.

Pics are here: http://openpilot.engineering.selfip.org/gallery.php?dir=RST_Racing
Length is 590mm, with 6 NIMH cells and Speed 600. Weight was about 930grams and construction was 3mm ply bulkheads and major longitudinals, 1.5mm spruce elsewhere and 0.4mm ply skin.

In the full-scale boat, you will not have quite as much power, and therefore you might want the shaft angled to help lift the boat (or at least the stern) out of the water.

How big are your models? The one you're holding looks about 900mm. I think you could remove some structure to reduce the weight. Lightening holes in the bulkheads, and perhaps reduce the amount of ply in the longitudinals. Low weight is everything in model boats, even if it sometimes causes other problems!

Good luck,

Tim B.

 

NiMh cells and speed motors are history!
Now if you dont run brushless you arent " for real".
My second one on 3S Lipo and 4800Kv:


I was experimenting with prop/shaft angle on it.
I increased the angle and the front of the boat started to dive in the wather!
And the speed was reduced immediatley when the boat gain some speed

*The one that I am holding on is 650mm long. It is going to run on a nitro 3,5ccm engine. I forgot the holes for weight reduction, yes.
But as you sad: power to weight ration is important.
I dont want to be the fastest out there. I just want to train my design and building for the BIG one

 

Unfortunately I was limited by the rules, as to what the power source was. I'm not sure I would have wanted to put any more power in it anyway, even with a surface drive. At least, not at that weight and size. However, the next boat will be faster. What sort of structural weight are you looking at? We were about 380g for the bare hull.

With a submerged drive you do get a significant bow down pitching moment, but this can be countered by moving weight aft. Note in the full-size pics that the engine and driver are as far aft as possible, the CG with driver in the seat will be around the back of the engine. This will split the loading about 2/3 forward, 1/3 aft. The exact location of the CG can only really be determined through trial and error.

Cheers,

Tim B.

 

The plan is to have motor behind the driver.
Something like this:

 

A submerged propeller operating close to an open surface will experience a vertical force component, even with zero shaft angle. This force is strong enough to cause "prop riding", ie the propeller is balancing in surface piercing mode at ~50% submergence.

As soon as air is entering the propeller disc area (the prop "ventilates"), the blades operate with an air "pocket" on their suction side, which completely changes the flow and thus the forces generated. Surface piercing propellers have been tested up to about 20 degrees shaft angle (~50 % submerged) with very good results. Normally, the blades are raked so that they are vertical in the "wet" operating zone. Most SP props operate in the 8 to 10 degree range.

So, in your case, design the setup with the prop hub at wl level with the dynamic lifting surfaces of the hull in correct operating trim and heave position, then the final adjustment of propeller lift is done via blade rake in combination with cg adjustments.

 

Great informations from you!
Thanks again

I saw some reading material based on propulsion here on this forum...
A plan for some other time.

Where did you get your knowlege on this kind of things?

 

I am a qualified Naval Architect. I mostly do software now though, and I love investigating new ideas.

Tim B.

 

Here's another DIY inboard drive that I'm building. Power is a honda motorcycle engine just forward of the motorboard. The boat is a 3 point hydro.

 

Here's another picture of the drive with paint and an engine.

 

Have a look at what the international racing professionals use - the use what they use because the competition is strong and the speeds are high... Look in my gallery for some on Marichimo... http://www.boatdesign.net/gallery/showgallery.php/cat/500/ppuser/22903 vertical adjustment only and very high lateral G forces in turning...

 

The front of a hydro needs to fly on a cushion of air while the rear end is supported by the forces of the propeller and the lift it produces so in reality theres hardly any part of the boat bearly touching the water ,so shaft angle as shallow to no angle at all !!. should ride with half the prop and the lower part of the rudder in the water , then you have the perfect ride . MAYBE !!