propulsion

[rasorinc]

Does a prop lose efficiency the steeper the angle of attack? If so would the flatest angle be the best for speed and power? Is there a formula?
Thanks much, Stan

[Rick Willoughby]

Does a prop lose efficiency the steeper the angle of attack? If so would the flatest angle be the best for speed and power? Is there a formula?
Thanks much, Stan

Stan
You are confusing shaft inclination angle and angle of attack of a blade. They are different things.

An efficient prop blade operates at a low angle of attack. If you mount such blades on an inclined shaft they will induce unbalanced forces because the angle of attack changes as the blade rotates through the full 360 degrees.

It is best to operate without shaft inclination if the geometry permits. Look at an outboard motor for example.

I do not have a formula for the impact of shaft angle as I avoid it. You can use the linked JavaProp to do prop calculations but it assumes the shaft is aligned with the flow:
http://colaco.freeshell.org/mhepperle/javaprop/jp_applet.htm
You need to set the Options page to water paramaters rather than air.
Density for saltwater 1025
Viscosity 0.0000013

Inclined shafts work but they are a compromise.

A surface prop is an exception as their blades are not working as foils. Their efficiency is low anyhow but give overall benefits at high speed due to low appendage drag.

Rick W.

[rasorinc]

thanks Rick, I meant shaft angle but could not come up with the right word.
I appreciate your response. What brought this question about was a V drive that showed a 15 degree down angle which I thought was severe.
thanks much, Stan

[Rick Willoughby]

Stan
If you do the numbers on these sort of drive set ups they show efficiencies around 60%. That means a lot of prop slip so the blades have a high angle of attack and the impact of the shaft inclination is not as severe.

Inevitably there are compromises. The ideal would be a very narrow drive leg with a horizontal shaft like a sail drive but that will not handle a lot of power. I have used a universal under water for low power application but it would need some serious engineering to get durability for high power.

15 degrees is not uncommon. I have seen 18 degrees on this forum.

The flatter the better but if it means reducing the prop diameter then this is also undesirable.

Rick W.

[FAST FRED]

Rick, I have seen verbiage on "pre swirler" some way of increasing the proip efficency with out the complexity of contra rotating propellers.

Any idea what that is , and if it would help an Atkin design?

MPG is the goal.

FF

[Rick Willoughby]

Fred
A feature of the Atkin design is that you can get the shaft horizontal and there is close to streamline flow over the prop. This is the best way to maximise efficiency. If you want a prop that does not go lower than the keel you can shroud it and this will improve the prop efficiency but you get extra drag from the shroud that offsets most of the gain.

The Atkin is a clever design. Just make sure you fit the biggest diameter prop you are prepared to tolerate. A 30X30 spun at 800rpm will get around 82% efficiency at 17.5kts. Requires 70HP.

If you reduce the prop to 16X18 spinning at 1400rpm, efficiency drops to 70% and power required goes up to 80HP.

I have been doing some optimisation for cat hulls to run at 15kts. These have a canoe stern like the Atkin hull but a lot narrower. If you like I could do one with the underwater shape optimised for 17.5kts just to see how it compares with the original Atkin style. I will post it on the SCALE MODEL thread.

The other little feature was to place the rudder in front of the prop. This means the canoe section can be almost as long as the whole hull and still shield the prop from ventillating.

Are you building a model yet?

Rick W.

[FAST FRED]

Are you building a model yet?

YES!

Have been busily cutting out the stations in 1/4 inch 5 layer ply.

At almost 80 inches a sheet of 3/4 ply with 2x4 stiffeners under is the construction strongback.

Looks interesting at the rear as I stick each station in place!

FF