S/P prop on a diagonal shaft

Good Evening All,

Although not by any means new to boating, I've not dealt much with surface piercing and was wondering if anyone could tell me what a prop might do when half immersed but on a diagonal, saying between 12 and 20 degrees. This is a permanent shaft and at all speeds where the propeller is able to ventilate.

Thanks!

 

You mean on a diagonal in the horizontal plane or the vertical ?

 

To the horizontal- essentially could a surface piercing propeller run (well) on an inboard shaft inclination, assuming it is half immersed as designed, or roughly so?

 

Hi Sassriverrat,

Forgive me, but your question has me very curious as to what hull form or application would take advantage of a surface piercing prop, but require the use of a shaft angle used by inboards?

Thanks
Dave

 

With no ability to change the inclination ?

 

Is it for a catamaran ?

 

It has been shown that a slight "dogwalking" ie a horizontal shaft inclination will give a very slight increase in forward thrust. You may also find a similar setup in inboard racing "proprunners". A Surface-piercing propeller does not have its thrust vector in the same direction as the shaft, and the trust vector is varying with propeller loading (advance coefficient).

 

So the question/concept stems from trying to build this hydrofoil conversion with minimal monies and maintenance add-ons.

So this being a monohull, she has traditional inboard propellers and after the foils are on, I'd like to put surface piercing props on the shafts instead of the old wheels, but I didn't know how surface piercing handled being inclined at heavy angles, such as maybe 12 degrees below the horizontal.

 

Sounds like an enormously ambitious scheme to me. Any diagrams or plans of how this will look ?

 

I do have some primitive drawings but my cad abilities/experience is quite limited. I guess I need to find someone that may be able to help clean up the cad drawing I have and show me how to add stuff like decks....

But yes. Soon

 

So then do we see issues mounting the propeller at a significant angle to the horizontal?

 

So, you are talking about a propeller with a vertical inclination then, I took it for a question on horizontal; my mistake (although the info I gave is correct per se). Provided the propeller rake is correct, a SP prop can work fine with vertical shaft angles up to 15 degrees (even above that).

 

Little confused by your vertical piece-

I'm intending for a shaft 15 degrees below the horizontal, the same as mounting such a propeller on a traditional, fixed inboard shaft.

 

In your answer to Mr Efficiency (posts no 2 and 3) you state "horizontal", referring to Mr E's question. You must understand the difference between the planes we are using in marine engineering. A horizontal plane is a plane parallell to the sea surface and a vertical plane is parallell to a wall. The angles we talk about lie in the respective plane. So, when you are talking about a "diagonal" shaft with "15 degrees below the horizontal", you actually mean a shaft with a declination in the vertical plane, not a diagonal in the horizontal plane.

 

Ah I understand what you're saying. Then yes, we are saying the same thing.