[hpv-boats] Right Angle Gearbox

[Rick Willoughby]

Ken
The efficiency of the propeller correlates with the aspect of the  
blades.  Long thin blades will be more efficient than short fat  
ones.  Inevitably there is a compromise based on draft or blade  
strength.

Mounting an efficient propeller at a slight angle will reduce  
efficiency by a small amount.  However the blade loading becomes very  
unballanced.  At a shaft angle around 5 degrees the the downgoing  
blade will be thrusting with high force while the upgoing blade will  
be doing next to nothing.  This condition puts the shaft in high  
bending load and creates vibration.  Increasing the shaft angle  
beyond 5 degrees really starts to cause problems.

You will see powered propeller shafts at higher angles like 15  
degrees but these props have high slip and usually three or more  
blades so the vibration is not as bad.  They usually have efficiency  
between 50 to 60%.

If you have a propeller pushing on a shaft the shaft can be  
completely unsupported as the prop is strongly self stabilising.   
When the shaft is inclined you are fighting these forces.

It is worth the effort to get spring steel if you can.  Machinable  
grade aluminium also works but it needs to be about 10mm thick and a  
little longer as it cannot be curved as tight.

Rick W



Thanks for the links Rick.  The flex shaft is fascinating.  Not sure  
were
I'd find some spring steel shaft, but I'll look around.

In the mean while I did figure out how else I could do this, and that  
is to
put the angle gearbox after, below, or on the swivel that the top end  
of the
drive shaft mounted to.  But this would point the propeller at a  
little bit
of a downward angle.  Not sure what that would do to the efficiency  
of the
prop.

Ken


Rick Willoughby
rickwill at bigpond.net.au
03 9796 2415
0419 104 821