[hpv-boats] Right Angle Gearbox

[GARZA, VICTOR]

Good to see your adaptations to the flex shaft have worked so well in your hpb, Rick. I particularly like your side mount and ability to quickly clear weeds. Wonder if there are considerations because the thrust being parallel rather than in line with the hull.... 
 
In my much lower power electric applications I use a much shorter 4' shaft made of fiberglass tube graduated in two sections. No strut or rudder. This seems to give better pivoting steering control from the stiffer upper section while allowing just enough flexing from the lower section to allow the prop to go where it wants. I need the lowest weight possible in my application so am always looking for the the lightest shaft and prop possible. That way the whole unit can be lifted up to remove weeds or even hand held for propulsion in extremely shallow water. I have quick release hex connectors for the shaft and quick release adaptations on the prop so that both can be switched out in seconds. That way data can be quickly gathered for different motor and prop combinations. Should the shaft or prop be damaged or lost on an excursion, switching to spare ones carried on board only takes seconds.
 
Hope this is useful to anyone working with flex shafts.
 
Cheers.
 
Vic

________________________________

From: hpv-boats-bounces at bikelist.org on behalf of Rick Willoughby
Sent: Tue 6/9/2009 4:39 PM
To: HPV Boat
Subject: Re: [hpv-boats] Right Angle Gearbox



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


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