[hpv-boats] Prop Shaft

[Rick Willoughby]

Ian
The angle does not affect efficiency greatly.  The main problem is  
the vibration and high bending loads.  If you have to go to a heavy  
shaft to reduce torsional compliance then it will be strong enough to  
take the bending loads.  However I once bent a 12mm aluminium shaft  
when going hard with a 1:3.5 reduction.

My main issue with the inclined shaft is the ever present vibration.   
It gets more noticeable as you go faster.  If this is not annoying  
you then live with the current inclination.  You are probably looking  
at hull number 3 to do the stepped deck.  I keep simplifying this  
arrangement so it is easier to make.

I have had a carbon fibre shaft wound for me with only diagonal  
fibres so it is stiff torsionally but compliant in bending.  I am yet  
to install it on anything.  It weighs a fraction of steel and even  
lighter than aluminium but 4 times stiffer torsionally than my  
current steel shaft.

I have a reasonably accurate model now for assessing the impact of  
shaft compliance.  A 12mm Al shaft that is 3m long, using your 1:3.3  
ratio, will be robbing around 8% of your power input due to the shaft  
torsional compliance.

I should see you at the lake tomorrow.


Rick
        Have been playing around a bit with shafts and
have got a straight 16mm aluminium shaft held in
place with a skeg. The angle is 16 deg which is way
off optimum but makes setting up easier. The flex
shafts always hit a dead spot at high revs and limits
the top speed. I can lower the gearbox another 100mm
by cutting out part of the hull for my feet and this improves
the angle to 11deg. Don't know if the effort of doing this
will greatly improve the performance, a bit of work and
more weight.
         Will be at the lake tomorrow.

Ian


Rick Willoughby