[hpv-boats] V15 Gearbox & Seating

[Rick Willoughby]

I am discussing a single person easily driven displacement hull.

The optimisation around a ratio of 4 to 5 considers many factors:
1.  I am running a 390mm diameter prop achieving 3.1m/s at 75rpm on  
the crank.  Pitch of current prop is nominally 620mm. So P/D is 1.6.
2. The optimum P/D is a function of the L/D of the blades.  The lower  
the L/D the higher the P/D for best efficiency.  I am achieving an L/ 
D around 20 at my operating condition over the working portion of the  
blade. The most efficient part of the blade is toward the outside of  
the blade.  My current blades have slightly larger chord toward the  
outside but it is not much better than just constant chord blades,  
which I often use.
3. With the large diameter to thrust, I have very low disc loading so  
this results in low slip - or high actuator disc efficiency.
4.  The large diameter enables me to use only two blade prop while  
still keeping high aspect blades.  This reduces induced drag on the  
blades.  These two blades have sufficient strength to transmit the  
thrust without flexing noticeably.  The minimum chord for a steel  
blade capable of taking full sprint thrust of say 100N is not less  
than 20mm.
5. The torsional stiffness of the drive train favours spinning the  
prop faster to avoid poor RMS to average power.  This was a key  
factor with my dislike for the 1:2 ratio.  For this reason I consider  
1:4 about the minimum ratio with a shaft drive system although there  
may be some layup of carbon fibre that would work at lower ratios  
without becoming unreasonably heavy.

I could not imagine the circumstances with a displacement boat where  
a ratio of 1:10 would result in the best efficiency.  There may be a  
situation with very easily driven boat such as a hydrofoil where 1:10  
could be close to optimum.  With my boat using a 250mm diameter prop  
I would throw away 7% in disc losses compared with 3% with the 390mm  
prop.

If anyone is interested in prop optimisation they might find these  
notes of interest:
http://ocw.mit.edu/courses/mechanical-engineering/2-23-hydrofoils-and- 
propellers-spring-2007/readings/kerwin_notes.pdf
There are some useful curves toward the end that can be applied even  
to the lightly loaded case of a pedal boat prop.

Rick




I think the advice on a step up of 1:5 or less only pertains to a
situation where the fluctuating torque of a single rider is used.  It
reduces the loss during the low-power parts of the cycle.  The high
pitch is a compromise, pushing water around as much as back.
Usually, the best efficiency is found when the pitch and diameter are
nearly the same, leading to a step-up of around 1:10 for a tandem
with the pedals out of phase.

Bob Stuart


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