[hpv-boats] V15-5m Update

[Rick Willoughby]

My sister finally got down to Melbourne to test the boat I built for  
her.  She is able to do 8kph at a relaxed, sustainable power lever  
and can push up to 12kph for a minute or so.  The corresponding  
speeds for effort in her Hobie flapper are 5kph and 7kph.  This is a  
clip from last weekend (3.3Mb):
http://www.rickwill.bigpondhosting.com/V15_Donna_1.wmv
Most of the noise is wind and my outriggers slapping the ripples.   
The gearbox noise on her boat can be heard when she pushes her speed  
up.  As a matter of interest she later tested with my cycling shoes  
rather than the rubber beach shoes and found those more natural  
although they are a couple of sizes too big.  The pedals can be used  
with or without cleated shoes or even bare feet.

I have modified the boat somewhat from the original build.  The main  
change is the gearbox.  The initial box was a Mitrpak T-102-M  
modified for cranks.  I broke one of these pushing the boat up to  
around 13kph and a friend broke one just accelerating - told him to  
stay below 12kph but did not say do not accelerate hard.  The boxes  
fail at the high speed shaft.  I have rebuilt one using higher  
strength steel in the shaft and Mitrpak are supplying redesigned  
shafts that will be stronger than the original.

Irrespective of the Mitrpak box improvements I decided there are a  
few disadvantages to swinging the 540mm prop required for a 1:2  
ratio.  The 1:4 box I have made has at least three times the torque  
rating of the Mitrpak box and allows me to swing a 400mm prop.  The  
box has straight cut bevel gears because these cost less than spiral  
bevel and I was a little uncertain I could achieve the required  
fabricating tolerances with my little mill and lathe.  If I make more  
boxes I will use spiral bevel gears mainly because they run quieter.   
They also have slightly higher ultimate strength and many times  
higher wear life although I doubt the latter will ever get tested on  
a pedal boat.  The box weighs 2.5kg.

I now have to make a few minor adjustments and clean the boat up  
before the road trip to the Gold Coast at the end of the year.


Rick Willoughby