[hpv-boats] V15-5m Update

[v garza]

----- Original Message ----- 
From: "Rick Willoughby" <rickwill at bigpond.net.au>
To: "HPV Boat" <hpv-boats at bikelist.org>
Sent: Tuesday, November 23, 2010 5:37 PM
Subject: [hpv-boats] V15-5m Update


> 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


Thanks for the report. On the gear box strength issues, I often wondered 
about some kind of slip clutch mechanism to protect from shock overloads. 
That could save someone from being stranded or being knocked out of a race. 
Something might be worked out with say, super strong magnets to be used for 
coupling the shafts without having the slippage drawback of friction based 
units. I can visualize someone going full tilt on the Murray and hitting a 
log or rock thus breaking or bending something that can be repaired 
rapidly....

Vic

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