[hpv-boats] V15 Gearbox & Seating

[Bob Stuart]

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

On 5-Jan-11, at 4:08 PM, Rick Willoughby wrote:

> Ian
> I made the gearbox from a purchased 1:4 gearset.  They are straight  
> cut bevel gears that cost USD100.
>
> The gearbox is noisier than spiral bevel gears but similar efficiency.
>
> This one weighed 2.8kg however I have since modified the design and  
> will use spiral bevel gears for the next one. The gearset is  
> heavier but the box will be lighter.
>
> I repaired one of the Mitrpak boxes I broke using higher strength  
> steel.  It will take at least 300W now.  In the end I did not like  
> the 1:2 ratio for such a light boat.  The big prop is a nuisance  
> for a variety of reasons. One less obvious problem is the induced  
> roll due to the higher torque.  It also has a greater tendency to  
> "walk" sideways than a higher speed prop.
>
> As far as gear ratio goes I determine a step up ratio between 4 to  
> 5 to be near optimum for an easily driven boat.  Your 1:3.3 is not  
> far off optimum - way better than 1:2.  You only problem is getting  
> a shaft that is stiff enough torsionally while still able to curve.
>
> Vic
> The height of the seat on V15 is not much different to V14.  The  
> step in the deckline gives the impression of the seat being higher.
>
> The low deck at the cranks means the seating can be more upright,  
> which is something my sister wanted.  I prefer being more reclined  
> but like the low deck at the crank to keep the feet lower.  Helps  
> with blood flow to the feet.  Once moving there is little tendency  
> for water to wash onto the lower deck in waves because the  
> divergent wave from the bow forms a trough along the side of the  
> hull. This will be less so with a 6m long hull because the  
> divergent waves are smaller.
>
> I have continued to play with the V15 design mainly to simplify  
> construction and improve load paths for strength.  There are a few  
> people building various alternatives of this design.
>
> I would not recommend the extended fairing behind the seat.  It  
> does reduce frontal windage but this can be achieved with a shorter  
> tail-box.  It increases the roll force when side on to wind.  With  
> wind on the bow quarter there is a noticeable tendency to sail but  
> this has limited application.
>
> The V15 design has a distinct advantage over V14 when pushing into  
> wind and waves.  There is no tendency for water to wash back over  
> the bow when it buries and the reduced windage is significant in  
> even slight wind.  It also surfs better because the bow does not  
> dive and it does not flop roll with the outriggers set aft.
>
> The flat rocker increases the turning circle over V14 and this  
> requires making allowance when in tight spaces.  For this reason it  
> does have quite good reverse because I have limited the angle of  
> folding on the prop.  It is quite easy to get up to 5kph in  
> reverse.  So in narrow canals she needs to execute a 3-point turn  
> and think about how the boat will be affected by any wind.  Her  
> Hobie turned much better but she averaged half the speed and had a  
> few scares in strong currents where she could not make headway.
>
>
> Rick Willoughby
>
>
>
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