[hpv-boats] V15 Gearbox & Seating

[Rick Willoughby]

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