[MHml] Prop Pitch for Multihulls (alternative solutions)

[Peter Nice]

I have 2 inboard 2 strokes (16hp) that belt drive 2 13inch 2 bladed
MaxProps.

Over the years I have experimented with both pitch and gearing (changing
pulley sizes). The compromise solution is to increase the gearing so the
shaft turns slower with higher torque and increase the pitch as far as
possible to bring the engine revs into the most economical range.

This technique is used dynamically by many large power boats with variable
pitch props. Once the desired cruising speed through the water has been
achieved using a fine pitch they gradually increase the pitch until the
engine revs are bought down to the most economical engine speed.

Since I dont have a variable pitch prop this only works well for average
winds and average seas. (should be sailing). With no wind the economy could
be improved and with lots of wind the speed and economy could be improved.
Any change in load carried also has a big effect. The same must be said of
any outboard with a fixed propellor. I have even thought of setting a fine
pitch on one engine for wind and waves and a coarse pitch on the other for
no wind.

Alternatives solutions

Power boats are overcoming this problem, manually, by using either a 2 speed
gearbox (Hurth ZF suitable for big engines only) or, automatically, using
Volvo's (sorry Gary) SpeedStep Drive on their DuoProp outdrives.

When a multihull is motoring it is much more like a car or a planning
monohull powerboat at displacement speeds, that is hugely affected by wind
and wave action, than a sailing monohull with its low windage, stablising
lead weight and waterline speed limitations. Unfortunately this is where
most of the small auxiliary prop/engine experience comes from.

What we should be looking at is marinising the engine and automatic
transmission from one of the new breed of Maxi Scooters Suzuki Burgman or
Honda DN-1. Nice efficient 4 stokes 400cc about 25 hp 700cc about 65hp
derated to half revs and you will get pretty good fuel economy too. Could
even be produced as a outboard package (bet that would be expensive). Set
the prop and let the transmission handle the changes in wind and sea
conditions. These gearboxes give upto 6 ratios for anything upto 100mph so
0-15 knots should be doddle.

Or what about grafting an auto transmission on a 40hp Smart diesel.

Peter Nice
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