The basis of the following guide is that the maximum displacement
hull speed is to be achieved by a clean and polished hull in calm sea
with no windage. Racing yachts may wish to opt for more limited horse power as the
engine is primarily only used to reach or leaving moorings, motor
sailors or yachts used in heavier seas and or tides may opt for up to
30% more horse power to ensure an adequate safety factor is available.
Engine power, rpm, gearbox reduction ratio, propeller diameter and
pitch are all variables that will affect a displacement hulls
performance and as a consequence is a specialist subject that for new
builds, the vessels designer should be consulted. For vessels which require repowering, there are existing guidelines
and or parameters that are already in place to help in making the
correct replacement engine choice. The existing engine, gearbox
reduction ratio and propeller performance compared to theoretical
maximum hull speed for the vessel will be a good indicator on whether it
can be improved or not.
Additional there may be some set parameters such as the distance
between the centre line of the vessels propeller shaft and the hull that
may or may not allow a larger more optimised propeller to be used. If
the propeller`s diameter is restricted, this can in part be compensated
for by changing the propeller pitch, but there are limitations. For more
detailed information on propellers visit Propeller Options.
- A general engine guideline for yachts with a displacement hull is the weight in tonne multiplied by 4 hp.
- Theoretical maximum displacement hull speed in Knots = 1.3 (or 1.4 for a sleek hull) x √ Water Line length (in feet).
