Operation Manual

Diesel fuel flow sensor kit Installation and Operation Manual

NAVMAN

Understanding your slip factor

Now, some slip is always to be expected. This

slip factor varies with different types of hull, and

typical slip factors are -

A propeller encased in an aperture behind

a keel, as in a yacht: 40%.

A hard chine planing hull with a full keel,

or deadwood: 30% to 35%.

A hard chine planing hull with little or no

keel: 25% to 30%.

Twin engined planing hulls: 18% to 25%.

If your calculations show you have a slip factor

that is much more than the figure above, then

further investigation is warranted. The propeller

may be the wrong size for the present engine,

or it might have got damaged, or there may be

a fitting on the hull that is disturbing the flow

ahead of the propeller. Propellers like to bite into

undisturbed water, and will not work efficiently

if water is aerated or churned up.

If the propeller is undersize, then it drills a hole

in the water, and the engine’s excess power just

makes frothy water.

If the propeller is oversize for the engine, the

engine will not be able to reach its maximum

operating RPM. The engines speed governor

will still try to achieve that RPM, and will open

up the throttle and pour as much fuel into the

engine as it can. If the engine can’t generate

any more power at that RPM, all that extra fuel

just ends up as heat, and that’s when things get

really expensive if the situation persists for any

length of time. Be warned - over propping an

engine will drastically shorten its life.

An excellent book with lots more information and

analysis of propellers is “Propeller Handbook”

by Dave Gerr (ISBN 0 7136 5751 0).