Extension
Some HF boats travel faster, jet engines enable skiing
on HFs when on: takes over from propellers, speed incr,
HF extend, ride on HFs, drag force reduce as hull out of
water, top speed = motive force eq/opp to drag force;
Jet engines off, HF drag force reduce, speed decr.
HF retract, Speed drops until drag force eq/opp propeller
motive force, constant speed (less than jet engine)