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Re: outboard

Bob Spencer wrote:

> Paul said:
> >Yes, and I was the one that said Chris was nuts, that
> >8 or 6 was plenty.  I appolige to Chris publicly now,
> >as I wish that I had a 15 or so, in the Gulf of MX,
> >and with more distance.
> Is there someone on the list who can explain to me the relationship between
> power and hull speed? A displacement hull can only be driven so fast, and
> as long as you have enough power to do that, what good does any excess do
> you? I was taught that all power above that is wasted. Is that not correct?
> Also, can anyone clue me in as to the best type of prop for a sailboat, and
> which pitch is best? The motor I'm considering is said to handle props with
> pitch from 6.5" to 9.5". I'm looking for power, not speed, so which pitch
> is best to drive a displacement hull? I'm guessing 6.5"... correct?
> Bob
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Hi Bob;

A planing hull rides on top of the water once it's moving at speed. It requires
some extra power to climb up on the step but once there it requires less power
to keep it there.

A displacement hull never gets up on the step as power is applied. (Unless you
apply enough that is overcomes all resistance, but this borders on explaining
why a bumblebee can fly!) Basically the boat pushes some water ahead of itself,
where it piles up, and leaves a vacuum behind itself, where it creates a hole.
The water can flow only so fast around and under the hull, from the pile to the
hole. The more draft and curve the hull has the worse this problem becomes.
This discussion can become very complicated but suffice it to say that both
forces slow the hull down.

However, if you push the hull to its theoretical limits, 6 knots for a SJ23
(See Tech Tip H05), the water can flow around the hull very easily and it takes
very little effort to push it through the water.  To push it faster than that
and it requires a significant amount of extra power because the hull is trying
to climb up the side of the hole it is in, onto the pile of water ahead of the
bow. It simply requires power to climb uphill.

Please keep in mind that this discussion centers around efficient speed through
the water, NOT ground speed over the bottom.  Ground speed is required to go
upstream or to get the hell away from conflicting traffic. This is what a
planing hull excels at.  They also make a lot of noise and mess up your hair in
the breeze, but that's another issue!

Hope this answers your question.

Bob Schimmel
(Always stay curious)

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