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From: Gene Gruender (no email)
Date: Mon Oct 01 2001 - 20:41:20 EDT
>It takes more speed to build up the troughs that the longer water line
>boat cannot climb over (in flat water).
>Some designers think of it as "suction". The stern wave sucks the boat
>back and will not allow it to climb over the bow wave unless there is a
> LOT more horsepower available. So the shorter waterline boat will
>encounter its stern wave sooner and at a lower speed than the
>longer water line boat.
It took me a long time to understand the relationship, now if I can only
write it so it makes sense!
First, waves are a set distance apart for any speed they are traveling, or
maybe it would be better to say that as the speed of a wave increases, it
gets farther apart. That formula that tells hull speed is the one that
tells how far apart waves are at a certain speed. It's no coincidence.
The faster a wave travels, the farther apart each is. The bow of your boat
makes a wave. The wave's speed is the same as your boat speed, just
because the front of your boat is pushing it at the speed it is going. The
next wave behind it is going to be a distance behind the first (and by
default, the same distance behind the front of your boat) that corresponds
to the formula. As long as that second wave is under the boat, the boat
is on flat water, supported front and rear.
Once the boat goes fast enough that the second wave is behind the boat
(exceeds hull speed) the rear of the boat falls into the trough. The
faster the boat goes, the deeper the trough becomes, the farther to the
middle of the distance between the waves the rear of the boat goes and the
farther the rear of the boat drops. Instead of sailing or motoring on flat
water, it's now trying to sail or motor on the face of a wave, a wave it
created it's self.
So you can see if the boat is longer, it will stretch across a longer gap
between waves and still be on level water (at least as far as the boat is
concerned) I'm sure it takes more power to push a bigger boat through
water, but it nothing compared to the power needed to push it up a steep
face of a wave.
Once a boat gets on a plane, it is riding on top of the water and the
waterline length doesn't come into play, since the waves no longer have
anything to do with it. But you'll notice that before it gets on a plane,
it has to ride up a very steep wave.
I'm sure most of us have sailed where there are sizable waves. Have you
noticed that as you go down the face of a wave your boat speed increases,
possibly far beyond hull speed? That is simply because you have the
reverse in play, the front is not supported level, it's in a trough and
it's sliding down the wave. Hull speed there would only come into play
once your speed is so great that the wave you're making inside of the big
wave leaves your boat dropping below level in the back (or the waves
catches up to you and you end up on the back side of it!)
Did I make any sense or just confuse things?
Gene Gruender
Rainbow Chaser
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