On Fri, 20 Mar 2009 16:25:42 -0700 (PDT), Frogwatch
wrote:

On Mar 20, 7:18 pm, "Calif Bill" wrote:
"Frogwatch" wrote in message

...



On Mar 20, 1:29 pm, John H wrote:
On Fri, 20 Mar 2009 12:25:30 -0400, HK wrote:
Jim Willemin wrote:
HK wrote in
:

Jim Willemin wrote:
Frogwatch wrote in
news:571e39b1-f812-459a-b9e6-
:

It seems that deadrise has a radical effect on the ability of a
boat
to get on plane quickly and thus use less fuel. More deadrise
makes
a boat punch through waves better but increases draft and reduces
fuel economy in smaller seas. I assume that variable deadrise has
been tried. Of course there are boats that try to use hull shape
to
accomodate varying conditions but not too successfully.
My Tolman has about 10 degree deadrise and is very light so is very
fuel efficient but pounds a lot in chop so I have to slow down to
about 12 kts. Why not some mechanism that would consist of another
outer variable hull layer that would be hinged at the chines
allowing the keel portion to move downward to increase deadrise.
It
would have a flexible stiff plastic piece at the front to keep
water
out of the area between the two hulls. Is this simply too complex
for too little benefit?

Having pondered this for a couple of days, I am struck by the
problem
of changing the area of the floor between the keel and the chine.
This might be tractable near the transom (overlapping leaves,
perhaps, kinda like a 'sliding lapstrake' construction), but as you
get near the stem you gotta worry about major changes in geometry as
well as area. I imagine a really clever designer could do it. I
suppose the question is where in the hull would variable deadrise be
most effective in the transition to planing? That might make an
interesting thesis for someone...

Bring a lifejacket.

I was thinking more of models in a wavetank. But seriously: where in
the hull is deadrise most important to achieving planing status? Is
it
in the forward third, the midships third, or the aft third? Since
planing is essentially the result of climbing one's own bow wake, or
the
wave whose wavelength is equal to your waterline length, my hunch is
that the geometry near the stem is probably most significant in
acheiving planing. But there has to be some kind of optimum - it
seems
to me that a flatiron skiff, say, would take more energy to achieve
planing mode than a runabout of identical length. So if the critical
region is in the forward third of the hull, where major changes in
both
hull geometry and hull area occur as you mess with deadrise, it seems
to
me probably not worth the effort.

Deadrise isn't important in achieving planing status: a flat-bottomed
boat will plane just fine. The more deadrise, the more power it takes to
get a boat up on plane.

...therefore, deadrise *is* important in achieving planing status. The
more deadrise, the more difficult to achieve plane.

I've been reading about "variable deadrise" hulls where the deadrise
changes as one goes from center of the keel area toward the chines.
The pictures all show a series of areas with reduced deadrise toward
the chines. Why not make this a continuous change? I am not sure
what the advantage would be however one can imagine a large deadrise
range at the bow area going to significantly less at transom.
Why does a hard chine boat ride diff from one with rounded chines?

Hard chine probably gives the boat more bottom width. On river jet boats
the difference between a 6'6" bottom width and a 7' width makes a tremendous
difference in planeing speed. Plus the draft on plane. Maybe an equivalent
to less dead rise while starting would be some wings that spread when at
moving in flat water and tucked in while in rough water.

Thanks Harry, even makes sense.

For the record. That wisdom did not come from Harry.