On Mar 21, 7:22 pm, Monkey Butler wrote:
- Show quoted text -

In theory the concept of mechanically variable deadrise should work
but in practice the complexity is too much of an obstacle in a small
boat so it makes more sense to have a different boat design for
different conditions and/or operational parameters. In a larger craft
the objective is more easily achieved with designs such as SWATH,
catamarans, or hydrofoils. Current high speed ferries are good
examples, and the Russians have built some neat hydrofoils .

I took the Westlawn course for my own entertainment and while much of
the facts and figures that I learned have been lost most of the
concepts are still clear. A couple of points… At rest your boat stays
above the surface of the water because of the hull’s buoyancy and by
virtue of its static displacement. Once you start moving your hull
will generate a series of waves in the medium that it is floating in.
At a point called hull speed your boat will be traveling in a trough
between the bow wave and the stern wave. As you increase speed further
the bow will rise as it attempts to climb the bow wave and at some
point Newton’s Third Law will kick in and the action of the water
meeting you hull surface will result in a equal reaction that will
lift the hull of your boat. Then, at some point the flow of water
against the hull will separate from one stream flowing under your hull
into two, one flowing under the hull and another flowing forward of
the hull as spray. This is the stagnation point, the point where
maximum lift is generated. Any water striking the hull at this point
is converted into 100 percent lift. Anything forward is wasted as
spray and anything aft will act on the rear planing surface and affect
the angle of attack. As speed increases the stagnation point will move
aft and at the same time the CG of the hull will move up. (BTW, This
is where trim tabs come in. They can be used to adjust the trimming
force and thus the angle of attack, but they will not create lift…).
Eventually the weight of your boat is no longer being supported by the
displacement of water but by the dynamic force of the water striking
its planing surface. A perfectly flat bottom means that the lift is
vertical. A vee bottom wastes a percentage of the energy as spray
deflected to the side.

If any of the above makes sense then it would seem that we would need
to adjust the deadrise at the stagnation point to avoid pounding. The
problem is that with any small boat running in a sea this point moves
fore and aft on the hull due to the waves that it encounters. Most
modern hulls deal with this by having a sharper vee at the bow and
less dead rise at the stern so I guess the answer to your original
question is that your radical idea has already been incorporated into
almost every hull design! In reality we know that every hull is a
compromise and the overall deadrise is always a tradeoff between sea
keeping and efficiency. I know that Seacraft for one advertised
variable deadrise as being a change in angle from keel to chine but in
normal marine architects language this is a term that refers to the
variation in dead rise angle measured at different points from bow to
stern. Constant dead rise hulls are referred to as Monohedron versus a
variable dead rise called a warped plane. Warped planes have their
limits as far as how much you can “warp” them before you trade off
performance especially if we are talking “Miami Vice” type boats.

Lobster boats or Downeast designs stretch the warped plane idea. They
have a very sharp entry forward but an almost flat bottom aft. This
limits their performance but it is the main reason for their famed
ride. Round bilges and full keels have nothing to do with it.

Steve P.

Steve:

Ok, I got most of that, but, why does a boat make a stern wave?