Re:Rocker in Hull
I too have been pondering a displacement hull that would also plane on
demand. I wonder what would happen if you moved the point of maximum
depth in the rocker forward, leaving the aft 1/3 of 1/2 of the bottom
flat in profile. I've uploaded a sketch of what I'm saying to Bolger2
files - rocker.png. It is an exaggerated profile of two hulls below
waterline. Perhaps she would set back on the flat section for planing.
Did I just reinvent the semi-displacement hull? ;)
What's wrong with this idea? 1 - the pivot point for turning would be
moved farther forward. Not bad on a short boat (see Dagger and Dart),
but possibly annoying on a longer vessel. 2 - too much trouble to build.
The flat-bottom box boats (Sneakeasy, Hawkeye, Fast Motorsailer) are
still efficient at low speeds. They plane easily, and heeling them in a
sailing situation turns the chine into a keel with rocker! So why
bother with the forward-shifted rocker? (gee, I just talked myself out
of trying that idea)
Clyde: One other option I've read about recently (Steward? nah. Gerr?),
but not seen in practice, may be what you are looking for. Putting some
rocker on the stern with a sharp lip where it meets the bottom (i.e. not
faired) will allow the water to break free of the rocker, enabling the
hull to plane. Caveat: it requires several more mph than a transom stern
to BEGIN planing. I imagine a 1 foot x 30 degree chamfer should be to
easy build on a boat with 6" draft at the stern, and it wouldn't take
too much planing surface away. This would have the effect of moving an
outboard motor back 1 foot. The powerboat folks often the add motor
mount extension on their sterns, we'll have to ask them why.
I've been wondering why the pontoon folks don't put pointy bits on their
transom sterns for hull speed efficiency. Most of their bows are not
faired into the tubes anyway (only one model at the recent boat show had
the bow faired into the tube), so they could just make the sterns
identical to the bows. They could still plane if they wanted to, but
they would get better mileage when idling about, and move faster to
boot. (of course, if they really want to plane they should be making the
bottoms of their pontoons flat, instead of adding a third pontoon to
their "sport" models to achieve more lift. But that's a different
issue)
Bruce, do you want to volunteer to try this out on Adagio and see if you
get better mileage when cruising?
Stew
demand. I wonder what would happen if you moved the point of maximum
depth in the rocker forward, leaving the aft 1/3 of 1/2 of the bottom
flat in profile. I've uploaded a sketch of what I'm saying to Bolger2
files - rocker.png. It is an exaggerated profile of two hulls below
waterline. Perhaps she would set back on the flat section for planing.
Did I just reinvent the semi-displacement hull? ;)
What's wrong with this idea? 1 - the pivot point for turning would be
moved farther forward. Not bad on a short boat (see Dagger and Dart),
but possibly annoying on a longer vessel. 2 - too much trouble to build.
The flat-bottom box boats (Sneakeasy, Hawkeye, Fast Motorsailer) are
still efficient at low speeds. They plane easily, and heeling them in a
sailing situation turns the chine into a keel with rocker! So why
bother with the forward-shifted rocker? (gee, I just talked myself out
of trying that idea)
Clyde: One other option I've read about recently (Steward? nah. Gerr?),
but not seen in practice, may be what you are looking for. Putting some
rocker on the stern with a sharp lip where it meets the bottom (i.e. not
faired) will allow the water to break free of the rocker, enabling the
hull to plane. Caveat: it requires several more mph than a transom stern
to BEGIN planing. I imagine a 1 foot x 30 degree chamfer should be to
easy build on a boat with 6" draft at the stern, and it wouldn't take
too much planing surface away. This would have the effect of moving an
outboard motor back 1 foot. The powerboat folks often the add motor
mount extension on their sterns, we'll have to ask them why.
I've been wondering why the pontoon folks don't put pointy bits on their
transom sterns for hull speed efficiency. Most of their bows are not
faired into the tubes anyway (only one model at the recent boat show had
the bow faired into the tube), so they could just make the sterns
identical to the bows. They could still plane if they wanted to, but
they would get better mileage when idling about, and move faster to
boot. (of course, if they really want to plane they should be making the
bottoms of their pontoons flat, instead of adding a third pontoon to
their "sport" models to achieve more lift. But that's a different
issue)
Bruce, do you want to volunteer to try this out on Adagio and see if you
get better mileage when cruising?
Stew
--- In bolger@y..., "Clyde S. Wisner" <clydewis@c...> wrote:
> Gregg, A long time ago, you posted something about the maximum rocker
one could put in the stern of a
> boat and still get it to plane. Would you do it again? I am messing
around with a 19' skimmer type
> jon boat with a dead rise bow like a Topaz I can't resist trying to
make it sail like a Fast Motor
> Sailer, even though I want it primarily for fishing. Clyde
From various things I've read, I have a pretty definite impression
that there's a lot of engineering knowledge regarding planing hull
design. If you decide you want to study it that hard, I can
probably get a few references to sources from bibliography in "Fluid
Dynamic Lift" and/or "Fluid Dynamic Drag", but they will be kind of
old as these books are not exactly new. Likely to be pretty technical,
tho. I suspect it would be easier than building a boat of any
considerable size that doesn't work, at least if you're used to that
kind of math, etc.
that there's a lot of engineering knowledge regarding planing hull
design. If you decide you want to study it that hard, I can
probably get a few references to sources from bibliography in "Fluid
Dynamic Lift" and/or "Fluid Dynamic Drag", but they will be kind of
old as these books are not exactly new. Likely to be pretty technical,
tho. I suspect it would be easier than building a boat of any
considerable size that doesn't work, at least if you're used to that
kind of math, etc.
--- In bolger@y..., wmrpage@a... wrote:
> In a message dated 3/20/02 6:51:36 AM Central Standard Time,
> clydewis@c... writes:
>
>
> > Gregg, A long time ago, you posted something about the maximum
rocker one
> > could put in the stern of a
> > boat and still get it to plane.
>
snipd]
In a message dated 3/20/02 6:51:36 AM Central Standard Time,
clydewis@...writes:
This is an intriguing idea, and I hope you will keep us posted in more detail
if you pursue this. I don't think that there is any easy rule of thumb that
you could apply to your conception, but I don't think your objective is
inherently unattainable. Getting there might prove a bit complicated though.
As far as planing under power goes, Dave Gerr ("Propeller Handbook", p.15)
states that "true planing" boats (S/L>2.9) must have quarter-beam buttock
angles of 2 degrees or less, with most modern planing boats having a
quarter-beam buttock angle of 0 degrees. In other words, very little to no
"rocker" aft.
However, the quarter-beam buttock angle is measured in comparison to the load
waterline at rest. On a small boat, that angle will vary considerably if crew
weight is shifted fore-and-aft. (i.e. on small sailing craft, a waterline as
shown on the plans something more in the nature of a Platonic abstraction or
notion or something than a representation of reality.)
It would seem conceivable to have a boat which would present a relatively
steep buttock angle (to avoid dragging the transom) when sailing, and a
relatively flat buttock angle (for planing under power) by shifting crew
weight fore-and-aft. The extent to which this "best of both worlds" scenario
would be practicable would depend a lot on the ratio of crew weight to
displacement, the distance that the weight can be moved in a fore-and-aft
direction relative to the (ever-changing) center of buoyancy and probably a
host of other considerations which do not immediately occur to me. My
intuition is that the shape of the bottom aft of amidships in profile must
approximate to a straight line and be trimmable to a very small angle if the
boat is to "plane".
This is probably a concept where it would be more feasible to build and test
than to try and calculate in advance. Go for it! (and keep everyone posted,
S.V.P.)
Ciao for Niao,
Bill in MN
[Non-text portions of this message have been removed]
clydewis@...writes:
> Gregg, A long time ago, you posted something about the maximum rocker oneAs our Moderator has not responded yet, I'll throw in my 2 cents worth:
> could put in the stern of a
> boat and still get it to plane.
This is an intriguing idea, and I hope you will keep us posted in more detail
if you pursue this. I don't think that there is any easy rule of thumb that
you could apply to your conception, but I don't think your objective is
inherently unattainable. Getting there might prove a bit complicated though.
As far as planing under power goes, Dave Gerr ("Propeller Handbook", p.15)
states that "true planing" boats (S/L>2.9) must have quarter-beam buttock
angles of 2 degrees or less, with most modern planing boats having a
quarter-beam buttock angle of 0 degrees. In other words, very little to no
"rocker" aft.
However, the quarter-beam buttock angle is measured in comparison to the load
waterline at rest. On a small boat, that angle will vary considerably if crew
weight is shifted fore-and-aft. (i.e. on small sailing craft, a waterline as
shown on the plans something more in the nature of a Platonic abstraction or
notion or something than a representation of reality.)
It would seem conceivable to have a boat which would present a relatively
steep buttock angle (to avoid dragging the transom) when sailing, and a
relatively flat buttock angle (for planing under power) by shifting crew
weight fore-and-aft. The extent to which this "best of both worlds" scenario
would be practicable would depend a lot on the ratio of crew weight to
displacement, the distance that the weight can be moved in a fore-and-aft
direction relative to the (ever-changing) center of buoyancy and probably a
host of other considerations which do not immediately occur to me. My
intuition is that the shape of the bottom aft of amidships in profile must
approximate to a straight line and be trimmable to a very small angle if the
boat is to "plane".
This is probably a concept where it would be more feasible to build and test
than to try and calculate in advance. Go for it! (and keep everyone posted,
S.V.P.)
Ciao for Niao,
Bill in MN
[Non-text portions of this message have been removed]
Gregg, A long time ago, you posted something about the maximum rocker one could put in the stern of a
boat and still get it to plane. Would you do it again? I am messing around with a 19' skimmer type
jon boat with a dead rise bow like a Topaz I can't resist trying to make it sail like a Fast Motor
Sailer, even though I want it primarily for fishing. Clyde
ghartc wrote:
boat and still get it to plane. Would you do it again? I am messing around with a 19' skimmer type
jon boat with a dead rise bow like a Topaz I can't resist trying to make it sail like a Fast Motor
Sailer, even though I want it primarily for fishing. Clyde
ghartc wrote: