On Wed, 24 Jun 2009 15:20:52 -0700 (PDT), Frogwatch
wrote:

Other than decreasing topheaviness and increasing beam, how do you go
about minimizing probability of capsize for a powerboat? Consider a
swamped boat, what factors cause it to want to float upside down?

Momentum of water, amount of water, vertical center of gravity, how
steep the Vee is, what the sea state is, how many people and located
where - whole host of factors, the primes being Vcog, Vee and sea
state. A swamped boat will stay upright in reasonable seas, but if you
had been moving in the ditch (the bottom point or trough, between two
waves) which is a completely insane idea in a small boat).

Decreasing the amount of top weight doesn't always translate to
decreasing the Vcog either. Increasing the beam of a boat doesn't
necessarily mean the boat automatically becomes more stable either
depending on how steep the Vee is. It's more the shape of the hull,
the type of hull (modified Vee vs Deep Vee) and it's draft that
determine stability.

If you design a boat with a moderate or semi-vee the boat will tend to
ride flatter on the surface which means that in a heavy sea state it
will pound or if you are unfortunate enough to get into a heavy
following sea or quartering waves when transiting actually cause more
pitch and yaw which can be very uncomfortable. On the other hand, a
steeper Vee adds in the roll component and now your dealing with roll,
pitch and yaw and that's also dangerous.

It's a guessing game in a lot of ways in particular if you are
modifying a plan that has some merit to it as it stands.

On Jun 24, 7:22*pm, Wizard of Woodstock wrote:
On Wed, 24 Jun 2009 15:20:52 -0700 (PDT), Frogwatch

wrote:
Other than decreasing topheaviness and increasing beam, how do you go
about minimizing probability of capsize for a powerboat? *Consider a
swamped boat, what factors cause it to want to float upside down?

Momentum of water, amount of water, vertical center of gravity, how
steep the Vee is, what the sea state is, how many people and located
where - whole host of factors, the primes being Vcog, Vee and sea
state. A swamped boat will stay upright in reasonable seas, but if you
had been moving in the ditch (the bottom point or trough, between two
waves) which is a completely insane idea in a small boat).

Decreasing the amount of top weight doesn't always translate to
decreasing the Vcog either. Increasing the beam of a boat doesn't
necessarily mean the boat automatically becomes more stable either
depending on how steep the Vee is. It's more the shape of the hull,
the type of hull (modified Vee vs Deep Vee) and it's draft that
determine stability.

If you design a boat with a moderate or semi-vee the boat will tend to
ride flatter on the surface which means that in a heavy sea state it
will pound or if you are unfortunate enough to get into a heavy
following sea or quartering waves when transiting actually cause more
pitch and yaw which can be very uncomfortable. On the other hand, a
steeper Vee adds in the roll component and now your dealing with roll,
pitch and yaw and that's also dangerous.

It's a guessing game in a lot of ways in particular if you are
modifying a plan that has some merit to it as it stands.

OK. lets consider a boat that has swamped but has a deck with
floatation below the deck. I think that in many (most?) cases she may
continue to swamp. How will she float if the water cannot get out?
How do we arrange subsequent floatation to minimize probability of
capsize? A cubic foot of floatation on the gunnels will give 57 lbs
of upward force if we try to immerse it in the water. Say we have 10
cubic feet for a total of 570 lbs of upward force on each gunnel.
That is a lot of force and we may need a lot less. This seems to
indicate that we should have floatation not only under the deck but
under the gunnels.
Would we be better with most floatation under the gunnels and no deck
so we actually stand on the bottom of the boat as in my current Tolman
(she has floatation in the bow and stern floatation tanks too). This
would lower the COG further minimizing the risk of capsize and getting
enough floatation on the gunnels so she cannot turn over even when
swamped.
Thoughts?

On Wed, 24 Jun 2009 17:19:51 -0700 (PDT), Frogwatch
wrote:

On Jun 24, 7:22*pm, Wizard of Woodstock wrote:
On Wed, 24 Jun 2009 15:20:52 -0700 (PDT), Frogwatch

wrote:
Other than decreasing topheaviness and increasing beam, how do you go
about minimizing probability of capsize for a powerboat? *Consider a
swamped boat, what factors cause it to want to float upside down?

Momentum of water, amount of water, vertical center of gravity, how
steep the Vee is, what the sea state is, how many people and located
where - whole host of factors, the primes being Vcog, Vee and sea
state. A swamped boat will stay upright in reasonable seas, but if you
had been moving in the ditch (the bottom point or trough, between two
waves) which is a completely insane idea in a small boat).

Decreasing the amount of top weight doesn't always translate to
decreasing the Vcog either. Increasing the beam of a boat doesn't
necessarily mean the boat automatically becomes more stable either
depending on how steep the Vee is. It's more the shape of the hull,
the type of hull (modified Vee vs Deep Vee) and it's draft that
determine stability.

If you design a boat with a moderate or semi-vee the boat will tend to
ride flatter on the surface which means that in a heavy sea state it
will pound or if you are unfortunate enough to get into a heavy
following sea or quartering waves when transiting actually cause more
pitch and yaw which can be very uncomfortable. On the other hand, a
steeper Vee adds in the roll component and now your dealing with roll,
pitch and yaw and that's also dangerous.

It's a guessing game in a lot of ways in particular if you are
modifying a plan that has some merit to it as it stands.

OK. lets consider a boat that has swamped but has a deck with
floatation below the deck. I think that in many (most?) cases she may
continue to swamp. How will she float if the water cannot get out?
How do we arrange subsequent floatation to minimize probability of
capsize? A cubic foot of floatation on the gunnels will give 57 lbs
of upward force if we try to immerse it in the water. Say we have 10
cubic feet for a total of 570 lbs of upward force on each gunnel.
That is a lot of force and we may need a lot less. This seems to
indicate that we should have floatation not only under the deck but
under the gunnels.
Would we be better with most floatation under the gunnels and no deck
so we actually stand on the bottom of the boat as in my current Tolman
(she has floatation in the bow and stern floatation tanks too). This
would lower the COG further minimizing the risk of capsize and getting
enough floatation on the gunnels so she cannot turn over even when
swamped.
Thoughts?

Interesting problem. And I'm not a naval architect nor do I play one
on TV.

However, it seems to me that the problem isn't so much the flotation
as it is positioning the Vcog such that you reduce the mechanical
leverage that causes the rollover. Floatation only allows for keeping
the boat on the surface - not necessarily from rolling or broaching if
that term is applicable to power boats - but it gets the point across.
Any boat, including mine which bobs like a cork and is designed to be
a level floater, can capsize if presented with enough leverage and/or
angular momentum. (Just so you understand, my boat is filled,
stem-to-stern, top-to-bottom with closed cell foam - there isn't a lot
of space left in my hull for anything except for a very small space in
the stern where all the pumps are.)

So your supposition of lowering Vcog is probably the best choice as
that is how you reduce lateral mechanical leverage given any
particular hull design. But again, it also depends on what type of
hull you have - at certain angles of Vee, you get inherent lateral
instability because of the design of the hull. For example, given the
same length my Ranger with it's 8 degree dead rise, wide beam (92") is
stable enough to me at 6'3", 255 to stand on the gunwale and there is
practically no movement in terms of roll even if I jump up and down.
Do the same thing on say a 20' Sea Pro with a 22 degree deadrise, and
you will notice that I'm standing there.

Conversely, and I've done this as an experiment, if I climb the T-top
on my Ranger adding weight higher up, the Ranger does become a little
dicey and I have no doubt that I could roll it sufficient to swamp it
if I really wanted to - which I don't. :)

Don't know if that helps any.