Bruce wrote, "I agree that the length to beam ratio is important but I don't
think that you can overlook how critical an oversized and effectively placed
rudder can be to enhancing directional stability. I think this is the most
important factor. The ability to steer effectively in a following sea is, in
my view, more rudder related. And, in a serious following sea being able to
use a quick blast of power to get steering response is helpful. Compound
rudders and KORT nozzles have been shown to be particularly efficient in
such scenarios."

You have a valid point, although if a vessel is truly stable then it will go
straight even without the rudder. If it is unstable then you need a big
rudder and fast reactions to keep it going straight.

OTOH, if we are talking full displacement hulls. When a wave over powers the
stern it wants to go faster than the bow and you'll need a lot of rudder to
fight it. This is one area where I think a semi-displacement hull has an
advantage. The semi-displacement boat will start to plane and surf an
overtaking wave. I think a long narrow semi-displacement hull could run in
conditions where a displacement hull would need to head into the sea.

Just my uneducated opinion, but if I'm wrong, I'd like to know about it
sooner than later.

Regards;
Mike Schooley
Designing "Portager" a 48' transportable trawler

I have found semi-displacing hulls do surf well to well some times. I find
this can be very dangerous if you end up surfing to fast and into the back
of the next wave. You then get what I call bow steer that will leave you
broadside to the following wave. Recovery from this position is hard and
very uncomfortable and can be unforgivable. I chose the slow and steady
approach even in a semi displacing hull letting the waves pass under from
the aft. The closer you  match the wave speed the smoother the ride. I have
also found in following seas that only a large rudder really helps. Semi-
displacement vessels seem to have small rudders that work best under thrust
from the prop unlike displacement hulls. My Opinion.

Willy
Invader No1
39 Kishi Conversion

OTOH, if we are talking full displacement hulls. When a wave over powers the
stern it wants to go faster than the bow and you'll need a lot of rudder to
fight it. This is one area where I think a semi-displacement hull has an
advantage. The semi-displacement boat will start to plane and surf an
overtaking wave. I think a long narrow semi-displacement hull could run in
conditions where a displacement hull would need to head into the sea.

Just my uneducated opinion, but if I'm wrong, I'd like to know about it
sooner than later.

Regards;
Mike Schooley
Designing "Portager" a 48' transportable trawler

Willy wrote, "I have found semi-displacing hulls do surf well to well some
times. I find this can be very dangerous if you end up surfing to fast and
into the back of the next wave. You then get what I call bow steer that will
leave you broadside to the following wave. Recovery from this position is
hard and very uncomfortable and can be unforgivable. I chose the slow and
steady approach even in a semi displacing hull letting the waves pass under
from the aft. The closer you match the wave speed the smoother the ride. I
have also found in following seas that only a large rudder really helps.
Semi-displacement vessels seem to have small rudders that work best under
thrust from the prop unlike displacement hulls. My Opinion."

Thanks Willy and also Bruce. Your inputs have provided valuable insight.

Your right, Willy, most semi-displacement hulls are optimized to operate at
2x to 3x displacement speed and their rudders are too small for adequate
control and slower speeds. Portager will only operate at about 1.5x
displacement speed and will cruise at displacement speed. She will have a
L/B ~5 and a large deep rudder so I think she will handle rough conditions
well. I just want to make sure that I am not compromising my rough weather
performance too much by going with a semi-displacement hull.

I have been thinking of reducing the draft by using a tunnel drive or twin
props, but this would also reduce the draft of the rudder.

Regards;
Mike Schooley
Designing "Portager" transportable Passagemaker

Chuck Shipley wrote, "Isn't it true that the hard chines of a
semi-displacement hull result in a vessel that is more susceptible to yawing
when waves are broad on your quarter than would be the case with the soft
chines of some full-displacement hulls?"

I believe that yours is a popular opinion, especially among displacement
hull builders, however it is not supported by any unbiased analysis or test
data that I am aware of.

Here is a good example of a direct comparison.
http://www.rina.org.uk/rfiles/IJSCT/Discuss/bailey.pdf .

The most relevant section 3.1 at the end of page 1 and top of page 2. They
performed side by side model testing of nearly identical round bilge and
hard chine hulls. The hard chine provided comparable or better performance
with better roll damping at low speed and much improved yaw resistance in
following seas.

Another good comparison is
http://www.formsys.com/Maxsurf/MSProductRange/SKPaper-Jul00.pdf . Again the
results are very close with the hard chine performing slightly better.

It is my opinion that the conditions you experienced are due to excessive
beam, not due to the hard chines of the semi-displacement hull.

Regards;
Mike Schooley
Designing "Portager" a transportable Passagemaker

Mike something to think about from my experience in the difference between
the two. When the hard chine gets slapped from under or down the side the
impact can be quite violent. With a good bang and a shake. To the point I
have seen it take out a window on an old Chris Craft 60 s era ( fun ride <
grin>) The soft chine seems not to react this way , smooth no bang or
shudder. One of the reasons I like the old fish boat as I hated that bang
and shudder. Someone else might be able to say if I am off base here or not.

Willy
Invader No1
39 Kishi conversion

Chuck Shipley wrote, "Isn't it true that the hard chines of a
semi-displacement hull result in a vessel that is more susceptible to yawing
when waves are broad on your quarter than would be the case with the soft
chines of some full-displacement hulls?"

I believe that yours is a popular opinion, especially among displacement
hull builders, however it is not supported by any unbiased analysis or test
data that I am aware of.

. The hard chine provided comparable or better performance
with better roll damping at low speed and much improved yaw resistance in
following seas.

Again the
results are very close with the hard chine performing slightly better.

It is my opinion that the conditions you experienced are due to excessive
beam, not due to the hard chines of the semi-displacement hull.

Regards;
Mike Schooley
Designing "Portager" a transportable Passagemaker

Passagemaking-Under-Power Mailing List

Willy wrote, "Mike something to think about from my experience in the
difference between the two. When the hard chine gets slapped from under or
down the side the impact can be quite violent. With a good bang and a shake.
To the point I have seen it take out a window on an old Chris Craft 60 s era
( fun ride < grin>) The soft chine seems not to react this way , smooth no
bang or shudder. One of the reasons I like the old fish boat as I hated that
bang and shudder. Someone else might be able to say if I am off base here or
not."

You raise a valid point. OTOH, not all semi-displacement hulls are created
equal. Some are almost planning hulls while others are closer to
displacement hulls. I have experienced conditions like you describe on
planning hulls, especially on my companies "Magnum". Could you tell me which
models you are referring to?

Regards;
Mike Schooley
Designing "Portager" a transportable Passagemaker

Willy wrote, "Mike something to think about from my experience in the
difference between the two. When the hard chine gets slapped from under or
down the side the impact can be quite violent. With a good bang and a shake.
To the point I have seen it take out a window on an old Chris Craft 60 s era
( fun ride < grin>) The soft chine seems not to react this way , smooth no
bang or shudder. One of the reasons I like the old fish boat as I hated that
bang and shudder. Someone else might be able to say if I am off base here or
not."

You raise a valid point. OTOH, not all semi-displacement hulls are created
equal. Some are almost planning hulls while others are closer to
displacement hulls. I have experienced conditions like you describe on
planning hulls, especially on my companies "Magnum". Could you tell me which
models you are referring to?

Regards;
Mike Schooley
Designing "Portager" a transportable Passagemaker

Agreed  there are lots of different designs on the same boats.

Some of my examples.

(Trawler)
The Grand Mariner 36 foot had hard chines and it banged and shuddered (
entered the water hard ).
The X fishboat has soft and rides and or enters the water soft.
(Semi displacement )
The Chris Craft Cavalier has hard chines and bangs and shudders or enters
the water hard.
The Chris Craft Sea Skiff has soft chines and enters the water soft.

The Uniflight 32 had hard chines and ran hard.
A Bayliner 32  had soft and entered soft.

So my take is the reaction to the chine or lack of. Not giving the water a
flat surface to smack gives a smoother ride. Again I might be off base here.

The Magnum type style needs chines for control at high speeds or the speed
they operate. I find any of these style boats bang and shudder including the
V bottom with miltiple lateral chines. But they need the chines for lateral
control and stability as they have no keel. Could be an interesting ride
without the chines and or keel when at the speeds they are capable, <grin>.

Willy
Invader No1
39 kishi Conversion

Willy wrote, " The Grand Mariner 36 foot had hard chines and it banged and
shuddered (entered the water hard ).
The X fishboat has soft and rides and or enters the water soft.
(Semi displacement)
The Chris Craft Cavalier has hard chines and bangs and shudders or enters
the water hard.
The Chris Craft Sea Skiff has soft chines and enters the water soft.
The Uniflight 32 had hard chines and ran hard.
A Bayliner 32  had soft and entered soft.

You make a good case for avoiding hard chines, but it appears that hard
chines are not necessarily mandatory on a semi displacement hull. Here are
some of the references I found.

http://www.boatus.com/goodoldboat/steelboat.htm

http://www.yachtworld.com/leeswilbur/leeswilbur_5.html

http://www.bruceroberts.com/public/HTML/POWERBOAT_DESIGN.htm

Intuitively I think soft chines probably entail some performance degradation
at high speed, but since I'm not really concerned about efficiency at high
speed, I think soft chines look like a good solution.

Then I decided to check the Dashews web site.

I found their page titles, "Directional Control"
http://www.setsail.com/dashew/dashew227.html most interesting. Here is a key
paragraph.

"The most important design characteristic controlling downwind steering - in
fact all steering - is beam-to-length ratio. The skinnier the boat, the more
directionally stable it is going to be, and the less it is apt to be
affected by changes in trim induced by waves. How the hydrostatic properties
of the hull change with heel is a corollary. If you get these factors right,
the hull will be inherently stable. And if they are wrong, no combination of
keel or rudder is going to be able to keep you comfortable, or under control
in big seas."

This correlates with what I have been saying, although I believe their lack
of confidence in the keel's contribution to directional stability is due to
their use on a modern sailboat keel. Since their keel is short in the axial
direction and centered at the center of the hull it has no control moment
and therefore makes minimal contribution to directional stability. A full
length keel (or better yet center to stern) would by much more effective.

Getting back to the point, the Dashew's continue,

"These design characteristics which promote safety in storms become comfort
factors in normal tradewind passaging. Get it right for one part of the
equation, and it will work for the other. A boat which "roots" (steers by
its deeply immersed bow) in normal passaging will not be able to run before
a storm, and will always have to slow down as the seas build. The motion of
a hull which is having difficult tracking is far more severe than one which
is headed in a straight line. One that steers better as it accelerates can
run off before a storm, opening an additional array of tactics (for more
information on heavy weather tactics see our Surviving the Storm)."

I think that this provides the answer to my original question. A well
designed semi-displacement hull with good directional stability can run
faster than a full displacement hull.

The other part of the solution is to use soft or radius chines. As far as I
can tell the Dashew's hull is a radius chine design also.

Regards;
Mike Schooley
Designing "Portager" a transportable Passagemaker