The float would have a hull speed limitation based on it's length. If
it was shorter than the main hull, it would be a big drag.

--

Roger Long



wrote in message
oups.com...
Roger:

Thank you for a very lucid explanation.
From this, is it correct to think that "hull Speed" is not some sort
of
value at which mathematics goes crazy and produces singularities but
simply represents a speed range in which necesary power to produce a
speed increase seriously increases?
Is Hull Speed defined in some way relating to the slope of the power
vs
speed curve?

Now, for the bizarre theory question. Consider a small boat that
has a
very long rigid extension on its stern that does not touch the water
except far from the boat where it has a rigid float. Would this
have a
higher hull speed than the small boat alone?
Could you arrange for this float at the end to gain back energy from
the trough behind it?
Could you arrange floats on this rigid extension at certain places
to
extract energy from the shorter period waves the boat produces?

David

No.

--

Roger Long



wrote in message
oups.com...
Can we alter the properties of the water surface to change hull
speeed.
What I have in mind is like spreading oil on water where oil is
spread
from the bow. I assume that what this does is to decrease the
amplitude of the shorter period waves. Even if it didnt increaqse
hull
speed, would it reduce the energy going into the shorter period
waves?

Roger, you sure know how to kill my fun, but thanks

However, there are games played with multihulls so that the waves from
one hull cancel the wave from the other. For one thing, this must be
considered to understand how the chop will slap on the underside.

However, advanced work has been done on more complex configurations of
three or four hulls with an eye towards high speed and efficiency. I
don't think this has led to any recreational sailboat designs.


Roger Long wrote:
The float would have a hull speed limitation based on it's length. If
it was shorter than the main hull, it would be a big drag.

I own a Newport 33 which has a waterline length of 27 ft. According to
the formula, the theoretical speed for the boat is 6.96 knots.
I have a 16 HP diesel with a 2 bladed impeller, and a maximum engine
RPM of 3300 RPM. Running the engine at 2700 RPM I can readily reach 6.5
knots.
In a good wind I can go to 7 knots. The maximum speed I have ever done
was 11 knots on the GPS surfing down a wave with full sails up on a
very broad reach in about 30 knot wind. Many other boats of the same
design ( relatively light displacemnt, fin keel and spade rudder)
report he same thing.
Racing boats in the around-the world alone race routinely exceeded hull
speed for long periods surfing down waves. The hull speed for a 60 ft
boat is 10.4 knots andthey were achieving more than 20 knots I seem to
remember. So that is the way to go faster than hull speed, find a wave
and then surf down.
Catamarans also go faster than hull speed all the time. So if you put
enough power into the boat in relation to the displacement and wetted
surface, you can exceed the Hull speed.

I think that traditional full keel boat with a high displacement would
have a lot of trouble getting close to Hull speed.
Rolf


Jeff wrote:
However, there are games played with multihulls so that the waves
from
one hull cancel the wave from the other. For one thing, this must be

considered to understand how the chop will slap on the underside.

However, advanced work has been done on more complex configurations
of
three or four hulls with an eye towards high speed and efficiency. I

don't think this has led to any recreational sailboat designs.


Roger Long wrote:
The float would have a hull speed limitation based on it's length.
If
it was shorter than the main hull, it would be a big drag.

Rolf wrote:

I own a Newport 33 which has a waterline length of 27 ft. According to
the formula, the theoretical speed for the boat is 6.96 knots.
I have a 16 HP diesel with a 2 bladed impeller, and a maximum engine
RPM of 3300 RPM. Running the engine at 2700 RPM I can readily reach 6.5
knots.
In a good wind I can go to 7 knots. The maximum speed I have ever done
was 11 knots on the GPS surfing down a wave with full sails up on a
very broad reach in about 30 knot wind. Many other boats of the same
design ( relatively light displacemnt, fin keel and spade rudder)
report he same thing.
Racing boats in the around-the world alone race routinely exceeded hull
speed for long periods surfing down waves. The hull speed for a 60 ft
boat is 10.4 knots andthey were achieving more than 20 knots I seem to
remember. So that is the way to go faster than hull speed, find a wave
and then surf down.
Catamarans also go faster than hull speed all the time. So if you put
enough power into the boat in relation to the displacement and wetted
surface, you can exceed the Hull speed.

I think that traditional full keel boat with a high displacement would
have a lot of trouble getting close to Hull speed.
Rolf

Hull speed is the absolute maximum that boat can travel through water.
All your examples have the water moving forward also so the boat is not
exceeding hull speed through the water.

Stephen

Actually, the water does not move forward in a wave but you are right
that the surfing examples are irrelevant examples since the waves are
pushing the boat forward in other ways.

The speed length ratio of the Newport 33 at 7 knots would be 1.35,
just a hair above the generally accepted displacement hull maximum of
1.33. If the hull has an easy run, the counter becomes part of the
waterline length as the stern waves rise up under it. Adding a foot
brings the ratio down to 1.32, exactly what you would expect for an
easy hull like that one.

--

Roger Long



"Stephen Trapani" wrote in message
...
Rolf wrote:

I own a Newport 33 which has a waterline length of 27 ft.
According to
the formula, the theoretical speed for the boat is 6.96 knots.
I have a 16 HP diesel with a 2 bladed impeller, and a maximum
engine
RPM of 3300 RPM. Running the engine at 2700 RPM I can readily reach
6.5
knots.
In a good wind I can go to 7 knots. The maximum speed I have ever
done
was 11 knots on the GPS surfing down a wave with full sails up on a
very broad reach in about 30 knot wind. Many other boats of the
same
design ( relatively light displacemnt, fin keel and spade rudder)
report he same thing.
Racing boats in the around-the world alone race routinely exceeded
hull
speed for long periods surfing down waves. The hull speed for a 60
ft
boat is 10.4 knots andthey were achieving more than 20 knots I seem
to
remember. So that is the way to go faster than hull speed, find a
wave
and then surf down.
Catamarans also go faster than hull speed all the time. So if you
put
enough power into the boat in relation to the displacement and
wetted
surface, you can exceed the Hull speed.

I think that traditional full keel boat with a high displacement
would
have a lot of trouble getting close to Hull speed.
Rolf

Hull speed is the absolute maximum that boat can travel through
water. All your examples have the water moving forward also so the
boat is not exceeding hull speed through the water.

Stephen

On 13 Apr 2005 20:55:30 -0700, "Rolf" wrote:

I own a Newport 33 which has a waterline length of 27 ft. According to
the formula, the theoretical speed for the boat is 6.96 knots.
I have a 16 HP diesel with a 2 bladed impeller, and a maximum engine
RPM of 3300 RPM. Running the engine at 2700 RPM I can readily reach 6.5
knots.

I have a Viking 33 with same waterline. My direct-drive Atomic 4 with
a two-blade can drive the boat at 5.8 knots in flat water at
half-throttle, but it's too damn noisy to get it to 6.4...that final
half-knot is simply not worth the gas or the noise, as the A4 is
quieter than a diesel at anything but full out.

In a good wind I can go to 7 knots. The maximum speed I have ever done
was 11 knots on the GPS surfing down a wave with full sails up on a
very broad reach in about 30 knot wind. Many other boats of the same
design ( relatively light displacemnt, fin keel and spade rudder)
report he same thing.

Your results match mine. I can hit 7.1 or 7.2 knots SOG sustained in
25 knots on the right point of sail, but she'll "surf" to 10+ briefly
on a run.

snip

I think that traditional full keel boat with a high displacement would
have a lot of trouble getting close to Hull speed.

Not necessarily, but generally, that's correct. Full keelers can surf
on a run as well, but they frequently can't helm quickly enough to
maintain the right angle. On the other hand, in a three-day blow, I'd
much prefer to heave to in a full keeler.

Personal preference, location and experience play a huge role in
getting the most out of your boat. In a full keeler, you may never go
as fast as theory, but you may sail longer because the motion is less
whippy and exhausting. Personally, I like cutaway forefoot, skeg hung,
semi-full keelers. Best of all worlds if designed right. I even like
the still rare idea of canted fixed dual bilge keels with extendable
centerboards, but it's not common (yet).

R.

Personally, I like cutaway forefoot, skeg hung, semi-full keelers.
Best of all worlds if designed right.

This boat designer agrees with you which is exactly why we bought the
Endeavour 32.


--

Roger Long

On Thu, 14 Apr 2005 16:39:41 GMT, "Roger Long"
wrote:

Personally, I like cutaway forefoot, skeg hung, semi-full keelers.
Best of all worlds if designed right.

This boat designer agrees with you which is exactly why we bought the
Endeavour 32.

OK, Mr. Designer...I am glad I am on the right track...I seem to be a
lone voice in the wilderness advocating a number of older Ted
Brewer/Bob Wallstrom/Robert Perry designs G.

On the used boat market, what models would you recommend "like this"
but in the 38-45 foot range? I also favour steel if well constructed
and coated originally, which is admittedly a big "if".

Your opinion would be most appreciated.

R.