The essential fact to understand about hull speed is that there is an
exact relationship between the length of a wave and how fast it moves
through the water. If you time the crests as they go by a fixed point
like a buoy, you can calculate the exact distance between the crests.
Longer waves move faster.

The hull makes wave as it disturbs the water. At low speeds, there is
room for several crests and troughs along the hull. You can see by the
large wave system even a small pebble sets up that it doesn't take a
lot of energy to create a wave train. Hull resistance at low speeds is
primarily skin friction.

As speed increases, the waves the boat makes must become longer in
order to maintain the speed / length relationship. Eventually there is
room for just one wave at the bow and one quarter wave at the stern.
When the speed length ratio is 1.0, there will be a crest at the bow
and another at the stern. The boat will be sitting fairly
symmetrically without trimming down by the stern and the wave
rebounding up under the stern will actually be pushing the vessel
ahead recovering some, but far from all, of the energy required to
produce the wave train. Vessels can thus get up to this speed with
fairly modest power.

To go faster however, the crest of the wave at the stern has to start
moving behind the boat. Two things happen. First, wave behind the hull
can not return energy to it. This pushes power requirements up.
Second, the hull now starts to squat by the stern which is moving into
the trough. The bow wave always remains about in the same place so the
boat has to start climbing up a hill that it is also making. The graph
of power required starts to go straight up as the stern wave moves aft
of the transom.

The basic relationship is that it takes four times as much power to go
twice as fast. If you graph this out, you'll see that hull speed is
not a precise point but is a fairly narrow band. You quickly reach a
point where doubling the size of the engine only gains you a quarter
knot.

If the boat is shaped so that water flow over the bottom creates
dynamic lift instead of suction, the hull will start to lift up. With
sufficient power, the vessel can be pushed up the hill of the bow wave
on to the top where it can again ride level. It will still be
producing a wave train but all the crests will be well behind it. A
deep hull like a sailboat or a tug boat won't do this. The suction of
steep flow lines in the stern will pull the stern down. Some hulls
will actually pull themselves below the surface if enough power is
applied. The waves created by hull will keep the water off the deck
but, if something suddenly stops the hull, it can be swamped by its
own wake.


--

Roger Long



Does it apply to non-rigid hulls (hulls that might flex in the
middle)
Very complex question. Can't be answered in general.

Does it apply to totally submerged objects?
No.

Does it apply to towed objects, like dinghies?
Yes.

What happens when an object exceeds hull speed?
See above.

Is there any way to "fool the water" into acting as if the boat is
longer than it is?
If anybody has figured out how to fool the universe yet, I'd like to
hear about it.

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

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

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

In article ,
Stephen Trapani wrote:

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.

I thought I mentioned this before. Hope I'm not repeating myself.

Hull speed is a suggestion for our boat, not the law. Though our
theoretical hull speed is 6.65 knots, we regularly exceed that with
aplomb, close hauled, close reach, broad reach, whatever point of sail.
Spent a wonderful afternoon with 6 other sailors last season. As long as
I was on the tiller, pushing her to where she likes to be, we were well
above the theoretical hull speed. As we pinched to get back into the
harbor, she insisted on doing over 7 knots directly into the wind (okay,
about 15 degrees off). That last was our lovely lady showing off, of
course, as what we did was clearly impossible.

1.34 was derived from observing boats about a century ago. Depending on
the hull, that constant can be quite a bit different. As I recall, some
multi-hull boats' K is in the 2 or 3 range. Xan's fat ass and sharp
transom keeps her driving towards a 1.7 or so constant.

--
Jere Lull
Xan-a-Deux ('73 Tanzer 28 #4 out of Tolchester, MD)
Xan's Pages: http://members.dca.net/jerelull/X-Main.html
Our BVI FAQs (290+ pics) http://homepage.mac.com/jerelull/BVI/

Stephen Trapani wrote:
Hull speed is the absolute maximum that boat can travel through water.

Not really. "Hull Speed" is sort of a convenient shorthand for
indicating where the graph of a vessel's speed vs power begins to get
inconveniently steep.


All your examples have the water moving forward also so the boat is not
exceeding hull speed through the water.

Even catamarans? How about planing types?

DSK

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.