On 12 Apr 2005 11:46:08 -0700, (Matteo) wrote:

I'm thinging of dirty hull (green slime), incorrect weight
distribution (bow tends to "point" upwards even when crossing small
waves).

==================================

A dirty bottom and/or dirty props will definitely slow you down. It's
also save to say that the effective waterline length of a 40 ft boat
is actually less than 40 ft. Another factor is something called
prismatic coefficient which if a fancy way of describing how sleek
your hull form is. Obviously it's going to take more power to drive a
40 ft square box through the water than a 40 ft sailboat. The
equation of 1.34 SQRT LWL is realy nothing more than an approximation
and is not written in concrete.

Roger Long:

One of these days I'd like to look into the derivation of hull speed.
Can you suggest a very basic explanation (a source thereof). I'm just
curious as to when it applies. Like:

Does it apply to non-rigid hulls (hulls that might flex in the middle)
Does it apply to totally submerged objects?
Does it apply to towed objects, like dinghies?
What happens when an object exceeds hull speed?
Is there any way to "fool the water" into acting as if the boat is
longer than it is?

Thanks

David OHara


Wayne. B wrote:
On 12 Apr 2005 11:46:08 -0700, (Matteo) wrote:

I'm thinging of dirty hull (green slime), incorrect weight
distribution (bow tends to "point" upwards even when crossing small
waves).

==================================

A dirty bottom and/or dirty props will definitely slow you down.
It's
also save to say that the effective waterline length of a 40 ft boat
is actually less than 40 ft. Another factor is something called
prismatic coefficient which if a fancy way of describing how sleek
your hull form is. Obviously it's going to take more power to drive
a
40 ft square box through the water than a 40 ft sailboat. The
equation of 1.34 SQRT LWL is realy nothing more than an approximation
and is not written in concrete.

On 12 Apr 2005 17:45:57 -0700, wrote:

One of these days I'd like to look into the derivation of hull speed.

=====================================

I'm guessing a bit but I'm pretty sure that you and James Clerk
Maxwell could figure it out with the aid of some wave equations and
the density of water.

wrote:
Roger Long:

One of these days I'd like to look into the derivation of hull speed.
Can you suggest a very basic explanation (a source thereof). I'm just
curious as to when it applies. Like:

Does it apply to non-rigid hulls (hulls that might flex in the middle)
Does it apply to totally submerged objects?
Does it apply to towed objects, like dinghies?
What happens when an object exceeds hull speed?
Is there any way to "fool the water" into acting as if the boat is
longer than it is?

Thanks

David OHara

I would think that if you remove the water from in front of the boat
and then replace it after the boat has gone by, there would be no
wake, or lost energy caused by the passage of the boat. Then, the
only restriction to speed would be skin friction. Energy left behind
a boat in the form of waves, or a wake, represents energy put into
propulsion which is inefficiently employed, causing a disturbance in
the water instead of increasing the boat speed. In normal hull
shapes, the full weight of water displaced by the hull must be moved
out of the way of the boat, and then it must return to the space
evacuated by the passage of the boat. That water will be disturbed,
containing eddies of water, the energy required for the formation of
waves and eddies being lost to the purpose at hand, propulsion.

I once envisioned a submarine built like a stove pipe, with a
venturi and prop inside a narrowing of the central tube. It should
produce no wake. Any such boat would never need to climb a wave, it
would rather always be going downhill, into the space where the
water was vacuumed out.

In such a sub, I also envisioned a ducted rim-driven propellor with
no hub. I wonder if there could be some advantage to this?

Terry K

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.

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?