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Old April 12th 05, 07:46 PM
Matteo
 
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Default Effective speed much less than theoretical hull speed.

This is the situation: My 40' LWL boat (15 ton displacement) has a 150
PS engine. From the formula for speed I calculated a hull speed of

sqrt(40)*1.34 = 8.47 knots

*but*: when i did trials last week (absolutely calm water, almost no
wind) those are the results:

800 rpm 5 knots no noticeable waves generated
1100 rpm 5.5 knots small waves
1800 rpm 6.5 knots (flat out) - huge waves generated, stern deep in
the water, boat "running uphill".

1100 rpm is around 50/60 PS (from the engine rpm/PS table).

Question: what could be the cause of the "slowness" of the boat ? I do
not pretend to reach 8.4 knots cruising but at least 7 knots should be
in.

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

Any experience ?

Thanks
Matteo

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Old April 12th 05, 08:22 PM
Roger Long
 
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Question: what could be the cause of the "slowness" of the boat ?

Expectations.

Hull speed is the theoretical limit for any practical amount of power
unless the hull shape is such that planing lift can start to reduce
the displacement. You have far from that amount of power.

The 1.34 figure would also be for a fairly slender (in flow terms, not
necessarily length to beam) hull. The number goes down as the hull
gets fatter. 1.25 is a more typical number for vessels as heavy as
cruising sailboats and heavy power vessels but they will take a lot of
power to get up to it.

If the hull is making the waves and trimming as you describe, cleaning
the bottom and fiddling with the prop probably won't make much of a
difference.

A lot has to do with the flow angles in the run. The bow makes
rebound up and push the stern ahead, recovering some of the energy
expended in making them and helping keep the stern up. Once the angle
between the run and the direction of motion exceeds 12 - 15 degrees,
the flow separates and the space between the smooth flow and the hull
fills with lazy eddies of water that largely move along with the hull.
Energy from the wave train can not be returned to the hull through the
zone. The effective waterline of your hull is actually from the bow
to the point where the flow lines (generally along the diagonals)
exceeds this critical angle.

--

Roger Long




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Old April 12th 05, 08:57 PM
Roger Long
 
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Oops. That's supposed to say, "The WAVES THE bow makes..." as
corrected below.


--

Roger Long



"Roger Long" wrote in message
...
Question: what could be the cause of the "slowness" of the boat ?


Expectations.

Hull speed is the theoretical limit for any practical amount of
power unless the hull shape is such that planing lift can start to
reduce the displacement. You have far from that amount of power.

The 1.34 figure would also be for a fairly slender (in flow terms,
not necessarily length to beam) hull. The number goes down as the
hull gets fatter. 1.25 is a more typical number for vessels as
heavy as cruising sailboats and heavy power vessels but they will
take a lot of power to get up to it.

If the hull is making the waves and trimming as you describe,
cleaning the bottom and fiddling with the prop probably won't make
much of a difference.

A lot has to do with the flow angles in the run. rebound up and
push the stern ahead, recovering some of the energy expended in
making them and helping keep the stern up. Once the angle between
the run and the direction of motion exceeds 12 - 15 degrees, the
flow separates and the space between the smooth flow and the hull
fills with lazy eddies of water that largely move along with the
hull. Energy from the wave train can not be returned to the hull
through the zone. The effective waterline of your hull is actually
from the bow to the point where the flow lines (generally along the
diagonals) exceeds this critical angle.

--

Roger Long






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Old April 13th 05, 01:45 AM
[email protected]
 
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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.




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Old April 13th 05, 05:11 AM
Terry Spragg
 
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Default

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


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Old April 13th 05, 11:18 AM
Roger Long
 
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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.


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Old April 13th 05, 04:19 PM
[email protected]
 
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Default

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

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Old April 13th 05, 04:23 PM
[email protected]
 
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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?



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