[MHml] Engine power

[Russell_P_Davis at cox.net]

Martin Schöön Said: "Something none of you have commented on yet is the 
'fact' that friction dominates over wave drag as the speed increases
above 10 knots."

Martin,

    It was that relationship shown in the graph on page #5 that provoked the 
musing on the dynamics of how and why fish appear to be such better mariners 
that we are.  If anyone can learn from the best mariners I figure you can.

Friction appears to be the largest power sink.

It seems that 'Friction' might be more open to increments of improvement if 
it could be divided into several sources of friction.

The question about the SWATH or submarine version of  the hull (doubled as 
one) was an attempt to split friction into two more manageable components: 
1(hull-skin:water friction in line with hull motion & friction internal to 
water shear on water displacement normal to the hull's displacing 
surface[first approximation]) and 2(hull-skin:water friction NOT in line 
with hull motion & friction internal to water shear on water displacement 
NOT a portion of that displacement required by the new position of the hull 
as it moves[first approximation])

I recognize that given a use for a vessel there are such good reasons why it 
is shaped the way it is - maybe not much can be changed.  I  personal 
operate in waters too shallow to allow a more fish-like SWATH hull - unless 
it was shaped like one of those clam eating sharks - but those type of fish 
are not known for their speed.

The part of fiction that can be addressed easily may yield an easy profit in 
reduced power requirements and fuel costs with higher payload at the same 
displacement.

Russell



----- Original Message ----- 
From: <Russell_P_Davis at cox.net>
To: "Informed discussion of multihull issues" <multihulls at steamradio.com>
Sent: Sunday, February 04, 2007 12:38 PM
Subject: Re: [MHml] Engine power


Martin,
That is an exciting piece of work you did - Very exciting to this uniformed
person.
I wonder how it is that fish like tuna can move so fast compared to most
sailors?  Is it possible for us to know how much better our designs could
be, so we could know what was worth attempting?

If  we could see the heat signature of the propulsion work on the water I
imagine that the heat that would show around the hull is fairly symmetrical
to the heat that  would show on the wave break at the shoreline.  Is that
somewhat true?  The environmental cost of those waves are just beginning to
be recognized.  Between that environmental cost and engine power cost
multihull design has got to have a great future.

I would love to see the same graphs for a version of the same hull in which
the water friction resulting from the air-water interface were removed - say
take the same length and displacement but run them as a submarine or a
SWATH.

Then see the same grafts for both the cat and the sub with skin friction
assumed away by a fantasy supercavitating or fantasy fish mucus surface
treatment so all we would see is as work and waves as would be the energy
spent in displacing the water of the hull cross section.

Then I wonder how much of the displacement energy returning as our 'fish'
moves from a spot could be captured and redirected by 'propeller' - kind of
analogous to the way the elastic tendons of running birds help them run
efficiently.  The fact that there is a stern wave at all shows that there is
something to capture - I bet some fish do that - possibly much as the
axial turbines counter vanes improve efficiency - But I must admit I cannot
visualize the flow where that would happen.

I hope you experience the questions as more interesting than vexing.  Given
your tagline I imagine that you will enjoy them.

Russell Davis


----- Original Message ----- 
From: "Martin Schöön" <martin.schoon at gmail.com>
To: "Informed discussion of multihull issues" <multihulls at steamradio.com>
Sent: Sunday, February 04, 2007 8:34 AM
Subject: Re: [MHml] Engine power


On 2/3/07, roy mills <rsirfj at shaw.ca> wrote:
<snip>
>
>     Are you quite sure you want to go at 20 knots?  14 would be twice as
> fast and would only need provide about 4.5 times as much thrust instead of
> 10. Besides 90 hp is not as big a challenge to find.  You were concerned
> that a tall standing outboard would bang up against the underside of AB.
> If
> you did not use an outboard that height would not be a problem. Sure
> Diesel
> is heavier, but fuel is heavy too and you would burn a good deal less.
> Prop
> efficiency is the key factor in converting hp to thrust.
>
>     Cheers anyway   Roy, who looks forward to a similar blizzard of
> technical advice to what you got with your initial query.

I agree with Roy when he questions your need for 20 knots. High speed
is costly in terms of power (and fuel cost).

I have done some (w)reckless number crunching. The results are
summarised in a pdf-dcoument that can be downloaded at:
http://hem.bredband.net/mschoon/Modelling_Afterburner.pdf

-- 
Martin Schöön              "Problems worthy of attack
                                         prove their worth by hitting back"
                                         Piet Hein
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