[MHml] Engine power

[Russell_P_Davis at cox.net]

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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