[MHml] Transom design for a cat

[Peter Southwood]

Hi Alan,
If your leeward transom does not immerse until the speed is high enough to 
give a clean flow break then I dont see much point in lifting it higher.
Cheers,
Peter

----- Original Message ----- 
From: "Alan Christoffersen" <alan at insatech.com>
To: "Informed discussion of multihull issues" <multihulls at steamradio.com>
Sent: Wednesday, January 31, 2007 10:11 AM
Subject: Re: [MHml] Transom design for a cat


Martin Schöön skrev:
> On 1/31/07, Alan Christoffersen <alan at insatech.com> wrote:
>
>> I am working on the design of a 45 foot performance cruiser - and need 
>> some
>> help as to the design of the transoms.
>>
>
> Are you designing a boat from scratch or modifying something?
> Well, anyways, have a look at
> http://sourceforge.net/projects/freeship/ (free)
> or if you want to pay for some added functionality:
> http://www.delftship.net/ (not expensive)
> and some free CFD that can use out-put from the above mentioned SW.
> http://www.cyberiad.net/michlet.htm
>
> I have played around with the Freeship/Michlet combination this
> winter and it is pretty impressive.
>
>> Should they be immersed at rest or not?  If not, what about when sailing
>> less than hullspeed or more than hullspeed?  I have looked at hundresds 
>> of
>> pictures, the vast majority have immersed transoms even at rest. What are
>> the advantages and disadvantages of a non immersed transom?
>>
>>
> I have suspect all those boats have submerged transoms because
> they are heavier than intended...
>
>
I am designing from scratch, but heavily inspired by some of the great
designers out there.

Hulls and freeboard inspired by John Shuttleworth
Forward cockpit, inside helm and fwd. facing saloon from Chris White -
but much wider and with steering position to one side.
Saloon/cabintop from Jeff Schionning.
Shades from Fontaine Pajot
The boat is a performance cruiser designed for EC Category A (> 200
nmiles from shore)
At present I have  200 mms or  8"  out of the water  at the transoms
with a lightship displacement of 6500 kgs.  Immersion is 200 kgs/cm, and
full load is 8000 kgs, so in theory there should be around 120 mms free
at full load if loaded correctly)

Anyone have an idea of what one should aim for? I guess this is another
compromise, between effective waterline length and non immersion of the
transoms.


Thanks for the freeship link, have downloaded and will play with it tonight.

Up to now i have done most of it on paper, then had a guy converting it
to 3D using Maxsurf.

Am at present moving on to the "smaller" bits using Rhinoceros, who give
you a free download with full functionality,  25 saves and full printing
capability, NURBS etc. It can read practically any CAD file format. (Som
my PC is swithed on for long periods with a good UPS). Some of the
internal design will be in  a traditional 2D CAD system.


I'm not sure that I will even bother with the CFD stuff - in my opionion
there are so many parameters that are not known, - that even a simple
thing like defining where the flow is turbulent, transitional or laminar
(Reynolds Number) - is doctorate material these days.  My approach is
fairly basic, in that I assume that a cat will never surf, and will
therefore always be in displacement mode.  IMO

Main factors in designing underwater hull shape are:
1.) Ensuring a comfortable "ride". Sharp bows, limited rocker, flatter
stern area, etc.
2.) Putting buoyancy under the weight - ensure that she lies at the
designed waterline.
3.) Limiting wetted area.


The rest is pretty much looking for tenths of knots, which are much
easier to find by building lighter,  bringing down airdrag  from the rig
and  all the other "comfort features", or increasing sail area/sail
efficiency (within reason)



Any input from all you experienced people out there will be welcomed!

P.S.
I am thinking about releasing the design as open source when its
finished - any ideas of where to do this. (Let more people enjoy the
time and money spent anyway..)

Regards

Alan

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