[MHml] Transom design for a cat

[Martin Schöön]

On 1/31/07, Alan Christoffersen <alan at insatech.com> wrote:
<snip>
> 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.
>
8000 kg on just under 14 m of total length makes it about as heavy as a Farrier
F27 for its length. My guess is you should to maximise water line length
a tweak hull lines to minimise wave resistance. How hard do you plan to
push: Early reefing, pushing to 50% stability or harder? This could affect
the hull style choice.
>
> Thanks for the freeship link, have downloaded and will play with it tonight.
>
<snip>
>
> 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

Someone else has done the PhD stuff for you. All you need to do is to
export to Michlet format from Freeship (you have to manually correct the
Lwl afterwards as a bug in Freeship makes it use Loa instead of Lwl). You
then name the file in.mlt and move it to the directory where michlet is located
and you start Michlet. A few seconds later you have curves of drag components
vs speed. Michlet can analyse single hulls, multihulls (up to five hulls and
their interaction) or you can compare up to five hulls simultaneously.

How accurate? Well there are some published comparisons between
tank test data and Michlet simulations at it is fairly impressive. I don't think
it is good enough for detailed optimisation but it should be OK
for comparing the merits of different hull styles (fat transom, narrow transom,
high/low prismatic, full/pointy bows, hull separation...)

> 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)
>
The difference between a good and so-so hull could actually be measured
in knots.

More important is sea kindliness and and an efficient and easy to control
sail plan.

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