[MHml] Stern shape

[Peter Southwood]

Hi Colin, I have been following this thread in some confusion
What you are doing to your boat could have extensive consequences, but I can 
not work out quite what it is that you are doing. If you could produce a 
sketch or a photo it would allow some of us who have design background to 
give useful advice based on what you are actually doing as opposed to what 
we might guess based on the descriptions so far.
Cheers,
Peter
----- Original Message ----- 
From: "Colin Owen" <allied at clear.net.nz>
To: "Informed discussion of multihull issues" <multihulls at steamradio.com>
Sent: Thursday, November 01, 2007 12:27 PM
Subject: Re: [MHml] Stern shape


>
> ----- Original Message ----- 
> From: "TomH" <tom at cruisingsailor.org>
>
>
>> Hi Colin,
>>
>> Without knowing the design/type of hull you're starting
>> with... could you tell is or show us a picture?
>>
>> Having said that... some general suggestions based on
>> conventional hydrodynamics and practical experience can be made.
>>
>> The caveat is that for a retrofit like you are doing you are
>> stuck with trying to transition from what you got to what
>> you want - as reasonably as possible.
>>
>> My first choice: the least complicated. low-drag
>> modification would be (looking at the profile view) would be
>> to continue the keel line straight/tangent from the current
>> keel slope at the transom. If this extension ends at a point
>> that is several inches ABOVE the load waterline you pretty
>> much have it knocked. Do the same for the sides of the hull.
>>
> The original hulls were rather sleek and narrow, so I have had to go flat
> and wide at the stern, in order to achieve what I estimate as necessary, 
> to
> bring the boat back to even keel (300 kg per side), so no can do.
>
>> Second choice: (if first choice does not terminate above the
>> waterline) is to follow any existing rocker/curve of the
>> keel until it terminates again several inches above the
>> waterline. If this results in a too short (less than your
>> 700mm) extension then fair it from the rocker curve at the
>> transom to a straight keel line toward the end of the
>> extension. (I would not ever slope the 'opposite' way unless
>> that area is well above the load waterline)
>>
> I have a slight rise to the stern, but not a lot, so I now have about 250 
> mm
> of draft under my planned water line, at the stern. Are you saying that if 
> I
> have a 45 degree slope, down and forward, that is OK?
>
>> Third choice for minimum drag -- if neither 1 nor 2 is
>> feasible for getting the trailing edge above the load
>> waterline with your hull shape (usually due to the transom
>> being deeply immersed to begin with) is to go in the
>> direction of fairing the sides of the hull inward to make
>> the immersed transom area as narrow and small as possible.
>> In this case you also make the radius of curvature as large
>> as possible and ending as straight or close to it as possible.
> Not sure what you mean, but as I am trying to add buoyancy, I want as much
> in the water as possible, but the cleanest exit. I guess it sounds like I
> just get to the stern and then make it flat. If I taper in any direction,
> then I am reducing the buoyancy. I have not studied the scientific
> properties of stern exit, but thankfully many kind people who have, will
> hopefully put me right.
>
> Thanks, Colin
>>
>> Hope that helps,
>>
>>  = = = = = = = = = = = == = = =
>> Colin wrote:
>>
>> I have been adding 700 mm and deepening my sterns (changing
>> transom, for
>> boarding platform) and I am about to glass over the foam I
>> have added. I
>> have shaped the sterns to be square (port to starboard), at
>> the end of the
>> boarding platforms, but I have a 500 mm radius, forward to
>> aft, going down
>> to the almost flat bottom of the hulls. No one has commented
>> on this until a
>> friend saw it last week and he said that was not good and I
>> should have a
>> flat, or even slopping the opposite way to my radius small
>> transom, to give
>> the best (least) drag through the water. Help please!
>>
>> Colin
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