[MHml] Stern shape

[Colin Owen]

----- 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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