[MHml] Stern shape

[TomH]

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.

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)

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.

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