Originally Posted by fstbttms
Is there a formula for determining wetted surface area?
No, there is no simple formula for determining wetted surface because each boat
has a unique geometry, and the results vary widely from boat
design to boat design. As we design boats on the computer, the computer will tell us pretty much instantaneously what the wetted surface is by using very complicated and advanced mathematics, and that's pretty accurate, but without a computer model, we have to estimate it in pieces.
For example, for most sailboats, the wetted surface is made up of the canoe body of the hull
, the keel
, and the rudder
, at least in most modern designs. So we can estimate the parts
. The Area of the canoe body (Acb let's say) can be estimated by: Acb = Lwl x B x 0.7.
The Area of the keel
: Ak = 2 x (root chord length + tip chord length)/2 x span. The 2's cancel out, so the wetted area would be Ak = (root chord length + tip chord length) x span. root chord is the fore-aft length of the top of the keel where it joins the hull, the tip chord is the fore-aft length of the deep end of the keel, and the span is the vertical distance between the two, i.e. the depth
of the keel.
The Area of the rudder
is calculated the same way as the keel: Ar = (root chord length + tip chord length) x span.
Then, the total wetted surface area is the sum of all three, typically called WS = Acb + Ak + Ar.
On a boat with a less distinct keel, say a Bristol Channel Cutter
, it will be harder to define the keel because it is more integral with the hull. So you kind of have to fudge estimating the area as best you can. Same thing applies to rudders that are not barn-door shaped, you have to make an educated guess as to the proper dimensions or area.
For a powerboat such as a trawler
, the same applies--you have to make educated guesses based on length, beam and draft
. Break the hull down into parts
that can be easily estimated, and then add the results together to get the final result.
I hope that helps.