Transom Drag
When my 1976 Tartan gets going I notice that my reverse transom becomes partially submerged. I think this must create quite a draft and actually slow the boat down. Is a better designed transom higher off the stern so that the transom does not become submerged? Also, it seems that a higher transom would also add to boat speed as the waterline would be extended at higher speeds as in the more traditional designs like Cape Dory, etc.
-Paul Kirshen
Concord, MA
We asked yacht designer Robert Perry of Seattle, Washington, to answer your question. Here’s what he had to say:
“Your Tartan is a medium-displacement boat with considerable fore and aft rocker or longitudinal curvature to the canoe body. As you approach hull speed, your boat sucks up a big quarter wave. The boat is not squatting-it is pulling that wave up.
“Ideally you would have enough transom overhang so you did not immerse that transom. This immersion causes drag. Just imagine dropping a handful of paper clippings into that eddy at the transom edge. The paper would follow the boat along. The energy provided to pull that paper along would be better served pushing the boat along.
“However, given LOA considerations and marketing targets, boats are often designed with truncated transoms. The drag incurred by the immersed transom must be balanced against the benefits provided by the additional volume aft, such as cockpit volume and volume for accommodations aft.”
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Why Hanks?
I notice that many well-equipped cruising boats owned by knowledgable sailors do not have roller furling. Is there a performance advantage? Can you shorten sail with a furler and still have a reasonably good shape?
-Philip Horsman
Via e-mail
Many cruising sailors, especially of the offshore variety, prefer hanks to roller-furling for safety reasons: Hanks are pretty foolproof, given regular lubrication-they allow different sails to be set and doused easily on the raw headstay or inner forestay, and if you intersperse hanks you can have a sail change waiting at the bottom of the stay.
Even though furling systems have been highly refined over the years, and have proven themselves in horrendous conditions, there are still lingering worries among some sailors about halyards and furling lines jamming at the worst possible times. A further concern is that when you do have to take a roller-furled headsail completely off, you have a big piece of loose sailcloth to handle on the foredeck (this is how racing crews spend a lot of their time), whereas a hanked-on sail can be hauled down and lashed to a lifeline and will stay put right there until needed again.
Unlike most things today, a hank-on headsail is desirable for all it doesn’t have-no furling drum to maintain, or to get in the way of anchor rodes and mooring lines; no furling line or fairleads for same. No extra hardware high or low.
A headsail foil is more aerodynamically efficient than hanks, so there’s a definite performance gain there. When you “reef” a roller-furling headsail you do lose some sail efficiency, because the sail is cut to fly full. That’s a natural compromise. The important thing is to move the leads appropriately as you roll the sail in and out, to keep the leech and foot properly shaped.
