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Transom Drag

PostPosted: Fri Feb 10, 2023 3:04 am
by GreenLake
Prompted by the thread on optimal heel angle, I poked around a bit to better understand the question of transom drag, and how it relates to fore-and-aft balance.

When boat designers approach this question, they tend to compare a planing hull, like the DS, to a "canoe" hull with a pointy stern. They then look for differences in the drag at below planing speeds, that is when the hulls are in displacement mode.

There's a penalty we incur in the DS at these speeds; offset by the promise of being able to plane and overcome the hull speed limit in the right conditions. That's the part that is built-into the hull design of the DS and nothing we can do about. However, the added transom drag changes when the transom is more and more immersed.

People suggest a whole lot of different mechanisms as to why a hull that is stern dragging can have up to double the transom drag. An immersed transom means the hull is trimmed down at the stern, and that means that water gets pushed down by the rear of the bottom and then separates turbulently at the transom. With a lot of energy absorbed on vortexes.

The precise details, or the relative contributions form each of these effects are perhaps less interesting than knowing what this means concretely.

I've written about this in a number of threads already, but I've taken measurements around the 3 knots range using a motor and GPS. I usually do these when I'm motoring in a calm and I get bored a bit...

Here's what I do. First I sit near the stern (as you would to get you hand on the motor throttle) and let the boat come to a steady speed on the GPS. Say 3.0 knots. Then, with tiller fixed, I move forward and wait for the boat to reach a new equilibrium speed at the same throttle setting. If I sit on the cuddy with my feet dangling into the cockpit I get the largest increase, to about 3.5 knots, but other forward seating positions also lead to significant speed increases.

Because the measurement is done under motor, it mimics level sailing (well, I'm not sitting 100% on the center line, but it looks level).

The absence of wind will change the results, compared to what you might see if you could do the measurement with steady wind, instead of fixed throttle setting. I reason the effect would be the following. If the sails are up under motor, the apparent wind increases, and continues to come from straight ahead (100% head wind). That increases air drag proportionally to the square of the speed. So the speed increase is a little less than otherwise.

With a true wind, while sailing, the apparent wind would also increase (but not as much as the magnitude is reduced because of the angle between true wind and head wind). But because lift, like drag, goes with the square of the wind speed, the boat will experience an increase in lift when it goes faster. So the speed increase should be a little bit more than achieved under motor.

Upshot is that, upwind, you want to sit as far forward as possible when going upwind at speeds below hull speed. If you can try this with another DS, you should be able to observe the effect by directly comparing two boats that only differ in fore-aft trim. The speed difference should be significant (for racing).