How much more efficient is a heeled boat? 35% more efficient!

[peterchech]

So my younger brother is in engineering school, and for his hydrodynamics class/lab he designed an experiment for my 1981 Hunter 25. He wanted to calculate the drag coefficient of the hull, when upright and when heeled. With his lab partners (about 4 students) and his lab professor (who wanted to come along and observe), with myself at the helm of the boat and my girlfriend's father driving the dinghy, we took about 3 hours in conducting the experiment.

Using my dinghy, we tied off to the bow and after several failed attempts figured out how to tow the boat in a straight line consistently. We then towed the boat with the dinghy, measuring water speed, apparent wind speed/direction and force (in pounds) required to maintain that water speed at the time my brother yelled "mark". We repeated the experiment about 20 times, going into the wind and then with it to reduce/average out the effect of the 5-15 knot winds experienced in our harbor. The water was almost completely flat. The experiment was done both upright and with the boat heeled about 10-15 degrees (he measured the angle of heel as well on each mark). It was funny seeing a bunch of young engineering students hanging off the shrouds to heel the boat over. They didn't look too comfortable

Other than my brother lighting up a cigarette at a fuel dock everything went well and usable data was obtained.

The drag coefficient of this boat was calculated to be .34 upright, and only .22 heeled. So, its shape is 35% more efficient when heeled (at between 3-4 knots in the water and between 10-15 degrees of heel) than when upright.

Well he got an A, and his professor was impressed enough to submit him for a scholarship due to this experiment (he got the scholarship BTW), so academically it was a success. It seems few engineering students are so creative in designing experiments.

Since the boat has not been bottom painted in 2 years, nor power washed in 1 year, it has some seaweed and minor barnacle growth on the bottom. He will be repeating the experiment in the Spring after new bottom paint for comparison. I can't wait to see how much more efficient the hull is then!

[bobperry]

Peter:
I'd need to know the speed you towed the boat to be sure why you got that reading. But here is my guess:
Say your boat has a DWL of 22' and that gives a reasonable hull speed estimate of 6.00 knots. To get a really good handle on the changing drag you would have to tow the boat at a variety of speeds, say one knot increments. When you are below about 3 knots your drag will be mostly due to wetted surface. Once you get above 3 knots the skin friction drag will be far less than the wave making drag as the boat works to push the water out of the way.

So, my guess is that you towed the boat at low speed and heeling the boat resulted in a reduction of wetted surface. This is the best way to increase boat speed in light air. A boat like yours with a fat fanny heeled over will see a large reduction in wetted surface. Watch a fleet on Lasers pr similar dinghies race in light air. They heel the boat over and trim the bow down to reduce wetted surface.

But "efficient" is maybe not the right word. You would have to check the performance of the fin keel at 30 degrees too. I think that if you are hard on the wind the keel is not working at its optimum so your overall efficiency is not better. Obviously the rig is also not working at it's optimum when the boat is heeled 30 degrees.

Great experiment! I congratulate your brother. I only wish I had been there to watch.

[rdw]

You always here that you can reef sails and heel less with a more comfortable ride and maintain your speed. I was also on a boat that was close hauled, heeled and had a fair amount of weather helm. Someone took over for me and I went to bed after which they said that they had reefed the sails, got rid of the weather helm, got rid of some of the heel, got a smoother ride and increased the speed from 7.3 to 8 knots. As you can tell for the question, I do not know enough. Give me meaningful instructions on what amount of heel is bad, what amount of reef is good and is there anything that contributes to weather helm other than balance of the sails. I will be interested in your answer. My friend says that the bow wave is what causes weather helm.
RDW

[paul323]

Good questions rdw - I am sure more experienced voices will be heard, but to simplify, generally a 10-15 degree heel is optimal - but this will depend on the boat. As I go over 20degrees, I reef (unless I want to give people the thrill of planting the rails!). Remember, the more you heel the more wind "spills" from the sails, resulting in more "excitement" but less power.

I really don't think weather helm is caused by the bow wave. A well-balanced boat typically has about 5degrees weather helm; if the boat is overpowered, or the sails poorly adjusted, it can get a lot heavier than that. If your weather helm is getting too much, adjust the sails or reef.

[bobperry]

rdw:
Your friend is mistaken about weather helm You should buy him my book for Christmas. He needs it.

[PCP]

I guess that the best performance in a sailboat regarding heel angle will have to do with hull design and it can be different for two different sailboats. I have noticed that sometimes very fast and light boats, the type derived from beamy solo racers are quite slow while motoring, comparing with slightly heavier sailboats of a different type (less beamier and more classical cruiser racers), both using the same power on engine that I believe is maximized in performance, I mean the gear relation and choice of propeller.

When sailing both types of boat use also about the same power (same sail area) but now they sail at about the same sail speed.

I think the beamier boats are just designed to sail heeled and even if the more classical type is also maximized to sail with some heel, the beamier ones are a lot more slow when not heeled. I am not talking about slow speed, but on motor cruising speed, were the differences in both types of boats (for a 49ft)n can be over 1K (5.5 to +6.5).

I have noticed this is several boats. This makes any sense to you Bob?

Regards

Paulo

[rdw]

what book and where do I get it.
You were thinking about a Christmas present for me!?
RDW

[bobperry]

rdw:
Go to Amazon, look for YACHT DESIGN ACCORDING TO PERRY, buy three dozen copies and give them out as gifts. Thank you in advance.

Paulo:
Yes, I think you are correct. I really think boats are designed to be sailed upright where the keel and rig can do the most work but different hulls can benefit on the change in wetted surface and heeled waterlines that some heel angle provides. A wide, flat boat tipped over 25 degrees can be a long skinny boat. But for most of the typical mom and pop boats that most us sail I don't think you need to worry about an optimal angle of heel. Of course if you are racing you need to worry about all of those details.

Look at a TP52. In any breeze at all they pile the crew on the rail to reduce heel angle. Boats have canting keels to reduce heel angle. Dinghies have a trapeze often. I'm just saying that for most boats once you get above half your hull speed zero angle of heel is optimal.

[sailordave]

Originally Posted by bobperry

rdw:
... once you get above half your hull speed zero angle of heel is optimal.

BINGO! And this ladies and gentlemen is why Sir Bob is a designer!

Which is why you sail a Laser (and other boats) flat as possible except in very light air.


Bob, as always, it is so nice to hear you chime in and 'splain things for us. I really get tired of hearing (here and in person) from the so called "experts" why things are the way they are.

[bobperry]

Dave:
I really like it when people call me "Maestro". Makes me feel important. And old.

I pitch in when I think I have something of value to add. Sometimes I like stirring the pot a lttle too. I can't tell you how many times I have been asked that question, "What is the optimal heel angle for this boat?" I always say zero degrees. Of course there are all kinds of caveats that could come next but they are all boat specific. And if you own a boat that comes from the CCA era there is probably almost nothing to be gained with heel or fore and aft trim changes regardless of how much wind there is. Of course some heel angle in very light air helps your sails to fall into the shape they were designed for.
Blah, blah, blah,,,,,