View Full Version : Weight vs Speed

Henry Webber
09-23-2000, 11:38 AM
All other things being equal (hull design and size, sail plan, etc), what is the relationship between boat weight and speed? Can it be expressed mathematically, i.e., double the weight = 1/2 the speed? I like traditional construction but I'd like to know if I'm giving up significant performance vs lightweight/modern construction methods. Also, does passenger and gear weight have the same effect? In a small boat (100-200 lb) that would far outweigh diffences due to construction methods.

Tom Lathrop
09-23-2000, 04:11 PM
All things are never equal Henry. The speed of a sailboat is the result of all the various factors acting in a given set of conditions. But still, horsepower-to-weight ratio is all important. To address your second question, weight placed properly in hiking position adds to both weight and horsepower at the same time.

Catamarans are generally faster than monohulls due to more power (more sail area due to greater stability) and lighter weight.

As a general rule, lighter is better. The fast boat designers spend a lot of time trying to improve the HP to weight ratio of their boats. That is why they cost so much. Exotic materials and super computer engineering run the costs of America's Cup boats into the stratosphere.

[This message has been edited by Tom Lathrop (edited 09-23-2000).]

09-23-2000, 06:37 PM
For otherwise identical boats, lighter will be faster, of course, but all other things are not ever going to be equal.

Thought experiment: Take two identical boats. Name one Photon and the other Electron. We need two skippers, Paula (150 pounds, 6' tall) and Elmer (250 pounds, 6' tall) and a 100 pound lead weight (actually, four 25 pound lead-shot filled bags.)

Now, if we put Paula in Photon with the 100 pounds of lead and Elmer in Electron, they'll have the same weight. The boats will displace the same amount of water, so they should sit on their lines the same ... until Paula wises up and realizes she doesn't have to sit on those bags, and puts one in the bow, one aft, and two amidships, out by the beams. Now she can scamper about without disturbing Photon's trim nearly as much as Elmer does Electron's.

Elmer responds by hiking out and Electron sails better because she's more upright; Paula grabs the bags, balances them on her nose, heels even less than Elmer, and Photon takes off.

Messier experiment: coat the inside of Electron's hull with a hundred pounds of lead, carefully distributed so that her center of mass remains unchanged from that of her identical sister Photon. When they're launched, Electron will sit lower in the water than Photon. When a wave comes along, she'll raise to it slower, and not as far, and she'll come down slower. This doesn't mean that Photon is "better"; it means she's different.

The problem is with the word "significant performance".

How are you going to measure that? If you're doing America's Cup racing, every gram is important, and the boat is designed to minimize the weight for the least strength needed to finish the series of races (among other optimizations) and it's considered very important. The other end of this is that as the boat gets lighter, it almost always gets weaker, which can mean that it becomes less rigid, which means (usually) slower.

You also risk breaking the boat -- and they do break them.

I'm inclined to think that if you're not match or fixed-design class racing it doesn't matter that much, and if you are, the decision is probably out of your hands anyway. If you are not racing and have a choice of methods, one which makes a boat that weighs 125 pounds, and one that weighs 150 pounds, there is probably not going to be much difference in their sailing because of that.

A boat is a study in balance, and you can't change just one thing.

09-24-2000, 10:30 AM
My two cents... length is more importian than weight. for the same weight the longer boat will be faster. the lighter boat will be faster for the same length because the draft will be less and so the cross sectional area willbe less.

the fancy tern coeficent of drag is the total drag devided by the number of square feet of cross sectional area. if doubling the weight of the boat doubles the number ofsquare feet of boat moving throgh the fater resistance will behigher but its not quite leaner

09-24-2000, 06:47 PM
Sorry I got called away with out proffing that last line. ment to say...

That if doubling the wieght then it will double the number of square feet of cross sectional area that is being draged through the water, but the effect is not linier becauseof relitively less surface friction of the newlt imerced hull in the water and the denser water the hull must move through.

09-24-2000, 09:04 PM
Is top speed a high priority? If it is, racing or not, then boat weight (in your boat size range) could play a major factor. While waterline length plays a major factor in displacement hull speed, it is no longer a limiting factor in a planing hull. The weight of the boat and its load (crew, rig, gear, etc...) play a large factor in the planing speed of the hull. If I am correct in my understanding of the size boat you wish to build, then crew weight could be your biggest factor. Depending on materials it is possible to build a wood boat to compete with more modern fiberglass versions. The International Lightning is a perfect example (recently shown in WB). I also know of a wooden 5o5 that weighed slightly more than its fiberglass counterparts, but it was still competative.

What are your typical sailing conditions?
If you sail in relatively calm weather, then favor a lighter boat. If you tend to venture out after the Coast Guard has raised their little red triangle then a heavier boat may be a bit more stable. Very dependant on the hull and sailplan design.

I used to sail Lasers but as I gained weight in college I grew out of my competiveness. At 200 lbs (in my Laser prime) I still outweighed the fleet average by 50lbs. While my weight excess could be considered a plus in high winds (more counter ballance), it was a negative in light winds (could not plane while lighter boats could). At my current 240lbs I am coming close to the combined weight for some of the two person dinghy classes, thus I have moved up to crewing on keel boats.

Hope this helps...
-YF Scott

[This message has been edited by scottek (edited 09-24-2000).]