I recently had an e-mail exchange with a well known windsurfer who is present on the Internet with a wealth of practical info regarding technique and equipment, as well as some of the more theoretical aspects of the physics of sailing. I had asked him why a traditional longboard performs so much better in subplaning conditions than modern planing-oriented shortboards. Indeed, even from my own experience, a longboard moves more swiftly and with smaller sails in light winds than a shortboard.

He explained this by saying that longboards, by dint of their V-shaped ,displacement-type hull produce an initial ( almost inertial, I’d say) hull speed and therefore require less wind power and smaller sails to be set ( and kept ) them in motion, whereas a shortboard, by not having the “ advantage” of this initial push created by its hull speed, requires stronger winds ( and larger sails). By the way, I searched the hull speed issue on the Internet and I have no reason to question its scientific ground, because it has a precise formula : H.S.= 1.34 x sqrt LWL , where LWL stands for Length of Waterline ( in feet). Hence, based on this formula, for a 12 ft longboard, the hull speed should be about 5 knts/hr. And here is where I have a problem, because this person also says Quote With longboards, sail size doesn't matter nearly as much. In non-planing mode a board has a "hull speed" based on its length. It doesn't take much power to get near to the hull speed, but it takes a lot of power to go faster than the hull speed. So you can cruise at a reasonable speed with a small sail like a 6.5, and a bigger sail like an 8.5 will only make you a little bit faster, unless it's windy enough to plane Unquote

The problem is that a 95 kg friend of mine is able to plane in only 15-16 knts winds on a Mistral Equipe with an 8.5 ! If he can plane, the speed attained by his longboard board must be quite considerably (and not only marginally !), above the puny 5 knts/hr hull speed. In fact my friend says that when he rides his Mistral in 15-16 knts winds, no way that he is going at 6-7 knts/hr. He knows he is going a lot faster and it makes sense, if he can even plane.

I wonder if anyone has any comments on this. Can we really say that a longboard cannot exceed by much its hull speed, NO MATTER the size of the sail?

Thanks for your input

Francone

Ted the Kiwi said..
In all seriousness if we wanted wind power we would hang next door In he wind power section. In this room we enjoy glide unpowered. Sails are for other folk. We like to use our arms n shoulders then let the board take over.

Sorry, your answer is quite cryptic and I don't see how it addresses my question : I was questioning that windsurfing expert's statement that with a longboard one is doomed to subplane at close the hull speed ( 5 knts?) if he doesn't plane, for whatever reason. My friend's experience with a Mistral Competition and mine tell me otherwise, but I would have liked to know what other people ( windsurfers!) have to say. Besides, I was addressing myself to the windsurfing community and I cannot understand why you say that sails " are for other folks "(!?).

Francone

Ted the Kiwi said..
In all seriousness if we wanted wind power we would hang next door In he wind power section. In this room we enjoy glide unpowered. Sails are for other folk. We like to use our arms n shoulders then let the board take over.

thats a little bit hypocritical ted with what room he should post in.

The hull speed rule of thumb is pretty useless actually.

Many craft can greatly exceed so called 'hull speed' without developing significant planing lift.