I have posted this thread by mistake in the surfing-longboarding room, but I got a kind of unwelcoming, even sarcastic answer, so I'll redirect it to the proper room.
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 ) in motion, whereas a shortboard, by not having the “ advantage” of this initial push created by the 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
I my practical experience with full displacement boats demonstrated that they are roughly limited to the waterline length in meters.
I.E. 1m of waterline lengths transalates into 1knots of speed.
I added more Horse Power, but could not increase the speed beyond 5% of extra speed.
After fiddling with the angle of the prop shaft, I managed to get 10% more speed from it, but that's it.
However, as soon, as you get a semi-displacement boat (flatter a the back) , new laws of physics kick in and the HP gives you more.
So... it would depend on the longboard hull shape.
...wonder if this response is relevant at all...
If you get the chance, have a read of frank bethwaite's book "high performance sailing" in particular the section on 2 mode sailing, it explains a lot about why a long displacement hull can outperform a 2 mode planing hull, but then gets left for dead by the planing hull as soon as planing is viable...
The hull speed is the limiting factor.
Short answer: if it's displacement only, then no it can't exceed the hull speed.
Francone, I know it sounds picky, but it's just knots, not knots/hr a knot is 1 nautical mph.
Back to your question, the limitation of hull speed occurs when the hull is trying to climb over it's own bow wave. The faster it goes the higher the bow wave gets, so more power just creates more resistance. That's why the statement, it takes a lot of power to go faster than hull speed. A planning hull develops lift that enables it to climb over it's bow wave, and why you can help pumping the sail by also bouncing the board a little. and why it takes less power to stay on the plan than it does to get there.
15 to 16 knots is a lot of wind, I think he's talking about much less, maybe 5 to 8 knots, when you'll be sub planing with a 6.5 and an 8.5 will only make you go a tad faster.
Once it 15 knots you can forget hull speed you should have left that behind long ago, (as long as you have a planing hull)
I might be wrong but the mistral equipe is not a displacement hull, therefore your friend is planning much before 12kt. Actually, I don't think we can say that even D2 board are fully displacement hull because given a certain speed they plane also.
my 2 cents.
Longboards DEFINITELY plane, although defining "planing" is opening a true can of worms - I've been through this with naval architects who design skiffs and yachts. The most accepted definition in the profession is something that you can't actually see in action - it can only be worked out on a computer or a test tank. But an Equipe with 8.5 should be doing what we call "planing" in a lot less than 15-16 knots - in that much wind the mast track should be all the way back and you'd be expecting to be hitting speeds in the teens, I'd think. As Plettil says, not even D2 boards are full displacement hulls; despite the rounded shape they definitely plane, as did the original Windsurfers.
The "hull speed" identified many years ago by Froude (who created that precise formula you mentioned) is not actually a hard limit - it is merely an area where the percentage of hull drag that is created by wave formation rises steeply. If I recall correctly, wave drag increases by the square of the speed. A short fat heavy hull creates so much wave drag that the steep rise in wave drag effectively forms a speed limit, as Froude's formula says.However, the effect is different with more efficient hulls that have lower wave drag. Hulls that are about 10 times as long as they are wide (like a longboard) create very small waves and therefore have very small wave drag. Therefore even if there is a steep rise in the percentage of drag that is caused by wave formation, it's no big problem - even if the drag increases by the square it's no big issue if the drag is very low to begin with.
In addition, the lift created by planing increases by the square of the speed. The skinny longboard shape isn't all that good in terms of creating planing lift, but you can still feel them start to rise on the plane pretty early; at a guess they are developing significant planing lift from about 4 knots. The planing lift is therefore reducing drag just as the "hull speed" limit comes into action, and the drop in one is cancelling out the rise in the other.
As Subsonic says, a lot of this is in HPS. Be aware that Frank didn't know much about longboards; we talked about doing some work on them together, but I never got around to it.
This is a messy answer - apologies.