Exceeding the hull speed on older longboards

[ittiandro]

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 anybody 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

Ittiandro

[techno900]

I am sure you will get some better answers, but the hull design on the longboard pretty much determines it's speed potential. For example, I still have a 1985 Mistral Superlight, which is a semi-displacement hull. It has a semi-round bottom and a lot of volume (260 liters). It glides well and is very fast in light winds (under 10 knots). Very fast means that it's 1-2 knots faster than other long boards that have planing type hulls. When the Superlight is in 15+ knots, it gets very squirrely and hard to control, the nose rides high and it does not plane. There is a huge amount of drag as the water wraps up around the rounded rails.

There are also what are called Division II boards, that have a more rounded hull than the Superlight and are faster in light winds, also non planing and really challenging to sail in stronger winds (15+ knots). I raced a friends Crit DII board once and did very well in the light winds, but it's like "log rolling", extremely tippy because of the round bottom.

I also raced an F2 Lightning and later a Mistral Equipe II XR. Both would plane and go pretty fast, and had similar volumes to the Superlight. I would venture to guess, that in the right hands, the Equipe II could hit 25+ knots of board speed. I know that I could blow away almost any intermediate short board sailor in 15-20 knots of wind. When I raced it, I used sails from 6.5 to 10.4 depending on the wind. Also three different fins, also depending on the wind and sail size.

On the Equipe II, the water has a clean release off the bottom with much less drag compared to the Superlight, which equals control and speed.

Not too unlike comparing skipping stones across the water. Try throwing a round stone and then a flat one. It's all about drag when it comes to speed.

[NickB]

I don't think the hull speed formula applies once you get a long board planing.

I've gotten my Starboard Phantom 320 (10'6) to 25+ mph GPS speed in 17mph wind (7m Ezzy Freeride sail).

And I've gotten my Bic WindSup (also 10'6) to about 18mph GPS speed in similar conditions with the same sail (see: https://vimeo.com/133869798)

So that's quite a bit more than the theoretical 4.3 knt hull speed per the formula...

Even in non-planing mode, both hulls glide up to 10 mph or so and then start planing...[/url]

[whitevan01]

the hull speed equation that was quoted only applies to full displacement hulls. just about all windsurf boards are planing hulls.

hull speed for a displacement hull is dependent on the distance between the bow and stern waves, the greater the distance the higher the hull speed. Planing is when the craft "overcomes" the bow wave and there is only a stern wave. There is more to it but those are the basics.

For a full treatment, see the Aero Hydrodynamics of Sailing by CA Marchaj.

[ittiandro]

[boardsurfr]

The hull speed discussion is completely irrelevant to longboards. I usually take a GPS when sailing a longboard, and often pick the longboard when the wind is just around 15 mph. I'm using either a Fanatic Ultra Cat or a Mistral Pandera; both behave similarly. The think I love about them is that they have "all the gears": a bit more wind, and they go a bit faster; a lot more wind, and they go a lot faster.

Check the GPS speeds from a session on the Mistral below. Wind was good at the start of the session and then dropped. You can see that the top speed in the runs varied from anywhere between 4 knots and 19 knots, with some runs topping out at 6, 8, 10, 12, 14 knots, etc.

If anything, short boards are more likely to display a "two zone" speed. The second image shows the speeds for a shortboard session (Fanatic Skate 110). Interestingly, the non-planing speed if often close to the calculated hull speed of 3.8 knots.

[boardsurfr]

It gets interesting when you use the modified formula to calculate hull speed that takes weight into account. With a 200 lb weight, you get to use factors of 1.66 instead of 1.34 for an 8 ft shortboard, and 2.42 for a 12 foot longboard. That gives you hull speeds of:
• 4.69 knots for a shortboard
• 8.38 knots for a longboard

If we say that 8 knots is about the minimum speed for staying on a plane (I typically use this for jibe analysis - if the speed never dropped below 8 knots, it's cleanly planed through), then we see that a longboard can smoothly transition from displacement to planing, while a shortboard has a noticeable hump to get over.

[boardsurfr]

I am, of course, ignoring the effect of length on planing. I think in first approximation, how fast you get to plane is proportional to the total planing area; a longboard will have a longer planing area than a shortboard, and thus can plane at lower speed (assuming similar width). This assumes somewhat similar underwater shapes, and similar rocker and water release. A rounded hull like on a Superlight will impede planing.

[ittiandro]

[DanWeiss]

The hull speed of any board or boat is the maximum achieved in standard testing conditions and assumptions before it crests its bow wave and can plane.

Simple.

What's less simple is why some boards are faster than others in displacement mode. Rocker, volume, rail design and foam flow generally dictate how a board reacts to energy at any given point. Thick, boxy rails with a sharp release or edge tend to be lower drag when planing and may be faster when subplaning but rarely so with small sails. The original Superlight and Windsurfer were very, very fast in lighter winds using sails between 5.8 and 6.5 compared to the Pan-Am boards and what eventually became the first generation Equipe. The Pan-Ams and Equipe spent upwind performance in light air with small sails for much faster planing speeds with larger sails. This carried forward at least until the 2010s with the advent of the huge Starboards and Exocets. Racing a tricked out Equipe XR with a 9.0 still didn't create upwind performance that would defeat my Superlight and its powerful "Regatta" sail when winds were light.

Sailing a lower angle would change the results, just as we see with the huge sails used for FW racing. Huge to go downwind, not upwind.