Get someone to photograph you while you're moving. You also need to know what your speed was when the photo was snapped. If there's only a single wave, that's your hull speed. That's how we measured the WD12's hull speed to be 6.4 mph.

Laszlo

Here is an on-line calculator:

http://www.sailingusa.info/cal__hull_speed.htm

 Hull Speed = 1.34 * (LWL)^1/2

LWL: length of the hull at the waterline.

The Skerry is roughly 13 feet at the waterline (Just went out and measured). This would give a speed of approx: 3.8 knots.

Chris

Hi Christine,

Here is a link to another site which does a better job explaining the calculation and the physics behind displacement hull speed:

http://www.cncphotoalbum.com/technical/hullspeed/hullspeed.htm

Using this and other calculators around the web, I am getting a result of 4.87 knots (5.6 mph) as the theoretical max hull speed for 13' LWL sailboat being powered by the wind. It is certainly possible to sail a light boat faster than its theoretical hull speed, by either planing or surfing. 

Planing is when the speed increases to the point where the bow rides up on the bow wave, lifting the hull on the surface of the water to have less drag.  This is where the bow wave moved aft, and only one wave is seen under the hull.

Surfing can happen when you are running downwind and the windblown waves push you from behind.  As they push, they also can create lift and a powerful forward surge.

It sounds like Laszlo was observing the WD12 while either surfing or planing.  The theoretical displacement hull speed for a 12' water line length is 4.7 knots (5.4 mph)

I just realized that my last post was unsigned.  I think I took too long to write it, and my session expired before I posted.  I just wanted to own up to it, just in case anyone thinks that I am providing incorrect information ;)

Ron P.

Thanks Ron!

That was a good link and a clear explanation. 

I know how surfing feels. A couple of weeks ago I went out and there were fabulous big rolling waves and quite alot of wind (about 15 knots according to the weather station just a mile away) . It was pretty scary but very exciting! A bit hairy to go up wind and turn around though.

I've not heard of Skerrys being a planing boat, but once I got caught in an afternoon squall inside the breakwater. It funneled the wind something fierce. No waves to speak of. I was going very fast downwind and all of a sudden the boat sort of gave a lurch and took off like a bat out of hell. I just hung on for dear life. I was quite lucky I think. At the time I was just happy I was able to stay afloat but now thinking about it, that lurch and sudden acceleration was probably planing. 

Thanks for the info

Christine

Actually, I was paddling like hell on flat still water, so definitely no surfing. Nor could a WD12 plane, no matter how hard it tried - the aft bottom is nowhere near flat enough nor wide enough. It's also shorter than 12 feet LWL, so it was exceeding the speed limit by quite a bit.

The answer is that formula is only good for a pure displacement hull, one which pushes the volume of water which it displaces away from where it's going. Hulls that don't push the water get to break the displacement speed limit  without surfing. The first, of course, are planing hulls which ride over the water, like a stone skipping across a pond.

The second type are long narrow hulls, like kayaks and catamarans, which slice through the water, rather than displacing it. Of course, there has to be some displacement or the boats won't float, but the slicing action raises the speed limit quite noticeably.

So by the time you add up the effects of planing, slicing, surfing, lifting, etc., that theoretical speed limit becomes very theoretical

Laszlo

L., you are an advocate of the--controversial theory--that the hull speed and the speed at which planing begins are the same.

Since you brought it up, and because this forum is always fair and balanced, I will give the opposing view.  I've no idea which is correct, but one of them is right and the other is wrong.

The question makes no difference to us as builders or boaters, of course.

So if any of you have unfinished boats waiting for you in the shop, stop reading and get out there.

The opposing theory is that the planing speed and the hull speed are in general independent of each other.  So, for a given boat, the planing speed may be higher than the hull speed. For such boats, wave drag increases until hull speed is reached, and then decreases after it is exceeded, as the bow wave and the stern wave pass out of constructive interference.

If the second is correct, and if the Skerry is one of those boats whose planing speed happens to be higher than its hull speed, it simply means that the Skerry will go faster than hull speed without planing or surfing.

Camper,

I never said anything of the sort. All I said is that was that the WD12 went faster than the formula (which doesn't consider the hull shape) predicted that it would go, and that it did so without surfing or planing. Your law-schooled daughter should be able to verify that for you (might as well get some use out of that degree, besides a snazzy Latin phrase or two :-) ).

Anyway, your counter-proposal is interesting. It sort of sounds like a variation on supersonic flight. You know, where flying in the trans-sonic region takes more energy than flying just beyond mach 1 (at least in some aircraft). There the cause is buffeting by many shock waves as some parts of the aircraft achieve supersonic speeds before the plane as a whole goes supersonic. Since the speed of sound in water is much higher than in air, it's a false analogy, though.

But, just for the fun of it, let's say it's true. The we can get into the realm of science fiction. That is, exceeding the speed of light. There's nothing that says you can't go faster than the speed of light. All the equations say is that at the speed of light you have infinite mass and energy. It takes more and more energy to go faster, until at the speed of light it's infinite, but beyond it it takes less and less. See the similarity?

So if your theory is true, it sounds as if boaters need a space warp to get them beyond hull speed. Fortunately, we can simulate that by simply putting the kayak onto a planing speedboat and then dropping the yak into the water. The  kayak will skip across the surface, well above its hull speed until resistance slows it down. So as long as the paddler can paddle fast enough to overcome the resistance, the kayak will continue skipping out over the horizon.

Camper, you volunteer for the experiment? Camper? Camper? Good, he fell asleep. Shhh. Everyone be very very quiet and leave this thread without waking him up again.

Laszlo

snrff...hmphh...Huh?

No, I wasn't sleeping, I was studying my physics of sailing book to learn more about this fascinating...[yawn]...subject.

Yes, please sign me up as the crash test dummy!

Anyway, once one finds oneself arguing with Laszlo or John or Kurt or whoever, the best thing for it is to press on and hope that an escape materializes when no-one's looking:

The formula for hull speed (given by the estimable Ron Paro above) merely gives "hull speed". which it did, and which is about correct for the Skerry, as far as we know. The formula is absolutely NOT predicting how fast the boat can go.

By definition, hull speed is simply...

the speed at which the bow and stern waves constructively interfere.

This means that the bow/stern wave crest height is maximized, and since wave drag is proportional to the square of the wave height, this is a bad place to be.  If you are not planing, you want to be going either faster or slower than this speed.

The misconception that hull speed is some sort of a speed limit comes from the unfortunate fact that, until we old-timers were exposed to stitch and glue kayaks, skin-on-frame kayaks, carbon fiber toys and the like, we had NEVER SEEN a boat that was so light (ok, and so poorly shaped aft for planing) that it would go right through hull speed without planing. 

Granddad's Chris Craft really wanted to keep trying to stay in the water and go faster than 1.34 times root LWL, but eventually the lift from that flat run of the hull said, "sorry Chris, you're heavy but not heavy enough to fight me and hold yourself in the water.  We're going to jump up on top of the water and run!"