Might be good to specify the boat under the rig. I'd also like to know what "avereage" chop is.

Max Hull Speed = 1.34 X The SqrRt of Length of Water Line.

Max hull speed for a vessel with a 16 foot water line  would be approximately 5.36 knots.  A small variance of lower speed would come into play for a head sea and a small variance for a higher speed may come in to play for a following sea .  Your slowest tack will be beating to windward while your fastest tack would be on a reach.  This is calculated by an whole other complex vectors and formulas. I would say a Kayak with a 16 foot water line and 40 square feet of sail on a beam or far reach could surpass 5.5 knots in 15 knot wind. . . Mr. Harris may have a better idea if he chimes in.

JP

Unless a boat is very lightly built, it will plane before its speed is much more than its "hull speed" as given by the above formula. So in repeating the (extremely common) misconception that hull speed is the theoretical maximum speed of a boat, JP is not far  off.

Nonetheless, I would quibble that it is a misconception, and it's better in the bigger scheme of things that we not perpepetuate it here.

Of course, the idea that increasing the power to a hull moving at hull speed will not increase its speed is ridiculous.  The more power you give it, the faster the hull will go.

A very common but more subtle error is to believe that the power required jumps suddenly at hull speed.  In fact, if you look at the speed/power curve for any hull you will see that there is no such discontinuity.

In  fact, for every speed there is a certain amount of power required to get a given boat to go that fast.  So, to achieve "hull speed" as given by the formula, there is a required power for a given boat.  If you supply that amount of power, it will go at hull speed.  If you give it less, it will go slower than hull speed.  If you give it more, it will go faster than hull speed.  

Another common variation on this fallacy is that hull speed is the theoretical maximum speed for *non-planing* motion.  The belief is based on the above misconception, that drag increases suddenly as boat speed passes through hull speed.

Even so, hull speed is a good rough guideline for estimating the maximum non-planing speed of all but the lightest boats.  

���Thanks for the reply (Harris?) For the record the hull is 17 feet and 5 inches. I whole heartedly agree that hull speed is a grossly misunderstood concept. I've bested my hull speed many times with just a 32 Sq ft sail and a fair wind. Its a west river 180 that ended up being a west river 175!!! I was hoping some one would have the numbers I asked for for a boat polar input. Grumpy, to get an average you add up the recorded wave heights then divide by the number of values entered. For me that averages 2 1/2 foot chop. We edit out the extreme or less common values to arrive at a mainstream sense of average. I can provide a link to Wikipedia if you wish. At anyrate guys the numbers I've found are roughly 8 knots for a reach, half that for a beating and 6.5 knots for a run. I was looking to see what others have noticed. I could be off a knot. At anyrate she seems to do a reach at half max wind speed maybe a little better. Pete

Questions about a sailboat's speed are very common.  To ask "how fast is the boat under such-and-such assumptions" is of course of interest to any serious designer, builder, or boater, and it is equivalent for a sailboat to "please provide me the polars, theoretical or experimental, for this boat".

It's a fundamental law of economics that one will never get a legitimate answer to this question. You will occasionally see a polar for some boat if you look long enough, but it will never be for anything like the boat you are interested in. 

Another law of economics states that you will receive an infinite number of replies falsely purporting to answer the question.

One reason is that it's expensive to model or measure polars, and the market value of the information is approximately zero.  We want to know how to choose or buy or design a good sailboat, but we won't pay anything for the knowledge.   It is one of those rare failures of free markets.  That doesn't explain why there are so many people who think they are answering the question but are not.

Similarly, on this forum one will often see questions about how to make a good filleted joint between panels.  What should be the radius of the fillet?  In all the years I have studied this question on the web, I've seen maybe one post that even addressed the question, and it was just a guy who bothered to make one sample and run over it with his pickup truck.  (The best scientific answer is, "well, I guess maybe they don't need to be real real thick").

Average chop''"............In Hawaii I used to race an open Rhodes 19 in 8 ft seas, ocean swells. Here in the east coast Atlantic you won't catch me in 4ft seas, steep bumps. What is "normal" changes from location to location.

Hello Pete,

If you can get your kayak on a plane at 7 knots or better with a MKII rig, a 40 square foot sail in 15knots of wind (supposing you don’t weigh less than 140lbs) while driving the hull and pontoons over a 1-2 foot chop I will buy you a steak dinner and your choice of beer or whiskey to wash it down.

Hello Grumpy,

I figured average wave height in a 15knot breeze at a Beaufort Force 3 which from my experience on all but open ocean would equate to roughly 1 -2 feet depending on FOW and unobstructed travel of steady wind speed over the surface.  By the way. . . I love your candor and would buy you a steak dinner, beer or whiskey just to visit with and ol’ salt like you. . .  

 Unknown 8:22PM Poster,

I’m not a very educated man but I do have experience I feel too presumptuous to share here. . . Not sure about all the economics analogies in regard to the topic but I would buy you a burger and fries if you could tell me exactly what all that meant in relation to Pete’s initial question. . . It just gave me a sore scalp from scratching my head trying to figure it out.

<<<"It's a fundamental law of economics that one will never get a legitimate answer to this question.">>> 

There is no right or wrong answer that anyone can give Pete on his question. Only an educated one or in some cases an experienced one. . .   The only right answer could only come from Pete himself after he sets sail on his vessel in  conditions he described and logged his experience. . . At which time I either buy him dinner or not. . .

Regards and respect y’all,

JP

 JP: I am the "anonymous" poster [though not intentionallly...I guess the CLC software will eventually recover from its apparently unrepairable defect and give my identity as the infamous Camper.]

 You wrote:

"| would buy you a burger and fries if you could tell me exactly what all that meant in relation to Pete’s initial question.

Pete's initial question was 

With 15 knots wind what speeds would you say are c had with the 40 sq.ft. Sail and "average chop" on a reach, run and tack or beat?

The answer to your question is that all that meant nothing in relation to Pete's initial question.  (See my text to understand that I was replying not to the original post but to a response to it.)

Doc says no fries for me, but I am looking forward to the burger.  With cheese if it's ok.

That max hull speed equation is assuming a pure displacement hull - one that has to push through the water. As the sail pushes the boat, the boat pushes the water. The water forms a wave as it is leaving the vicinity of the boat. As long as the wave can get out of the way of the boat faster than the boat itself is moving, adding more power to the boat will move the boat faster. Eventually, though, the boat and the wave get to the same speed and the water can't get out of the boat's way. So it piles up into a bigger wave which the boat then tries to climb over. The bigger the wave, the steeper it gets. The steeper it gets, the more power it takes to climb it. The more power you add, the bigger the wave. The vicious circle continues until the power source is maxed out. The equation comes from the math describing how quickly a wave can move through an incompressible fluid. It's basically the solution to how fast does a wave of a certain wavelength propagate. That is the theoretical basis for the hull speed equation.

Another way of saying the same thing is that the water that the boat is pushing out of the slot that it is trying to occupy piles up in front of the boat because it can't get out of the way fast enough. The faster the boat goes, the bigger the pile gets. Eventually the pile gets too big to push any faster with the existing power source. Adding a bigger power source will move the boat a little bit faster, but mostly make the wave bigger.

That equation, however, has a lot of simplifying assumptions, such as the boat is level, the boat has a fixed waterline, the boat has a certain shape, etc. It's a good approximation, but there are many ways to "cheat" it.

If the boat is not a displacement hull, it doesn't try to push through the water. In that case there is no wave, so there is no wave speed limit. The most obvious example of this is a planing  hull. Because of its shape (wide flat back end) it can actually pop up on top of the wave if enough power is applied and the amount of power can be realistically provided by whatever power source is on the boat.

The other way to "cheat" the limit also involves changing the hull shape. A boat that is narrow for its length will slice through the water instead of pushing through it. It will still form a wave, but the wave will be much smaller and provide a smaller obstacle to the boat's motion. The slot that the boat occupies is smaller so there's less water to get out of the way. These boats include kayaks, sharpies and catamarans.

Since the CLC rig and the kayak form 3 "slicer" hulls sailing in close formation, I'd expect them to "cheat" the wave equation. The exact speed depends on so many factors that the best answer to the question is - try it and tell us.

Have fun all,

Laszlo

>>>Max Hull Speed = 1.34 X The SqrRt of Length of Water Line.

The "1.34" is the "speed/length ratio," and it's a variable in the equation, not a fixed figure.  It's a reliable number for non-planing hulls with a length-to-beam ratio hovering around 2.5:1 to 4:1.  Very roughly.  PocketShip fits the description of a boat that ought to be sailing to a speed/length ratio of 1.34.  However, with GPS in hand and witnesses aboard, I have observed it sailing at an S/L of 1.94.  Clearly it was climbing up over its bow wave and starting to plane.

As hulls get narrower, "1.34" creeps up to 1.7, 1.8, 2, and onwards.  Fast multihulls aren't planing, but are often observed sailing at S/L's of 4 or more.  Madness has managed 3.65.

This tri is definitely NOT planing, but it's clearly ignoring the "1.34 X The SqrRt of Length of Water Line" rule!

Anyway, this is all somewhat academic and doesn't answer the question at hand.  A kayak with the CLC SailRig is not going to sail like a Nacra 5.2, even with the big 70-square-foot sail.  Just doesn't have the horsepower.

With the 40-square-foot sail and a West River 180 as the center hull, I would expect broach-reaching speeds in 15 knots of wind and smooth water to be a relatively modest 6-7 knots.  Hard on the wind, the small sail is really fighting to overcome a lot of windage and I wouldn't expect to see more than 4-5 knots.  Better than you could do paddling, to be sure...

On this photo shoot, I was sailing a relatively heavy Chesapeake 17, 24 inches of beam, with the 40sqft sail, and occasionally touched just over 9 knots on the GPS in a gusty 20-25 knots of air:

By the way, on the timing-out thing when posting here:  It's a safety issue.  If you're signed in to this site, you might have an account with associated billing information.  Wouldn't want to walk away from your computer and leave billing info accessible.  So just like a banking website, after a certain amount of time it clocks you out.

If I think I've dallied too long, I just "Control-C" my entire post, log in again, then "Control-V".  Takes seconds.

I really have no way to measure the wind speed, but using the 70sq ft sail on my 17lt and the MKII rig I do see 9 kts without a lot of effort at the local lake. Whatever the wind conditions for that are, they seem nowhere extreme and are not causing white caps to cover the lake. I also saw 9kts in big seas on a crossing but that was racing down waves 9kts - 0kts, big rinse, repeat. Something that allows the boat to get to speed quicker, although I have not perfected the system yet, is using a climbing ascender attached to the top of the mast. Now the mast is tied to the hull at the bottom of the mast, and the sail is held up by the ascender at the top of the mast. This way the the halyard is not trying to shoot the mast through the bottom of the kayak The mast is somewhat relaxed and only stressed from the wind in the sail. I swear that this increases my ability to pick up speed and it sails easier under lighter wind now. The part I am not finished perfecting is the release mechanism for dropping the sail. I have one modification to make this summer and it should be good. Anyway, I 'm off topic. I just wanted to say the MKII can move your boat to 9kts. In my case easiy because more sail area.