Rethinking Hull Speed

[Gary Wyngarden]

As the theory goes, hull speed for a displacement-hull sailboat is approximately 1.32 times the square root of the waterline length. For my Hunter 37.5 that would be about 7.4 knots. The highest number I've ever seen on my knotmeter is 9.9, but we were surfing down a large breaking wave off Estevan Point on the West Coast of Vancouver Island. That got my attention--it wouldn't have been a good time to broach. Ok but that's surfing.But then last week on a return trip across the Strait of Georgia (about 20 miles of open water) we recorded consistent speeds of 8.2 to 8.3 knots for over an hour on both the knotmeter and GPS. We had 22 knots apparent on the wind gauge with the wind just aft of the beam which means the true wind was probably in the upper 20's. We had two reefs in the main and the genoa rolled up to about 80% of the foretriangle. We were flying, but had the boat under control.Then yesterday we were in an informal race with a catamaran in San Juan Channel with true winds of 18-20 knots. Our knotmeter topped out at 7.9 again supported by the gps and we were consistently running at speeds above 7.5 knots on a close reach. (BTW we passed the cat.)I understand the theory behind hull speed and I also understand surfing. But I'd be interested in others experiences with sailing above hull speeds. What were the circumstances and conditions and how much sail did you have up? Did you feel any loss of steering control at those speeds.Thanks for your input.Gary WyngardenS/V Wanderlust h37.5

 

[Ross]

The formula for hull speed has a lot

of qualifiers attached to it. If you can find a speed/horse power curve for displacement hulls it will be apparent that with enough power you can push above apparent hull speed. With the wind speeds that you had there was a great deal of power available and you used it well. While surfing down a wave you had gravity, wind and water all in your favor.

 

[Don]

On the edge

Gary, I have a 35.5 and I beleive my hull speed is 7.3 Knots. On a trip back from Sucia in May we were hitting 10 knots on the knot metre and higher on the GPS due to the flood tide. We were on broad reach with full main and the 130 up. We were definitely overpowered as in the gusts (high 20's) it was a huge struggle to keep it from rounding up. I do manage speeds above hull speed quite regularily but I think it is the added power of the big winds. The sails are 14 years old, the bottom needs work and we normally carry full tankage and a ton of cruising supplies. When we manage to exceed hull speed by any great amount we are normally on the edge.

 

[David]

Probably more complex than it looks

I had a very similar experience with my Beneteau 373 a couple of weeks ago, also with wind in the mid 20's abaft the beam. We sailed at 8.4 knts on the GPS for about 20 minutes in pretty protected waters, also with a reduced rig.As I understand the theory, the hull speed limitation for a displacement boat arises due to the physics of the standing wave pattern the displaced water creates. I am guessing that this is strictly true only if the water is absolutely flat. Anything that creates turbulence on the surface probably impacts the standing wave to some extent, and may allow the hull to escape the standing wave in front of it. This is different than surfing, obviously. Any physicists out there?

 

[Scott]

Happy 4th of July, Gary

There was a discussion about this in the Cruisers Forum and a number of guys basically argued that the formula isn't true for the hull designs of todays boats. They said it is a basic formula for the ships in the days of old.I don't remember the specifics but, basically, the speed is determined by the length of the bow wave; however, todays boats are not limited by the waterline length. These guys were saying that the length of the bow wave can extend beyond the stern of the boat (before forces start to effect the boat), thus the speed of the wave increases and the hull speed of boats can exceed the hull speed predicted by this formula (even though the hull is not planing).I have to agree with them based on my experience on our 27' Starwind. The theoritical hull speed of the 22' waterline length is about 6.3 knots. We sail on a small lake so surfing is not possible! We often have very flat water in high winds, particularly because the power boats have often gone in and the wind flattens the wakes as well.I've known that our knot meter is out of calibration and I haven't bothered to correct it since we did't have GPS. But, I just got one for a present! We were out sailing on Sunday with winds often gusting into the 20's and we were single reefed and had a 110% jib up. We had a steady reading of 6.6 knots on a reach just aft of beam. Based on our knot meter we did not come all that close to our highest readings. Earlier this year, with the same sail area up, we reached speeds that I am approximating at possibly over 7 knots based on some rough estimates. I am anxious to get more experience with the GPS, and get the knot meter calibrated (it was fun to see 9 knots registered on that high wind day, even if we knew it was not true!). And, no, my knot meter is set for knots, not MPH ... I checked!So, yes, I believe that you are exceeding the theoretical hull speed. I think the formula is outdated based on the arguments I was reading in the Cruisers Forum.

 

[BarryL]

Waterline Length

Hello,The water line length you used is based on static measurements - ie the boat sitting in a tank of water. When you are sailing, that measurement is no longer true. As the boat heels the water line length changes. When the boat moves through the water the water line length changes.More than likely, as your boat heels the LWL increases. As the boat accellerates the LWL increases. Add up the increases and there you have it.Barry

 

[Warren Milberg]

Theory vs reality

I, too, have frequently exceeded the theoretical hull speed for my Hunter 28.5. I have always attributed this to the belief that the constant needs to be adjusted, up or down, to fit the hull type of your boat. Once I re-jigger the constant, and add in what I believe to be the square root of the real waterline length of my heeled boat, the forumla sort of works. In the end, it is theoretical, but the rush of exceeding the maximum is always quite real. It's one of the reasons sailing is such fun.

 

[Dan McGuire]

LWL Theory Does not Hold Water

The additional length required to attain the speeds described just are not there. For example, with Gary's 37.5' length, the absolute maximum length is 37.5". That gives a maximum hull speed of less than 8.1K and that is if every inch of his length is available which is not likely. Certainly Don's 10k is not available with a 35.5' hull.It is my understanding that the hull speed is based on the movement of a wave of that wave length. When sailing, you establish a bow and stern wave and you are sailing in the trough. In order to exceed that speed, you must move up the bow wave or push the bow wave faster. I believe that under certain circumstances, you may be planing which requires some lifting of the bow. The biggest problem with this theory is that while the water is lifting the bow, the force on the sail is trying to push it down.Also I believe, as someone mentioned above, that this is not an exact science down to the nearest 1/10th knot.I have exceeded hull speed in my MAC 23. The speed was measured using a GPS and there is virltually no current in the lake.

 

[abe]

I think that the formula is more accurate for..

heavy full keel type of displacement boats and not the lightweight boats that may tend to "surf" a little.abe

 

[jim kolstoe]

hull speed not a limit

the formula for hull speed does not describe a speed limit, only the speed at which you have to begin climbing your own bow wave to go faster. That requires a greater increase in power than a similar acceleration below hull speed. Consequently, it is, as your experience shows, very possible to exceed the hull speed if you have enough power (wind speed) to make the climb.Jim Kolstoe, h23 Kara's Boo

 

[Anchor Down]

Welcome to the 21st Century

The traditional formula is the square of waterline length (whatever that is at any given moment, as has been pointed out: LWL lengthens as boat heels) x 1.34, not 1.32.However, I've read that modern underbodies should be considered, and the 1.34 multiplier adjusted to a more generous 1.5.Using your numbers, I figure your boat has a LWL of approx. 32ft:The square of 32 = ~5.665.66 x 1.5 = 8.49We must remember that design changes will require a re-examination of tried-and-true formulae. There is no law of physics that demands the use of the 1.34 figure. It was just a number that was arrived at from experience that seemed to be pretty useful to apply broadly at one point in time. Now, with fin keels providing plenty of lift with less drag, spade rudders, shallow underbodies, 1.5 seems to be a better descriptor.Welcome to the 21st Century:Hull speed = the square of LWL X 1.5

 

[Ross]

A small but significant correction:

Welcome to the 21st Century:Hull speed = the square of LWL X 1.5 Should read the square ROOT of LWL X 1.5The square of 32 equals 1024

 

[Dan McGuire]

21st Century Has Little to do With It

I have heard of all sorts of factors all of the way from 1.3 to 1.4. I have never heard the 1.5 figure before. I would like to hear the source of that number. Many years ago, I saw how the 1.3X number was derived. It is an incredibly complex equation in which a number of factors are ultimately ignored for simplicity and because they add little to the equation. Part of that equation is a number of constants which were empirically derived and subject to some dispute. I don't believe that changes to the keel and rudder affect the basic assumption that requires the boat to climb out of a trough.I believe there may be a number of factors which allow the boat to beat the number. Those include the built in inaccuracies of the equation, the possibility of limited planing and possible exaggerations of speed.I also believe that lengthening of the LWL by healing may be one factor, but by itself cannot account for all cases. I think that there may be the possibility that while heeling, the boat may lend itself to planing because of the slab side. A better keel and rudder might help in this regard because of less weather helm.Just an aside. I do not consider myself any where near an expert in this area or any other area, for that matter. I do consider myself fairly analytical and love this type of mental exercise. As such the views stated above are strictly my considered opinion.

 

[Anchor Down]

Called my hand

All right, I just pulled 1.5 out of the air. Just trying to make the original poster feel good.Actually, I did read it somewhere in passing, but can't give the source. It was from a recognizable name in sailing, though. But I'm neither a mathematician nor a naval architect, so I will bow to the wisdom (even if it is sometimes nothing more than common wisdom) of others.As Ben Franklin said… (what was that, again, Ross?)Of course, the square ROOT was what I should have written. Good thing I don't work for NASA…

 

[Ross]

when I lived in Gulfport ,Miss I did a lot of

harbor fishing at watched the wooden sail boats motoring outof the harbor. This was summer of 1960 and my vantage point was a row boat. The wave making characterics of those old boats ( built in the 30's,40's,and 50's) placed a distinct hollow at midships and a high stern wave. I have not seen any fiberglass boats make waves like those old wooden boats did. Those hull shapes don't fit the new racing rules so the builders have been making changes to build faster boats within the new rules. This doesn't mean that the new boats are better just different.

 

[Joe]

You are missing the obvious...

your hull is not a true displacement hull. It is somewhere between a displacement hull and a planing hull. The best thing you can do is throw that stupid formula out, because it doesn't mean squat.

 

[Anchor Down]

Google to the Rescue

I may have found something.___ ___ ___From Royce's Sailing Illustrated, page 140:The maximum efficient hull speed for a heavy displacement hull is 1.34 times square root of the waterline length (WLL)". The key here is "heavy displacement." This is a simple formula keyed to "heavy displacement" and a range extending all the way to 1.6 times square root of waterline length and above is possible for medium and light displacement vessels.___ ___ ___Hmmm…

 

[Scott]

Joe, I think you're right!

Ross, what you describe in the older boats pretty much sums up the origin for the formula to begin with. The speed of the bow wave can be predicted by measuring wavelength. As the wavelength increases so does the speed. I think with the old timers, it was recognized that the speed of the hull was limited by the effect of the stern wave, which was like a shackle. So the speed was pretty much limited by the waterline length. Jim, I don't think that it is additional power that causes the boat to lift out of the bow wave (as a planing hull does). I think it is the design of the modern boats which enable the wavelength to increase beyond the stern(thus increasing speed) and what you find is the stern wave behind the boat. I've often seen this. The stern wave is no longer the prohibitor of speed as it was in the older ship designs and waterline length is no longer the limiting factor.Do I sound like I'm full of shit, or what?

 

[Don]

Possible hull damage

I have read somewhere that when you are being towed by a powerful boat you must be careful not to exceed hull speed as it could cause damage. The theory was that to exceed hull speed needed great force and that could be a problem. Does this mean that exceeding hull speed by wind force and not surfing can also be damaging?

 

[Ross]

YES! any time you are stressing the boat

past its design limits you run the risk of breaking something. Probably not the hull but the tow rope has to pull on something. If ten HP will push the boat to hull speed will the cleat that the tow rope is on with stand a twenty HP pull?