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Old 08-11-2010, 13:11   #16
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Originally Posted by capt_douglas View Post
Can your vessel exceed the maximum theoretical speed? Yes.
And not only as a result of the heeling motion, which makes the waterline longer.

The fact is that almost ALL hulls will plane to some extent. Some much more than others. What does it mean for a boat to plane? It simply means that it is moving fast enough that, rather than pushing a volume of water that is fully equal to the displacement of the boat around it as it moves, it is riding up to some extent, pushing some water underneath it, and pushing less water around it than it's full displacement.

A full-planing hull gets up so that hardly any water is pushed out of the way. A displacement hull pushes a lot of water out of the way, but can also ride up to some extent. Modern sailboats with very flat bottoms will ride up relatively easily into a semi-planing mode, and as a result can exceed hull speed relatively easily.

Hull speed is NOT an absolute maximum speed beyond which the boat cannot possibly go. It is simply a point where it begins to take a dramatically larger amount of energy to make it go faster. Put a big enough motor on it and I have no doubt that you could get a Westsail 32 to plane--at least to some extent.

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Old 08-11-2010, 13:26   #17
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It would depend on the area in which you sail, too. Areas that have lots of light wind days favor fin keel/lighter displacement boats. Lighter boats go faster in lighter air for the same LWL compared to heavy displacement boats. You (usually) need less wind to reach hull speed in fin keel designs compared to full keels. Once you're at hull speed, that's it, your speed plateaus.

You may ask "If you could be assured of hull speed conditions all the time while sailing for a given LWL, why not get a full keel heavy displacement design?" Because that is a big "if" on those conditions being present, and because the 1.34 doesn't work for all fin keels. A full keel may reach hull speed at 25-30 kts, a racer at 15-18 kts.

A good way to find out the wind speeds that a given boat reaches hull speed is not necessarily by math- try asking the owner at what wind speeds does he usually reef!


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Old 08-11-2010, 13:59   #18
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The 1.34 rule seems so far off that it's pointless to remember two significant digits.

My LWL is 22'4".

The 1.34 x (sqrt LWL) rule says I max out at 6.33 kts.

In reality, I regularly exceed 8 kts and max out just under 9 kts (confirmed by a speedo and 2 GPS's).

It seems almost everyone can go faster than their max speed as calculated by the 1.34 rule. If every boat is the exception to the rule, maybe it's time to throw the rule out the window.
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Old 08-11-2010, 14:01   #19
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Originally Posted by SkiprJohn View Post
Your actual hull speed will depend a little on design and weight vs sail area but you won't be exceeding hull speed (once it is determined) in flat water no matter how much power you put to it.
Not quite.

The problem is that a lot of people have gotten it into their heads that "hull speed" is some absolute maximum number beyond which the laws of physics do not allow a "displacement hull" to go. But it's not like that. Put enough power behind it and most any boat can exceed hull speed. Given a boat with a fairly flat bottom, it will be able to exceed hull speed relatively easily.

"Hull speed" is nothing more than a theoretical number that represents the fastest that the boat will go while still displacing a volume of water that represents the full mass of the boat. That is, the speed it will go when it is not planing to even the slightest degree.

The thing is that most modern sailboats WILL plane, at least to some small degree. And when they do they exceed hull speed.

So, "hull speed" is a theoretical number that has nothing to do with sail area, or hull shape, or anything else like that. It has only to do with the actual waterline length at any particular moment. How easily (or if) you can exceed hull speed, on the other hand, is very much dependent on hull shape and sail area. A flat bottom hull carrying lots of sail will easily exceed hull speed. A very deep, rounded and heavy hull with only a modest amount of sail will find it quite hard to exceed hull speed.

Hope that helps.
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Old 08-11-2010, 14:44   #20
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Originally Posted by houseboy View Post
Ok, so the theoretical speed of a hull is based on the hull length....there is not much of a difference in realized speed between boats ranging +/- 5'.....which is @ .5 nm/h. That is assuming you are compaing the same make/model's different lengths.

So, take out the variables that would affect things, like skipper experience, sail area,...etc. Is there really that much realized speed between different "popular" monohulls of the same length? Given you are at @ 5-7 nm/hr, does the hull design, make a noticeable difference in the speed? Or does the design impact more of the handling and the ride?
Consider a nice flat/wide bummned hull going downwind. It will come to the plane a lot faster than a boat with no Back side and an ass the side of a pea. By getting to the plane faster it will be faster.

Going upwind, a thinner hull with a fin keel will usually kick the pants of a fatter long keeled boat of the same size.

So Yes. It Matters.

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Old 08-11-2010, 16:16   #21
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The width of the boat hull plus the angle of entry will make a significant difference to speed potential.
Narrow monohulls go faster (particularly upwind) but suffer from lack of interior room.
The "waterline length rule" is a guide only.
It is based on the formation of a bow wave above a certain speed and when the boat hull tries to lift up over the bow wave which requires more power. A lot more power. The stern wave also is a factor
As a general rule displacement hulls require 5hp/ton to reach displacement speed "rule"
Planeing hulls require a minimum of 50 hp/ton to raise the boat (hydrodynamically) and get in front of the induced bow wave.

However modern catamarans which have narrow hulls 16-1
(for a 40 ft waterline, the width of each hull is only 2ft 6inches) defy the "rule" and cut through the water without forming a bow wave and can go much faster.
Then the "new rule" is when you double your speed you need four times more power.
This is the concept now used on Catamaran ferries all over the world, including the latest multihull in the US Navy.

By the way, S/V Beaujest. My aluminium 42 ft trimaran motor sailor is performing better with the new motors.
I replaced the 2 X 30 hp Honda's with 2 x 90 hp Tohatsu outboards.

I can now cruise at 12 knots with a maximum of 15 knots. However I normally use one motor only at 4,000 rev's and cruise at 9 knots.
No bow wave and the stern wave flattens out after 8 knots.
Boat weight is 6 1/2 ton
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Old 08-11-2010, 16:41   #22
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Hull speed is when you have a wave at the bow and stern and a trough in between.

What's different is the amount of energy it takes between the various hull forms to travel below or above hull speed.

Hull speed is determined by physics, not a naval architect.

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Old 08-11-2010, 16:53   #23
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Originally Posted by beau View Post
The "waterline length rule" is a guide only.
I think what confuses people is that "1.34 x the square root of the LWL" sounds really exact and scientific when really it's at best a loose estimate.

It makes no sense to use two significant digits for something that isn't even 70% accurate.
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Old 08-11-2010, 17:50   #24
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WAIT/ have all missed the major influencing factor. Theoretical Hull speed relies on the boat being level and driven by an engine. Hull speed is the point where the engine is at its best revs/power for the given boat/length, this point is where even if you increase the revs you gain little or no extra speed as the boat struggles to get over the hump of the hulls driven wave. (Only on displacement hulls). When under sail the energy driving the boat will at most times be greater than that able to be supplied by the engine. Also as the boat heels the water line is increased, thus giving a longer wave making the hull easier driven. This was why the 70s IOR rules gave the designers the chance to play with the rules and use extreme overhangs that gave extra long heeled waterline lengths.
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Old 08-11-2010, 20:38   #25
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The advantageous heel factor

As the boat heels, not only does the wateline increase, (particularly with narrow, long over hanging hulls like mine) but the hull looses symmetry and the keel and hull begin to add upward lift allowing the boat to exceed it's theoretical hull speed. I have a D/L ratio of about 230 with a S/D ratio of about 18. Plenty of power.Using a time speed distance calculation (as my knot meter is inoperable), on a crossing from Edmonds to Kingston, (just under 5 Kn miles) on a close to beam reach, we averaged 8.4 knots with my hull speed being only 7.3 (7.9 heeled at 15-20 deg). That was in around 18-20 knot winds with higher gusts. My avitar pic was taken that day during the sea trial before sale.
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Old 08-11-2010, 21:12   #26
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Good discussions, thank you. This maybe a rookie question, but what is best way to determine your "loaded" LWL. Is it as simple as using a long measuring tape along your hull surface from the bow to center of stern, holding the tape at your observed waterline? In other words, for a given LOA, a beamier boat will have a longer LWL?
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Old 08-11-2010, 21:36   #27
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Originally Posted by Hud3 View Post
Maybe this is more to the point the OP was getting at, since boats rarely sail at hull speed.

When we sold our boat the GPS showed our average SOG to be 6 kts, which is 75% of the boat's calculated hull speed. On offshore passages we averaged about 85% of hull speed. Our boat was relatively heavy, full keel, and cutter rigged.

Anyone have similar stats for a lightweight racer-type boat, based on actual experience?
I agree with everything written above. Most sailing is done in light air and less than optimal wind and sea state, in currents, and in heavy conditions it is often best to deliberately slow the boat down for comfort and so things don't break.

The image below is our wake at 14 knots in flat water. We sail in a cruising trimaran above "hull speed" (~38ft LWL) regularly and if we can get around 14+ knots apparent wind speed over our sails we hit 8-10 knots easily or hit 12+ knots in 20+ apparent (depending on wind angle and sea state, bottom cleanliness and load), but on a roughly 600nm vacation cruise this year we averaged (according to our GPS) a hair over 6 knots. Not impressive, but most days few sailboats would have kept up with us. (Just keeping it real.)

BTW just to explain the image -- you can see the wake from our starboard ama (leeward float) just to the left of the rail seat in the foreground corner of the image. It combines with the wake from our center hull to form the standing wave, and there is no wake from the port ama because it is windward and is out of the water. (Note also the angle of the top rail -- we're heeling less than 10 degrees.) Ye-ha! Wish we could do that every day.

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Old 09-11-2010, 02:51   #28
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Originally Posted by chris07732 View Post
what is best way to determine your "loaded" LWL. Is it as simple as using a long measuring tape along your hull surface from the bow to center of stern, holding the tape at your observed waterline? In other words, for a given LOA, a beamier boat will have a longer LWL?
No, the LWL measurement is a straight line.

The best and easierst way to work it out is with Google Just put in the boat and model and lwl and it will pop up (usually!)

beneteau 393 lwl - Google Search

Now if you really want to be a cheat just click on this calculator!
Boat Speed | Hull Speed | Speed Calculator

The calculator says my hull speed is 7.94 where I thought it was 8.5. I feel much slower today... glad I'm at anchor

What it does indicate is... in my earlier post I said I always reef at 8 knots no matter what... thats because I feel the boat is becoming a handfull after that. As you approach hull speed the boat gets more unstable: Its trying to jump outta the water and plane...

Also we havent talked about surfing the boat... in the open ocean you can go far faster than hull speed surfing. Fine most of the time, but if the waves are getting too big and brraking and your boat has lost its stability because its over hull speed then you are getting into dangerous situation that could culminate in a pitchpole.
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Old 09-11-2010, 03:44   #29
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I always understood the problem of sailing faster than the calculated max speed was that as a displacement hull moves through the water it pushes water and creates a bow wave. The size of waves - length between crests and height from crest to trough increase proportionately. Small waves travel slower, their crest to crest height is shorter and their depth is less.

As the hull move faster through the water it creates a bigger wave - longer and deeper and this is directly proportional to wave speed. And wave size is also dependent on the depth of water. You can't have large waves in shallow water! Wave form changes as waves "sense" the bottom and this is why large ocean sine like waves will break and overfall (get shorter and steeper) in shallower water.

So as your boat move creates a wave with the crest at the bow and the trough aft. As it approaches and reaches design hull speed the trough is mid ship and the boat has a crest at the bow and at the stern and this wave is as long as the boat and obviously has a proportional depth.

To move the boat faster it would then create an even longer wave, since wave size is proportional to speed. Now the trough would be aft of mid ship and the wave length longer than the boat and since the wave is created at the bow, the hull is essentially trying to sail up and out of the crest... the stern has fallen and is no longer supported by the aft crest. The boat is literally trying to sail "up hill".. up the back of the bow wave.

What can happen depending on bow / hull shape as the boat sees long and longer waves (assuming it can create these waves) is that the boat cannot sail up hill and instead sails into the the wave... or broaches. The hull shape reaches a point where falls into the face of back of the crest and it now in "flat water" situation but trying to sail steeply up hill. So the process is self limiting... and the factors are somewhat variable depending on hull shape as well as the direction of the wave. Sailing upwind you have the wind driven wave coming at some angle to the CL and the wave created by the boats motion co-linear with its motion... and toss if leeway effects on the angle of attack to the bow wave.

This is dynamic and complex.

You'll notice if you are sailing in relatively calm seas and light breezes and the wind pipes up, the sea state hasn't had a chance to catch up (wind creates waves) your boat will accelerate very quickly and get up to speed because it isn't dealing with the wind created waves. But over time the wind driven waves will increase and the boat will begin to slow as it has to drive through those waves as well as its own bow created displacement wave.

Any standard formula is OK but it applies to a single set of conditions for your hull.
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Old 10-11-2010, 05:55   #30
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Originally Posted by Hud3 View Post
When we sold our boat the GPS showed our average SOG to be 6 kts, which is 75% of the boat's calculated hull speed. On offshore passages we averaged about 85% of hull speed. Our boat was relatively heavy, full keel, and cutter rigged...

I think there is considerable wisdom in the 75% rule… generally that’s what I use for planning; sometimes I beat it marginally and sometimes it beats me. Even though I’m generally of the reef-early school, I find that on my smallish little chunk I only average about one knot less than I did with boats of at least double the waterline and nearly five times the displacement…

Maybe it has something to do with my innate laziness, but I’d guess that neither length nor displacement tell the whole story – how easy is it to keep moving, and how easy are sail-changes are huge factors for me – although I reef early, I’ll also throw up the drifter in a heart-beat… in any case, while one vessel or another may be able to run the mach numbers up under ideal or racing conditions with a seasoned, beefy crew – the more likely scenario for many of us is what does it do under small/family crews on a passage with aunt Minnie aboard…


Worry: misuse of imaginationů
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