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allanpeda · 28-05-2011, 18:32

Quote:

Originally Posted by daddle

Going faster than hull speed is not the same as planing. Kayak's can easily exceed their hull speed because their bow waves are so small. The stern wave is some distance behind the kayak in this case. It is not planing. Ultralight monohulls and cats will do the same thing as they make small waves which make it easy for the hull to travel up the trough between the bow and stern waves, leaving the stern wave some feet behind the boat. In a ULDB planing is a completely different effect. The planing boat does not make significant displacement-generated waves and the hull speed formula therefore does not apply. Also planing feels and sounds completely different.

1.34 is the only proper value for the formula on this planet. It is the value it is because of the physical properties of water and gravity. People using other values are computing some speed other than what is known as "hull speed" - which is the speed of a wave system having a wavelength equal to the hull length. They are probably trying to define some 'maximum speed' which is not really what 'hull speed' is.

A poster did mention that the waterline changes length as speed increases. This is usually true, and is especially true for hulls designed to old racing rules that penalized at-rest measured waterline length.

Any hydrodynamics references?

daddle · 28-05-2011, 18:40

Quote:

Originally Posted by allanpeda

Any hydrodynamics references?

http://legacy.sname.org/newsletter/Savitskyreport.pdf

allanpeda · 28-05-2011, 18:55

Quote:

Originally Posted by daddle

http://legacy.sname.org/newsletter/Savitskyreport.pdf

These two seem more to the point:

Kayak Speed and Hydrodynamic Designs
Heavy Boats, Light Boats, and Hull Speed - Antrim Associates Naval Architects

SaltyMonkey · 28-05-2011, 19:06

Nay, not so...

Crunching Numbers: Hull Speed & Boat Length

"...This all seems very tidy, but in fact the concept of hull speed is viewed skeptically by many yacht designers. “It’s total ********,” is how one designer friend of mine, Jay Paris, puts it. For in reality many boats, even those with honest-to-God displacement hulls, can easily exceed their nominal hull speeds.

One reason this happens is that a boat’s effective waterline length will often increase as it goes faster, particularly if it has long overhangs, and thus its speed potential under the formula will necessarily increase. Another reason this happens is that a boat’s stern can be designed to significantly suppress its stern wave, which helps to make the hole created by the wave trough when traveling at speed smaller, and also to increase buoyancy aft, which helps keep the stern from falling into the hole in the first place. Stern sections capable of doing this have overhangs that exit the water at a steep angle, usually 15 degrees or less (this helps suppress the stern wave), and are beamy with lots of increased volume aft (which increases flotation)...."

and more online...

allanpeda · 28-05-2011, 19:16

Well then it's merely a case of unfortunate choice of words, using the term "hull speed", as per Fround this number is really simply the speed of the bow and stern waves generated by the forward motion of the hull.

denverd0n · 30-05-2011, 08:57

Exactly. It is an unfortunate choice of words, and different meanings being applied to the term "hull speed". Some use the term "hull speed" to mean the maximum speed a displacement boat can go. Others use it to mean the speed the boat is going when the bow wave equals the length of the hull. The latter is also the maximum speed the boat will go while still displacing its full weight in water.

As it begins to climb up the bow wave the boat also begins to displace less than its full weight in water. It then goes faster than the hull speed. In the right conditions almost all boats will do this, even full-keel, heavy, "displacement" boats. That doesn't mean that their "hull speed" has changed (unless you are using the term to mean "absolute maximum"), it only means that they are now exceeding it.

1.34 is the correct number for any given LWL. The problem is that sailboats heel and as they do so the LWL almost always changes. Hence, you would have to know the precise LWL at each different degree of heel to precisely determine the hull speed at that particular point. But why?

As was mentioned within the first couple of postings, hull speed is a moderately interesting number to know, but in practical terms it is pretty useless. If you're trying to determine how many miles you can go in a day then hull speed is only a very rough number to begin from anyway. If you are trying to determine the absolute maximum that you can go, "hull speed" won't tell you that.

So the bottom line, really, is that it's not worth worrying about. Calculate it for your boat if you want, but remember that in the real world of sailing it tells you nothing very useful.

allanpeda · 30-05-2011, 09:09

Quote:

Originally Posted by denverd0n

...

So the bottom line, really, is that it's not worth worrying about. Calculate it for your boat if you want, but remember that in the real world of sailing it tells you nothing very useful.

Well, I would add that in engineering, a ball park number is good to have, even if only as a seed value for an iterative refinement. Reality changes this value so much, it's good to have something to toss out for discusion.

**chris07732** · 30-05-2011, 09:54

I've been told that the LWL is the straight line distance between the bow and stern at the water line. The "fineness" of the hull apparently doesn't come into play using the 1.34 formula. A fat tub has the same "hull speed" as a kayak of the same LWL. Intuitively, this doesn't make sense to me. I appreciate that it will take more power to move the tub at hull speed, but the onset of drastically more power to move past hull speed is the same kayak to tub. Not sure why this is true, if it is true.

**David M** · 30-05-2011, 10:27

As one poster already said, hull speed is hull speed unless you move to a different planet where gravity or the medium you are floating in is different.

It's where you have a wave peak at the bow, a trough in the middle and a wave peak at your stern. Your boat can be shaped like a toothpick, a whale or a cube and it makes no difference. The amount of power it takes to drive your boat through the water to reach hull speed for a given length has no relevance to your hull speed. Your hull speed given your LWL is a fixed number on this planet.

Show me where in a naval architecture book written for professional naval architects it says different. This is what I learned in my naval architecture class.

denverd0n · 31-05-2011, 05:39

Quote:

Originally Posted by David M

Your boat can be shaped like a toothpick, a whale or a cube and it makes no difference.

You are exactly right. Given a cube and a toothpick with the same LWL they will have the same hull speed. The only difference is that getting the toothpick up to hull speed, and then exceeding it, will be a LOT easier! Nonetheless, their hull speeds will be the same.

**Dockhead** · 31-05-2011, 05:54

Quote:

Originally Posted by denverd0n

You are exactly right. Given a cube and a toothpick with the same LWL they will have the same hull speed. The only difference is that getting the toothpick up to hull speed, and then exceeding it, will be a LOT easier! Nonetheless, their hull speeds will be the same.

Right! Hull speed is by no means the only thing which influences maximum speed of a vessel. It is merely one concept (albeit an important one). It is merely that speed at which the wavelength of the created wake exceeds the length of the vessel such that the vessel is no longer supported by the wake and starts to climb its own bow wave.

How much energy is required to do that varies greatly, and so the effect of hull speed is very different with different hull forms and displacement. Very thin light hulls might hardly notice the bow wave climbing process and go right past hull speed with just a little extra energy. Fat heavy hulls might experience hull speed as a very harsh limitation. But the hull speed itself is directly related to waterline length without any consideration for hull form or displacement.

There's a great article here: Wave making resistance - Wikipedia, the free encyclopedia

**Dockhead** · 31-05-2011, 06:03

Another great explanation of hull speed: http://myeerah.server101.com/Hull%20Speed.pdf

capn_billl · 31-05-2011, 06:30

The hull speed is not really a law, it is more like a guideline.

It is good to know for planning. You're not going to get good efficiency over this speed. If you're designing a hull, it is good to know a long skinney hull is going to be better than a short fat one for efficiency and speed.

**Dockhead** · 31-05-2011, 06:48

Quote:

Originally Posted by capn_billl

The hull speed is not really a law, it is more like a guideline.

Well, actually, it is a law -- a rigid law of physics.

The thing is, however, that the law applies in different ways to boats with different hull forms, displacement, and power available. Some displacement hull boats can exceed hull speed fairly easily; for others it's like hitting a brick wall. Most are somewhere in between.