Theoretical Boat Speed

[Graeme Colmer]

Can anyone advise what is the theoretical hull speed for a 2009 36 Ft Hunter?

How is this generally calculated?

Any thoughts appreciated.

[daddle]

Hull speed is not a theoretical number. It is a value derived directly from the hull's waterline length, see:

Hull speed - Wikipedia, the free encyclopedia

For your boat it will be around 7kts. It has very little to do with the expected speed of a cruising boat on a typical journey. That will be closer to 5 for you.

Heavy full hulls may rarely reach hull speed, while light well formed hulls exceed it effortlessly. Therefore Hull Speed is not a maximum speed. It's actually rather useless...

Hull speed is simply the speed of a wave that is as long as the waterline. The 1.34 scaling factor cannot be altered unless you move to another planet no matter how long this thread becomes.

There are many cool calculators at:

Hull Speed

[Dockhead]

First, Google "Hunter 36 LWL" and get the length of the water line. Result: 30'7"

Second, Google "hull speed calculation". Result: 1.34 * square root of LWL. There's also a handy online calculator here: Hull Speed

Third, input your LWL. Result: 7.41 knots.

Simples.

[barnakiel]

And to go faster, the boat must get on the plane. But not all of them will do!

b.

[marshmat]

Hull speed has a clear definition: the speed of a freely travelling wave whose length is equal to the boat's waterline length. Hence V_hull = 1.34 * sqrt (LWL), for LWL in feet and V_hull in knots. 1.34 is the speed to length ratio.

Hull speed is not a very useful number from a performance standpoint. A better, but still very rough approximation for the top speed of a displacement hull is Dave Gerr's modified hull speed formula:
(Speed to length ratio) = 8.26 / (Displacement to length ratio)^0.311
then
Speed = (speed to length ratio) * sqrt (LWL)
where displacement to length ratio is in long tons per cubic foot, LWL is in feet, and speed in knots.

If you want a more useful estimate of your boat's speed potential, you need to look at the rating rules under which she'd compete, or any number of more complex formulae (S-number, for example), or- ideally- look at her VPP polars.

[Paul Elliott]

To a point, even a heavy displacement boat can exceed hull speed if you push it hard enough. The theoretical hull speed is the point where the speed/power curve becomes steep. Above hull speed the boat may become hard to handle, it will be plowing a huge hole in the water, and the loads on the rigging will become quite high. Unless the boat gets up and surfs, the excess speed will be just a few knots, and these are hard-to-get knots.

Some hulls also have an actual hull speed greater than the theoretical hull speed you get from published LWL calculations, because of hull shape changes as the boat heels, or is more submerged by the stern wave (and thus the LWL increases).

I agree, in practice your cruising speed will be less than your calculated hull speed. 75% of hull speed seems to be a reasonable guess, but this obviously depends on your sailing style.

[daddle]

Originally Posted by barnakiel

And to go faster, the boat must get on the plane.

Not true. To go faster the boat must start moving up the backside of the bow wave and leaving the stern wave some distance behind. Planing is entirely different.

[perchance]

Hull speed for a Hunter? I think it's about 120 mph if you drop it from a great enough height.

[Adelie]

Originally Posted by Graeme Colmer

Can anyone advise what is the theoretical hull speed for a 2009 36 Ft Hunter?

How is this generally calculated?

Any thoughts appreciated.

Couldn't find your model on sailboat data.com , but assuming your waterline length is about 31'

Hull speed = 1.34 x LWL^.5 = 1.34 x (31)^.5 = 7.5kt

Hull speed is a decent indicater of the point at which adding engine power to go faster becomes almost pointless for most displacement boats. You could double your power and get an extra .5-1.5kt speed out of the boat at significant extra fuel usage. For a stock boat boat with stock engine, count on max speed being 0-1.0kt greater than theoretical hull speed.

Using a lightship displacement of 15,500lb and LWL of 31', 61hp should push you at about 8.45kt consuming 3.4gal/hr diesel, or 2.5 nm/gal, 33.6hp will get you 7.5kt at 4nm/gal, 5kt will give you about 8.5nm/gal. Best fuel economy will be about 10nm/gal at about 3.5kt. Milage may vary a bit depending on exact hull configuration. Speeds and milage assuming calm water.

Hp required calc'd using Gerr's formulas as implimented at:
Boat Speed | Hull Speed | Speed Calculator

Gal/hour calc'd by dividing HP required by 18hp/gal diesel per hour to get gallons per hour,

MPG calc'd by dividing speed by gal/hr.

[allanpeda]

Originally Posted by daddle

Not true. To go faster the boat must start moving up the backside of the bow wave and leaving the stern wave some distance behind. Planing is entirely different.

How is the onset of planing different from the transition described?

[SaltyMonkey]

1.34 is a suspicious constant and you need to adjust depending on hull shape etc.

[allanpeda]

Originally Posted by SaltyMonkey

1.34 is a suspicious constant and you need to adjust depending on hull shape etc.

Yeah, I read that this number is not cast in stone. Considering all the other variables, and the application of the number (that is to say, it is safe to assume the asker is not designing a guided missile cruiser), I think it sufficient for most purposes. Once you start adjusting for hull shape, you might as well start testing a scale model's Froude number.

[Adelie]

Originally Posted by SaltyMonkey

1.34 is a suspicious constant and you need to adjust depending on hull shape etc.

How do you decide when numbers are suspicious? Can you tell me which ones to be wary of and which ones are above reproach?

1.34 is the generally accepted constant, so that is the place to start. If you want to dig into the underlying math to get a better result you replace the constant with several more variables and a different constant.

[SaltyMonkey]

Just sayin...when you look at more modern fin or cruiser/racer designs, you need to consider this number is not high enough. Therefore, be optimistic if you continue to use the old 1.34 formula and what you get out of it. For older F/keel boats 1.34 is guud enuf.

[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.