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post #1 of 11 Old 06-17-2010 Thread Starter
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Larger boats faster

Just something I was wondering and may have a simple answer. From what i've read, it seems that larger boats are always faster than smaller boats. Now I understand that with a larger boat comes the ability to use larger sails and that in turn means more wind in the sails, higher speeds. But with a larger boat size also comes more weight. Maybe i'm thinking too much in terms of cars but couldn't a smaller boat be built to be faster than a larger boat (less weight, etc)
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post #2 of 11 Old 06-17-2010
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Hull speed - Wikipedia, the free encyclopedia

I cheated but in essence a longer hull should be faster, with exceptions of course...

Hull speed, sometimes referred to as displacement speed, is a rule of thumb used to provide an approximate maximum efficient speed for a hull. It is only ever an approximation and only applies where the hull is a fairly traditional displacement design. It is usually described as a speed corresponding to a speed-length ratio of between 1.34 and 1.51 depending on which of the limited sources one refers to.
In English units, this may be expressed as:

where:
"LWL" is the length of the waterline in feet, and
"v" is the speed of the vessel in knots
The constant may be given as 1.34 to 1.51 knot·ft−˝, or in metric units, 4.50 to 5.07 km·h–1·m (where LWL is measured in metres and v is the speed in km/h)








Wave making resistance begins to increase dramatically in full-formed hulls at a Froude number of about 0.35, which corresponds to a speed-length ratio of slightly less than 1.20. This is due to a rapid increase of wave-making resistance due to the transverse wave train. At a Froude Number of 0.40 (speed-length ratio about 1.35) the wave-making resistance increases further due to the increase of the resistance caused by the divergent wave train which is added to the transverse wave train resistance. This rapid increase in wave-making resistance continues up to a Froude Number of about 0.45 (speed-length ratio about 1.50) and does not reach its maximum until a Froude number of about 0.50 (speed-length ratio about 1.70).
This very sharp rise in resistance at around a speed-length ratio of 1.3 to 1.5 probably seemed insurmountable in early sailing ships and so became an apparent barrier. On the other hand, these values change dramatically as the general proportions and shape of the hull are changed. Modern displacement designs that can easily exceed their 'hull speed' without planing include hulls with very fine ends, long hulls with relatively narrow beam and wave-piercing designs. These benefits are commonly realised by some canoes, competitive rowing boats, catamarans, fast ferries and other commercial, fishing and military vessels based on such concepts.
Since the wave amplitude increases the energy transferred to the wave for a given hull length the wave drag can be very sensitive to the vessel's weight.

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post #3 of 11 Old 06-17-2010
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there are so many variables it's mind boggling. length of water line, wetted surface, keel size and type, rudder type, high or low freeboard, bow design, transom design, and many more things!

Long narrow boats are fast. wide short boats are slow. Yet a long boat heeled over may be slower then a short wide boat. One example on the small side is a canoe with 2 people paddling.. and they cannot keep up with a single paddler in a long narrow sea kayak.

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post #4 of 11 Old 06-17-2010
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Displacement hulls deonstrate a power to speed relationship based on the length at the waterline. The result is a speed at whichlarger increases in power are required to increase boat speed. /This is termed hull speed and is applicable under sail or power.
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post #5 of 11 Old 06-18-2010 Thread Starter
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So does weight play a major role in speed? Or is it more based on displacement?

It doesn't seem like some of the multi hulls that are used in say the America cup racing have large displacements but seem extremely lightweight. Would a heavier/larger displacement boat perform better?

For instance, if you took 2 similar boats, with similar equipment and same wind conditions, but one was say 500 pounds lighter, the obvious winner should be the lighter one correct?
So if you took that same lightened boat and put it against a larger displacement boat that was heavier, would it still win because of the larger size?
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post #6 of 11 Old 06-18-2010
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Quote:
Originally Posted by spirit54 View Post
So does weight play a major role in speed? Or is it more based on displacement?

It doesn't seem like some of the multi hulls that are used in say the America cup racing have large displacements but seem extremely lightweight. Would a heavier/larger displacement boat perform better?

For instance, if you took 2 similar boats, with similar equipment and same wind conditions, but one was say 500 pounds lighter, the obvious winner should be the lighter one correct?
So if you took that same lightened boat and put it against a larger displacement boat that was heavier, would it still win because of the larger size?
Your prior posts have mixed in a lot of variables, but if you change "similar" to "identical" displacement hull boats, you are correct that the lighter one will have an advantage. If you start changing variables by giving the heavier boat a longer waterline then the equation becomes more difficult.

If you start looking at dissimilar boats then the advantage may not be so clear as the the sea conditons change which might tip the scales, since the heavier boat may carry more speed through waves. If you change the wind conditions the advantange goes to the lighter boat as it will accelerate faster in the puffs. If you change the course to be sailed the lighter boat will accelerate faster out of the tacks, but the heavier longer boat may have the advantage on a long reach. So the short answer is weight is one factor, but there are a lot of others, though weight gets a good deal of attention since its one factor you have some control over.

PHRF attempts to control for all these variables and is probably as good a rough yard stick as there is to compare the speed potential of disimilar boats.

As a FWIW, small planing hull sailboats can reach speeds most displacement hull boats can only dream of.

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post #7 of 11 Old 06-18-2010
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I'd point out that the whole equation changes when you consider multihulls. My 28' trimaran can often run down 40' monohulls with relative ease. The traditional estimate of hull speed doesn't apply to multihulls, as the hull forms are too radically different and the constant multiplier is not 1.34 but something much higher.

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post #8 of 11 Old 06-18-2010
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There are a gazillion varibles, but basically....

Hull shape, Hull type (Displacement vs Planing), displacement, Sail area, SA/D, and keel or ballast depth, are the prime factors of a boats speed.

In general:
  • Planing hulls are faster than displacement hulls
  • Boats with similar/identical hull shapes, performance is base on displacement and SA/D. The higher the SA/D value, typically the faster the boat

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post #9 of 11 Old 06-18-2010
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For quite a few years we owned a 24 footer that gave 40 footers fits. But it definitely depended on the conditions. An easily driven light boat will accelerate and get moving again much sooner than a heavier longer boat despite the latter's advantage in LWL. However once the breeze is up, esp beating the larger boat will walk away due to her power and displacement length potential. Off the wind the smaller boat may have an advantage again if it's a planing type of design, at which point all the displacement-based LWL vs speed calcs go out the window....

The newer sport boat designs are really pushing the limits and the traditional measure of boat speed potential often does not apply anymore.

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post #10 of 11 Old 06-18-2010
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complicated

Hello,

By now I'm sure you have learned how complicated determining performance. Years ago this was much simpler
- the longer the waterline, the faster the boat. Now, with planing hulls this is no longer true. It used to be that only small boats could plane, now even big boats (50' and up) can plane and have sustained speeds over 20 kts.

A few more comments. Speed often depends on conditions. A big heavy boat will be faster than a small light boat in higher winds, but in light winds, say nder 7 kts, a small light boat will easily outperform a larger heavy boat. In light wind there isn't enough 'horsepower' for the heavy boat to go.

You asked about two very similar boats, one 500 lbs lighter. There are many boats like this. On some new sport boats, you can buy a standard aluminum rig (mast and boom) or a carbon fiber rig. The CF boat will be 100 lbs lighter and will outperform the aluminum rig. Other boat has different keel options - deep draft or shallow draft. The boats will typically weight about the same but the deep draft boat will outperform the shallow draft boat (right up until it hits the ground )

Again, this gets complicated!

Barry

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