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Old 04-22-2010
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SailBoat Speed

How do you determine speed of a sailboat before you buy it? When I say speed of a sailboat I mean in comparison to wind speed. What can you do to enhance the performance of a sailboat?

Thank you
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Check the polar diagrams. That will give you a rough idea of how a well-sailed boat will perform under various wind speeds at various points of sail.

Another way to compare boats, at least on a rough basis, is to look at their PHRF ratings. The higher the rating, the slower the boat.

There really isn't a lot you can do to enhance the performance of a boat. Other than keeping excess weight off the boat, fairing and smoothing the bottom and keeping it free of growth, keeping your sails in good shape, etc... but the basic speed of a given boat won't change much.

Some designs are just going to be much faster than others... Multihulls are a good example of that... trimarans, and to some degree catamarans, are going to be faster than their monohull counterparts.
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Quote:
Originally Posted by Sailboat101 View Post
How do you determine speed of a sailboat before you buy it? When I say speed of a sailboat I mean in comparison to wind speed.
Wow! What a far-reaching question! But, let me start off the discussion, and there'll be more contributions from others.

Generally, sailboats have displacement hulls, and their speed capabilities are traditionally thought to be limited by their waterline length, but that's not exactly true. Sailboats can, and often do, exceed their theoretical hull speed, especially when sailing off the wind. Therefore, you can get a very limited idea of a boat's speed capability by determining its hull speed, which can be calculated by using the following formula: Hull Speed = 1.34 x the square root of the LWL (length of the waterline)

However, the length of the waterline doesn't tell the whole story. Some sailboats with a waterline of a certain length will be faster than other sailboats with the same waterline length, due to other design variables, such as the beam (width) of the boat, and the shape of the keel or rudder, the amount of sail area, and the displacement (weight) of the boat, among other things.


The differences in the design features of a sailboat can cause one boat to exert much more drag than another boat as it moves through the water, reducing it's speed. Design differences can also enable one boat to plane more easily than another boat, which greatly reduces the amount of drag and increases speed.

Quote:
What can you do to enhance the performance of a sailboat?
The "performance" of a sailboat includes not only it's speed capability, but also it's ability to sail close to the wind, among other things. I assume your question refers to speed.

Generally, you can enhance a boat's speed by maximizing the ability of it's sails to generate driving force, and by minimizing drag. You should use the biggest sails that the boat can carry efficiently, and trim them well. You can reduce drag by placing the crew's weight in a position that will usually keep the boat as upright as possible, and will result in the least amount of wetted surface. There are a gazillion other factors that often take a lifetime to learn, but I'll let others try to think of them all.

You might ask, why would anyone use design elements in a boat that don't maximize it's speed? The reason is because speed isn't the only quality that people find desirable in a sailboat. Sailboat's are designed for a particular purpose. Some are designed for all-out speed and pointing ability, and some are designed for safe, reasonably fast and comfortable cruising. People buy the boat that best meets their needs.
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Old 04-22-2010
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Quote:
Originally Posted by Sailormon6 View Post
Generally, sailboats have displacement hulls, and their speed capabilities are traditionally thought to be limited by their waterline length, but that's not exactly true. Sailboats can, and often do, exceed their theoretical hull speed, especially when sailing off the wind. Therefore, you can get a very limited idea of a boat's speed capability by determining its hull speed, which can be calculated by using the following formula: Hull Speed = 1.34 x the square root of the LWL (length of the waterline)....
That's a good point to make out diferent types of cruising boats, in what concerns speed:

There are some sailingboats that downwind are pratically limited to their hull speed, other that can go easely 2 or 3k over hull speed (allmost all modern designs) and there are some new generation fast light cruising boats that can sail safely downwind at almost two times their hull speed.

Regards

Paulo
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Old 04-22-2010
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In addition to the US Sailing's 'polar' diagrams, you can also get such info from the US Sailing "VPP programing" - will differentiate between different equipment, differing sails used, etc. etc.
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So I looked up data PHRF data on the two boats in question. I nearly fell
over when I found that the Bahama Islander Older and Heavier boat is faster...
Is this right?


















































Class\Type
Lowest

Handicap
Highest

Handicap
Average

Handicap
ISLANDER
BAHAMA 24
246 270 258
ISLANDER 24
BAHAMA
258 273 264
CATALINA 22 FK 249 279 267
CATALINA 22 ODR
270 276 270
CATALINA 22 SK 263 279 270
CATALINA 22 WK 267 285 273




Quote:
Originally Posted by sailingdog View Post
Check the polar diagrams. That will give you a rough idea of how a well-sailed boat will perform under various wind speeds at various points of sail.

Another way to compare boats, at least on a rough basis, is to look at their PHRF ratings. The higher the rating, the slower the boat.

There really isn't a lot you can do to enhance the performance of a boat. Other than keeping excess weight off the boat, fairing and smoothing the bottom and keeping it free of growth, keeping your sails in good shape, etc... but the basic speed of a given boat won't change much.

Some designs are just going to be much faster than others... Multihulls are a good example of that... trimarans, and to some degree catamarans, are going to be faster than their monohull counterparts.
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Old 04-22-2010
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A larger handicap number, means a slower boat.

PHRF numbers as I recall are based on a triangle (upwind, downwind, beam wind) and also locally adjusted for local wind speed range, so they are not an absolute comparison, they must be taken in context. Some boats, usually heavier ones, will be disadvantaged in light air but become relatively faster in bad weather when they can stay upright and there's plenty of wind. (Again, within limits.)

To make any boat faster, you clean the bottom. You get rid of excess weight. And you add sail area, and new sails as opposed to blown out old sails. And, of course, you learn to trim the sails for maximum speed made good on course.
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Old 04-22-2010
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Hello,

Those boats are basically all the same speed. PHRF is good for determining differences in speed, but for the information to be useful, you need to have a LARGE difference in the number. If one boat was 150 and other 100, that would mean something, or one was over 250 and other under 200. The difference between 258 and 270 is basically nothing.

This assumes you are NOT racing the boat. If you are racing, then every second counts. For cruising, the difference between the two boats might be 5 minutes after a few hours of sailing.

Barry
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Old 04-23-2010
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I had an Islander Bahama 24 for a year and sailed it a lot. It was great in over 5 knots. Under that it was sooooo slow. Not a good light air boat, lots of skin friction, low wavemaking resistance.
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Old 04-23-2010
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The B24 has a longer waterline and thus a higher hull speed. But they are only about 10 secs per mile apart which isn't much. Esssentially equivalent speed.
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