What exactly does hull speed mean ? Will the boat NEVER go above that, or is it MOST of the time, or what ? I had the H23 up to 6.4 knots yesterday measured via GPS, many times over 6 knots actually. The wind was howling though at 18-22 knots.
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Jon Bastien
Hull speed is a theoretical number
Hi Mike, I'm not sure of the formula for computing it, but I do know that the "hull speed" of a boat is a theoretical number, based on wetted surface area, friction, rig size, and a few other things. The speed you see on your GPS is Speed Over Ground, and may be affected by currents, tides, etc. If you're sailing in the same direction as a 3-knot current, you'll make 6 kts through the water, but the water is moving at an additional 3 kts, making your speed over ground 9 kts. So, in reality, a boat may exceed it's theoretical hull speed easily (Formulas are not always accurate), and as an additional boost, the currents and tides may be pushing your speed over ground number even higher. Clear as mud? --Jon Bastien H23 '2 Sheets to the Wind' H25 'Adagio'
Hi Mike, I'm not sure of the formula for computing it, but I do know that the "hull speed" of a boat is a theoretical number, based on wetted surface area, friction, rig size, and a few other things. The speed you see on your GPS is Speed Over Ground, and may be affected by currents, tides, etc. If you're sailing in the same direction as a 3-knot current, you'll make 6 kts through the water, but the water is moving at an additional 3 kts, making your speed over ground 9 kts. So, in reality, a boat may exceed it's theoretical hull speed easily (Formulas are not always accurate), and as an additional boost, the currents and tides may be pushing your speed over ground number even higher. Clear as mud? --Jon Bastien H23 '2 Sheets to the Wind' H25 'Adagio'
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Jim Kolstoe
Hull speed calculation
Mike, Hull speed is a calculated maximum displacement mode speed, based on the waterline length of the hull. I think the formula is 1.4 times the square root of the waterline length, although I may be misremembering the constant. I once calculated the hull speed for my h23 at about 5.7kts, based on waterline length of about 19.4ft. Again, its been awhile so the exact numbers may be off. The significance of hull speed derives from our boats being displacment hulls, efficient at low speeds, but not designed to plane. As the hull (or anything else) moves through the water, it generates both bow and stern waves. The wave length is a function of speed through the water. The faster the speed, the longer the wave length, until the hull is going fast enough to generate a wavelength equal to its waterline length. This is the hull speed. At this point the hull sits in the trough between waves. To go faster, it has to climb up the bow wave, which requires a lot more power. Sail boats generally are unable to generate that additional power, except in very strong winds or surfing down a wave. This is why longer sailboats tend to be faster, and why the America's Cup "J" boats had such long overhangs - to create a longer waterline when heeled over. Although Jon is correct in distinguishing speed over the ground from speed through the water, there were some intriguing postings about gettng the h23 up on a plane and hitting 10+kts, check the archives. Its not impossible to exceed hull speed in a displacment hull, it just takes a little extra omph. Jim Kolstoe, h23 Kara's Boo
Mike, Hull speed is a calculated maximum displacement mode speed, based on the waterline length of the hull. I think the formula is 1.4 times the square root of the waterline length, although I may be misremembering the constant. I once calculated the hull speed for my h23 at about 5.7kts, based on waterline length of about 19.4ft. Again, its been awhile so the exact numbers may be off. The significance of hull speed derives from our boats being displacment hulls, efficient at low speeds, but not designed to plane. As the hull (or anything else) moves through the water, it generates both bow and stern waves. The wave length is a function of speed through the water. The faster the speed, the longer the wave length, until the hull is going fast enough to generate a wavelength equal to its waterline length. This is the hull speed. At this point the hull sits in the trough between waves. To go faster, it has to climb up the bow wave, which requires a lot more power. Sail boats generally are unable to generate that additional power, except in very strong winds or surfing down a wave. This is why longer sailboats tend to be faster, and why the America's Cup "J" boats had such long overhangs - to create a longer waterline when heeled over. Although Jon is correct in distinguishing speed over the ground from speed through the water, there were some intriguing postings about gettng the h23 up on a plane and hitting 10+kts, check the archives. Its not impossible to exceed hull speed in a displacment hull, it just takes a little extra omph. Jim Kolstoe, h23 Kara's Boo
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Dave Condon
Other Factors
Mike; The previous posts are good responses. Often I am asked this question and of course there is a mathmatical equasion but there are other factors to. This includes condition of the boat/hull, knowledge of your sailing area, experience, tuning, etc.. The boat design is only one factor. A friend of mine competitivly sails a slug and wins alot of races due to all factors not the design alone. Crazy Dave
Mike; The previous posts are good responses. Often I am asked this question and of course there is a mathmatical equasion but there are other factors to. This includes condition of the boat/hull, knowledge of your sailing area, experience, tuning, etc.. The boat design is only one factor. A friend of mine competitivly sails a slug and wins alot of races due to all factors not the design alone. Crazy Dave
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Dave Condon
Other considerations
Mike; The previous posts are good responses. Often I am asked this question and of course there is a mathmatical equasion but there are other factors to. This includes condition of the boat/hull, knowledge of your sailing area, experience, tuning, etc.. The boat design is only one factor. A friend of mine competitivly sails a slug and wins alot of races due to all factors not the design alone. Crazy Dave
Mike; The previous posts are good responses. Often I am asked this question and of course there is a mathmatical equasion but there are other factors to. This includes condition of the boat/hull, knowledge of your sailing area, experience, tuning, etc.. The boat design is only one factor. A friend of mine competitivly sails a slug and wins alot of races due to all factors not the design alone. Crazy Dave
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Been there
Here's the straight dope
Hull speed depends only on waterline length, LWL. It is DEFINED as the speed of a wave whose length equals the waterline length of a vessel. As a matter of physics, this is K*sqrt(LWL). For DWL measured in feet, gravity at the earth's surface, and the properties of water, K is about 1.34. Hull speed is NOT a maximum speed for your vessel. Some sailboats never will reach hull speed. Others regularly exceed it. Most of the wrong answers below confuse hull speed with velocity prediction. To maintain a particular speed through water, a vessel must have enough propulsive force to overcome several forms of resistance, including one form called wave resistance. Wave resistance grows non-linearly, and its curve has a knee upwards near hull speed, for most hull shapes. The intuitive explanation for this is that at or after hull speed, the vessel must "climb up" its bow wave, while at hull speed it sits between its bow and quarter waves. Note that skinny hulls don't do so much "climbing up," which is why a well designed catamaran can sail significantly beyond its hull speed. (Almost all catamarans use displacement hulls, and their speed does NOT come from planing.) There are other forms of resistance; racers sand their bottoms for a reason. The propulsive force generated by a sailboat depends on windspeed and angle, and on the sailboat's rig, stability, quality of sails, and skill of its crew. Sailboats suffer leeway, countered by their keel. There are velocity prediction programs that will calculate theoretical polars for a sailboat. The better ones require significant design information, including hull shape, keel shape, center of gravity, and sailplan.
Hull speed depends only on waterline length, LWL. It is DEFINED as the speed of a wave whose length equals the waterline length of a vessel. As a matter of physics, this is K*sqrt(LWL). For DWL measured in feet, gravity at the earth's surface, and the properties of water, K is about 1.34. Hull speed is NOT a maximum speed for your vessel. Some sailboats never will reach hull speed. Others regularly exceed it. Most of the wrong answers below confuse hull speed with velocity prediction. To maintain a particular speed through water, a vessel must have enough propulsive force to overcome several forms of resistance, including one form called wave resistance. Wave resistance grows non-linearly, and its curve has a knee upwards near hull speed, for most hull shapes. The intuitive explanation for this is that at or after hull speed, the vessel must "climb up" its bow wave, while at hull speed it sits between its bow and quarter waves. Note that skinny hulls don't do so much "climbing up," which is why a well designed catamaran can sail significantly beyond its hull speed. (Almost all catamarans use displacement hulls, and their speed does NOT come from planing.) There are other forms of resistance; racers sand their bottoms for a reason. The propulsive force generated by a sailboat depends on windspeed and angle, and on the sailboat's rig, stability, quality of sails, and skill of its crew. Sailboats suffer leeway, countered by their keel. There are velocity prediction programs that will calculate theoretical polars for a sailboat. The better ones require significant design information, including hull shape, keel shape, center of gravity, and sailplan.
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Been there
By the way ..
Your 23 will plane in the right conditions, and then your wave resistance drops. A good breeze, a beam reach, sail trimmed just right, crew on weather rail, and then YEE HAH. It's hard to sustain for long, but lots of fun. Of course, you have to exceed hull speed on the way to planing speed.
Your 23 will plane in the right conditions, and then your wave resistance drops. A good breeze, a beam reach, sail trimmed just right, crew on weather rail, and then YEE HAH. It's hard to sustain for long, but lots of fun. Of course, you have to exceed hull speed on the way to planing speed.
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Greg Stebbins
Oh crap!
Hey Been There, You really think a 23 will plane? Ha, can't happen! Oh, and by the way, you should replace those puny factory jib blocks if you play this game (which can't happen). Mine started to come out of the deck. I replaced with 2 1/4's with big bases and huge back plates. Rated at 2000 lbs, they're easier turning and stay on the deck where they belong. Planeing 23's - give me a break! Greg H23 Faster- p.s. J22 will plane also but – the 23 comes up sooner and is Faster- (12+ knots once by my GPS)
Hey Been There, You really think a 23 will plane? Ha, can't happen! Oh, and by the way, you should replace those puny factory jib blocks if you play this game (which can't happen). Mine started to come out of the deck. I replaced with 2 1/4's with big bases and huge back plates. Rated at 2000 lbs, they're easier turning and stay on the deck where they belong. Planeing 23's - give me a break! Greg H23 Faster- p.s. J22 will plane also but – the 23 comes up sooner and is Faster- (12+ knots once by my GPS)
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Russ King
Displacement, Semi-displacement, and planing
Lasers, sailboards, and most small sailing craft (12' to 16') have planing hulls. Most larger sailboats have displacement hulls (they are not designed to plane; you are likely to drive the boat under trying to plane). The go-fast powerboats have displacment or semi-displacement hull shapes. That's why they can go fast (that and the big engines). There are powerboats that have small engines and displacement hulls because that combination is more fuel efficient. Trawler yachts, and all commercial boats (including the Titanic) have displacement hulls. Theoretical hull speed applies to displacment hulls (and planing or semi-displacement hulls operating as a displacement hull). I have heard many different numbers for "theoretical hull speed" constant, ranging from 1.2 to 1.5 (times the square root of the waterline length). The number I use is the square root of 2 (about 1.4). Large ships get some pretty impressive speeds from the 1.4 times sqrt(waterline length). Plug in the numbers for a 600' freighter, and you'll see what I mean. One final note: hull shape and beam, not just length, affect the actual hull speed you can expect. Narrower hulls are usually faster, but a really bad design can make even a narrow hull slow.
Lasers, sailboards, and most small sailing craft (12' to 16') have planing hulls. Most larger sailboats have displacement hulls (they are not designed to plane; you are likely to drive the boat under trying to plane). The go-fast powerboats have displacment or semi-displacement hull shapes. That's why they can go fast (that and the big engines). There are powerboats that have small engines and displacement hulls because that combination is more fuel efficient. Trawler yachts, and all commercial boats (including the Titanic) have displacement hulls. Theoretical hull speed applies to displacment hulls (and planing or semi-displacement hulls operating as a displacement hull). I have heard many different numbers for "theoretical hull speed" constant, ranging from 1.2 to 1.5 (times the square root of the waterline length). The number I use is the square root of 2 (about 1.4). Large ships get some pretty impressive speeds from the 1.4 times sqrt(waterline length). Plug in the numbers for a 600' freighter, and you'll see what I mean. One final note: hull shape and beam, not just length, affect the actual hull speed you can expect. Narrower hulls are usually faster, but a really bad design can make even a narrow hull slow.
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