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Dafinition of Hull Speed
The Hull speed is the maximum theoretical velocity of a given boat through the water. It is calculated by multiplying 1.3 times the square root of its waterline length in feet divided by the distance to port in nautical miles, minus the time in hours to sunset cubed, if more than one hour or divided by the time to sunset cubed, if less than one hour.
The Hull speed is the maximum theoretical velocity of a given boat through the water. It is calculated by multiplying 1.3 times the square root of its waterline length in feet divided by the distance to port in nautical miles, minus the time in hours to sunset cubed, if more than one hour or divided by the time to sunset cubed, if less than one hour.
I don't get it??? *o
This is an intriguing discussion and some guys(gals) don't like it.....sooooo move on to another and let the juices here flow!!!
This is an intriguing discussion and some guys(gals) don't like it.....sooooo move on to another and let the juices here flow!!!
Profoundly stated, Dan.....
...apparently this discussion has enabled you to grasp the subtle nuances of the theoretical hull speed formula and how it may apply to the yachtsman's specific circumstance.
...apparently this discussion has enabled you to grasp the subtle nuances of the theoretical hull speed formula and how it may apply to the yachtsman's specific circumstance.
Can't think of a Title
I have never been one to retreat just because I didn’t like what someone had to say or because a subject was controversial. There were a few times that my comments were wrong or stupid, which taught me a lesson. There have been several responses to this thread, which said that the subject was boring or everything had been said and we should move onto something else. That is probably true. Many of the subjects in this forum are boring to me, but they are interesting to someone. I take a look at the subject and maybe a few responses and then decide whether I want to read it. It is just like a TV. There is an off switch. The bottom line is that, if a subject is boring or the subject has been exhausted and no one has a further interest, it will die a natural death.
I have never been one to retreat just because I didn’t like what someone had to say or because a subject was controversial. There were a few times that my comments were wrong or stupid, which taught me a lesson. There have been several responses to this thread, which said that the subject was boring or everything had been said and we should move onto something else. That is probably true. Many of the subjects in this forum are boring to me, but they are interesting to someone. I take a look at the subject and maybe a few responses and then decide whether I want to read it. It is just like a TV. There is an off switch. The bottom line is that, if a subject is boring or the subject has been exhausted and no one has a further interest, it will die a natural death.
key word : theoretical
seems a lot of people have spent there ' weekend here instead of on their boat.!!!!!! in my orig reply I said theoretical. the formula i gave is still pretty much the same as it always was. people that on a regular basis exceed their hull speeds are more than likely on a semi disp hull. or have a hull design that is designed to do such a thing. ie wide flat bottom light disp or whatever. obviously different designs can get up and surf a lot easier than others. my particular boat an O'Day 30 cb takes a lot more power to get up and surf than say a J 24. I have attained speeds far in excess of its theoretical hull speed. in one Md. governors cup race about 10 or 12 yrs ago we reached speeds in excess 14 knots. we arn't sure how much in excess because my knot meter at the time was an analog and the numbers on the dial only went to 14. we were pegged at about 1" beyond 14. we estimate about 16 knots. However here's the biggie. we were w on w jib poled out with spin pole, locked out and down. main out and prevented. wind 30knots+ gusting to 40+. great ride if you like "white knuckle" sailing. we finished race "70nm" with avg. speed of 10.5knots. obviously way beyond hull speed. in a typical disp hull you have to have enough power to get it up so to speak, but it won't stay there for long before it settles back in. to disp. mode until the next gust and a lift on the stearn. obviously you have to be able to overcome you bow wave.
seems a lot of people have spent there ' weekend here instead of on their boat.!!!!!! in my orig reply I said theoretical. the formula i gave is still pretty much the same as it always was. people that on a regular basis exceed their hull speeds are more than likely on a semi disp hull. or have a hull design that is designed to do such a thing. ie wide flat bottom light disp or whatever. obviously different designs can get up and surf a lot easier than others. my particular boat an O'Day 30 cb takes a lot more power to get up and surf than say a J 24. I have attained speeds far in excess of its theoretical hull speed. in one Md. governors cup race about 10 or 12 yrs ago we reached speeds in excess 14 knots. we arn't sure how much in excess because my knot meter at the time was an analog and the numbers on the dial only went to 14. we were pegged at about 1" beyond 14. we estimate about 16 knots. However here's the biggie. we were w on w jib poled out with spin pole, locked out and down. main out and prevented. wind 30knots+ gusting to 40+. great ride if you like "white knuckle" sailing. we finished race "70nm" with avg. speed of 10.5knots. obviously way beyond hull speed. in a typical disp hull you have to have enough power to get it up so to speak, but it won't stay there for long before it settles back in. to disp. mode until the next gust and a lift on the stearn. obviously you have to be able to overcome you bow wave.
Another wrinkle
If you have a paddle-wheel sending unit, it can emit some interesting signals when the boat is in the right attitude. I have found that indicated speed can increase by 1.5 to 2.5 knots if the wind is brisk, the boat is heeled some, and the sending unit is then in turbulent, perhaps foaming water.
If you have a paddle-wheel sending unit, it can emit some interesting signals when the boat is in the right attitude. I have found that indicated speed can increase by 1.5 to 2.5 knots if the wind is brisk, the boat is heeled some, and the sending unit is then in turbulent, perhaps foaming water.
TRUE!!!!!!
If your paddle wheel is not centered and pointed straight down you will see a slight difference in indicated speed on port vs. starboard tack if the boat is heeled. of coarse the amount of diff. will vary depending on the amount of heel and speed. i see as much as 0.5 knot going up wind at a boat speed of 6.5 to 7.0 knots.. going down wind with the boat flat you won't see any differance. on a reach you will see a slight diff but very small since there is a lot less heel. capn jim sv "Que Pasa"
If your paddle wheel is not centered and pointed straight down you will see a slight difference in indicated speed on port vs. starboard tack if the boat is heeled. of coarse the amount of diff. will vary depending on the amount of heel and speed. i see as much as 0.5 knot going up wind at a boat speed of 6.5 to 7.0 knots.. going down wind with the boat flat you won't see any differance. on a reach you will see a slight diff but very small since there is a lot less heel. capn jim sv "Que Pasa"
Never heard that one before!
Your theoretical hull speed is about 7.46knots +-. You can exceed this speed, but as explained before, you have to have enough power to get the boat up on the water, instead of in it. given the disp. of a sail boat that has lead(ballast)under it, it isn't easily put up there and its next to impossible to keep it there for very long. usually only a few seconds. at the end of a surf you will hear a definate differant noise of the water around the boat. sort of a "whoose" sound. then you will feel the boat settle back into the water. when this is all is happening you'll see the speedo surge to maybe 9knots or something greater hopefully, then slow back down to your hull speed of about 7.5 knots. you will also feel the boat sort of rise up when it starts to surf or plane. then just wait for the next gust and wave and do it all over again. btw its extremely difficult to get anything other than a sled to surf upwind. most typical boats that have a ballast to disp ratio of 40 to 50% can't surf upwind. the easiest way is downwind. on a beam reach its also very difficult but can be done, but no where near the same amount of surfing. on a reach when you get that much power in the sails you have a heeling problem to deal with.
Your theoretical hull speed is about 7.46knots +-. You can exceed this speed, but as explained before, you have to have enough power to get the boat up on the water, instead of in it. given the disp. of a sail boat that has lead(ballast)under it, it isn't easily put up there and its next to impossible to keep it there for very long. usually only a few seconds. at the end of a surf you will hear a definate differant noise of the water around the boat. sort of a "whoose" sound. then you will feel the boat settle back into the water. when this is all is happening you'll see the speedo surge to maybe 9knots or something greater hopefully, then slow back down to your hull speed of about 7.5 knots. you will also feel the boat sort of rise up when it starts to surf or plane. then just wait for the next gust and wave and do it all over again. btw its extremely difficult to get anything other than a sled to surf upwind. most typical boats that have a ballast to disp ratio of 40 to 50% can't surf upwind. the easiest way is downwind. on a beam reach its also very difficult but can be done, but no where near the same amount of surfing. on a reach when you get that much power in the sails you have a heeling problem to deal with.
I think Clyde & ben310 have it right
With a lot of good explainations from Gord May & David Foster. The traditional Hull speed calculation for my boat with a 30.5' static waterline length is about 7.3-7.4 knots. But I really like that Clyde mentioned Dave Gerrs theoretical hull speed calculation. As I think that is **really** the theoretical max (not counting true 'surfing'). We all know that we exceed the so called "Classical theoretical hull speed". Hell we do it all the time, and just look at the polars of most boats (not counting a full keeled crab crusher). The polar of my boat (C36) for a 20knot wind and ~110 degree apparant wind is about 8.6 knots (and this is not surfing) . ....way more than "Classical theoretical hull speed" of 7.4. I have acheived this speed once on a beautiful sail with high winds and flat seas, (no I wasn't surfing). Here is a picture of my max speed from knotmeter when I got back to the boat, but I will admit to never seeing the actually speed above 8.6 with my eyes. (Click the link below) http://mysite.verizon.net/vze32p3k/sitebuildercontent/sitebuilderpictures/.pond/118-1879_web.jpg.w300h225.jpg ...and for all you GPS people, this speed was confirmed with my GPS and with the known tidal current at that spot and at that time (which was actually just a little) ps Paul of Toronto, how/where did you get 10 knots as your max speed calculation?...thats way off
With a lot of good explainations from Gord May & David Foster. The traditional Hull speed calculation for my boat with a 30.5' static waterline length is about 7.3-7.4 knots. But I really like that Clyde mentioned Dave Gerrs theoretical hull speed calculation. As I think that is **really** the theoretical max (not counting true 'surfing'). We all know that we exceed the so called "Classical theoretical hull speed". Hell we do it all the time, and just look at the polars of most boats (not counting a full keeled crab crusher). The polar of my boat (C36) for a 20knot wind and ~110 degree apparant wind is about 8.6 knots (and this is not surfing) . ....way more than "Classical theoretical hull speed" of 7.4. I have acheived this speed once on a beautiful sail with high winds and flat seas, (no I wasn't surfing). Here is a picture of my max speed from knotmeter when I got back to the boat, but I will admit to never seeing the actually speed above 8.6 with my eyes. (Click the link below) http://mysite.verizon.net/vze32p3k/sitebuildercontent/sitebuilderpictures/.pond/118-1879_web.jpg.w300h225.jpg ...and for all you GPS people, this speed was confirmed with my GPS and with the known tidal current at that spot and at that time (which was actually just a little) ps Paul of Toronto, how/where did you get 10 knots as your max speed calculation?...thats way off
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