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In article ,
Rodney Myrvaagnes wrote: On Wed, 20 Apr 2005 05:25:11 GMT, Jere Lull wrote: In article , DSK wrote: Usually making hulls non-rigid makes them slower for given power. One notable exception: a PortaBote, but that's not practical for most hulls. The hull deforms unbelievably, which is disconcerting. It is hard for me to believe that is an exceoption. It would be notable if so. Indeed, it would be a miracle. Obviously, you haven't run around on a PortaBote. 4 HP gives a solid 10+ knots speed with lots of load. When the boat goes on plane, the "floor" under the helmsman's feet drops a bunch of inches. Unlike a solid hull, the PortaBote expands into areas of low pressure to limit the drag (suction). Rodney Myrvaagnes NYC J36 Gjo/a "Religious wisdom is to wisdom as military music is to music." I *LIKE* military music, aka Marches. -- Jere Lull Xan-a-Deux ('73 Tanzer 28 #4 out of Tolchester, MD) Xan's Pages: http://members.dca.net/jerelull/X-Main.html Our BVI FAQs (290+ pics) http://homepage.mac.com/jerelull/BVI/ |
On Thu, 21 Apr 2005 05:40:38 GMT, Jere Lull wrote:
In article , Rodney Myrvaagnes wrote: On Wed, 20 Apr 2005 05:25:11 GMT, Jere Lull wrote: In article , DSK wrote: Usually making hulls non-rigid makes them slower for given power. One notable exception: a PortaBote, but that's not practical for most hulls. The hull deforms unbelievably, which is disconcerting. It is hard for me to believe that is an exceoption. It would be notable if so. Indeed, it would be a miracle. Obviously, you haven't run around on a PortaBote. 4 HP gives a solid 10+ knots speed with lots of load. When the boat goes on plane, the "floor" under the helmsman's feet drops a bunch of inches. Unlike a solid hull, the PortaBote expands into areas of low pressure to limit the drag (suction). If you say so. The age of miracles is not past. Rodney Myrvaagnes NYC J36 Gjo/a "Curse thee, thou quadrant. No longer will I guide my earthly way by thee." Capt. Ahab |
On Sat, 16 Apr 2005 22:22:00 GMT, Jere Lull wrote:
In article , Stephen Trapani wrote: Hull speed is the absolute maximum that boat can travel through water. All your examples have the water moving forward also so the boat is not exceeding hull speed through the water. I thought I mentioned this before. Hope I'm not repeating myself. Hull speed is a suggestion for our boat, not the law. Though our theoretical hull speed is 6.65 knots, we regularly exceed that with aplomb, close hauled, close reach, broad reach, whatever point of sail. Spent a wonderful afternoon with 6 other sailors last season. As long as I was on the tiller, pushing her to where she likes to be, we were well above the theoretical hull speed. As we pinched to get back into the harbor, she insisted on doing over 7 knots directly into the wind (okay, about 15 degrees off). That last was our lovely lady showing off, of course, as what we did was clearly impossible. 1.34 was derived from observing boats about a century ago. Depending on the hull, that constant can be quite a bit different. As I recall, some multi-hull boats' K is in the 2 or 3 range. Xan's fat ass and sharp transom keeps her driving towards a 1.7 or so constant. Jere, It sounds like your speed-length parameter is higher than 1.34 - a testimonial to your hull designer. The 1.34 comes from the fact that speed-squared of a wave = g/2*pi times wavelength. If your hull's stern really places the stern wave a distance back from the bow wave equal to your design waterline length, then 1.34 is pretty accurate as the point where the curve of additional HP to yield additional speed for a displacement hull becomes almost vertical. However, with sweet butock lines, stern reflexes and other sophistications of hull design, the stern wave can actually be moved a bit aft of your transom. The wavelength thus becomes greater than your DWL. Since speed-squared is proportional to wavelength, and since your boat speed and the wave speed must match, you get a speed-length parameter that's higher than 1.34 as the effective multiplier times the square root of your DWL (since DWL is now less than the wavelength). At least that's my simplified understanding of a very complex subject. Al s/v Persephone Newburyport, MA |
In article ,
Albert P. Belle Isle wrote: On Sat, 16 Apr 2005 22:22:00 GMT, Jere Lull wrote: In article , Stephen Trapani wrote: Hull speed is the absolute maximum that boat can travel through water. All your examples have the water moving forward also so the boat is not exceeding hull speed through the water. Hull speed is a suggestion for our boat, not the law. Though our theoretical hull speed is 6.65 knots, we regularly exceed that with aplomb, close hauled, close reach, broad reach, whatever point of sail. Spent a wonderful afternoon with 6 other sailors last season. As long as I was on the tiller, pushing her to where she likes to be, we were well above the theoretical hull speed. As we pinched to get back into the harbor, she insisted on doing over 7 knots directly into the wind (okay, about 15 degrees off). That last was our lovely lady showing off, of course, as what we did was clearly impossible. 1.34 was derived from observing boats about a century ago. Depending on the hull, that constant can be quite a bit different. As I recall, some multi-hull boats' K is in the 2 or 3 range. Xan's fat ass and sharp transom keeps her driving towards a 1.7 or so constant. Jere, It sounds like your speed-length parameter is higher than 1.34 - a testimonial to your hull designer. Full agreement. The 1.34 comes from the fact that speed-squared of a wave = g/2*pi times wavelength. Yes, I agree with the derivation of the formula -- as long as we include that wavelengths can differ. Swells have wavelengths 100s of feet and periods many seconds from crest to crest, while wind-driven waves have quite a bit shorter wavelengths and periods. And wind-driven waves have different periods and wavelengths. If your hull's stern really places the stern wave a distance back from the bow wave equal to your design waterline length, then 1.34 is pretty accurate as the point where the curve of additional HP to yield additional speed for a displacement hull becomes almost vertical. However, with sweet butock lines, stern reflexes and other sophistications of hull design, the stern wave can actually be moved a bit aft of your transom. The wavelength thus becomes greater than your DWL. Our resting WL is 24'. To maintain a 1.34 constant and get the speeds we've verified while definitely not surfing, our effective waterline would have to be greater than 35'. That's a LONG way behind our transom! Since speed-squared is proportional to wavelength, and since your boat speed and the wave speed must match, you get a speed-length parameter that's higher than 1.34 as the effective multiplier times the square root of your DWL (since DWL is now less than the wavelength). It feels like you left a bit out and mixed a couple of things here. Again, I agree that it probably has something to do with the wave speed, which has a certain value when the constant is 1.34. Change the wave's speed and you change the constant, and wave length. At least that's my simplified understanding of a very complex subject. Al s/v Persephone After having chased several NA's explanations for a few years, I finally gave up trying to explain and simply accept that it's more complex than 1.34, since other hull shapes have much higher observed constants. -- Jere Lull Xan-a-Deux ('73 Tanzer 28 #4 out of Tolchester, MD) Xan's Pages: http://members.dca.net/jerelull/X-Main.html Our BVI FAQs (290+ pics) http://homepage.mac.com/jerelull/BVI/ |
On Mon, 25 Apr 2005 04:06:45 GMT, Jere Lull wrote:
In article , Albert P. Belle Isle wrote: On Sat, 16 Apr 2005 22:22:00 GMT, Jere Lull wrote: In article , Stephen Trapani wrote: Hull speed is the absolute maximum that boat can travel through water. All your examples have the water moving forward also so the boat is not exceeding hull speed through the water. Hull speed is a suggestion for our boat, not the law. Though our theoretical hull speed is 6.65 knots, we regularly exceed that with aplomb, close hauled, close reach, broad reach, whatever point of sail. Spent a wonderful afternoon with 6 other sailors last season. As long as I was on the tiller, pushing her to where she likes to be, we were well above the theoretical hull speed. As we pinched to get back into the harbor, she insisted on doing over 7 knots directly into the wind (okay, about 15 degrees off). That last was our lovely lady showing off, of course, as what we did was clearly impossible. 1.34 was derived from observing boats about a century ago. Depending on the hull, that constant can be quite a bit different. As I recall, some multi-hull boats' K is in the 2 or 3 range. Xan's fat ass and sharp transom keeps her driving towards a 1.7 or so constant. Jere, It sounds like your speed-length parameter is higher than 1.34 - a testimonial to your hull designer. Full agreement. The 1.34 comes from the fact that speed-squared of a wave = g/2*pi times wavelength. Yes, I agree with the derivation of the formula -- as long as we include that wavelengths can differ. Swells have wavelengths 100s of feet and periods many seconds from crest to crest, while wind-driven waves have quite a bit shorter wavelengths and periods. And wind-driven waves have different periods and wavelengths. If your hull's stern really places the stern wave a distance back from the bow wave equal to your design waterline length, then 1.34 is pretty accurate as the point where the curve of additional HP to yield additional speed for a displacement hull becomes almost vertical. However, with sweet butock lines, stern reflexes and other sophistications of hull design, the stern wave can actually be moved a bit aft of your transom. The wavelength thus becomes greater than your DWL. Our resting WL is 24'. To maintain a 1.34 constant and get the speeds we've verified while definitely not surfing, our effective waterline would have to be greater than 35'. That's a LONG way behind our transom! Since speed-squared is proportional to wavelength, and since your boat speed and the wave speed must match, you get a speed-length parameter that's higher than 1.34 as the effective multiplier times the square root of your DWL (since DWL is now less than the wavelength). It feels like you left a bit out and mixed a couple of things here. Again, I agree that it probably has something to do with the wave speed, which has a certain value when the constant is 1.34. Change the wave's speed and you change the constant, and wave length. At least that's my simplified understanding of a very complex subject. Al s/v Persephone After having chased several NA's explanations for a few years, I finally gave up trying to explain and simply accept that it's more complex than 1.34, since other hull shapes have much higher observed constants. The physics says that extra-long-wavelength swells just propagate more slowly. The c-squared = lambda*g/2pi is pretty fundamental for surface waves (as opposed to deep pressure waves, like tsunamis). There's a passage in John Craven's "The Silent War" where he gleefully chants the formula from the sail of a nuclear submarine as he watches her bow and stern waves demonstrate the validity of something he had drilled into his head in grad school. Maybe you have to be a geek to appreciate it g. However, the oversimplified nature of my "push-the-stern-wave-aft" explanation is, of course, quite true. IANA. If you haven't already read it, Jere, van Dorn's "Oceanography and Seamanship" has a pretty good discussion of the speed-power curves for planing, semiplaning and displacement hulls - as well as, among other things, a nifty nomograph for predicting sea-state from duration or fetch of sustained winds. Good sailing. Al s/v Persephone |
On Mon, 25 Apr 2005 20:16:09 GMT, Albert P. Belle Isle
wrote: Our resting WL is 24'. To maintain a 1.34 constant and get the speeds we've verified while definitely not surfing, our effective waterline would have to be greater than 35'. That's a LONG way behind our transom! Jere - As a quick calculation, 24ft DWL would yield a hull speed of about 6.6kt with a speed-length coefficient of 1.34. To get to 7kt, the effective DWL at 1.34 would be a little over 27ft - not 35ft. My previous boat had a DWL of 28ft, for which a speed-length coefficient of 1.34 would predict 7.1kt. I easily got 7.4kt on beam reaches, which would say my real coefficient was almost 1.4 (or that my effective DWL at 1.34 was a little over 30ft - a two foot "push-back" of the sten wave, which was roughly how the peak of the stern wave looked from my cockpit. Regards, Al |
On Mon, 25 Apr 2005 04:06:45 GMT, Jere Lull wrote:
In article , Albert P. Belle Isle wrote: On Sat, 16 Apr 2005 22:22:00 GMT, Jere Lull wrote: In article , Stephen Trapani wrote: Hull speed is the absolute maximum that boat can travel through water. All your examples have the water moving forward also so the boat is not exceeding hull speed through the water. Hull speed is a suggestion for our boat, not the law. Though our theoretical hull speed is 6.65 knots, we regularly exceed that with aplomb, close hauled, close reach, broad reach, whatever point of sail. Spent a wonderful afternoon with 6 other sailors last season. As long as I was on the tiller, pushing her to where she likes to be, we were well above the theoretical hull speed. As we pinched to get back into the harbor, she insisted on doing over 7 knots directly into the wind (okay, about 15 degrees off). That last was our lovely lady showing off, of course, as what we did was clearly impossible. 1.34 was derived from observing boats about a century ago. Depending on the hull, that constant can be quite a bit different. As I recall, some multi-hull boats' K is in the 2 or 3 range. Xan's fat ass and sharp transom keeps her driving towards a 1.7 or so constant. Jere, It sounds like your speed-length parameter is higher than 1.34 - a testimonial to your hull designer. Full agreement. The 1.34 comes from the fact that speed-squared of a wave = g/2*pi times wavelength. Yes, I agree with the derivation of the formula -- as long as we include that wavelengths can differ. Swells have wavelengths 100s of feet and periods many seconds from crest to crest, while wind-driven waves have quite a bit shorter wavelengths and periods. And wind-driven waves have different periods and wavelengths. If your hull's stern really places the stern wave a distance back from the bow wave equal to your design waterline length, then 1.34 is pretty accurate as the point where the curve of additional HP to yield additional speed for a displacement hull becomes almost vertical. However, with sweet butock lines, stern reflexes and other sophistications of hull design, the stern wave can actually be moved a bit aft of your transom. The wavelength thus becomes greater than your DWL. Our resting WL is 24'. To maintain a 1.34 constant and get the speeds we've verified while definitely not surfing, our effective waterline would have to be greater than 35'. That's a LONG way behind our transom! Since speed-squared is proportional to wavelength, and since your boat speed and the wave speed must match, you get a speed-length parameter that's higher than 1.34 as the effective multiplier times the square root of your DWL (since DWL is now less than the wavelength). It feels like you left a bit out and mixed a couple of things here. Again, I agree that it probably has something to do with the wave speed, which has a certain value when the constant is 1.34. Change the wave's speed and you change the constant, and wave length. At least that's my simplified understanding of a very complex subject. Al s/v Persephone After having chased several NA's explanations for a few years, I finally gave up trying to explain and simply accept that it's more complex than 1.34, since other hull shapes have much higher observed constants. It isn't a different constant. It is just that many boats nowadays are light enough so they aren't limited to the length of the wave they make. If you observe a tug traveling without a tow, you will see very easily what wave we are talking about. Rodney Myrvaagnes NYC J36 Gjo/a "Curse thee, thou quadrant. No longer will I guide my earthly way by thee." Capt. Ahab |
Hi All,
i post this update after changes to the trimming of the boat. I said the boat is poorly trimmed and seem to "sit" too much with the stern in the water. So i put around 3/4 of tons of water (750 liters) in the anchor room (the small cabin in the bow which contains the chain - is quite large and sealed). This had the effect to sink the bow around 8 inches and lift the stern around 3 inches. The boat is still NOT level but at least is getting closer. Now the effect on travelling speed: I could reach 5.5 knots at 900 rpm instead of 1100 rpm ! And my fuel consumption went down 40% at once (for the same speed). The benefits would be less and less trying to drive the boat faster and faster - basically the speed improvement at full throttle would be insignificant. Also waves were greatly reduced at 5.5 knots (almost reduced ripples). My theory: with the bow "lifted" too much the physical boat lenght starts quite some distance from the actual entry point in the water. Adding weight at the bow would push it down and effectively lenghten the WL. Now i have a long-range tank in the bow (1.5 ton) which is empty now (i normally use other two tanks, in the middle of the boat, each 1/2 ton). Next week i will bunker diesel and fill ALL the 3 tanks to the top - this should push down the boat another bit and eventually set it level - then make new trials. I also looked for green slime and the like on the hull - not much there. I will post the results in another 2 weeks or so. Matteo (Matteo) wrote in message om... This is the situation: My 40' LWL boat (15 ton displacement) has a 150 PS engine. From the formula for speed I calculated a hull speed of sqrt(40)*1.34 = 8.47 knots *but*: when i did trials last week (absolutely calm water, almost no wind) those are the results: 800 rpm 5 knots no noticeable waves generated 1100 rpm 5.5 knots small waves 1800 rpm 6.5 knots (flat out) - huge waves generated, stern deep in the water, boat "running uphill". 1100 rpm is around 50/60 PS (from the engine rpm/PS table). Question: what could be the cause of the "slowness" of the boat ? I do not pretend to reach 8.4 knots cruising but at least 7 knots should be in. I'm thinging of dirty hull (green slime), incorrect weight distribution (bow tends to "point" upwards even when crossing small waves). Any experience ? Thanks Matteo |
In article ,
Albert P. Belle Isle wrote: As a quick calculation, 24ft DWL would yield a hull speed of about 6.6kt with a speed-length coefficient of 1.34. To get to 7kt, the effective DWL at 1.34 would be a little over 27ft - not 35ft. We've sustained much higher speeds, but don't feel like opening myself up for someone saying that it's impossible, that I'm surfing, my knotmeter's off, there's a current, or some such. I would agree except that I eliminated all those things. We really have done "impossible" things. Would love to know how, but gave up and simply enjoy. -- Jere Lull Xan-a-Deux ('73 Tanzer 28 #4 out of Tolchester, MD) Xan's Pages: http://members.dca.net/jerelull/X-Main.html Our BVI FAQs (290+ pics) http://homepage.mac.com/jerelull/BVI/ |
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