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#11
posted to rec.boats.cruising
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Hobby Horse?
"Larry" skrev i en meddelelse ... "Roger Long" wrote in news:wrHgh.4913$nq5.3130 @twister.nyroc.rr.com: Asking what? Who... Wa... ? -- Roger Long I knew your "waves" answer was gonna cornfuze 'em....(c; Larry "All complex problems have at least one simple solution ... that doesn't work" ... (who said that?) -- Flemming Torp |
#12
posted to rec.boats.cruising
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Hobby Horse?
"Roger Long" wrote in message ... What I really meant was: Insufficient damping combined with first or second order correspondence of excitation cycle with natural pitching period. The significance of Radius of Gyration and rotational inertia may be greater in terms of resonant carry over of motion than gross value and counter intuitive results may be experienced when adjusting weight distribution. Translation? The natural bobbing frequency of the boat is the same as (or a multiple of) the frequency with which waves strike. Answers? 1. Damp the boat's bobbing - make the forward pitch a different stiffness from the rearward pitch. Then it won't bob so much. Since that means changing the boat's lines, is a bit difficult to do under way . . . 2. Change the bobbing frequency - shift weight from the centre to the ends to slow it, shift weight from the ends to the centre to speed up the bobbing. Leads to a lot of moaning from the boat's crew, who specially don't like being dunked at the bow. And hauling a couple of them up the mast, though more effective, isn't popular either if you're going to windward - it's that lost stiffness. And sometimes this doesn't work anyway (Roger's 'counterintuitive' thesis) if you try to slow the bobbing frequency. 3. Change the wave frequency - alter course. Mind you, it might take longer to get where you want to go, but just occasionally the extra speed gained my cancel the penalty of shifting off course. Do the sums . . . -- JimB http://www.jimbaerselman.f2s.com/ Comparing cruise areas within Greece and N Spain |
#13
posted to rec.boats.cruising
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Hobby Horse?
Flemming Torp wrote:
"All complex problems have at least one simple solution ... that doesn't work" ... (who said that?) The Iraq Study Group? -- Roger Long |
#14
posted to rec.boats.cruising
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Hobby Horse?
JimB wrote:
1. Damp the boat's bobbing - make the forward pitch a different stiffness from the rearward pitch. I used to be a great believer in asymmetry until sailing my present boat which is fairly symmetrical and has a surprising full entrance for a 1970's design. She has a cruising weight mast, a large anchor hung out over the bow, water tank under the V berth, batteries and lots of gear aft, everything that urban legend says should promote wicked pitching. She has about the nicest motion I've encountered in a sailboat. It used to be thought that spreading weight out into the ends made for a more comfortable boat. This was back when boats tended to be more symmetrical. Now, everyone "knows" that moving weight from the ends to the middle reduces pitching. The shape of the typical boat has changed but so may have the conventional wisdom to some extent. Does the boat really become more comfortable after you have spent a long, hot, afternoon moving all that heavy stuff from the ends to under the midship berths? Send three people up to the bow sometime when beating and compare the results immediately. The physics of pitching are pretty much the same as rolling, a subject I've pondered enough to have produced some powerboats that are considered remarkable for their comfort. Like props, it's a complex subject and the common rules of thumb, 3 blades better, move weight to the middle, are often correct for a narrow range of typical boats but wrong as general principles. -- Roger Long |
#15
posted to rec.boats.cruising
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Hobby Horse?
Here is something I want to try sometime, maybe. Maybe someone has
tried it already or knows why it will not work. Drop a big ol Fortress anchor over the stern with about 30 feet of line. It would be heavy enough to hang pretty near straight down, maybe? When the stern comes up the flukes act as a break slowing the rise. When stern goes down the flukes just sink. I think it would change the harmonic frequency, but it may just make things worse. Any thoughts? Howard Larry wrote: Rich Hampel wrote in news:151220061439253714%RhmpL33 @nospam.net: Better to to store heavy weight near the center of rotation (in the middle of the boat). Its a phenomenon of rotational inertia. Yeah, but who can stand all that wife bitching every time she has to crawl over those big tool boxes on the main salon deck to get to the head? Larry Isn't she the reason they were stowed in the ends in the first place?...(c; |
#16
posted to rec.boats.cruising
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Hobby Horse?
It will really change the motion when the water depth gets down to about 20
feet. Up here in Maine, it will quickly pick up a bunch of lobster pots and become are really effective hydrodynamic damper. A common anti-rolling device for use at anchor consists of something like plywood triangles with rope bridles hung over each side. The bridles pull them flat through the water. One edge is weighted so that the sink edgewise and quickly when the rope is slackened as the boat rolls towards that side. Very effective from what I've heard. The metal fish that you see fishing boats towing on outriggers work the same way but are designed to tow at speed. No similar solution for pitching though. If you are experiencing hobby horsing, your solution would probably bring relief making you feel clever. The result would probably be because of the change in speed though, something you could have done more conveniently and safely with the throttle. -- Roger Long |
#17
posted to rec.boats.cruising
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Hobby Horse?
On Sat, 16 Dec 2006 14:00:23 GMT, "Roger Long"
wrote: It used to be thought that spreading weight out into the ends made for a more comfortable boat. This was back when boats tended to be more symmetrical. Now, everyone "knows" that moving weight from the ends to the middle reduces pitching. I think there are really two issues related to pitching; comfort and speed. Putting weight in the ends will slow the pitching moment, and that tends to be more comfortable. Removing weight from the ends will allow the boat to pitch at a faster rate, and that tends to be faster because it keeps the bow from digging into the waves and increasing drag. Light weight ends also dampen pitching motions more quickly because there is less angular momentum to deal with. I can tell you from personal experience that moving fast through big waves on a lightweight boat with a fast pitching motion is not very comfortable even though it gives you considerably more speed. |
#18
posted to rec.boats.cruising
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Hobby Horse?
On Sat, 16 Dec 2006 09:12:13 -0500, Howard wrote:
Drop a big ol Fortress anchor over the stern with about 30 feet of line. It would be heavy enough to hang pretty near straight down, maybe? When the stern comes up the flukes act as a break slowing the rise. When stern goes down the flukes just sink. I think it would change the harmonic frequency, but it may just make things worse. Any thoughts? Yes. My thought is that it would be a lousy way to sail. Why not go with full flopper stopers off the bow and stern? |
#19
posted to rec.boats.cruising
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Hobby Horse?
"Roger Long" wrote in message
... JimB wrote: 1. Damp the boat's bobbing - make the forward pitch a different stiffness from the rearward pitch. I used to be a great believer in asymmetry until sailing my present boat which is fairly symmetrical and has a surprising full entrance for a 1970's design. She has a cruising weight mast, a large anchor hung out over the bow, water tank under the V berth, batteries and lots of gear aft, everything that urban legend says should promote wicked pitching. She has about the nicest motion I've encountered in a sailboat. It used to be thought that spreading weight out into the ends made for a more comfortable boat. This was back when boats tended to be more symmetrical. Now, everyone "knows" that moving weight from the ends to the middle reduces pitching. The shape of the typical boat has changed but so may have the conventional wisdom to some extent. Does the boat really become more comfortable after you have spent a long, hot, afternoon moving all that heavy stuff from the ends to under the midship berths? Send three people up to the bow sometime when beating and compare the results immediately. The physics of pitching are pretty much the same as rolling, a subject I've pondered enough to have produced some powerboats that are considered remarkable for their comfort. Like props, it's a complex subject and the common rules of thumb, 3 blades better, move weight to the middle, are often correct for a narrow range of typical boats but wrong as general principles. -- Roger Long I've sailed a Yamaha 30 for a number of years. One of the things that's interesting about this boat is the old one-lung diesel and fuel tank are under the v-berth. I don't find it to hobby much at all compared with similar size boats. -- "j" ganz @@ www.sailnow.com |
#20
posted to rec.boats.cruising
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Hobby Horse?
"Roger Long" wrote in message ... I used to be a great believer in asymmetry until sailing my present boat which is fairly symmetrical and has a surprising full entrance for a 1970's design. She has a cruising weight mast, a large anchor hung out over the bow, water tank under the V berth, batteries and lots of gear aft, everything that urban legend says should promote wicked pitching. She has about the nicest motion I've encountered in a sailboat. Sounds like a lovely old 8m I used to sail. Great long overhangs, lots of weight - not only in the ends, but all over. Heavy long keel, tall mast, very slim hull. Went like a demon to windward, but modern racing handicaps put the end to that type of design. Hobbyhorsing? That boat hadn't heard of it. For all her overhangs, her pitching inertia was so great that she just creamed along dead level (well, relatively) through all seaways. Very comfortable. Except for one thing. She had little freeboard, and once the waves got above 1.5m or so, they just rolled along the deck. With 2m waves, she made like a submarine half the time. So, there's comfort, and comfort. Motion versus soaking. -- JimB http://www.jimbaerselman.f2s.com/ Comparing cruise areas within Greece and N Spain |
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