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Go Back   SailNet Community > Skills and Seamanship > Seamanship & Navigation
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Seamanship & Navigation Forum devoted to seamanship and navigation topics, including paper and electronic charting tools.


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  #21  
Old 02-22-2004
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Factors influencing boat balance

So, if one were to create a pivot point at the maststep and install a hydraulic forestay and a hyd. backstay. Then when the boat heels over one could ease off on the forestay and pressure up on the backstay (tilting the mast back) and this would compensate for the jacking effect. True or false?

I heard it said that on conventional boats, particularly fractional rigs, that tighten up the backstay bends the mast, flattening the mainsail. I can see how that would help with the jacking effect. But how could that flatten the mainsail. By creating an arc in the luff, wouldn''t that give the sail more of a balloon effect? Or, have I been mislead?
Del
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  #22  
Old 02-23-2004
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Factors influencing boat balance

The answer to you question is false....

If you wanted to offset the affects of ''jacking'' you would want to shift the rig foward in the boat since the center of lateral resistance moves forward as the stern rises.

As to the effects of tightening the backstay on a fractional rig, when you tighten the backstay you do a number of things on a Frac. The masthead moves aft a little but the majority of the mast bends forwards which actually shifts the center of effort slightly forward. That part is pretty minor. The main impact comes from depowering the sails. As the mast bends it pulls fabric horizontally out of the center of the sail and opens the leech a little so you end up with a flatter mainsail with a shallower angle of attack. The resulting geometry moves the camber forward a little as well. Combined that reduces heel and reduces weather helm.

Tightening the backstay also tightens the forestay. When you tighten the forestay you take the sag out of the stay which pulls fabric horizontally out of the middle of the jib and opens the leech relative to the luff of the sail, again depowering the jib and moving the Center of Effort forward which decreases heeling and weather helm.

Jeff
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  #23  
Old 02-23-2004
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Factors influencing boat balance

Thanks for your post. Nice clear explanation of jacking, by the way. Note again my use of the phrase "Everything else being equal." In real life, everything else *isn''t* equal: all the forces interact
to a degree that makes it sort of silly to try to tease them apart and consider them in isolation, but then again this is merely a thought experiment, right?

Remembering that the question at hand focused very narrowly on the question of how heel itself influences helm, I am excluding other factors (like distortion of the sailshape).

In my head I''m trying to separate the effects of heel into two components: what goes on above the waterline, and what goes on below it.

Above the waterline, it seems (again, reasoning intuitively without an expert''s knowledge) that heel just moves the whole sailplan to leeward, (and twists a couple of force vectors around in ways that are too complicated for me to think about.) Everything else being equal (!) this alone would create weather helm.

Now take away the rig and look below the water, and I get confused. I can see that for the circular hull cross section,("submarine"), heeling would have no effect on helm. (so the only effect would be the rig, and hence the net effect would be weather helm.)

But what about the flying saucer with a keel? My gut tells me that as it heels, there''s now more wetted surface on the low side of the centerline, and so the whole thing is going to try to steer towards the weighted side. But I really don''t know.

I have available the necessary experimental materials: one sunfish hull and one kid. If the pond weren''t frozen solid, I''d float the sunfish (sans rig), sit the kid on one rail, shove the whole thing forward and see which way it turned.
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  #24  
Old 02-23-2004
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Factors influencing boat balance

If you sit the child on starboard and pushed the Sunfish, it will turn to port. (Make sure that you give the kid a paddle to get home.) When racing small boats steering can often be accomplished by shifting weight rather than turning the tiller. When I used to single hand my Laser 28 under spinacker, I would steer the boat from the bow while jibing by shifting my weight as necessary.

Jeff

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Old 02-24-2004
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Factors influencing boat balance

Yes, you''re right, but the question I''m asking isn''t *quite* as dumb as the one you answered. (Your answer wasn''t dumb at all)

Of course you can steer a boat by shifting weight to alter the angle of heel. The question is: how much of that effect derives from the fact that heel moves the rig and how much has to do with hull asymmetry.

In other words, if you had your Laser 28 out with the sails furled or the rig out and the boat driven by a prop, could you also, under those circumstances, steer it as effectively by shifting your weight as you can while the sails are up and drawing?
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Old 02-24-2004
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Factors influencing boat balance

This is strictly a hull effect. In light air it is not unusual for racers to rock a small keel boat home. Often the sails are dropped to protect them from harm and the boat is simply rocked back and forth by crew members at the shrouds. The helm is generally tied amidships. To turn the crew lean out to one side to lean the boat hard over and that turns the boat almost as quickly as turning the helm. The turn of course has nothing to do with the sails which are stowed at the time.

Jeff
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Old 02-24-2004
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Factors influencing boat balance

Yep, now that you mention it I have seen that. I can''t quite figure out why, though. I''m imagining the shape of the submerged portion of a heeled-over hull, and it is not intuitively obvious to me why it wants to turn to "weather" (i.e. toward the high side). I guess this is why computer modeling has changed naval architecture -- it''s hard to visualize this stuff.
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Old 02-24-2004
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Factors influencing boat balance

Perhaps this will help you visualize why as a beamy boat heels it tries to turn to the to windward. If you think of a 40 foot beamy boat, the canoe body might be 12 feet in width at the maximum beam at the waterline. If you measure vertially it might only be two feet deep in the water. So if you think about it there is roughly 1/3 the curve vertically as there is horizonatally. As the boat heels the windward side rapidly gets to be a straighter line than the leeward side. Now sketch that shape on paper and notice that it also cocked off axis toward the leeward side. When you think of it as a giant rudder in the turned position it gets pretty easy to visualize why the boat would turn.

Jeff
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  #29  
Old 02-24-2004
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Factors influencing boat balance

Thanks much. And without resorting to graphics, too.

I''m guessing that a boat that had a very blunt entry and pretty straight topsides, and that carried a lot of its beam pretty far aft (geez, I''ve just described a barge -- or a loaf of Wonder Bread) would not suffer so badly from this particular form of heel-induced weather helm (leaving aside any other bad habits it may have).
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  #30  
Old 02-25-2004
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Factors influencing boat balance

A boat with blunt bow and stern will have tendancy to develop more weather helm when heeled. A boat with a narrow waterline beam in relationship to its depth of section will develop the least amount of turning force when heeled.

Jeff

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