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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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  #11  
Old 05-16-2003
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Factors influencing boat balance

Have to think that that late 70''s/early 80''s was when the IOR was creating major problems with squirrelly boats that were bears to handle because of their shapes when they heeled. That''s why they sailed with so much meat on the rail all the time- to keep them from heeling. The canoe body, whose "in-water" shape changes less when the boat heels, developed as the IOR self-destructed in the early 80''s, no?
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  #12  
Old 05-17-2003
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Factors influencing boat balance

I think there are a lot of factors that lead to an improvement in balance as a boat heels, but it is a "which came first the chicken or the egg thing" with the IOR rule.

In the late 1970''s, in the wake of the Fastnet disaster there was a lot of light shined on the subject of seaworthiness and motion comfort. Piles or research took place and teh yachting community became quite aware that the trends in then ''modern'' race boats were getting quite unhealthy and that these trends were also affecting the design of normal cruising boats as well (For example high production boats like the Catalina 38, Hunter 34 and 40 of that era, and of course the Beneteau line was full of IOR influenced designs.)

At the same time, there was a whole range of racer/cruiser type boats being introduced that were not designed to any rule. (Boats like the J-36/35, Express 37, or Farr 11.6) These boats proved to be much faster than similar sized IOR boats of that era. People really began to question the validity of the IOR rule.

At the same time, improvements in computers allowed a lot of progress on VPP (velocity prediction program) based rating rules and in the early 1980''s the IMS (a VPP based rule actually called the MHS at that point) was just coming into being. Since the IMS did not use measurement points in the same way that early rules used measurement points, there was no gain in distorting hulls and rigs to beat a rule. In the absense of a rule that over or under rates any characteristic, a simply faster boat will still have some advantage because its speed can be used to place the boat more stratically on the race course.

Designers also began to look for unrated advantages. So for example, a boat that was in balance when upright and also could heel without developing as much weather helm would be faster than one that had its rudder dragging through the water. A boat that rolled or pitched through smaller angles, dampened roll and pitch quickly, and without a jerky motion would maintain attached flows longer and so be faster and make less leeway. The rule could not rate that advantage so huge efforts were made to improve these coincidentally important seakeeping charactisteritics.

Computers also allowed testing with full sized boats being instrumented for roll, pitch and heave characteristics. Computers allowed more detailed analysis of sailing hull form at various heel and trim angles. Computers even allowed more precise recording of the data generated in testing. Out of all of that came better race boats and that filtered down into better cruising boats as well.

Beyond all of that, there was a movement away from high form stability dependent boats. I believe that high form stability boats were rejected both for thier less comfortable motion but also because of the greater awareness of ultimate stability.

Also in my opinion, it is hard to say whether IOR died out because the non-IOR boats got so much better or whether the IOR died out and that allowed its successors to be better boats. I had the chance to talk to Geoff Stagg with Farr''s office after the first year that Goucho was on the race course and winning a large percent of the races she entered. I had written an opinion supporting the direction that Goucho represented that was published in Sailing World. In talking with Geoff, one thing that has always stuck with me was his repeating throughout the conversation how comfortable and easy these new boats were to sail especially when compared to other grand prix level boats. Of course they have only gotten better since Goucho.

The other aspect of this is the Open Class boats. I personally really do not like the direction that they have gone with thier extreme beam, but with computer modeling they too can be made to balance out quite neutrally.

I need to get out of here and go cruising.
Regards,
Jeff

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Old 07-17-2003
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Factors influencing boat balance

Yah know,
This Saturday I had the great fun of piloting a 65-footer around and up the bay and I must say that if I had a theoretical knowledge of why the boat had such weather helm, I might have saved myself a few blisters from holding the helm over so hard.

We were looking for speed and were healing 15-20 degrees close hauled on a starboard tack for a couple of hours and had a big, billowing drifter winched in as tight as we could. We also had flattened the mailsail as the wind freshened. Early on, I thought the mizzen was pushing the stern to lee and, in fact, slacking the mizzen did help me run closer to the wind. I figure that the mizzen wasn''t adding forward thrust so much as feathering the boat into the wind.

Nonetheless, I would be curious what to do next time to minimize the weather helm.
Any opinions?

Chas
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Old 02-02-2004
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Factors influencing boat balance

Reduce Sail? Jeff''s answer was on the mark,he really knows sail theory. My experience has been that weather helm starts getting tough when you approach hull speed. Just as the power needed to increase velocity increases exponentially above hull speed I believe weather helm does also. Were you near theoretical hull speed?
My physics is pretty much limited to remembering to move my beverage to the lee side when I come about. When I need about 1/4 to 1/3 turn on the wheel, constantly, I think about reducing sail. I do it because I''m uncomfortable, the boat don''t care.
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  #15  
Old 02-03-2004
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Factors influencing boat balance

On a well designed and properly sailed modern boat, there is no connection between approaching or exceeding hull speed and the amount of weather helm. In fact on a well designed boat, the helm will often get lighter and more balanced as hull speed is approached and exceeded.

Jeff
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Old 02-15-2004
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Factors influencing boat balance

Ahoy, Pirate of Pine Island ere, me thinks a couple of fat ladies outten to balance out yer rig just nicely!! AARRGGHH.
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Old 02-20-2004
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Factors influencing boat balance

On the other hand, How about a stripped down old submarine with a mast and keel? No matter how far she heels it''ll never lose it''s shape at the waterline. Thought of building a model for a try out. I think the big problem with weather helm is that the rudder is no longer vertical to the direction of movement through the water and in order to steer, the rudder has to drag sideways a bit. If one could keep the rudder vertical with the vessel heeled over then weather helm may not be so much of a problem. On a submarine shaped vessel (torpedo) one could create a swiveling rudder mount, kind of like a gear type feathering prop., to keep the rudder vertical. Bilge keels may even benifit in this application.

This should really confuse the formulas.
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Old 02-22-2004
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Factors influencing boat balance

The original poster''s question (asked in May 2003!) was whether the movement of moving the COE well to leeward off the CLR during strong heeling produces a prominent weather helm effect.

Since none of the many subsequent posts in this thread appear to have addressed this question, let me answer affirmatively. In fact, an excellent article by Steve Colgate on this website directly deals with this force component in the horizontal vector plane.

Interestingly, the term CLR would appear to be less than adequate in discussing this mechanism, as the fore-and-aft resistance rather than the lateral resistance is one of the driving forces here. It would appear that in 3-dimensional vector space the center of fore-and-aft-resistance does not necessarily coincide with the center of lateral resistance.

Have fun!

Flying Dutchman
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Old 02-22-2004
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Factors influencing boat balance


Intuitively (and reasoning from intuition with only a little knowledge is always risky) the Flying Dutchman''s point makes sense.

I imagine the thrust vector as though there were no sails, but the boat is being towed through the water by another boat, with the tow rope attached (magically) to where the center of the sailplan would normally be -- a bit aft of the mast, a fair bit above the boom. The towing boat is heading off a bit to leeward of where you''re trying to steer.

As the boat heels over, this attachment point moves way out to leeward, creating a lever arm wanting to turn you up into the wind.. You''re partially compensating for this by also mashing a bit more hull into the water on the leeward side and pulling some out on the windward side, which moves the center of a cross section off to leeward, but the center of the sailplan is moving off to leeward faster than the center of resistance of the hull.

From this thought experiment, everything else being equal, it would seem to me like the "submarine-with-rig" (circular cross section hull) would suffer much worse from heel-induced weather helm than would a flat dish "flying saucer" kind of hull shape.

I''d love to hear the results of the experiment. One bathtub, one window fan, one 2liter soda bottle.... etc. etc.
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Old 02-22-2004
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Factors influencing boat balance

I am sorry to report that you are a bit off in your conclusions about why weather helm builds with greater winds and heel angle with regards to the ''submarine with a sail vs flying saucer'' conclusion.

There are a whole range of actual reasons that boats develop weather helm as they heel. They include, as a higher wind load increase on the sail, the fabric stretches moving the center of effort of the sails aft. As a boat heels its water plane changes shape, typically getting more assymentrical. That assymetry tends to be flat on the windwards side and rounder on the weather side steering the boat to windward.

With all due respect, ccboston, you have it backward. A cylindrical section does not develop as much assymetry as a flat section so the ''submarine with a sail'' will develop less weather helm than the ''flying sauser''. That said with careful modeling this negative effect can be minimized.

Then there is ''jacking''. Since most boats are finer in the bow than the stern, as they heel the stern lifts and the bow goes down. This moves the center of Lateral resistance forward in two ways. The first is a lifting of the rudder from the water (most pronounced on a spade rudder boat) and the second is submersion of the bow overhang, most pronounced on a boat with long overhangs.

And that is just a couple of the main suspects.

Jeff
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