[Rhodes22-list] Mast Bending & PHRF Rating
Toad the Wet Sprocket
rhodes22-list@rhodes22.org
Sat, 24 Aug 2002 12:17:35 -0400
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<P>Well All it seems than Roger has got the hot set-up. The proof is in the <STRONG>P</STRONG><FONT size=1>HRF</FONT>-<STRONG>UDDING</STRONG>.</P>
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<P>Banana pudding LMAO!<BR><BR></P></DIV>
<DIV></DIV><BR><BR><BR>Please respond to sprocket80@hotmail.com
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<DIV></DIV>From: "Roger Pihlaja" <CEN09402@CENTURYTEL.NET>
<DIV></DIV>Reply-To: rhodes22-list@rhodes22.org
<DIV></DIV>To: <RHODES22-LIST@RHODES22.ORG>
<DIV></DIV>Subject: [Rhodes22-list] Mast Bending & PHRF Rating
<DIV></DIV>Date: Sat, 24 Aug 2002 07:23:00 -0400
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<DIV></DIV>Alex,
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<DIV></DIV>Yes, the effect of bending the mast & having a fully battened mainsail, which is cut to respond to mast bend, is in those PHRF ratings somewhere. But, It's difficult to separate out the effects of individual changes. I really couldn't do back-to-back comparisons between my present mainsail & my original partially battened standard mainsail, because the original was pretty much worn out when I bought the new mainsail. Comparing an old worn out cruising mainsail with a new racing mainsail isn't a very useful exercise. But, the seat-of-the-pants feeling between the two is like night & day.
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<DIV></DIV>I had the backstay adjuster & standing rigging setup to be able to reproducibly bend the mast while I was still using the original mainsail. Bending the mast had a noticible flattening effect on the original mainsail shape. However, the sail cloth was so blown out that the point of maximum draft was too far aft no matter how much luff &/or foot tension I cranked in. When the wind piped up, the point of maximum draft moved even farther aft, so the whole exercise became rather futile. I don't know if a crispy new standard mainsail would behave better. I suspect it requires full battens to really get control of the mainsail shape.
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<DIV></DIV>If you take a mainsail, which has been designed to respond to mast bend, & lay it out on the ground, you will see the luff is not cut straight. Instead of being straight, the luff bows outward in the middle. When this curved luff sail is flown from a straight mast, this "extra" material from the curve allows the airfoil shape to get deeper just aft of the mast. The sailmakers refer to this sail shape as having "deep draft" & it is what you want for light air performance. Now, when the mast is bent, the "extra" material from the curved luff fills in the bend. This results in the airfoil shape getting flatter, which desireable for heavy air performance. My rig tuning procedure sets up the standing rigging to enable the backstay adjuster to simultaineously take the sag out of the forestay & flatten the mainsail shape as the wind speed picks up. Thus, the backstay adjuster becomes a very important sail shaping control, almost like the flaps on an airplane.
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<DIV></DIV>If someone has a standard mainsail, which is in good shape, & they are willing to loan it to me for a couple of weeks, I would be willing to do some back-to-back testing & report back to the list.
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<DIV></DIV>Remember, none of this discussion has any relevance for those of you with IMF mainsails. Don't try to bend your IMF mast or you may damage your IMF mainsail &/or the furling mechanism.
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<DIV></DIV>I used my old mainsail for a paint tarp once & then threw it away. FYI, old sails don't even make good paint tarps. Man are they ever slippery, especially if you ever get a fold of cloth where there is Dacron on Dacron!
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<DIV></DIV>Alex, thanks for putting this whole rig tuning discussion into some kind of perspective.
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<DIV></DIV>Roger Pihlaja
<DIV></DIV>S/V Dynamic Equilibrium
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