[Rhodes22-list] Mast Bending & PHRF Rating
Roger Pihlaja
rhodes22-list@rhodes22.org
Sat, 24 Aug 2002 07:23:00 -0400
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Alex,
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.
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.
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. =20
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.
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. =20
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!
Alex, thanks for putting this whole rig tuning discussion into some kind =
of perspective.
Roger Pihlaja
S/V Dynamic Equilibrium
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<DIV><FONT face=3DArial size=3D2>Alex,</FONT></DIV>
<DIV><FONT face=3DArial size=3D2></FONT> </DIV>
<DIV><FONT face=3DArial size=3D2>Yes, the effect of bending the mast =
& having a=20
fully battened mainsail, which is cut to respond to mast bend, is in =
those PHRF=20
ratings somewhere. But, It's difficult to separate out the effects =
of=20
individual changes. I really couldn't do back-to-back=20
comparisons between my present mainsail & my=20
original partially battened standard mainsail, =
because the=20
original was pretty much worn out when I bought the new =
mainsail. =20
Comparing an old worn out cruising mainsail with a new racing mainsail =
isn't a=20
very useful exercise. But, the seat-of-the-pants feeling between =
the=20
two is like night & day.</FONT></DIV>
<DIV><FONT face=3DArial size=3D2></FONT> </DIV>
<DIV><FONT face=3DArial size=3D2>I had the backstay adjuster & =
standing rigging=20
setup to be able to reproducibly bend the mast while I was =
still using=20
the original mainsail. Bending the mast had a noticible flattening =
effect=20
on the original mainsail shape. However, the sail cloth was =
so blown=20
out that the point of maximum draft was too far aft no matter how much =
luff=20
&/or foot tension I cranked in. When the wind piped up, =
the point=20
of maximum draft moved even farther aft, so the whole exercise=20
became rather futile. I don't know if a crispy new =
standard=20
mainsail would behave better. I suspect it requires full =
battens to=20
really get control of the mainsail shape.</FONT></DIV>
<DIV><FONT face=3DArial size=3D2></FONT> </DIV>
<DIV><FONT face=3DArial size=3D2>If you take a mainsail, which has been =
designed to=20
respond to mast bend, & lay it out on the ground, you will see the =
luff is=20
not cut straight. Instead of being straight, the luff bows =
outward in=20
the middle. When this curved luff sail is flown from a straight =
mast, this=20
"extra" material from the curve allows the airfoil shape to get deeper =
just aft=20
of the mast. The sailmakers refer to this sail shape as having =
"deep=20
draft" & it is what you want for light air performance. Now, =
when the=20
mast is bent, the "extra" material from the curved luff fills in the =
bend. =20
This results in the airfoil shape getting flatter, which desireable for =
heavy=20
air performance. My rig tuning procedure sets up the standing =
rigging to=20
enable the backstay adjuster to simultaineously take the sag out of =
the forestay & flatten the mainsail shape as the wind speed =
picks=20
up. Thus, the backstay adjuster becomes a very important sail =
shaping=20
control, almost like the flaps on an airplane. </FONT></DIV>
<DIV><FONT face=3DArial size=3D2></FONT> </DIV>
<DIV><FONT face=3DArial size=3D2>If someone has a standard mainsail, =
which is in=20
good shape, & they are willing to loan it to me for a couple of =
weeks, I=20
would be willing to do some back-to-back testing & report back =
to the=20
list.</FONT></DIV>
<DIV><FONT face=3DArial size=3D2></FONT> </DIV>
<DIV><FONT face=3DArial size=3D2>Remember, none of this =
discussion has any=20
relevance for those of you with IMF mainsails. Don't try to bend =
your IMF=20
mast or you may damage your IMF mainsail &/or the furling=20
mechanism. </FONT></DIV>
<DIV><FONT face=3DArial size=3D2></FONT> </DIV>
<DIV><FONT face=3DArial size=3D2>I used my old mainsail for a paint =
tarp once=20
& then threw it away. FYI, old sails don't even make good =
paint=20
tarps. Man are they ever slippery, especially if you ever get a =
fold of=20
cloth where there is Dacron on Dacron!</FONT></DIV>
<DIV><FONT face=3DArial size=3D2></FONT> </DIV>
<DIV><FONT face=3DArial size=3D2>Alex, thanks for putting this whole rig =
tuning=20
discussion into some kind of perspective.</FONT></DIV>
<DIV><FONT face=3DArial size=3D2></FONT> </DIV>
<DIV><FONT face=3DArial size=3D2>Roger Pihlaja</FONT></DIV>
<DIV><FONT face=3DArial size=3D2>S/V Dynamic Equilibrium</FONT></DIV>
<DIV><FONT face=3DArial size=3D2></FONT> </DIV>
<DIV><FONT face=3DArial size=3D2></FONT> </DIV>
<DIV><FONT face=3DArial size=3D2></FONT> </DIV>
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