Lewmar guidelines says the sheet load is about 200# for a fractional 20
footer. Then you have to multiply by a factor for the angle of deflection
(from straight thru) at which the sheet passes thru the block.
Angle of deflection, load factor
30, .52
45, .76
60, 1.00
75, 1.22
90, 1.41
105, 1.59
120, 1.73
135, 1.95
150, 1.93
160, 1.97
180, 2.00
Alternatively, Harken gives a formula for genoa sheet load at the back of
the catalog:
SL=SA x v^2 x 0.00431
where SL =sheet load in pounds
SA = Sail area in sf
V = wind speed in knots
Examples:
a 70 sf lapper in 20 knots of wind creates a sheet load of 120 pounds. If
the jibsheet makes a 30 degree deflection, the load factor is .52 and the
load on the block is 62 pounds.
A 99 sf genoa in 15 knots creates a sheet load of 96 pounds. passing thru a
block at 30 degrees deflection, with a load factor of .52, the load on the
block is 50 pounds.
A 32 sf storm jib in 25 knots -- sheet load of 86, passing thru a block at
45 degrees deflection (due to higher clew and angles), with a load factor of
.76 --> 65 pounds.
If you get a block with a working load of at least 200# (breaking strength
will be 2 or 3 times that), you should be plenty safe for the jibsheets.
---->>> More than you ever wanted to know about blocks on a P19:
Harken bullet blocks #166 have a SWL of 300 pounds, swivels, ball bearings,
and cost about $14 each. Great, strong, lightweight compact block.
(Doesn't look very impressive if you're into big, heavy stainless steel
blocks.) The wire version of that block (Harken #183) has a SWL of 500# and
makes a great turning block for running lines aft. IM uses these blocks all
over the Potters. Great choice, in my opinion.
For a P19 main halyard load is about 200#; if you need a block to run it
aft, taking a 90 degree turn to run aft, go with 350-500# safe working load.
For all other halyards taking a 90 turn to run aft, 250-300 SWL is
sufficient. For endboom mainsheeting on a P19 , the total sheet load is
about 200#, the individual block loads depend on the geometry and purchase
of your system -- you may want to go with bigger blocks just to reduce
friction so the sheet runs easily in light winds.
For blocks that take static loads, there's no need to spend the money for
ball bearings; race bearings or cetner bearings are less likely to deform
under constant load and cost less. For blocks lthat aren't bearing a static
load, like sheetleads, you may want to spend the extra money to get ball
bearings to reduce friction.
Fair winds,
Judy B
-----Original Message-----
From: Scott Pack [mailto:scott.pack@pictureiq.com]
Sent: Thursday, May 04, 2000 11:57 AM
To: WWPotter@egroups.com
Subject: [WWP] P-19 jib sheet loads
Anyone know the loads in pounds that a std. lapper and/or genoa would
generate on a P-19? I need to pick some blocks for handling these sheets,
and need to know a ballpark number. Thanks.
-Scott
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