> What is the minimum size block ( I guess in max work load) that should be
> used at the base of the mast when running (a) main halyard and (b) a single
> line jiffy reef line to the cockpit. This on a P15. Thanks for the info.
> Owen Shaw Nashville #2216 still no name.
Hi Scott,
I use Brion Toss' book on rigging, and it has tables which show loads on
halyards and sheets.
Main sail area of the P16 is about 65 square feet. The graph on page 23
shows a mid-boom sheet load (I know, that's not the halyard, have patience,
we'll get to the halyard) on a mainsail of about 100 square feet (the
smallest they even worry about!) is between 300-350 pounds. Your sail is
smaller, so this is a high estimate.
To arrive at the load on the halyard, the rigger's rule of thumb is to take
80% of sheet load. So we estimate your sheet load at 240-280 pounds.
On another graph I have (from the Lewmar company) it shows the typical load
on a halyard for a 20 fooot boat to be under 250 pounds.
so both of these sources say the load on your halyard should be under 250
pounds.
So far so good. We have two reliable sources that say the halyard load
should be under 250 pounds. Now lets talk about the block we need to make a
90 degree trun.
When a line takes a single 90 degree turn around a block, the block "feels"
141 percent of the load. So a line with a 250 pound load on it requres a
turning block with a safe working load of 352 pounds.
350 pounds would be the minimum rating for a block to turn your mainhalyard
90 degrees.
The load on a reefing line can easily match the load on the main halyard in
very high winds, even though you have made the sail smaller (lift generated
by the sail is proportional to the velocity squared.). It should be about
the same rating as your halyard block.
So, If you got a block with a safe working load of 400# for your halyard and
reefing line, it would be more than adequate. In fact, when I calculated the
load for my P19 main halyard, I used a 500# SWL. So you'll be fine with
that.
Harken makes a series of "small bullet blocks" that are wonderfully compact
and inexpensive with working loads of 300#. Those are marginal but they
also come in a "wire" block version that has a SWL of 500#. The ones I used
(Harken #183)had swivels on them and cost about $14 each.
Have you considered using a maststep plate to mount the turning blocks? It
eliminates putting holes in your cabintop and gives you the flexibiltiy to
add up to 8 blocks total for running lines aft without putting any holes in
the cabintop.
You can see pictures of some maststep plates on my Potter website at
http://hometown.aol.com/jblumhorst/HomePage/Rigging/maststeps.htm
Good luck with your project.
Judy B
1985/1999 WWP-19 #266 "Redwing"