Spreader tip is 2 inches too far aft

[art heyman]

Spreader tip is 2 inches too far aft( a replacement spreader) . How to calculate torque at the mast brackets , and what to expect if I sail her like this ?

I matched the holes and tip on the old spreader exactly as I drilled the new ones. I noticed the old port spreader had more for and aft play than the starboard. On examination this was due to wider holes for the pins ; clearly these had worked open during 38 years. I ran the drill through more and added some slop but was warned off adding enough, it seems. Essentially I trusted the center points of the old spreader holes; wrong .

As I look up the mast from abeam on the port side, and hide the mast head with the spreader, the shroud clearly runs forward to the chain plate from the spreader, unlike the port side.( all in line there)

The pins are steel, the spreader aluminum; does the mast get a dimple if I sail her ? Do the pins give ? Do the pins open some nice harmless slop in the aluminum ? ( 1/8 inch thick , with welded 1/8 inch plates as support out 5 inches from the mast on the inside - so the pins go though 1/2 inch of metal. (these spreaders are rectangular "tube" 3 inches by 1.5 by 41 inches long))

Do I run up the mast, pull off the spreaders and drill out the holes (on a calm day!) Is it a mistake to add more "slop" ? Do I have another made and bring the mast down to put it on ?

This is on a 30 foot Pearson - the mast is about 38 feet; not a very small boat and not so cheap to have the mast go down and up again - and I want to get this done and go sailing !

A shot of the rig here: www.sailboat-delivery.com/sany0854.jpg

[csandys]

Just an idea, but could you add a shim under one side to bring it around? Maybe a couple of washers. I do not know what the compression load is and if this idea would cause that section of the mast to bend in. Along the same lines, have a plate made that could carry a bigger load.

[allene]

My spreaders don't match (55 year old boat) and the guy down the gangway with a somewhat newer fiberglass boat don't match by even more. We both sail. Probably good to have them match but also good to go sailing. Oh, and in the process of trying to figure out what was going on with my rigging I discovered that the mast was off to starboard by an inch. Cut that way in the cabin top and that way for 55 years. Fleet champion and way faster than most of the L-36s around. Go figure. My point: it is a boat , not an astronomical telescope.

Allen

[art heyman]

Here's a reply on the force from another newsgroup: (it suggests that the spreader tip out of line with the mast head and chainplate can be worth dealing with. )


How to calculate the torque? Well, if you know what the tension in the
shroud is going to be (and given that the spreader is hinged, I guess we
can assume the tension in the upper and lower parts will be the same),
call it T, the shroud will exert a force F at the spreader tip equal to
2*T*cos(A/2), where A is the angle between the two parts of the shroud
(probably around 140 degrees). The direction of the force will be in
the plane defined by the two shroud parts, and will be in a direction
bisecting the angle between them.

Now identify the plane defined by this force direction and the axis of
the spreader, and note the angle B between them (this will be about
3 degrees). The force will be resisted by the spreader in two
components, one compressive along the spreader, equal to F*cos(B),
and one seeking to wrench the spreader sideways, equal to F*sin(B),
at right angles to the spreader and in the plane just identified (which
will be horizontal (and the wrenching force will therefore be directed
forward) unless the spreader is at the incorrect vertical angle (in
which case the wrenching force will also have a vertical component
(probably down))). The torque will then be 41 inches times that but
the action of the torque on the socket, if its mountings are perhaps
4 inches apart, will be to amplify the wrenching force at the spreader
tip by a factor of 10. Your rivets (if that's what holds the socket
to the mast) will be pulled out with a force equal to about 1/3 the
shroud tension (1/3 being roughly 10*sin(3)*2*cos(70)), so I reckon
it's definitely worth correcting the problem before you pile on the
pressure. But you could be lucky. Having pulled out the aft rivets,
the forward ones could still hold, as the system would by then have
re-balanced itself. There's a good chance the mast will stay up. :-)