The loads I would be most concerned about are not so much those on the shrouds
and stays but the compression loads that the mast is placing on the boat (due to
the angles, the rigging pulls "down" harder than fore/aft or side to side). As
rigging tension is increased, either intentionally or by strong winds, mast
compression loads increase trying to drive the mast through the bottom of the
boat bow-and-arrow style. The difference when pushed by the wind is that the
boat can heel and "give", limiting ultimate tension. All that downward force has
to be supported by the cabin top, the compression post and eventually the hull.
Looking at the relative size of things, the compression post may be more likely
to fail than the mast is. And how is that force being transmitted to the hull?
Is there enough fiberglass in that area to adequately spread the additional
load?
With the paranoia only an engineer can muster, I'd encourage people to be
cautious about making modifications to their rigs until a full understanding of
the "whole" picture is at hand. The more I get to know my P-15 the more I
realize that both Stanley Smith and Herb Stewart were smart guys, capable naval
architects, and put a lot of thought and calculaton into both of the Potter
designs.
Dave Kautz
P-15 #1632 Tilly Lucy
Palo Alto, CA
______________________________ Reply Separator _________________________________
Subject: Re: Shrouds
Author: Non-HP-ttursine (ttursine@gnt.net) at hp-boise,mimegw7
Date: 5/19/99 2:12 PM
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West Wight Potter Website at URL
http://www.lesbois.com/wwpotter/
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> The upper and lower shrouds are positioned so
> you can tune a curve into the mast safely if you want to. (Most fractional
> sloops have adjustable backstays, I'm assuming there's one in this
> discussion)
Have you done this w/ your boat, Judy? Has anyone else with P19s from
the era of ours?
You are indisputably correct about the tuning (static and dynamic) of
a fractional rig, but I have always been hesitant to try dynamic mast
bending on Ursa Minor. Have you noticed the section on that mast? It's
positively _massive_ for the application, far larger and stiffer than
on comparable boats. I wonder if it wouldn't take entirely too much
stay tension to effect an appreciable bend. Or at least way too much
to be categorized as safe. Did your rigger take this into account?
Regards,
Bill Combs
WWP 19 #439 (Aug 1987)
"Ursa Minor"
Fort Walton Beach FL
ttursine@gnt.net