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West Wight Potter Mailing List maintainer
dfarrell@ridgecrest.ca.us
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Jim,
Thanks for the data and calculations. My 1985 boat has a single bolt system
for keel retention, with the bolt passing sideways thru the trunk and keel.
I use a 5/16th inch nylon bolt, because there's some rust inside the hole
drilled in the keel, decreasing the size of the hole. I could get a 3/8" in
there if I clean up the rust with a drill bit.
Considering the acceleration due to gravity that would occur in a capsize,
what size nylon bolt would suffice to hold the keel and prevent it from
falling out of the inverted trunk? I think that's the minimum requirement
for strength. It's fine with me if it yields upon a fairly light bump. I
carry spare bolts.
When I damaged my keel trunk using a steel bolt as a retainer, it was a very
odd set of circumstances. First of all, I usually use only bungies to hold
down the keel but seas were very rough that day, so I used a bolt to prevent
any water from entering. Secondly, I used a steel retaining bolt, because I
couldn't find a nylon one.
It wasn't your typical situation of the keel hitting a rock with just the
leading edge. The boat actually sailed up and over a rock shaped like a
ramp, lifting the boat up a few inches with the boat resting on the keel. A
moment later, we sailed off the far side of the "ramp" and back into the
water. In this situation, only part of the boat weight was on the keel,
because the rest of the boat was still supported by water. The bolt didn't
break, and the keel rotated around the bolt thru the keel from the drag on
the rock, damaging a 1" x 1.5" area at the top front corner of the keel
trunk.
For my lockdown design (one bolt sideways thru the trunk and keel), I think
it would be better to have the single bolt break well before 1200 pounds of
weight it on it. For others who have the keel with the 4 flip up bolts, do
you think multiple small bolts would be best?
BTW, I usually use trucker-strength bungies to hold down my keel, and dont
even bother with the bolt unless the seas are very rough. I worry that
they're not stong enough. I think an anchor snubber for a small boat would
be a better idea. It's designed to absorb shock loads without breaking and
yet stretch to absorb the impact. Or a short length of line with a little
slack in it, but that wouldn't keep water from entering when seas are rough.
Just a thought.
Fair winds,
Judy B
> -----Original Message-----
> From: Jim Nolan [mailto:panache426@hotmail.com]
> Sent: Thursday, January 13, 2000 10:41 AM
> To: wwpotter@tscnet.com
> Subject: Beware - Nylon bolts for keel retention
>
>
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> West Wight Potter Mailing List maintainer
> dfarrell@ridgecrest.ca.us
> List hosted by www.tscnet.com
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> Some warnings for nylon bolts to hold down the keel on the P-19.
>
> Nylon has a wide variance in yield strength. For cheap nylon
> I've seen 9,000
> to 12,000 psi yield.
>
> The nylon bolt should hold the keel weight, 400 lbs and break
> at about the
> weight of the boat, 1200 lbs. This is just rule of thumb.
>
> The breaking force (lbs)of the nylon bolt is its yield
> strength (psi) times
> its minor cross sectional area. From the cross sectional
> area you can
> calculate the diameter of the bolt.
>
> The minimum diameter nylon bolt (9000 psi)to hold just the
> keel weight is
> .240 inches (this is diameter on inside, not outside of the thread).
>
> The minimum diameter nylon bolt (9000 psi) to break at the
> boat weight(1200
> lb) is .41 inches.
>
> So if you want a bolt (9000 psi)that will hold your keel in
> the event of an
> overturn, but yet yield when you crash into a sandbar/
> submerged SUV you'll
> need at least a 3/8 or 1/2 inch nylon bolt.
>
> Please note that nylon varies considerably in its yield
> strength, so this is
> just a nominal calculation. Also I hope someone checks my
> calculations for
> possible errors.
>
> Bungee consideration - how far does a bungee have to stretch
> to hold 400
> lbs? May want to consider multiple bungees under alot of
> tension to hold
> keel.
>
> Jim Nolan P-19 #426 Panache
>
>
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