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Keel Cable Load

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JamesBird
1st Mate

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USA
68 Posts

Initially Posted - 04/03/2016 : 06:04:41

I know the swing keel weights 1500 lbs but does anybody know how much of that weight is on the keel cable? Just curious

Jim
Danvers, MA
79 Cat25/SR/SK/(1185)

Dave5041
Former Mainsheet Editor

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USA
3758 Posts

Response Posted - 04/03/2016 : 08:15:44

Probably nearly all of the keel's weight is on the cable when the keel is retracted. Much of the mass of the keel is in the bottom third where the cable attaches. This next estimate is speculation based on the increasing thickness of the lower part of the keel and the location of the pivot and cable attachment, additionally, the pivot end of the keel is not difficult to position when replacing the pivot. The cable attachment appears to intersect very close to the center of mass of the keel in the retracted position. In the lowered position, of course, the load can be from zero to whatever tension you choose to apply. As the keel is lowered, the intersecting angle moves toward the bottom of the keel and more load is absorbed by the pivot. Some tension is important to prevent the keel from swinging while underway and banging against the housing. I usually crank back 2-3 turns from hanging free, that results in cable hum at about 3 - 4 knots.

Dave B. aboard Pearl
1982 TR/SK/Trad. #3399
Lake Erie/Florida Panhandle

Lee Panza
Captain

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USA
465 Posts

Response Posted - 04/03/2016 : 22:39:09

This is a question I've wondered about myself, so this was finally the excuse I needed to figure it out. I'm a civil engineer, not a mechanical or structural, but this isn't rocket science. The challenging part was breaking the keel into component parts and calc'ing the volumes and centroids of each. A few hours (would've taken less time when I was younger) and I've come up with a weight of about 855 pounds on the cable when the keel is fully raised and the boat is out of the water. That puts about 645# on the pivot pin. Interestingly, the spec I found for 1/4 inch 7x19 cable puts its WLL at 1280# and its min. breaking strength at 6,400#. Keep in mind that in water that lump of cast iron weighs "only" around 1,275#, with around 725# hanging from the cable.

I guess I'll sleep just a little better from now on.

(break)

Actually, having taken a look under there this evening I see that the cable is not vertical where it connects to the lifting fitting, so the stress on the cable will be a little higher. More like 900 - 910 lbs. (dry). I'll still feel a lot more secure when I'm cranking it up before hauling out now that I've seen these numbers. Thanks for asking, Jim.

BTW I'll still change that cable every other year anyway, and I STRONGLY encourage everyone else to do so, especially in salt water. Flexing around the drum and around the turning ball (even if the ball is turning freely, which sometimes isn't the case) adds a little fatigue to the situation, and crevice corrosion ESPECIALLY IN SALT WATER (but even in fresh) weakens stainless steel insidiously. These are factors that don't lend themselves to a straightforward engineering analysis, and we engineers like to use really large safety factors when we can't figure things out analytically. So, change that cable every other year!

The trouble with a destination - any destination, really - is that it interrupts The Journey.

Lee Panza
SR/SK #2134
San Francisco Bay
(Brisbane, CA)

JamesBird
1st Mate

Members Avatar

USA
68 Posts

Response Posted - 04/05/2016 : 06:51:37

I was thinking the load on the cable might have been about half the 1500lbs or a little more. I was hoping someone with an engineering background would have the answer. Thanks Lee. I'll sleep better also.

Jim

Jim
Danvers, MA
79 Cat25/SR/SK/(1185)

Dave5041
Former Mainsheet Editor

Members Avatar

USA
3758 Posts

Response Posted - 04/05/2016 : 19:26:22

That doesn't agree well with my observation that with the keel pivot off and the entire mass of the keel hanging from the cable it is not too difficult to lift and reattach the pivot assembly.