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Keel dynamics

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steephen
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Initially Posted - 04/11/2004 : 21:28:29

A general question. In working over my 1980 swinger I see the carriage is held to the boat with four (4) 3/8" machine screws, each about two inches long. Doesn't seem like much, even though the keel's been up there for 25 years. Suppose the boat's heeled over to port with the keel all the way down. It seems to me there should be a ton of stress on the two starboard (uphill) bolts as the keel tries to right itself and the hull. On the other hand, isn't water pressure trying to "lift" the keel, which helps (along with the sails) create forward drive? And thus there wouldn't be stress on the bolts because the whole keel/hull/mast configuration moves as a single unit?
Stephen on "Little Wing"

Edited by - on

ClamBeach
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Response Posted - 04/11/2004 : 21:39:32

I'm in agreement with you that a few 3/8" bolts are not adequate to support 1500 lbs of iron
acting on what is probably a 4' lever arm.

IMHO, the fit between the keel trunk and the keel is what keeps things from flopping around on a tack... which is why Catalina Direct offers a kit to maintain the proper clearance (snug) up there. The swing keel MacGregors also use the keel trunk to control side to side flop, they have a really small keel pivot that would never take the strain.

Edited by - on

Dave Bristle
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Djibouti
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Response Posted - 04/11/2004 : 22:50:26

Not having a swinger (but having looked at a few), I'll agree that the trunk provides most of the support against lateral the lateral pressure. There are a few mitigating factors:
(1) 1500# becomes something like 1300# when immersed in water. (Still a lot of iron.)
(2) The mass is distributed along the length of the keel--not at the end of the lever (so think of it as equal to 1300# at the end of a 2' lever)
(3) If your maximum heel angle is 20 deg., the lateral force from that mass is some small fraction of what it would be at 90 deg. (I don't remember the trig on that one.)
(4) As the heel angle increases, more of the weight is supported by the trunk and less by the hanger. So you aren't more likely to overstress the hanger at extreme heel.

Now, for some perspective: Has anybody heard of any swing keel falling off a C-25 or a C-22? (I suppose if one did, its owner would probably not be a member of this association.)

Edited by - on

rford
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Response Posted - 04/12/2004 : 00:10:29

I have read stories of a swing keel falling off a C22 at www.catalinaowners.com. A link to one of those stories is below.

http://archive.sailboatowners.com/archive/archivepview.tpl?sku=2003306195900.75&forumabr=sb&fno=25&_ptitle=ptitle%2Brtitle&&model=22&forum=none&andor=wo&keyword=keel%20&pr=p&ssite=CO&srt=pdate&start=7&db=2003

Apparently it has also happened to a C25 ... that one seems to have sunk.

http://archive.sailboatowners.com/archive/archivepview.tpl?sku=2003306120438.2&forumabr=sb&fno=25&_ptitle=ptitle%2Brtitle&&model=25&forum=none&andor=wo&keyword=keel&pr=p&ssite=CO&srt=pdate&start=2&db=2003

There are probably lots of keel stories around.

Edited by - rford on 04/12/2004 00:20:33

Leon Sisson
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Response Posted - 04/12/2004 : 01:11:16

Stephen & others,
<blockquote id="quote">quote:<hr height="1" noshade id="quote">In working over my 1980 swinger I see the carriage is held to the boat with four (4) 3/8" machine screws, each about two inches long. Doesn't seem like much,...
-- Stephen<hr height="1" noshade id="quote"></blockquote id="quote"><blockquote id="quote">quote:<hr height="1" noshade id="quote">I'm in agreement with you that a few 3/8" bolts are not adequate to support 1500 lbs of iron acting on what is probably a 4' lever arm.
-- ClamBeach<hr height="1" noshade id="quote"></blockquote id="quote"><blockquote id="quote">quote:<hr height="1" noshade id="quote">The mass is distributed along the length of the keel--not at the end of the lever (so think of it as equal to 1300# at the end of a 2' lever)
-- Dave Bristle<hr height="1" noshade id="quote"></blockquote id="quote">Actually the lever arm is somewhat longer than that. The whole swing keel is about 8' long. The pivot is about 17" from the end. The 1,500 lbs. isn't evenly distributed -- the keel is quite bottom heavy. It's about 3" thick near the pivot end, and widens to about 6" thick at the lower end.

Here's a couple of really long quotes on the subject of keel bolt strength. The first is extracted from a response on the subject of ballast keel attachment safety factors by Dave Laux (whose technical explanations I miss alot).<blockquote id="quote">quote:<hr height="1" noshade id="quote">... The rule of thumb is that any one keel bolt should support the keel.

If you have 5/8 bolts, 5/16 rad squared x pi x yield strength of the metal bolt is .3125x .3125=.098 x 3.14=.3066 x 36,000 (the yield strength of normal commercial bolts)= 11,000 lbs per bolt ultimate strength divided by safety factor of 5 still gives each bolt enough strength to hold the keel plus some. Which is why four 3/8 bolts will hold a similar weight keel on a swinger though with a great deal less margin for duckups. Our boat is a wing keel. In fact while the ballast keel has fallen off some boats its an unlikely accident and with your situation if you can examine most or all of them I would find something else to worry about.
-- Dave Laux<hr height="1" noshade id="quote"></blockquote id="quote">Which lead to...
<blockquote id="quote">quote:<hr height="1" noshade id="quote"><center>Leon's calulations for C-25 swing keel bolt strength and safety factor</center>

If you start with 3/8" bolts, 3/16 rad squared x pi x yield strength of the metal bolt is 0.1875 x 0.1875 = 0.035 x 3.14 = 0.110 x 36,000 (the yield strength of normal commercial bolts)= 3,976 lbs. per bolt. 3,976 lbs. / 1,500 lbs. = 2.65. So each bolt is strong enough to support over 2.5 times the weight of the keel. The whole set of bolts, if evenly loaded, would support over 10 times the weight of the keel. That's a safety factor I can live with.
-- Leon Sisson<hr height="1" noshade id="quote"></blockquote id="quote">

And here's some interesting info from Don Casey on keel bolts. It's from a long disertation on fixed keel attachments, so I'm going to cut to the part I think is of most interest to C-25 swing keel owners.
<blockquote id="quote">quote:<hr height="1" noshade id="quote">Stainless Steel Keel Bolts

To the dismay of some authorities, iron ballast keels are also attached with stainless steel bolts. Stainless has two advantages over galvanized: the life expectancy is about twice as long, and stainless bolts are less likely to seize in their holes, making them easier to extract.

This second advantage points out a disadvantage of using stainless bolts in an iron keel. Because the iron is less noble than the steel alloy, galvanic action between the two tends to enlarge the hole in the keel. To mitigate this, stainless keel bolts are typically coated with some insulating material before they are installed.

{now pay attention to this next part -- L.S.}

Stainless keel bolts have one more drawback—the most significant one. Stainless steel loses its resistance to corrosion when immersed in stagnant seawater. Because of this, it is an inappropriate material for underwater fittings. It is used for keel bolts based on the optimism that the bolts will remain dry. If they do, the bolts are good for half a century. If they don't, life expectancy can drop to half a decade.

In seawater, stainless steel is subject to pitting, a particularly virulent form of corrosion that literally destroys the metal from within. If you have ever snapped a stainless steel chainplate in your hand—I have—you will have a healthy concern about stainless steel keel bolts.

Here again, X-ray or ultrasound may be employed to determine the condition of the bolts without extracting them. "Ringing" the bolts with a rap of a hammer can reveal a badly corroded bolt to the discerning ear, but I have little doubt that many bolts suffering from corrosion would "pass" this test.

If the tops of your keel bolts are encapsulated in resin, they are almost certainly stainless steel. The encapsulation is there to seal the bolts from bilge water. The first step in inspecting stainless keel bolts is to chip away this resin cap. Everything inside should be as shiny as a new dime. If not, water has found its way to the bolt, presumably from the keel joint since the top was sealed.

It is a common (and questionable) practice to use mild steel washers on the stainless steel bolts. These can lose thickness to rust and loosen the keel, but they do dramatically confirm water intrusion. If the washer is rust-covered, the bolt ought to be inspected. If the washer looks new, the bolt is less likely to reveal damage—unless the washer is sitting on a bed of sealant. In that case, a clean washer may only confirm the effectiveness of the sealant.

Be sure you re-encapsulate the bolts after inspection.

Stainless steel keel bolts are used extensively in lead keels. Except for being more compatible, galvanically, they face all the same problems, plus one more. In lead keels, the keel bolts are more often actually keel studs, cast in place when the lead is cast. They typically have an L or J shape inside the lead keel, so they cannot be extracted. And the part of the bolt in the keel cannot be x-rayed because it is shielded with lead.

Bronze Keel Bolts Bronze cannot be used in a cast-iron keel (unless the bolts are electrically insulated from the iron) but it is a superior choice for attaching a lead keel. Unlike stainless, bronze likes stagnant seawater. A good marine bronze can lose less than one thousandth of an inch to corrosion in 40 years of immersion. Bronze keel bolts generally succumb to overtightening rather than to corrosion. Under normal circumstances they are good for the life of the boat.

-- Don Casey<hr height="1" noshade id="quote"></blockquote id="quote">And so I replace my stainless steel swing keel pivot hanger bolts every two years. I also added an all bronze swing keel pivot support reinforcement using four 1/2" bronze bolts through the hull into the bilge. There are four sacrificial zinc anodes connected to all parts of my C-25 swing keel pivot assembly. There are another two sacrificial zinc anodes bolted to the cast iron keel near the cable attachment point.<blockquote id="quote">quote:<hr height="1" noshade id="quote">Not having a swinger (but having looked at a few), I'll agree that the trunk provides most of the support against lateral the lateral pressure.
-- Dave Bristle<hr height="1" noshade id="quote"></blockquote id="quote"><blockquote id="quote">quote:<hr height="1" noshade id="quote">IMHO, the fit between the keel trunk and the keel is what keeps things from flopping around on a tack...
-- ClamBeach<hr height="1" noshade id="quote"></blockquote id="quote">How the righting moment of the Catalina 25 swing keel is transfered to the hull depends to a great extent on the radial clearance between the pivot pin and the pivot hole in the keel. It the hole is worn hourglass shaped, then as the boat heels, the keel tilts towards upright until the head of the keel contacts the walls of the keel trunk. If the pivot pin is a close fit in a cylindrical pivot hole in the keel, then the righting moment of the swing keel is transfered to the hull through the keel pivot pin, hangers, bolts, threaded inserts, etc. There was a lengthly discussion of this issue here a while back. I started out arguing for letting the trunk take the thrust. After much debate, I became a convert to the heeling forces through the pivot pin believers, where I remain today.<blockquote id="quote">quote:<hr height="1" noshade id="quote">Has anybody heard of any swing keel falling off a C-25 or a C-22?
-- Dave Bristle<hr height="1" noshade id="quote"></blockquote id="quote">Yes. It's more likely to happen to the C-22, because a given amount of crevice corrosion has a proportionately greater effect on their thinner bolts. The only incident I'm aware of in which a C-25 swing keel fell off involved a borrowed boat being left to pound on a rock breakwater in large waves. Under those circumstances, I can forgive the keel bolts for letting go after a while. When I first began to scratch my head about the C-25 swing keel attachment hardware, I called the Catalina Yachts factory and talked to one of their technical wizards. To paraphrase his response, it came down to, "Trust me Dude, C-25 swing keels don't fall off at the pivot. If you want to worry about something, worry about a neglected lift cable letting go."<blockquote id="quote">quote:<hr height="1" noshade id="quote">The swing keel MacGregors also use the keel trunk to control side to side flop, they have a really small keel pivot that would never take the strain.
-- ClamBeach<hr height="1" noshade id="quote"></blockquote id="quote">That's all true for swing keel MacGregors. It's the way they're designed to work. (I had one.)

I just finished doing my taxes, so I hope I don't come across as snarly.

This post was further interupted by a call from my neighbor, after midnight, to say that their housemate had just drifted out of sight in a peddle boat with no rudder, paddles, anchor, lights, etc., in 10 knots of wind, all in the pursuit of a drowning baby dove. Of course they now needed to be rescued. I retrieved the peddle boat and occupant. The dove drowned, and they didn't even have the consideration to save the fresh carcass for my cats. Some people just don't think clearly under pressure.

I'm just going to post this mess as-is, and edit it later if it needs it. G'night all.

-- Leon Sisson

Edited by - on

ClamBeach
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Response Posted - 04/12/2004 : 12:56:18

To continue the engineering discussion... (if anyone's interested)

Leon's calculations...
"If you start with 3/8" bolts, 3/16 rad squared x pi x yield strength of the metal bolt"

Remember that the effective cross-section of a 3/8" bolt is not 3/8"... the root diameter
of the thread is considerably less. (the smaller the bolt, the greater the percentage loss).

A 3/8 - 16 bolt has a minor diameter of around .310 inch which reduces the results of the calculation quite a bit. This gives a yield strength of 2,715 lbs/bolt versus 3,800 ( + - ) using the full diameter as the basis.

Now let's talk leverage.. as it's the leverage that I'm concerned about...
you're not just talking about 1500 lbs of keel hanging from a pin.

Some guesstimated assumptions for discussion (worst case scenario).

Boat at 90 degrees (full knockdown)
Center of keel mass: effective load of 1,200 lbs acting on a 4' load arm
against a 4" working arm (4" = the approx axis of the keel to the pivot hinge)

That yields a ratio of 12:1 under static conditions for an approximate tensile loading
of 4,800 lbs on the top two bolts. Note that the hinge 'on the bottom' in this situation
will be (mostly) under comression. 4,800 lbs is starting to get pretty close to
the 5,430 lb yield point of (2) 3/8" stainless bolts.

Throw some dynamic loading into the picture and the situation gets
worse. While I don't think the bolts will fail under that
loading, I'd be concerned about the supporting structure yielding if
in fact, the entire load was being carried by the hinge assembly.

My notion is that even if you have a very tightly fitting hinge/pin
and there was clearance between the keel and the trunk,
the hull structure will begin to distort under such loading and
load will be transferred to the keel trunk.

All the arm-waving aside, time has proven that the design works... as was pointed out, these boats usually don't fail at the hinge, so the proof is in the pudding.

Leon also wrote...
"I also added an all bronze swing keel pivot support reinforcement using four 1/2" bronze bolts through the hull into the bilge"

Leon, is there an article that you published on this? I'm interested in hearing what/how you did this. If I'm going to take my boat to the Sea of Cortez I'm either going to beef up the keel assembly (on both ends) or convert to a wing.

Edited by - on

Gloss
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Response Posted - 04/12/2004 : 18:18:00

Hey Leon,
I have some squirells in my back yard that I would love for your cat to take care of. Send him over.

Edited by - on

dlucier
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Virgin Islands (United Kingdom)
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Response Posted - 04/12/2004 : 18:48:10

Is there going to be a test on this???

Edited by - on

steephen
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Response Posted - 04/12/2004 : 20:01:05

Hope there's no quiz, as I'm already overloaded in school as it is. Where did you guys learn all that stuff? Thanks for the info, folks. When catalina sent me the new keel hanger assembly it came with new bolts. My main interest was in the "lift" on the face of the keel as it's under sail. Isn't most of the keel weight being lifted by the water passing over it (hydrofoil) as the boat moves forward?
As far as the bolts go, mine were 25 years old (I think they were the originals and the boat's a 1980) and came out with no corrosion on them. I bought new everything this year, just for the sleep factor. Which brings me to another question. I got a new Fulton winch from CD and it has literally no wear, as I installed it last September. In removing the cable this year, I rotated the drum without any load on it and seem to have discombobulated the brakes. Does anyone know about why one is always supposed to have a load on the drum before turning the crank? I sent it back to Fulton for adjustments, no longer trusting my mechanical skills.

Stephen Z

Edited by - on

dlucier
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Response Posted - 04/12/2004 : 20:19:44

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by steephen
 ...My main interest was in the "lift" on the face of the keel as it's under sail. Isn't most of the keel weight being lifted by the water passing over it (hydrofoil) as the boat moves forward?...<hr height="1" noshade id="quote"></blockquote id="quote">

Actually, the weight of the keel is not being "lifted". The lift is generated by the shape and angle of the keel as it moves through the water and this lift is what prevents sideslip when under sail.

For more information, check out this SailNet article titled [url="http://www.sailnet.com/collections/articles/index.cfm?articleid=colgat005"]The Balance of Hull and Sails.[/url]

Edited by - dlucier on 04/12/2004 20:21:27

ClamBeach
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Response Posted - 04/12/2004 : 21:40:44

Stephen asked...

"In removing the cable this year, I rotated the drum without any load on it and seem to have discombobulated the brakes. Does anyone know about why one is always supposed to have a load on the drum before turning the crank?"

Disclaimer - "if your winch is like my winch"

There's really nothing to go wrong in there. Nothing to adjust.
The action of the winch is really quite simple.

When the drum over-runs the handle (the keel is trying to rotate it), the relative motion of the drum to the drive shaft causes a brake pad to advance on a lead thread... applying a brake between the drum and the winch frame.

When the handle over-runs the drum (lifting) the relative motion of the handle and drum causes the brake pad to 'retract' down the lead screw slightly and the brake releases, allowing the drum to turn.

Unlike standard boat trailer winches... the pawl on the keel winch only works one way... raising. The only way the keel is lowered is by release of the brake. Only way that happens is by turning the handle counter-clockwise relative to the drum.

If there's no load on the drum, the drag on the shaft will probably make it turn with the handle and you won't get any braking action.

Only four moving parts of significance. Drum, two shaft segments, and the pawl. Keep them clean, corrosion free and properly greased and the winch should last virtually forever. One thing to watch is keeping oil off the brake surfaces. Which means you shouldn't spray lubricant wildly around in there.

I have some pictures of my disassembled winch if anyone is interested. Should probably pass along to somebody who's keeping a website so they can be published.

Edited by - on

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