Association Forum - Swing Keel load path & calculations

Assn Members Area · Join

Home.

Association Forum

Ships Store

Cruising

Racing

Technical Tips

Photo Gallery - New

Boat Search - Updated

Manuals & Brochures

Swap Meet

Links

Officer Flag Line & Staff

Bylaws & Constitution

Members Area

Join the Association

Notice:
The advice given on this site is based upon individual or quoted experience, yours may differ.
The Officers, Staff and members of this site only provide information based upon the concept that anyone utilizing this information does so at their own risk and holds harmless all contributors to this site.

Association Forum

Username:	Password:
Save Password
Forgot your Password?

All Forums

Catalina/Capri 25/250 Sailor's Forums

Catalina 25 Specific Forum

Swing Keel load path & calculations

New Topic

Topic Locked

Printer Friendly

Author

Topic

gbward
Deckhand

Member Avatar

USA
12 Posts

Initially Posted - 07/28/2014 : 13:29:22

Hello all,

I am a recent new owner of a 1978 C25 swing keel and a mechanical engineer.

I have read much about the design and maintenance of the 1500# swing keel. And I am planning to overhaul my keel this winter. One thing I am curious about is the load path for the righting moment of the boat.

Catalina Direct sells a retrofit kit with plastic pads that are supposed to be glued to the keel near the top so that the keel reacts some of the moment through the keel trunk. CD advises that this retrofit should be installed whenever the the keel is centered (either via cheap centering shims/washers or the hanger casting retrofit.)

This leads me to wonder what was the original design intent? Was the fit between the keel and the pin a close fit? In other words, was the entire righting moment load supposed to be directed through the keel/pin interface, through the pin, into the hanger casting, and then into the 4, 3/8 inch bolts?

That appears to be a huge leverage / force multiplier. And, is the surrounding fiberglass (that encases the custom stainless nuts) thick/strong enough to carry that load?

Or, as in the retrofit kits, is the some portion ofthe righting moment supposed to be distributed high up into the keel trunk?

I did a quick hand calculation (with many assumptions) that showed the force in one bolt at 5775 lbs- if the pin take the whole moment. Which is near or above the proof strength of 3/8 bolts depending on the grade.

The hole in my keel is pretty worn, so I expect that the moment load is not going completely through the pin - and, to some extent, is being transferred to the upper keel trunk.

Just trying to figure out what the original condition was so I can get a better sense of the numbers.

Thanks in advance,

-Geoff

Edited by - on

CateP
Navigator

Members Avatar

USA
108 Posts

Response Posted - 07/28/2014 : 15:46:08

We had serious keel thunk and bought the CD spacer kit. Waste of money. It didm;t come close to filling the gap between the keel and the trunk. We ended up having a new pivot hole drilled in the keel because the old one had gotten too big from thunking. Starboard marine lumber was attached to keel sides to fill gap between keel and trunk. $700+ job, but worth every penny.

Edited by - on

OJ
Master Marine Consultant

Members Avatar

USA
4382 Posts

Response Posted - 07/28/2014 : 15:51:18

Naturally you are speaking above most anyone not possessing a mechanical engineering degree. Every once in a while I'll see a component drawing from Catalina that isn't a part of the parts catalog. You might try Jerry Douglas at Catalina Yachts . . . he will certainly be able to answer your question and possibly supply a drawing as well.

Edited by - on

Stinkpotter
Master Marine Consultant

Members Avatar

Djibouti
9013 Posts

Response Posted - 07/28/2014 : 20:01:07

Do your calculations include the premise that the keel is righting the boat in water, not in air? The specific gravity of water is about 750 times greater, and 1/7 that of iron, so the effective weight of the keel is reduced by about 1/7. I haven't tried to figure out the end effect in terms of leverage...

You'll notice that the head of the swing keel includes a projection that, when the keel is down, adds some horizontal area in the trunk. I suspect Catalina expected that to spread some of the leverage as the keel leans a little in the trunk. And the mass of that head is partially above the pivot point... But all of this is just a suspicion--not close to something I know, and I escaped Mechanical Engineering at Purdue after two years.

Edited by - Stinkpotter on 07/28/2014 20:12:44

sethp001
Mainsheet C-25 Tech Editor

Members Avatar

814 Posts

Response Posted - 07/28/2014 : 21:52:49

Geoff, I think you're onto something. The recommended retrofit kits reduce the lateral force on the fiberglass keel trunk and transfer some of that force to the hanging hardware. Apparently some boats had so much space between the keel and the keel hangers that most, if not all, of the lateral force was on the fiberglass keel trunk and there was little, if any, torque from the keel on the pin going to downward moment on the keel hangers and keel hanger bolts. Since there aren't a plethora of failures, it seems that the fiberglass keel trunk can take these forces. The primary goal in the retrofit was to prevent the keel from banking back and forth in the trunk (which is unsettling) and to reduce wear on the pin and the keel from side-to-side movement.

Also, not all boats can benefit from the recommended retrofit as the manufacturing tolerances were apparently wide. I bought the retrofit kit for my boat, but discovered that the space between my keel and the keel hangers was already within the specifications to be achieved by the retrofit. So I just installed the spacing discs to make sure some of the lateral force was getting transferred to the fiberglass keel trunk, and sent the retrofit kit back.

Edited by - sethp001 on 07/28/2014 22:39:19

sweetcraft
Admiral

Members Avatar

USA
816 Posts

Response Posted - 07/29/2014 : 04:44:56

I took the info from the 22's and installed twice until all thunk was stopped. Used heavy carpet over entire top of keel glued with construcction adhisive. First attempt used contact cement. My carpet is still there and I think this experiment was before CD. I have removed keel several times and repaired the voids when the Bondo from factory fell out. The keel is not perfect but the sand blasting, epoxicoating and micro ballon faring gave a good base that the constuction glue still holds the carpet. I do check each season before launch.

Edited by - on

gbward
Deckhand

Members Avatar

USA
12 Posts

Response Posted - 07/29/2014 : 06:00:55

Hi Stinkpotter,

I didn't consider the difference of SG in my calculations. Honestly, I'm having difficulty figuring how that factors into the problem. I can say that my calculation was for a worst case condition, where the boat is on it's side and the keel is horizontal. So in that condition, it's right on the edge of the water /air interface. Generally speaking, I neglected the bouyancy of the keel.

Thanks for the reply.

-Geoff

quote:
Originally posted by Stinkpotter

Do your calculations include the premise that the keel is righting the boat in water, not in air? The specific gravity of water is about 750 times greater, and 1/7 that of iron, so the effective weight of the keel is reduced by about 1/7. I haven't tried to figure out the end effect in terms of leverage...

You'll notice that the head of the swing keel includes a projection that, when the keel is down, adds some horizontal area in the trunk. I suspect Catalina expected that to spread some of the leverage as the keel leans a little in the trunk. And the mass of that head is partially above the pivot point... But all of this is just a suspicion--not close to something I know, and I escaped Mechanical Engineering at Purdue after two years.

Edited by - on

Stinkpotter
Master Marine Consultant

Members Avatar

Djibouti
9013 Posts

Response Posted - 07/29/2014 : 06:53:21

quote:
Originally posted by gbward

I can say that my calculation was for a worst case condition, where the boat is on it's side and the keel is horizontal. So in that condition, it's right on the edge of the water /air interface.

Ya, at 90 deg., I guess the keel would be essentially out of the water. Some have been concerned that at that point it might slide back into the trunk, substantially reducing the righting moment and allowing the boat to turtle. I'm pretty confident it would take big, breaking seas on the beam to get her that far over--just one reason why we consider the C-25 to be a coastal, not blue-water, cruiser--especially the swinger. (There are several other reasons.)

Regarding the keel trunk taking some of the pressure, in my memory, the only damage to trunks reported here has been due to free-falls caused by a broken attachment of the cable to the keel. (There have also been a few small free-falls due to the keel riding up over a submerged object.) You can find discussions on that as well as worn turning balls and pivots, but I don't think you'll find any reports of keels falling off. (Then again, with all the activity on this forum, a fairly small percentage of the C-25s built are represented by participants here.)

Edited by - Stinkpotter on 07/29/2014 06:54:41

Dave5041
Former Mainsheet Editor

Members Avatar

USA
3754 Posts

Response Posted - 07/29/2014 : 19:02:50

My opinion, FWIW. There is so much space between the keel and trunk and a snug fit of the pivot that the keel should not contact the trunk. If the keel is not fixed in the lateral center, then it would slide to the lower side and still not transfer load to the trunk. The pads are added only to prevent lateral movement. I installed nylon fender washers (shaved for thickness) against the rough keel and polished stainless fender washers against the pivot housing instead and my keel does not shift. I also repaired the enlarged pivot hole with the sleeve from CD and high strength epoxy instead of a press fit. If the hole is to bir for the sleeve, it should be filled by weld and re drilled. I would still be inclined to over-drill and use the sleeve as a press fit. The nuts, weldments, are deeply bedded in a heavy fiberglass layup. The system appears to be designed for the loads to be distributed entirely through the pivot assembly.

Edited by - on

Lee Panza
Captain

Members Avatar

USA
465 Posts

Response Posted - 08/01/2014 : 19:47:08

Keep in mind that when the boat is heeled excessively it is also making substantial leeway. In addition to the wind pressure pushing the rig to leeward, through transverse drag forces on the sails, spars and lines, the same wind force is pushing against the hull. The combined effect is to push the boat sideways through the water, in addition to the forward component that we're primarily aware of. Indeed, the reason the boat is heeled is that the water is reacting by pushing laterally to windward on the keel.

That lateral force on the keel is overcoming the gravitational force pulling it downward - in effect, it is partially supporting the keel in that position.

Generally speaking, if the boat is heeled there is some amount of counter-force against the side of the keel.

In theory, this wouldn't be true in an extremely rapid build-up of air pressure (from an explosion, for instance), which could actually cause the rising keel to be resisted by the water, with that force adding to the gravitational force. Wind, however, even the sudden puffs we sometimes experience, doesn't impart that kind of shock loading on the keel-hull joint. If the boat was to experience that kind of sudden lateral pressure the rig might fail first - the shrouds and chainplates aren't sized for that kind of loading.

Of course, this is all complicated by the vertical movement of the entire system in response to surface undulations, both swell and chop. For instance, as a wave passes under the boat beamwise, there are times when the boat is rising under the approaching crest while the water beyond the crest is already dropping. With a long keel extended sideways from a boat heeled way over, this could be a significant factor.

In short, the dynamics of the forces on the keel-hull joint are a lot more complicated than just the simple balance of heeling-force versus righting-force as shown in the diagrams in the books I've looked at. Just the same, I've never heard of a Catalina 25 swing keel having one of the brackets pull out, and I don't think it's ever likely to happen from wind-induced heel.

I could conceive of a situation where the upright boat is being carried sideways in a rapid current when the side of the keel suddenly encounters a submerged rock. The angular momentum of the hull and rig would then be opposed to a lateral shock loading on the keel, and I could see the possibility of those bolts being sheared or stripped. A sailboat just isn't designed for that.

So, I generally wouldn't be worried about the size of those bolts.

Edited by - on

Stinkpotter
Master Marine Consultant

Members Avatar

Djibouti
9013 Posts

Response Posted - 08/02/2014 : 10:05:23

"Past performance is not an indicator of future results..." However, 38 years without any reports from the 400+ swingers registered with the association is a pretty strong hint.

Edited by - on

Dave5041
Former Mainsheet Editor

Members Avatar

USA
3754 Posts

Response Posted - 08/02/2014 : 19:33:31

The several engineers that I have known were really good at making the simple complex.

Edited by - on

Lee Panza
Captain

Members Avatar

USA
465 Posts

Response Posted - 08/02/2014 : 22:41:12

quote:
Originally posted by Dave5041

The several engineers that I have known were really good at making the simple complex.

We take classes in that in Engineering School!

Edited by - on

Boomeroo
Navigator

Members Avatar

Australia
128 Posts

Response Posted - 08/04/2014 : 15:26:17

A great topic for us engineers. But for the accountants .....
Compare the load point for the fixed keel and the large multible bolts at the hull keel joint . You cannot expect to replace that with the very small bolts in the swing keel joint. Thus it must be that the very strong keel case takes the load transfer to the hull.

My rebuilt keel case ussumes that and the upper keel is a tight fit in the case with never a clunk. The keel and keel case drawings are available somewhere in this forum to down load .

Edited by - on

Boomeroo
Navigator

Members Avatar

Australia
128 Posts

Response Posted - 08/04/2014 : 15:26:57

Edited by - on

Topic

New Topic

Topic Locked

Printer Friendly

Jump To:

Association Forum

Notice: The advice given on this site is based upon individual or quoted experience, yours may differ.
The Officers, Staff and members of this site only provide information based upon the concept that anyone utilizing this information does so at their own risk and holds harmless all contributors to this site.