A Discussion of Bolt-on vs Encapsulated Keels
A discussion of Bolt-on versus Encapsulated Keels
Anyone who has read my articles on this subject over the years knows that I strongly favor bolt-on keels over encapsulated ballast keels. I know that many in the offshore cruising community prefer encapsulated ballast keels so I thought it might make sense to try to address both sides of this discussion.
Let’s start with the basics, neither bolt on keels or encapsulated keels are inherently stronger or weaker. It all comes down to how any given example of each is actually engineered. Either approach has to deal with a variety of vertical, lateral and longitudinal loads, they both have to spread these loads out into the hull and they both have to deal with the stresses of a heeled boat dropping off a wave, or running at speed into an immoveable object.
If you look at the way side the forces work, the maximum force occurs where the keel stub hits the hull and not necessarily where the ballast meets the hull. Structurally the keel is a simple cantilever and maximum bending moment occurs at the fold in the hull where it turns down toward the keel. In terms of side load, what happens structurally at the bottom of the keel is almost irrelevant to the discussion.
So it is that the area that turns down into the keel needs to really have a lot of strength and that area needs to be reinforced to distribute the loads out into the hull. In a properly designed hull, there are a series of athwartship frames that take the loads out into the hull and often there is a longitudinal frame that distributes these loads from the frames ends fore and aft. It does not matter whether we are talking about a bolt on keel or an encapsulated keel, these framing elements are critical to achieving the necessary strength in this area of the boat almost no matter how thick you make the fiberglass.
And in either type of keel, these athwartship members have to be connected to solidly to the ballast keel so that they can take the sideward moments (bending forces) from the ballast keel and connect them to the rest of the hull.
Here's where it gets harder (but not impossible for an encapsulated keel). If you visualize a boat with an encapsulated keel at an extreme angle of heel, the weight of the ballast keel has a lot of leverage pushing against the sides of the encapsulation. The high side of the keel where the ballast ends is trying to pry the encapsulation away from the ballast. All that glues the ballast to the encapsulation typically is a polyester resin slurry. Polyester resin is not a great adhesive especially to lead or iron, and so over time that adhesion fails.
You might think that is no big deal. The ballast is in there and where can it go. But as I said earlier it is critical to transfer the side loads from the ballast to the athwartships frames. In a properly engineer encapsulated keel, this is done through a series of bonds.
To explain, once the ballast keel is 'glued' in place in the fiberglass keel cavity, the top of the keel is glassed over. Normally this layer is pretty thin and is only intended to keep water out of the keel cavity, and in the case where there are athwartships frames, provide bonding for the athwartships frames. The athwartships frames are bonded to this membrane. Properly constructed and still intact, the ballast transfers its loads to the sides of the keel cavity, which transfers most of the loads to the athwartships frames through the membrane at the top of the keel. And there in lies the problem, once the bond between the F.G. keel and the ballast has loosened the ballast is prying the top of the keel membrane away from the side of the keel cavity and with that goes a weakening of the athwartships connection to the hull.
Walk around most any boatyard with a Lucite hammer and tap out the area about 6" to a foot below the top of the ballast on any older boat with an encapsulated keel and you will find that many, if not most, have a void in this area.
Making this problem worse is that this is an area that is next to impossible to repair because once water gets in there (and it does) it is very hard to re-establish a connection between the ballast keel and the hull.
Now then, what happens when that boat runs aground hard? There are a couple things that normally come into play and a number of things that generally happen. Starting at the bottom of the keel, it is very hard to do a proper glassing job at the bottom of the keel. The workers are laying up glass in a narrow cavity that a person really can't get into very well. Although there are lots of tricks to doing the work in these tight areas, having repaired quite a few keel bottoms in my day, the glass work at the bottom of keels is very heavy, from the laps in the cloth, but not very sturdy. There are usually large lenses of un-reinforced resin, voids, and sections of dry cloth.
When you run aground hard, the force generally crushes the poorly laid up cloth and creates a force that tries to drive the ballast upward against the membrane to the top of the keel and the athwartships frames and pulls downward at the forward end of the keel. If the ballast shifts at all, which it typically will in a hard grounding, you now have a leak; maybe small, maybe large, but not one that can be easily repaired to full strength.
Proper repair often means cutting away the membrane at the top of the ballast keel, drying out the encapsulation cavity,, attempting to rebond the ballast keel to the encapsulation envelope, grinding away portions of the transverse frames, and the damaged glass at the bottom of the keel, and reconstructing the keel bottom, watertight membrane at the top of the keel and repairing the transverse frames. That level of diligence rarely occurs, leaving a weakened keel structure out there waiting for disaster.
So, here is the other side of the story; bolt-on keels. In the case of a bolt on keel you really do have greater maintenance. The fairing materials tend to be short lived. This is a pain in the neck process and even using epoxy, it has to be repaired or redone every 7 to 10 years. (That said it is no worse than fixing the blisters that can come from moisture trapped between the keel cavity and the ballast working its way out through the not so great glasswork in the keel cavity. But then again, not every encapsulated keel boat gets blisters while every bolt on keel boat sooner or later needs re-fairing.)
At some point, on most production boats with bolt on keels the SS keel bolts need to be replaced. (I have never understood why for just a little more cost the boating industry does not use Monel Keel bolts but that is another story.) I see this quoted as 20 to 50 years depending on who you believe, and how the boat was built, used, and maintained.
When we talk about the relative strength of a bolt-on keel, we are really discussing the ability to make a mechanical connection vs. a glued connection. Obviously, I'm a mechanical connection kind of guy, but I also understand that the loads get transferred through narrowly prescribed load paths and as such the engineering and build quality of those load paths need to be skillfully engineered and executed, and in many ways more skillfully designed and executed than an encapsulated keel.
In some ways a quality execution is easier. For example, the keel stub of a bolt on keel is generally shallow enough that you can get really good glass work and the overlaps occur so that you have the thickest amount of glass right where you need it most, right at the top of the keel. Often, on better built, low volume boats, the athwartships frames are often laminated right in as the hull is being laid up. (On less expensive boats and mass produced boats the frames are generally molded separately or as part of the liner and glassed or glued in.) The keel bolts then pass through the outward turned tabbing or flanges on the frames making a solid mechanical connection between the keel and the frames.
In the same grounding as above, the metal (especially lead) absorbs much of the shock of the impact before ever distributing the loads to the keel bolts. The bolts have to distribute all of the loads in a compact area. As mentioned, here is where engineering is critical and why there is nothing worse than a poorly engineered bolt on keel, and nothing better than a properly engineered one.
In that impact, the aft end of the keel pushes up. In a properly engineered bolt on keel, that upward force is distributed through a solidly engineered transverse frame at the aft end of keel which carries the loads into a wide area of the boat. Often there will be a bulkhead (seat face or other cabinetry) or massive longitudinal member tied to this frame that distributes the loads fore and aft, and a athwartships bulkhead that distributes the loads into the hull and deck. (That same structure should be there in an encapsulated keel and often isn’t, but it absolutely needs to be there with a bolt- on keel.)
At the forward end of the keel there needs to be a similar frame as well. This is the frame is perhaps easier to engineer. It needs to have sufficient bearing area to not slice down through the hull and to withstand the withdrawal forces of the forward keel bolts. Lastly there is shear through the hull and keel area, but that's the easy one. All it takes is enough hull thickness, surface area on the bolts and enough bearing plate area to prevent the bolts from acting strictly in single sheer.
Because a proper bolt on keel requires better engineering and execution it is generally a more expensive keel to build. The keel casting must be made carefully. The bolts and bearing plates are expensive. Great care must be taken in boring the holes in the hull for the keel bolts and there is a lot of labor and handwork in fairing the keel.
This is why a lot of manufacturers take the short cut of doing an encapsulated keel. This is especially true of smaller production yards where the cost of precision tooling a keel casting can be excessive.
In the end it is a trade off between the higher maintenance but nearly infinitely maintainable (bolt-on) and the lower initial maintenance but limited lifespan. Encapsulated keels are low maintenance but once they have failed, they virtually can no longer be repaired.
To address the frequent comment that encapsulated keels are better for offshore because the keel cavity is integral with the hull, while it is true that the keel cavity is integral with the hull, the ballast in not integral with the with the keel cavity. In the end, it comes down to whether you trust a not extremely good glue (polyester resin slurry) or mechanical fasteners. (Heck, once upon a time, wooden boats had internal ballast, but they went to bolt on keels for the same reasons outlined here.)
For those of us, like myself, who tend to own older boats, I would think that the ability to repair a bolt on keel far outweighs the potentially lower maintenance of an encapsulated keel.
At least that's how I see it.
Sailing "Synergy", my Farr 11.6 (AKA Farr 38 design #72)) out of Annapolis, Maryland
Thanks, Jeff.
Excellent analysis, as always; whether you agree or not, this is a very well reasoned article.
I wonder if you could suggest a couple of good and poorly engineered examples of each type of keel.
I know this is putting you on the spot; and feel free to decline if you think it's too politically hot, but as someone in the early planning stages of a bluewater purchase this topic is of great interest to me.
Thanks
encapsulate keel, with keel bolts
Dear Jeff,
I just read your discussion of keel construction. My Hallberg P28 has an encapsulated lead keel, but the keel is also bolted on. At least, I see keel bolts in the bilge. Does this mean that my keel is not really encapsulated as per the definition that you use in your discussion?
I'm a bit worried about keel bolt corrosion. Do you know a way to check the keel bolts? I know that the previous owner grounded the boat so hard that the fiberglass was in fact punctured. So, there must have been water in the keel cavity. I wonder what I should do in order to check the keel? The ballast is lead.
Any ideas?
Best,
Alex
PS thanks for being such an asset to us!
A friend lost his 1986 Oday 322 2 years ago while sailing on Lake Ontario near Cape Vincent N.Y. when his bolt-on wing keel ripped free of the hull leaving 3" holes where the keelbolts pulled through the fiberglass hull.The boat capsized and eventually washed ashore where it was salvaged.The owner spent some time in the water clinging to his damaged kayak before he was rescued by a passing boater.
I realize this isn't an everyday occurrence but I wonder how many other boats like this are going to have similar problems as they age and how do you recognize a weakened keel stub before it fails.
Phil(with a bolt-on keel)
I am a strong proponent of encapsulated keels for cruising boats but don't think it is an important enough issue to worry over. If the boat is otherwise what you want...any properly engineered keel will do. Given a choice though...a good encapsulated keel leaves you with nothing to worry about... and this is especially true when buying a used boat as most of us do.
Cam
No Longer Posting Here. PM me to say Hi or if you need a hand.
So we've had boats with both types of keels but each of them seem to be attracted to mud bars. Is this attraction stronger in one over the other? I'm thinking of getting a kickstand for Nikko.
Ray
S.V. Nikko
1983 Fraser 41
_____________________
Boating for over 20 years, some of them successfully.
The Oday 322's have suffered a number of keel failures. The keel stub in them is very thin when compared to other boats of similar size..Originally Posted by Philsboat
Many old wooden boats have cast-iron 'floors' through-bolted to each of the ribs in the mid-section. The keel was then through bolted to the floor and the floor was designed to take all the grounding and bending forces Jeff mentioned.
I do think sometimes that maybe the importance of a really good structurally-impregnable floor has been lost over the years in the pursuit of lighter/faster.
"The wind may be free, but the sails bloody well aren't!!"
Hey...just was reading this thread and saw you never got an answer from Jeff who is indeed a gem! The presence of keel bolts means you do not have an encapsulated keel. The bolts hold it on. There is probably a seam on your keel near the top that has been faired in and smoothed and painted over. With a lead keel...chances are you are looking at nuts on the keel bolts rather than hex heads on bolts so the only way to check them is to drop the keel. With NO external separation visible and no rust on the bolts you can see...this is probably an unnecessary expense.
By all means repair any laminate damage on the keel itself and tap around with a hammer to see if you HEAR any differences in the keel area that would indicate water intrusion while it is out of the water this winter.
Come back at us and we can help further if you find issues. Sorry we missed your original post and welcome aboard!
Cam
No Longer Posting Here. PM me to say Hi or if you need a hand.
alomper:
You may have lead ballast that is external but covered in a glass skin. This is often done in order to sane tiume and money getting a nice fair finish to the keel itslef. The lead casting can be rougher but the molded grp skin can be near perfect. This type of keel would have to be bolted on. I would not consider tis type of keel an "encapsulated" keel the way we tend to use that term. Although your keel us certainly encapsulated.
I think "interna; ballast" is maybe the best way to describe a keel where the ballast is dropped into a keel envelope that is part of the monocoque hull molding. "External ballast" would be the term I would use for a keel that relies upon bolts to keep it attached to the hull. I think both ways can work just fine providing they are well done. Internal ballast does put a lot of restrictions on the actual shape of the keel as the keel has to be of a shape that allows it to be removed from the mold. Then comes the problem of sliding the big chunk of lead or iron down into the envelope.
As a designer I prefer the freedom of design that external ballast allows. Builders ofte go with internal ballast because it is cheaper, easier and has less potential warrantee issues. If you look at the hundreds of medium sized crusing boats that were built in Taiwan in the 70's and 80's you will find that probably at least 85% of them had internal iron ballast. Lead was quite expensive in Taiwan. The only boat I did in Taiwan that had external lead ballast was the Lafitte 44.
Bob...recognizing that from a design/performance standpoint you can do better with an external keel... for strength/safety in a cruising boat, can you make an external keel that has the same safety factor in a hard grounding compared to a well designed, lead ballasted internal keel? If so...can you give a couple of examples of production boats where this has been done? Thanks!
Cam
No Longer Posting Here. PM me to say Hi or if you need a hand.
Bob, thanks for the post and explanation, from the point of view of a designer. Most excellent.
Here in the photos below are the 2 FG halves that compose my torpedo. The torpedo is then attached to the keel, that has a metal structure inside. The keel skin is just for shape..and has little structural value. The strength of my keel is all inside.
To fill the torpedo with lead, it first was finished, and attached to the keel. Then a hole opened on the top, and the torpedo buried in sand 3 feet down, to prevent deformation when lead was poured in, and to allow slow cooling.
Lead was then measured for weight, melted and poured into the keel, through the whole.
We left about 15% of the lead outside and this was cut in small 2 inch chunks.
The remaining lead was then mixed with cold wax, into a paste, and the mixture filled the rest of the torpedo.
The mixing with wax, allows the removal of lead, in an easy way, as we adjusted keel weight.
We ended up removing weight.
Photo of torpedo halves. The form is just for shape and hydrodynamic purposes.
Keel complete, ready to install in the boat.
big keel
Jeff- thanks for the good info. You mention that many internal ballast keels will have a void just down from the attachment point and I'm wondering about recommendations to repair it. I don't have any reason to believe that our boat has a void as the surveyor did a thorough job of tapping everywhere around that area (I'll be checking it carefully at next haulout), but if I found a void in the future would the "fix" be to drill holes and fill with epoxy (after letting any water drain out) like you would do with a rudder, or would something more extensive be required? Thanks for your thoughts and ideas.
Cam: For the sake of argument,,, Giulietta's can be the very strongest and most resistant to failure during grounding.
IF,,,IF and it's a big IF,,,the keel goes thru the hull and is attached at the top of the cabin trunk or deck. Securing a high aspect ratio fin at both the hull and the deck or cabin trunk top can result in a very strong keel. Some are done this way but most due the the imposition of the keel fin trunk on the interior layout are not unless they are Gran Prix race boats.
So, in the real world the most "durable" keel is probably the internally ballasted, low aspect ratio fin with generous leading and trailing edge fillets that will spread out the impact loads. If,,,and it's a big IF the boat is laid up with a generous layup AND there are floors throughout the keel area that will transfer the loads out into the hull shell over a large area. I also like to double up the floor at the trailing edge where the keel will try to shove itself up thru the hull when you impact something at the leading edge.
The "ifs" are very important.
Best not to hit things.
Pragmatic building issues also come into play with an internally ballasted keel. Look at the trailing edge of a keel. See how thin the "envelope" is aft? Now imagine trying to lay fibergalss mat and wroving down into that thin, tapering adge while not allowing any voids. It's a hard job to do correctly and not end up with several layers of thin grp. This actually may be what JRD is talking about.
Last edited by bob perry; 11-27-2009 at 06:47 PM.
Bob- In re-reading Jeffs post I misinterpreted where he was referring to about a void (see below). If a void is found in the ballast area is it possible to repair it with epoxy I wonder? Jeff makes it sound like it is not. I found what you said about doubling the floors at the aft end of the keel interesting, sounds like a very good idea. In our Brewer we have floors, but the aft one is not doubled. Thanks.
Thanks Bob... I've actually had both a good encapsulated keel and a poorly done one.
(Your Tayana52 was the Good One!) ... so I understand the difference! Interesting point about trailing edge voids.
The T52 was driven ashore in Grenada/Ivan...and stayed on its' keel with several other boats piled on us...for two weeks. The keel took it all with no damage save a few scrapes in the glass...ditto the skeg hung rudder...though we did get water ingress there.
Anyway... looking at all the bolt on boats with big damage, reinforced my faith in spreading the load out!
(Sometimes mother nature rises up to meet ya!)
Thanks!!
Last edited by Camaraderie; 11-27-2009 at 08:31 PM.
Cam
No Longer Posting Here. PM me to say Hi or if you need a hand.
Bob, not that it makes much difference to you, as a designer, but here is how my keel is attached to the hull, for reference..and since we we're talking about keels..I like keel talk.
The hull is as you know cored with Divinycell, except in a 40 cm (more or less) center section all along the hull's axis.
The boat has a Kevlar reinforced structure bonded to the hull in the form of an "H" with many arms..that runs from the forward bulkhead to the area of the engine more or less, and extends to the stern, passing load onto what is the two aft cabin walls.
Inside one of them, in the center, there is the keel box, it's as if it is an inverted box with a cut out with the profile of the keel.
That box is solid FG and fits inside the space created by the structure. This box has a cut with the shape of the keel's profile.
see bellow
Then, the keel enters the hull from bellow and passes through the box. Once inside 2 4 inch bars cross the long beams, the keel and attach it to the structure. These studs pass through the keels internal beams, that are one piece.
Then, on top, 3 bolts, tighten the keel vertically, with a lid in steel that closes the box, on all 4 sides and top..it also prevents water and any movement of the keel.
This system was chosen because the keel is very thin, long and narrow, so attaching it to the hull was not a good solution. And almost impossible.
This system is very solid against groundings and allows the keel to flex without stressing the hull, since the loads are applied to the beams.
In a grounding, the whole keel training edge side may want to go up, but can't because the horizontal struts hold it, and the steel plate too.
The bolts force the keel down into the studs.
The photos bellow show the keel that is fully encapsulated and water tight.
In the photo bellow you may see a thin water line a foot and half down from the top of the keel blade, that is how much of the keel is inside the boat in the structure grid that is there.
The torpedo is shaped with 2 fiberglass halves, that enclose the lead. A door allows adding and removing lead, that is mixed with wax in the top, so you can scoop the lead out with some heat.
Shark for looks only and to scare the dolphins...
Alex
Alex, I understand how the plate on top of the keel box would not only be gasketed but would be compressed by the weight of the keel itself, but do you not get water seepage from those traverse "bolts" holding the keel box to the "H" frame? Or are those bolts simply siliconed to reduce leaks?
It's a very strong system, but that's the only issue I can see.
Can't sleep? Read www.alchemy2009.blogspot.com for fast relief
Yes..they are siliconed to prevent water intrusion.
as a matter of fact I did have one weep very lightly in 2008 (maybe a cup of coffee a week)..I did not worry too much because it was going into the compartment that had the bilge pump..and I had it weep for 8 months..never got beyond that.
One day, I got tired of it, and just got the old stuff removed and new applied.
There is a seal around the keel inside the box too..
It's not a big issue
Alex
Giulietta:
Wrong!
It makes a big difference to me that you posted such an informative description of your keel attachment. Thanks. I like to gain as much information about this subject as I can.
I'm still learning everyday.
Bob, if you ever meet Alex, you'll understand how informed a man can be about a boat he has a hand in designing! Giulietta still stands as the boat with the best deck layout and line organization I've ever been on. It also helps that he's got diagram-making skills that remind me of the airplane model kit instructions I used as a kid.
Having the ability to sketch out your ideas is a huge plus, as I'm sure you realize every day...but since the advent of the computer, I see less good draftsmanship and little indication that it's being taught. My Polish welder, who is about 40, was able to sketch with a few lines several variations of the solar arch/bimini we were building, and I was able to alter them without too much damage. Frankly, this worked far less ambiguously than trying to use words...although he liked learning the English for the parts of a boat, even though he said "this is a crazy language with six names for everything".
Can't sleep? Read www.alchemy2009.blogspot.com for fast relief
Good, thank you. so what do you think of thios design? What would you do different or improve?
be honest..my feelings won't be hurt.
Alex
Giulietta:
I am warming up for writing an articile in GOOD OLD BOAT (GOB) on keels. I would sure like to use your diagrams in the article as an example of how this type of keel can be attached. Could you email me pdf's or jpegs please? Or can I just lift the images off the post here?
Ok, I saved one pic and the drawing in my GOB folder. Now may I please have your permission to use them? I will give credit to you.
Last edited by bob perry; 11-29-2009 at 12:35 PM.
Sure Bob, please do..by all means..I will more than happy to send you the originals if you want to. When I post these photos and images, its for everyone to see, for everyone to use..I just like to share..that's all
When that edition comes out, could I have a copy of the magazine? please? or two?
I will also gladly provide any more drwaing or photos you may require.
I will look for them tonight in my computer and send them to your email.
Alex
Giulietta:
You should contact GOB and see if you can get some copies. I get two and I can send you one if you can't get one where you are. I'll send you a magazine and you can send me a nice bottle of Rioja.
I think I will be fine with the one pic and the drawing you posted. I think my srticle will be better if I can use some images that I don't do myself. It will addd credibility to what I am going to try to say.
Bob, thanks..yes, please..send me one of them but make sure you autograph it..OK??
Now...I will not send you a bottle of Rioja, because that is Spanish wine, and everyone knows that the wine of my dear Portugal, is much much better..unless off course you insist in having that vinegar they sell there in my neighbouring country...
So instead of Rioja, how about some real wine? Red or white? From Portugal.
Asking me to send you Rioja, is like me asking you to send me a Mexican hat
So the photos you got are good, that is fine
Thanks
Alex, the Portuguese
Wait a minute, I think I have a Mexican hat! It's yours.
Sorry Giulietta. I thought you were Spanish but you kept your boat in Portugal.
My bad. Could you please tell me the exact model of tyour boat?
Bob, my boat is a custom built one off named DC 1200CrWait a minute, I think I have a Mexican hat! It's yours.
AI AI AI AI..ANDALE!!! ARRIBA!!! ARRIBA!!!!
Sorry Giulietta. I thought you were Spanish but you kept your boat in Portugal.
My bad. Could you please tell me the exact model of tyour boat?
made by Delmar Conde, in Portugal
Alex
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