Re: Modern Hull forms and Motion Comfort
Thank you for the generous offer to join you for a sail. The boat with the strange interior plan was at the Annapolis Boat Show probably 5 years ago, and I think was at the show a couple years in a row.
I did want to circle back to the discussion regarding my comment on encapsulated keels. I apologize that some of this was cut and pasted from an article that I had written on the subject a few years ago, and also apologize for the length ( should put that in my signature line). To put my comments into perspective, I will honestly admit that my comments reflect my own personal viewpoint of boats and my own internal filters; a viewpoint that sees boats as a tool; an ideally very complex and sophisticated tool- one whose role in life is to do a wide range of specific tasks exceptionally well. My opinion on specific boats and their details tends to derive from my understanding of the technical, engineering and science based side of issues. As much as I can draw a springy sheer with the best of them, and admire yachts of all eras, I tend to analyze boats at a very technical level.
And in that vein, I see distance cruisers as the pick-up trucks of the sailing world; needing to be purpose oriented, practical, rugged, low maintenance, hauling a small number of people, and a proportionately large amount of cargo. As much as anything else, they need to take a serious beating and keep functioning safely. And that is the foundation on which my opinion of encapsulated keels is built.
I know that are a whole lot of folks who think that encapsulated keels are okay, and to some, encasulated keels may even be preferable, but when I look at the technical implications of how an encapsulated keel is actually constructed from a structural and practical standpoint, and having observed and tried to repair boats with encapsulated keels after they have been damaged, I personally see no reason except cost savings to use an encapsulated keel. I understand that to properly engineer a bolt on keel to withstand the kinds of heavy blows that a keel needs to take, requires a lot of care and a very robust internal structure, but because of the physical location of that structure, it is easier to construct this sturcture in a manner that is reliable over the life of the vessel.
And while an encapsulated keel can get by without the kind of robust internal structure required to make a bolt on keel reliable, if long term reliability and saftety is the goal, then encapsulated keels should have almost the same strength bilge membrane and the same transverse and longitudinal framing as a bolt on keel. But few boats with encapsulated ballast get that level of internal framing.
My other gripe with an encapsulated keel is the difficulty in doing a proper repair once an encasuilated keel gets damaged. Typically, in a hard grounding a number of things happen to a boat with an encapsulated keel. Typically the skin of the keel encapsulation gets ruptured and separates from the ballast. Sometimes this is a small slit, but often its a larger tear. This allows water into the small cavities between the keel and the ballast and once wet it can mean the ‘beginning of the end’ for the boat as this permanently wet fiberglass blisters itself from the interior and the wet areas spread around the ballast. This is especially a problem on a boat that is hauled out for cold winters where freeze/ thaw cycles can really pry the skin loose from the ballast.
Since the connection between the ballast and the envelope is portion of the keel structure, transmitting lads from one side of the keel to the other much like the web of an I beam, and keeping the ballast from shifting, the loss of connection between the ballast and the skin should be taken seriously.
In an earlier discussion of encapsulated vs bolt-on keels, the question was raised as to how frequently does the ballast actually delaminate from an encasulated keel. Out of curriousity, I walked around a boat yard tapping keels and taking notes on encapsulated keel boats. My recollection is that something approaching 50 % had separated areas in excess of a 1'-6" radius. Most of the delaminated areas were in the upper third of the keel which makes sense when you consider the forces acting on the bond between the ballast and skin. On some of the wost cases, you could literally flex the delaminated area with your hands or could see the bulge away from the ballast near the support blocks.
But getting to the unique aspects of this discussion, I completely disagree with the idea that a distance voyager should worry less about damaging the encapsulation envelope than coastal cruiser. When you go offshore, it is amazing what is floating out there; shipping containers have gotten a lot of press lately, but debris big enough to sink a boat goes far beyond that.
In my sailing career, I have sailed past whole trees, a dumpster, a billboard, a wrecked dinghy complete with outboard motor, what looked like a 2,000-3,000 gal., iron tank of the type that might get used by a gas station, a steel outrigger that we concluded came from a shrimp boat and so on. And the very nature of distance cruising is that you cannot keep the kind of 24/7 watch that would guarantee that you don't hit something just below the surface at speed.
Similarly, distance cruisers, almost by definition, are entering unfamiliar ports of call and so the likelihood of 'messing up big' becomes a greater risk.
So to me, while the odds of any one distance cruiser getting into trouble may be small, the risk of serious consequences is much bigger. And adding to the risk is slicing the encapsulation envelope well below the surface, driving the ballast up through the bilge membrane, and leaving an un-repairable leak while at sea. And on that basis, I don't think that given those risks, an encapsulated keel is the right technology.
I cannot speak for why any individual boat company would chose to use an encapsulated keel. In the case of the Cherubini 44's, to me these boats are a fashion statement and not about the sailing. Fashion statements do not need to be designed for offshore passage making. Consequently, the costs associated with building boats like the Cherubini seem to derive from high levels of fit and finish, vast quantities of material, and exclusivity, rather than from thoughtful structural engineering. The use of encapsulated keels on Shannons is a more complex question in my mind. Since their mission seems more focused on serious cruising, I have no idea why they would elect to use encapsulated keels.
With regards to the description of how the bulbs are added to your Outbound, that gets to my point, that there is not a great way to add bulbs to an encapsulated keel. The solution that you describe is very complex and yet does not succeed in creating as reliable a solution as a bolted keel, especially on a boat which has a generous keel root like the Outbound.
The problems start with the way that an encapsulation envelope is constructed. It is very difficult to lay glass with any precision at the very bottom of cavity the depth of a keel. When I have repaired or observed the bottom of damaged encapsulation envelopes, the quality of the glass work at the bottom of the keel has consistently been dismal. Typically, you see a mix of resin rich cloth with lenses of unreinforced resin, and conversely, dry glass. And that is the surface supporting the ballast and coming between the unstoppable ballast and the immovable submerged object.
Unlike a bolt on keel, the ballast is then just ‘glued’ in, and usually that is done with polyester slurry or worse. This is a brittle material with dubious sheer and tensile strength. On a concentrated impact, this ballast often shifts sheering away from the walls of encapsulation and rotating up against the bilge membrane above. Few boats with an encapsulated keel, have a bilge membrane designed for that kind of load.
The failure of the membrane at least on the one boat that I looked at, occurred in part because of the nature of the construction of the membrane which is added after the keel has been set, and so typically has a secondary bond between the membrane and the hull. The membrane in that case was actually surprisingly thick, but it peeled away from the hull on the line of the secondary bond. I suppose that this could be improved if the membrane were laid up in epoxy with its much greater secondary bond strength, but that is almost never the case.
Moving onto the bulb, while through-bolting the bulbs to the keel makes sense from the bulb side, the question in my mind is what is actually supporting the bolts. By that I mean, are the bolts supported by the encapsulation skin? Or are they supported by the lead ballast that is glued into the encapsulation skin.
If supported by the skin, then that places a concentrated load on a very small bearing surface of a material that does not do great with those kinds of loads. A direct impact to the bulb would try to sheer those bolts through the skin creating a point of entry for water. If the bolt loads are placed on the ballast, then it places a load on the slurry adhesive, not a great way to go long term. If the “glassed over” skin is adding to the support strategy, you are counting on the secondary bond peel strength of the polyester-glass matrix where the load is concentrated at the point that the outside glass skin joins the hull.
In the end, what you describe is probably about as good as can be done to add a bulb to an encapsulated keel, and yet, it is a cobled together solution that would have been far easier design as a properly engineered, bolted in place, keel in the first place and in my opinion, end up with a more reliable and safer keel design over the life of the boat.
To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts.
Curmudgeon at Large- and rhinestone in the rough, sailing my Farr 11.6 on the Chesapeake Bay
Last edited by Jeff_H; 04-12-2013 at 10:20 AM.