Join Date: Feb 2000
Location: Annapolis, Md
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what keel for offshore
This issue of impact resistance and durability is a tough one. It comes up often and is a popular topic amoungst the steel boat crowd. Hitting that immovable object at speed is a very real possibility that is expeienced by a relatively few sailors but it does happen. If I had to design a boat to resist impacts it would be structured as I describe below. (I apologize, I wrote most of this for another purpose and so may have items that are not totally relevant to your question but are included because I really don''t have a lot of time these days.
There are a number of things that can be done to minimize damage on impact to a composite hull. Starting from the bigger scale and working down to the smallest scale, composites (like most structural systems) really benefit from a fairly closely spaced framing system that allows loads to be quickly distributed through the largest area of the hull. By transmitting the force to the largest area possible the unit loads come down very quickly. In a head on collision closely spaced longitudinal stringers serve to limit buckling while distributing the loads aft. The absence of longitudinals in the forward sections is one of the shortcomings found in most production boats. Longitudinals should be carried into the topsides for impacts with docks, and when heeled over but can typically be wider spaced as these loads are generally smaller in force and larger in contact area than a collision at full speed with something like a submerged container.
A fine bow is important in designing any boat to minimize impact damage. This minimizes the target and increases the likelihood of a glancing blow. (A fine bow also helps with motion comfort and windward performance but that is off the topic.)
In designing a composite boat for frontal impact, it is a good idea to include often a crush block at the stem. This is a glassed in box filled with a hard but compressible material. The shell of the box is heavily tied into the hull and the system of stringers so as to distribute the loads into the stringer system. It is water tight to contain any water that might breech the outer skin.
Athwartship framing and bulkheads are also important to distributing side the loads both from sailing (as well as rig, rudder and keel torsion loads) and from impact loads. They do this in two ways, they distribute loads directly into the hull and deck, and they distribute side loads into multiple longitudinal members so that side loads are distributed over as large an area as possible. One of the niceties of this grid is that it limits the damaged area in an impact to a comparatively small portion of the skin and limits the span of the material within that area. One of the short comings of this system is that it concentrates a lot of load into a small piece of the hull that must be able to withstand the impact without relying on the inherent energy absorption that is found in deformation of a larger area semi-ductile material as might be found in a steel boat. As a result these panel strengths needs to be actually stronger than the panel strength of steel or else find other ways of absorbing energy or some combination of the two. This is actually reasonably easily accomplished in a number of ways not generally found on production boats. One of the easiest ways to increase the impact resistance of a composite hull is to eliminate non-directional fibers (mat and chopped glass) from the laminate.
Conventional polyester resins tend to be pretty brittle. The crash and military helmet industry uses a surprisingly ductile vinylester resin and because it is used in such large industrial quantities it is surprisingly reasonably priced. Vinylester resin also offers very high resistance to passing water molecules pretty much on a par with epoxy and so acts as a very effective barrier coat and of course is immune to blister problems. The only problem with vinylester resins is that they do not form very good bonds with wood, certainly not as well as epoxy. If you are planning on using a wood core then you are better off using epoxy. If you are going with vinylester resin then you are better using a medium to high density closed cell foam coring. These can be purchased with a memory, meaning that they can withstand some crushing without losing strength and return to their uncrushed shape. This property allows the absorption of the impact forces in much the same way that a ductile material like metal absorbs energy by distorting. These materials were invented for automotive and aeronautical applications and are also reasonably inexpensive and easy to work with. They also offer very high sheer stress characteristics.
Lastly comes the resistance to point load impact and abrasion. This is where Kevlar really shines. That is the reason that Kevlar is used for bulletproof vests and for military helmets. It had the ability to absorb an extremely highly concentrated point load and absorb the energy without failing. Kevlar has tremendous abrasion and sheer resistance as well. (It quickly dulls steel and hardened steel instruments that are used to cut it) Combining Kevlar in the outer laminations, coupled with the ductility of the vinylester resin used above and the energy absorbing characteristics of the foam core, you have a hull material that can stand up to enormous abuse, pound for pound considerably more than any other boat building material. I posted the numbers the other day so I won''t go through that again.
Then there is the issue of keel attachment and resistance to impact. I strongly prefer a cast lead bolted on keel in terms of resistance to impact with minimal risk of sinking. I believe that the production boat manufacturers push the idea of encapsulated keels because they are cheaper to mass-produce and not because they are inherently better. Going to a bolt on keel allows one of the most vulnerable impact points, the leading edge of the keel to be moved away from the water tight envelope of the hull. A bolt on keel requires much more careful engineering than an encapsulated keel since the loads get concentrated in comparatively small areas. Here again, the goal is to quickly distribute the loads to as large an area as possible. To do so, large glassed in transverse frames that are tied into the main bulkhead and longitudinal system become highly critical to the load distribution. In an impact the aft end of the keel tries to drive upwards through the bottom and the forward end tries to pull down through the bottom. It is critical to have mechanical connections, wide flanges and multiple transverse frames at both ends of the keel, to tie these frames into over sided longitudinal stringers and to use solid glass in this area where sheer and tension stresses can be enormous in a collision. A keel sump allows deeper webs on the transverse frames right where the strength is needed.
This brings up what is both the weakness and the strength of composites. Unlike steel plating, you really need to vary the laminate and core throughout the hull depending on the kinds of loading expected in a particular part of the boat. By the same token, the good news about composites that they are pretty easy to tailor to specific the loads in that section of the boat. You are not stuck making a decision between an overweight plating that is sized to the worst spot on the boat. Nor using a lighter weight plating that is undersized at the most extremely loaded area of the boat, or have the stress concentration and fatigue issues at the point where a lighter weight plate meets a heavier weight plate.
There are also design subtleties that can also help in a really bad impact situation. No matter how a boat is built, and no matter what material it is built out of, there will be some situation where the hull can be breached. Being able to contain the breech can be critical to survival. This business of completely lining a hull is one of the biggest problems with newer boats in an impact situation because it prevents you from reaching the interior of the hull and making any kind of a temporary repair from the interior or even being able to see where the leak is located. In terms of structure, closely spaced framing can limit the spread of a small tear in the hull. Compartmentalizing the damage can help prevent flooding. One such compartmentalization system happened to come on my current boat. In the case of my current boat aft of the crush block, I have a centerline bulkhead that extends significantly above the waterline and which is roughly 20% of the length of the boat. This bulkhead is heavily bonded into the centerline of the keel at the bow and into the transverse frames and bulkheads. The transverse bulkheads are rigidly bonded to the centerline bulkhead and the longitudinal stringers, and are arranged so that they also extend significantly above the waterline as well. This creates a series of 6 small watertight compartments albeit open at the top. The volumes are such that at less than a 20 degree heel, if any two were flooded they would still be above the waterline. While this is not an ideal first defense, and it did not work on the Titanic, which had a similar scheme, it does give you half a chance to fight the leak without sinking. The arrangement of stringers and frames in my boat would allow you to screw a watertight panel, with bitumen sealer strips, into the tops of the stringers and frames without breaching the hull which potentially stops or slows the leak further.
Anyway, I hope that this is at least food for thought.