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  #1  
Old 03-30-2004
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what keel for offshore

First let me thank Jeff H and the rest of you who have shared so much knowledge with those of us who strive to become more familiar with the many intricacies of a sailboat’s design and purpose.

I have gathered from your many posts and answers that some of you, Jeff H included, favor the more modern hull/keel designs for coastal and offshore cruising due to their inherent advantages in the following areas.

1) Speed
2) Maneuverability
3) Sail plan congruency (less sail area required to go faster)
4) Light wind performance
5) Safety (ability to get out of harm’s way a bit quicker)

My question is, how much more vulnerable would the fin keel of a Dehler or Hanse quality type boat be to a submerged container, log or the like, compared to the full keels found on say, a Nor’sea, BCC, Nauticat or Cabo Rico?

Also, how would these 2 type of keel designs comparatively fare in the worst of storms, and in groundings, would it make much of a difference?


Thanks in advance.
Mike
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Old 03-31-2004
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what keel for offshore

I commend your attention detail, particularly contingency planning - ie: the worst-case-scenario (container collisions, storms & groundings).
I would, however, like to temper the discussion with the observation that: most cruising consists of “interminable stretches of utter boredom, punctuated with moments of shear terror.”
By which I suggest that you may be expected to spend 60% of your cruising time at anchor, 30% sailing in light air, 4% in heavy air, and perhaps <1% “in harm’s way” [OK, someone offer better proportions ].
Using Failure Mode Effects Analysis (FMEA) we multiply the likelihood of a failure occurring by the severity of the consequences, to arrive at a risk quotient.
Risk = Likelihood x Severity
Hence, even a fairly drastic consequence (of damage or failure), presents a small(er) risk, when multiplied by a very low likelihood.

I’d consider many other characteristics of any particular keel configuration (as you have already summarized), ahead of hitting a semi-submerged container.

Regards,
Gord
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Old 03-31-2004
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what keel for offshore

An intersting article:
IT’S HOW WELL IT’S DONE THAT REALLY COUNTS - By Jeremy R. Hood
He says, in part:
“...For many years books on the subject of cruising expounded the idea that a long keel with a keel hung rudder was the only suitable underwater shape for offshore sailing, yet if you head on down to Islas Mujares or Antigua or any other cruising destination, you will find relatively few cruising boats with full keels. It is clear that whatever has been written about the desirability of a keel running the full length of the vessel, few who set out and actually go places actually adhere to this. Probably the majority of cruising boats have modified fin keels and of these some have a skeg for the rudder, others do not...”
Full article at:
http://www.hshyachts.com/html/it_s_how_well_it_s_done_that_r.html

HTH
Gord
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Old 03-31-2004
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what keel for offshore

To some extent the choice of a keel comes down to your own specific goals, prejudices, and fears. The basic physics make it harder to engineer a fin keel to withstand a major impact. It is a simple product of the longer lever arm of the fin vs the smaller lever arm of the fin attachement. That is not to say that a fin keel cannot be made strong enough to withstand anything that you are likely to hit, just that it is harder to do. Since most fin keel boats are designed to meet the higher performance requirements of a coastal cruiser, most fin keels are not engineered around the severe loads that are implied in a really hard grounding or a collision with a shipping container. So while fin keels can be as strong and safe in an impact as their longer keeled sisters, they generally aren''t.

That said, few full keeled boats are either. Today, most full keeled fiberglass boats use encapsulated keels. While encapsulated keels are cheaper to build, they are generally much more vulnerable to a sinking in a hard grounding or collision with that immoveable objects. In most cases, the ballast becomes the immovable object when the rock collides with the boat, allowing the skin to be ruptured at the point of collision and the ballast to be driven up through the comparatively thin membrane that occurs over the top of the keel on most full keel boats. We had that happen to a Pearson that my family owned years ago.

When you ask,"how would these 2 types of keel designs comparatively fare in the worst of storms.....would it make much of a difference?" that question is too simplistic. Boats are a system. They survive heavy storms or not, they are comfortable in a blow or not, not only because of their keel types but because of a wide range of other factors that are more significant and to a great extent related to weight, windage, foil, and buoyancy distribution, and robustness. neither a full keel or a fin keel has an inherent advanatage in heavy going. Many of the notable attributes attributed to full keels come from factors that have less than zero relationship to whether the boat has a fin or a full keel.

In the end it comes down to the specifics of the particular design and its engineering. It also comes down to the boat''s proposed use which can greatly shift the priority for one particular trait over another.

Jeff
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Old 03-31-2004
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what keel for offshore

Jeff, you mentioned that fin keels
"Can be as strong and safe in an impact as their longer keeled sisters"
How would you go about this task?
Can they be made that way?
Are some allready made that way from the factory?



Are there reinforcement members that can be added to the axis of the hull proper and fin keel?

Is the likelihood of making contact with a submerged container so minimal that it should not require drastic mods?

I understand that there is no substitute for sound navigation, updated charts and the good old eye on the ball, but all other things being equal,
Would the correct implementation of forward looking depthfinders/sonars make said collisions less likely?


Thanks again for your thoughtful answers.
In my many years surfing the web I have not seen a more informed and willing to assist bunch of folks.

Mike
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Old 03-31-2004
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what keel for offshore

Mike

Q: “Are there reinforcement members that can be added ...”
A well designed keel attachment might include a bilge sump box (what’s it called Jeff?), which acts like a keyway spline for the keel-hull joint.

Q: “...Is the likelihood of making contact with a submerged container so minimal that it should not require drastic mods?”
Yes, unlikely enough that I wouldn’t let the possibility affect the boats basic design.


Q: “...Would the correct implementation of forward looking depthfinders/sonars make said collisions less likely?”
No.

OMHO
Gord
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Old 03-31-2004
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what keel for offshore

Mike:

"Jeff, you mentioned that fin keels ''Can be as strong and safe in an impact as their longer keeled sisters...'' Can they be made that way? Are some allready made that way from the factory?"
It isn''t practical or cost-effective to try and reengineer an existing keel/hull structure so as to make it stronger re: a grounding. The answer is to buy a boat that is as well engineered for the various potential threats as you can. We viewed a newish H-R 36 with a bolted on lead keel on England''s South Coast this past summer, after it had been rammed dead-on into a rock at hull speed. A large chunk of lead was taken out of the keel plus a number of smaller ''missing and/or rearranged'' pieces, yet the hull showed no other damage. When you reflect on the quality of the H-R yard and the nature of the rocky Scandinavian cruising waters, I guess this isn''t a surprise...but it was impressive.

To attempt the ''Cliff Notes'' answer to your ''keel for offshore'' question, you will likely end up with an extended fin and separate (partially or fully skeg-hung) rudder with encapsulated ballast. This is simply because most of the boats that will otherwise meet your needs and which were built in the 80''s and 90''s fit this description. If you don''t, you''ll instead have a bolted-on (hopefully, lead) keel. Yet predicting this little piece of the future doesn''t help much, as it''s the overall nature of the design and build you need to consider (a point Jeff''s already made) vs. picking out a few of the nits and concentrating on those.

I realize ''collision bulkheads'' are in vogue these days, and you can even modify some existing designs to attempt to create this feature. But in reality, there are only two ways in which I would have confidence when dealing with a collision at sea with e.g. a container (tho'' keep in mind it could be something else like a large, unlighted sea buoy that''s gone adrift, such as the one we sailed right past at hull speed enroute the Azores, easily big enough to damage WHOOSH): either well-engineered, built-in floation (as e.g. in a Sadler 34 or any Etap) - or a good liferaft and 406 Epirb.

One follow-up on what I would call the Keel Sump, the box section created by the hull sides, the extended fin''s trailing edge (aft) and the ballast segment (forward): this can be a source of structural weakness given how hulls are sometiomes laid up in halves and joined/glassed over (perhaps not robustly) at their centerline. Boats that are unintentionally grounded, and work & pound a bit on their keel bottoms, often will split the centerline joint in the area of this box-section keel sump. Bristol 40''s are one example who have this problem. My point is that this nice, deep keel sump, while an advantage in some respects, can be a weak link in other circumstances.

Forward-looking ''sonar'' (depthsounder) gear is getting praise from some cruisers but in circumstances where they are feeling their way into shallow waters (e.g. via a cut in a reef) amidst murky waters. This kind of gear won''t protect you at sea or, most likely, along a foggy shoreline either.

Jack
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Old 04-01-2004
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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.

Respectfully,
Jeff
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Old 04-01-2004
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what keel for offshore

It appears that I spoke too soon, replying that forward-looking SONAR was of no use in avoiding sem-submerged obstacles. The March issue of “Marine News” has an interesting article on Obstacle Ovoidance Sonar.

From “FarSounder” at:
http://www.farsounder.com/

The FS-3 is FarSounder''s flagship product. This 3D forward looking sonar has been designed as a surface ship navigation device capable of detecting whales, shipping containers, and other in-water obstacles. Unlike other FLS products, the FS-3 is capable of generating a 3-dimensional depth map over its entire 90 degree field-of-view with a single ping. This system is not only capable of telling you how far away targets are but also at what range (max. 1,000 Ft.) and how deep the target is located.

Regards,
Gord
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Old 04-01-2004
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what keel for offshore

wow.

Thanks for that, Jeff. Definately some food for thought.
Would it be possible to get the complete article of this?


Sasha
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