What is the difference between a coastal crusier and a bay b - Page 2 - SailNet Community

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post #11 of 15 Old 09-08-2002
JeffH
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What is the difference between a coastal crusier and a bay b

I agree that we can throw around anecdotal info all day long and not prove anything. BUT there actually has been a tremendous amount of work done on composites and the loadings imparted into various parts of a boats hull. This research has been performed for a wide range of reasons from military, to EU and Insurance standards, to various racing campaigns. With the ability to instrument full size boats and measure real loads there is an enormous amount of data becoming available. I disagree that" Only an extensive statistical analysis, or full-scale, carefully controlled tests will conclusively prove which underbody type is inherently safer, and then only for the exact configuration actually tested." As research is providing more precise information on strengths of materials and loadings felt by individual components of a vessel, it is not all that hard to actually calculate the loads imparted into a keel. Byond that there is a large sampling of boats out there of both types of construction. In talking to a fairly large number yacht designers and surveyors who see alot of both kinds of keels after the extreme impacts I''ve concluded this about encapslated vs external ballast:
What seems obvious is that properly engineered and constructed either encapsolated ballast or external ballast can work adequately in most grounding situations. Both can in fact be designed to withstand very extreme groundings. The problem is that few production boats are engineered as well as they should be if one of the prime design criteria is to withstand an extreme grounding and survive. Usually the prime design criteria includes a lot of issues such as budget, performance, ease of construction and so extreme grounding resistance becomes a bit more secondary.

To touch on some of the other points, for an equal keel shape and equal ballast, an external ballast keel should have a deeper more voluminous bilge than an encapsulated keel because the ballast occupies the volume of the encapsulation. The nice thing about externally ballasted boats is that they can usually be repaired to their original condition. The same cannot be said for encapsulated ballast boats.

In my experience I have not encountered an externally ballasted keel that can''t withstand substantially more than a 4 knot grounding. I have been aboard quite a few boats with externally ballasted keels that have run aground at pretty high speeds (in quite a few cases approaching 6 to 7 knots)and in each case the boats have come through with little more than superficial damage to the ballast (mushroomed lead in the case of rocks). That has not been the case in encapsolted keel boats.

In the most extreme case that I know of an externally ballasted Contessa 33 was dropped from a crane while being offloaded as deck cargo from a ship. While there was clearly damage to the internal framing, it was a pretty straight forward repair that is still intact and invisible after 15 years pretty hard years of useage. But of course that''s just another set of anecdotes.

Jeff
 
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post #12 of 15 Old 09-08-2002
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What is the difference between a coastal crusier and a bay b

NautTwilight,if your statement is entirely correct,we all might as well ''pack up & go home''.
im learning a lot from this messege board,the info ive gathered will greatly help me to weigh up the options,then decide for myself & make my own decisions .(ie on a boat purchase).

it sounds a little like your suggesting no-one say anything,or offer any observations or opinions,in fear of being sued,
your not a lawyer are you? =0)
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post #13 of 15 Old 09-09-2002
JeffH
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What is the difference between a coastal crusier and a bay b

A couple more points, while you are right "If you want the best hull for a grounding, it won''t be wood or fiberglass." but on a pound for pound basis, it would be a composite. On a pound for pound basis a kevlar sheathed, cold molded wood core would be right up there as one of the best materials for impact and abrasion (which is why kevlar is being used for bullet proofing).

To me, two of the most interesting sets of testing done to date have been done in Europe. The first dealt with fairly large steel hulled boat that was sunk a few years back due to wave action. The boat was recovered and the bow sections on one side were found to have been crushed inward by the impact. The boat was donated for research purposes and an experiment was performed in which the other side of the bow was mounted so that a controlled water pressure could be exerted and recorded. The pressure was increased until the bow section crumpled to the same shape as the side wrecked by the sea. Until that time it was pretty much assumed that the greatest load that a boat might encounter would be the concentrated loads of a small area impact with a fixed object at speed (i.e. shipping container or a rock). These are pretty easy loads to calculate and design for. The experiement with the crushed bow, completely altered the way the structural design of bow sections by showing that simply de to wave action there can be are extremely high loads that can be imparted over a large area and that merely designing for a point impact is not enough.

The second set of experiments deal with instrumenting full sized vessels. Probably the most interesting uses a specially constructed boat in which the hull and rig have a certain amount of flexibility built in. Within the hull is a very rigid but comparatively light weight frame that is tied to the hull and rig with are a series of precise instruments. As the boat is being sailed detailed measurements of loads on and movement of the hull are being read and recorded along with precise measurements of wind and boat speeds as well as the wind and water pressure felt at a buch of locations. This data is being compared with computer generated finite element calculations to improve the ability to mathmatically predict the actual loads that a boat will experience in a wide range of conditions. While these kinds of measurements are beyond the rhelm of most small boat builders, the kind of data that is coming out of this type of work has allowed designers to purchase comparatively simple structural design software that provides more reasonable empirical design imput.

Jeff
 
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post #14 of 15 Old 09-09-2002
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What is the difference between a coastal crusier and a bay b

JeffH,
Thanks for reinforcing my point that there is probably no conclusive experemental nor empirical proof of either type keel being superior.
bullseye,
Thank you for illustrating my point about jumping to unsupported conclusions, though doubtless you didn''t intend to.
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post #15 of 15 Old 09-10-2002 Thread Starter
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What is the difference between a coastal crusier and a bay b

There''s proof.All you have to do is go hang around boat yards.You''ll see it.

Dennis
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