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Old 16-11-2006, 19:32   #16
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Originally Posted by 44'cruisingcat
This boat didn't even hit anything, and it sank quite quickly. It was very fortunate that another boat was nearby, or loss of life could very easily have occurred.
60 minutes? You call that quickly. A lot of shoring can be done in 60 minutes just not with a rudder sticking thru the hole. Besides that was a rudder/hull failure not a hole from hitting something. Good reason not to buy an f-boat....................._/)
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Old 16-11-2006, 19:56   #17
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I wrote this for an earlier discussion of structural design for offshore use:

"No matter what you do you just about can't put enough conventional fiberglass in the hull of a boat to prevent it from being pierced in a collision with a floating container or other heavy, solid, small contact area object. For that matter, if you are going to end up with a reasonable weight boat, you typically don't end up with enough steel or ferrocement either. For a serious 'go anywhere cruiser', the key to building a safe go anywhere structure consists of a variety of factors.

Small panel areas, by that the boat should have a series of longitudinal frames and athwartship frames. Forward of the main bulkhead these should be quite closely spaced. [I will use my boat as an example (which was after all built for offshore work despite her light weight) the biggest unsupported hull sections below the waterline forward of the main bulkhead are about 4" by about 16" in area.] There should a ‘crush block’ at the stem at the waterline. [On my boat the crush block extends 6" above the waterline and extends back 16" to the first transverse frame.]

The area forward of the main bulkhead should be compartmentalized with watertight bulkheads that extend vertically above the waterline that would result if the boat had at least two of the compartments flooded. [On my boat these bulkheads appear to extend over a foot above the flooded waterline.] Ideally the tops of the longitudinal frames and the athwartship frames are on the same plane so that you can screw plywood into the tops of the frames to slow or stop the flooding. Ideally one of these bulkheads are on the centerline of the boat because should the boat ride up on something the sharp ridge at the centerline of the vee’d sections at the forward end of an offshore boat would really have to stand up to a lot of abuse. That whole bulkhead system should be heavily glassed into place. [Except for heavy glass work, that pretty much describes the construction of my boat.)

In the area of the keel there should be massive and closely athwartship ‘floor frames’ (this applies on fin keel or full keel, encapsulated or bolted on). [On my boat the ‘floor frames’ are over 8” deep and 4” wide and taper out to 4” deep above the waterline terminating at the waterline longitudinal except on the areas near the two main bulkheads where they extend to the rail.] On a boat with an encapsulated keel, the membrane across the top of the ballast needs to be as heavy as it would be on a boat with a bolt-on keel.

There should be no liners blocking access to the skin of the boat (at least forward of the main bulkhead and on the leading edges of the keel) up to the height of the flooded waterline mentioned above. All decks and flats in this area should be quickly removable so that access to make repairs can occur. [Here my boat gets a ‘B’ I can get to everything under the berths and forepeak quite quickly but the deck of the forward cabin is not removable. That is something I plan to change if I ever take the old girl offshore.]

Seacocks should not rely on backing blocks. Instead the hull should be built up to a thickness that locally reinforces the area under the seacock and distributes this localized stiffness out into the hull.

Once you have done all of that, coring or non-coring becomes less important. But coring tends to produce a better offshore hull. To quote from the Shannon website,” The most important feature of Shannon's fiberglass work is the use of composite core construction techniques. Composite core construction uses a layer of structural foam sandwiched between two thicknesses of laminated fiberglass. A composite hull can be both lighter and stronger than a conventional hull made with only solid fiberglass laminates. Cored hulls can remove unwanted weight above the waterline and have tremendous impact strength to absorb a blow from a piling, another boat or in a grounding. Solid laminate hulls are heavier in the topsides and when hit, tend to fracture and fail along the filament lines of the laminate. Shannon hulls use 1/2" to 1" semi-rigid PVC closed-cell linear foam material. Linear foams do not shear internally under impact, as has been found in the less expensive cross-linked PVC foams. Extensive testing has proven that foam core materials have better memory than balsa wood cores, enabling them to spring back into shape after a concussion. Unlike balsa wood, foam cores not allow water migration and rot if water penetrates into the core material from a skin fracture.”

In a composite boat, I believe that a couple layers of Kevlar, ideally in a vinylester or epoxy resin, and located in the outer plies, is critical to achieve the kind of abrasion resistance exemplified by steel but at a tiny fraction of the weight.

Then there is the main bulkhead. I don't care how a boat is constructed, at the mast and shroud area there needs to be either a massive ring frame or bulkhead to address the kind of loads that come from the rigging and keel loads. Without some kind of athwardships rigidity the boat will flex in a way that will ultimately weaken it through fatigue.

Then there is the rudder area. Again, I don't care what kind of rudder you have, the area around the rudder post should have sturdy knees or bulkheads extending transversely and fore and aft. In my opinion, the rudder tube should extend well above the waterline and should have support at or near the deck level. In my opinion the rudder tube (and perhaps the shaft log), should be in thier own watertight compartments or at least a compartment that is tight against the hull but extends above a partially flooded waterline. (This may require two shaft seals for the prop shaft or a flooded engine compartment neither of which is too easy to achieve.)

There should be substanial knees or bulkheads at shrouds attachment points and these should be tied into substantial longitudinal framing. Hull to deck joints are another area of concern. I am a big believer in a belt and suspenders approach. I personally like a large inward facing hull flange with frequently spaced, comparatively small diameter bolts, that is backed up by a slightly resiliant adhessive caulk. The bolting should pass through an uncored section of the deck. At that point, glassing the interior of the joint becomes extraneous and makes future repairs much harder to perform.


This should be a good starting point for a discussion. "

Respectfully submitted,
Jeff
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Old 16-11-2006, 20:14   #18
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So I am starting to see a pattern here. Do you think we all should ditch the Boat hulls and take up sailing shipping containers ;-) :-)
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Old 16-11-2006, 20:22   #19
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Good point. But then the question becomes; is it safer to cruise in one shipping container or to fasten 2 together as a catacontainer?
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Old 16-11-2006, 21:05   #20
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On the other hand, monohull sailors wishing to refute a multihullers argument should simply purchase an Etap. Their 46 is gorgeous and will sail with all the seacocks open and the hull flooded. They've achieved this through a double hulled construction (foam filled) and bouyancy compartments (likewise foam filled). The boat is insulated, designed by Bertone and sails well! And, on the inside you're none the wiser - it looks like any other 45 footer.
I looked at an Etap at the boat show a few years ago. I think the extra floatation is a great idea, but the boat I looked at had a much smaller interior than any other boat with the same external dimensions. The analogy I used at the time was that it looked like a 40 foot hull on the outside and a 35 foot hull on the inside. I bet it stays warm in the winter, though.

If the 46 looks just like any other 45 footer, they must have found more places to put the foam that don't take up space that could be usable for other purposes. I know my boat has many cubic feet of odd-shaped spaces that I can't use for anything.

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There is one approach to make a boat unsinkable as an after market product called YachtSaver which are inflatable bags which are kept inside the cabin and provide flotation. They would make movement within the cabin pertty difficult, but it beats sinking!
I seem to recall that the company that made Yachtsaver disappeared abruptly a few years ago. As far as I know, the only public statement was that it was a bankruptcy. At the time, I heard rumors that there was some problem with the product, but I never saw any confirmation or any details on what the problem might be.
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Old 16-11-2006, 21:56   #21
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Glad to see a positive turn here. Very good question IF in questionable context. Always carry plywood, splash zone and lots of spare line. If the hole is accessable, stuff anything in it that you can grab IF you can safely do so without getting trapped inside. Chances are, with a hole this big, unless it happens at the back of the settee where you happen to be sitting, you are sunk. The fact is, any catastrophic hull failure of this magnetude will sink a cruising boat (mono or multi) so fast your priority will need to be escape, not salvage. Take a look at the "windwalker" website if you have doubts. Realistically, it probably will not happen, but if it does... If you want a concept of how much water will come in that 10" hole, take the hose off your 1 and 1/2" through hull for 30 seconds. Consider that the volume of water that will enter a 10 inch hole is approx 80 times as much as that through hull.
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Old 16-11-2006, 22:35   #22
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go the catacontainer, you should have posted this in my thread though
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Old 16-11-2006, 22:44   #23
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Originally Posted by delmarrey
60 minutes? You call that quickly. A lot of shoring can be done in 60 minutes just not with a rudder sticking thru the hole. Besides that was a rudder/hull failure not a hole from hitting something. Good reason not to buy an f-boat....................._/)

That's right, the boat never even hit anything, but it's still down there with the fishes. It took about an hour from when the rudder post broke, but it was some time after that that they realised they were sinking - they didn't know until the water level was above the floors. They could have been in serious trouble with a faulty life raft, if the other boat hadn't been nearby.
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Old 17-11-2006, 00:37   #24
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Originally Posted by 44'cruisingcat
That's right, the boat never even hit anything, but it's still down there with the fishes. It took about an hour from when the rudder post broke, but it was some time after that that they realised they were sinking - they didn't know until the water level was above the floors. They could have been in serious trouble with a faulty life raft, if the other boat hadn't been nearby.
I didn't go to the link because it was identified as an identity theft site,but,If you were sailing along happy and content and the rudder post broke wouldn't you notice there was a problem fairly quickly?Mudnut.
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Old 17-11-2006, 02:04   #25
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Isn't there a discription for sailors that know little about their own boat and emergency procedures?
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Old 17-11-2006, 12:45   #26
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question about Kevlar

Jeff,

As usual a thorough and well-considered precis. With regard to Kevlar reinforcing - I seem to recall the Gozzard that went down near Bermuda a while back, after presumably hitting a container, was reinforced with Kevlar, but IIRC the Kevlar was on the opposite side to what was recommended by the builder - ie. it was inside rather than outside, or vice versa. There was also the catamaran Emerald Jane that disintegrated rapidly on a reef in the S Pacific, that also was reinforced with Kevlar. My concern with Kevlar might be that it is too strong to be used within the composite. I don't know how 'stretchy' it is, but if it resists stretching then an impact in a small area would tend to pull the Kevlar fibres towards the impact point along the plane of the laminate. That would probably cause more damage, or at least delamination. Can you comment on whether the Kevlar should be laid up on the inner or outer surface of the laminate? If only there for abrasion resistance, then perhaps it would be better to layer the Kevlar layer on the outside of the already-cured hull, with nothing more than a secondary bond. Your comments please.

Kevin
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Old 17-11-2006, 15:34   #27
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I don't recall hearing that the Gozzard used Kevlar but she may have. For a while you'd hear all kinds of claims of using Kevlar for reinforcing it is hard to say what these mean specifically.

To answwr your question As I understand it there are two ways that it seems to be use best for impact. From what I gather, if used on the exterior of the hull, it quickly distributes the impact loads over a larger area than a more stretchy laminate. It also helps limits the spread of cracks beyond the immediate impact area. Kevlar is hard to pierce and has very high abrasion resistance especially when used with epoxy or vinylester resin, as is the case in military and higher quality motorcycle helmets.

Kevlar used on the interior of the laminate further increases the distribution of the impact to a larger area and also further helps limits the spread of cracks beyond the immediate impact area.

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Old 17-11-2006, 15:35   #28
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is kevlar itchy to sand like fibreglass??
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Old 17-11-2006, 15:48   #29
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Originally Posted by mudnut
I didn't go to the link because it was identified as an identity theft site,but,If you were sailing along happy and content and the rudder post broke wouldn't you notice there was a problem fairly quickly?Mudnut.
Yes they noticed they had no steering. They didn't immediately realise they were sinking.
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Old 17-11-2006, 19:36   #30
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. Kevlar is hard to pierce and has very high abrasion resistance especially when used with epoxy or vinylester resin, as is the case in military and higher quality motorcycle helmets.

Jeff
Unfortunately I'd disagree.
Some of the cats i've worked on have had foam / kevlar hulls and it's easy to punch holes in with a screwdriver or similar.

Infact in Gavin Le'suierr's excellant book "Multihull Seamanship Illustrated" he recomends having an icepick or tomahawk in a accessable bag outside your multi in case of capsize, so as to chop a hole in the hull to allow air in for trapped crew or to get in side for protection from the elements or to get supplies etc.

Dave
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