Azimut 68-S; Hull Laminate Problems...

I am amazed at some of the misconceptions being put forward here...

There's nothing wrong with a cored hull, even below the water line, and in fact a properly done cored hull will have more strength and far better proprties in terms of strength, and damage tolerance than an equivalent weight solid hull. Remember that in any surface that is stressed in bending, the high stress is only in the outer fibers, the middle of the surface (or beam) is only stressed in shear. So long as the core is sufficiently strong in shear and the outer skin is strong enough to withstand local impacts a cored hull is far superior than a solid gfrp hull.

Problems come in when the outer skin is not thick enough to prevent the core from being damaged by deflections of the outer skin. Look at an old surfboard and see the delaminatons from getting beat around and you will see what happens to a brittle core with a thin skin. Some builders are saving a lot by thinning out the skins, and such a hull will be stiff, but won't have the damage tolerance of a hull done right. The core material makes a huge difference too, with PVC materials like Airex being much stronger and more damage resistant than some of the cheaper materials.

Bottom line is know your builder and if he uses a cored design, find out how thick the skins are and what the core material is and you will be fine.

I know one thing. I have seen a lot of pictures of cored production boats with major issues in the last year or two. I've never seen a Cabo with a hull issue (or other solid glass hull) that I can remember in a long long time.

Very well said Pascal. I'd like to add that even though you get more speed with a cored hull, you can usually rarely run at that speed. I delivered a new 52' Cabo with a cored hull that cruised at 35 knots, I rarely and I mean rarely could run at that speed. Most of the time I had to back it down to somewhere in the mid 20's knots speed and this was in pretty moderate seas. With the cored hull, you heard every wave through the boat and the lighter weight made it so you'd have to back it down a lot earlier due to it not riding as smoothly. On the other hand I've run the 45' Cabo extensively (solid glass hull), and you can almost always run at it's 31knot cruising speed.

I agree. The " bleach Bottle" boats are attractive because they are often built from the inside out.
Pack every concievable accutrament into the boat, build the hull around it and then add a 6 foot aft deck and call it a " Yachtfish".
When I see these type boats I just figure the owner has more money than sense.

I am not sure if this thread is still about this Azimut or cored boats in general, but may I add that the first cored hull I was involved in building was also the largest GRP yacht of that time, a 130 foot 3-masted schooner.

The technique was developed in Sweden for naval ships and when we tested it for strength and impact resistance at DNV, they said they had never measured a stronger hull.

This was 30 years ago and the yacht is still going strong: http://southerncloud.com.au/sydney

In all this debate it seems that some people overlook the point that monolithic layups can disguise the true extent of damage from impacts. Many of us will have seen 'proper' repairs being undertaken. Commonly, once ground/skimmed you see the inner layers of of solid FRP boats heavily fractured and water logged extending for considerable distance even if the outer skin seemed reasonably unaffected by impact damage. Remember the osmosis years. There is no real strength remaining. On the otherhand most times you see a quick surface grind, a bit of filler and repaint being the only attempt at repair with the encouraging 'good as new' assurance from the repairer. When the professional experts say that the damage may catastrophic but not obvious from normal impedence testing, their views should be noted in my humble view. With the best naval architects and consultant FRP specialists saying that properly engineered composite lay-ups may be superior by comparison maybe us old boys need to reflect on as to whether what looks OK may be weak as. An old tart that has had cosmetic surgery is still a well worn woman underneath!

Pascal, Thank you, I think you just made my earlier point exactly....

I looked up sandwich construction in Gerr's book, "The elements of boat strength" and he recommends a thickness of skins that totals 70% of the total weight of a similar conventional hull. If you see a similar size cored boat that weighs half that of a conventional version you have to figure that maybe you could be buying into problems....

Gerr also shows a sample calculation for a 40' 25 knot hull which has a planing surface outer skin thickness of 0.190 inches, which is a lot thicker than what we see in this Azimut, even more so when one considers that the Azimut is faster and bigger, so to my mind it does indeed look a bit thin skinned.

OTOH, we don't have any idea of how this boat was operated to create this damage. It may just be falling apart due to "normal usage", or some fool (or maybe even this owner) may have pounded it to pieces for all we know. No matter how strong a boat is, if somebody wants to smash it's guts out he can do it without a lot of effort by bending the throttles forward in too big a sea....

Right now this is only one data point.....

that is a good post..

I'd say 20% to 30% of all cored boats over 35' will be living 10 years from now.. You know at least the ones left in dry storage will be around...

These are 73 bertram hull plugs cut after the boat was in service 33 years, at the keel she is 1 1/4" at the transom she is 1 1/2" and at the sink drain she is 1" this is the thinnest part of the bottom of the hull, the hull sides are 1/2" below water and no thinner than 1/4 any where. I fully peeled the hull and know it will not blister after the first layer of mat, I did put carbon and epoxy on the bottom but there was no need for more then just an epoxy coat but un-till I put in 10 larger thro hulls and 10 under water lights, I had know idea how thick she rely was......

I do not care how well it is built. The ocean will find a way in to a core hull at some point.

I think we'll never see the return to solid glass hulls it will bring the price of a new boat up 50% maybe 100% it takes a lot of glass and resin to make up a panel 1 inch 4x8 and a core is $40 retail in south fla... It takes to gallons of resin per side per layer of glass at $12 dollars a gal for the brown boat yard not green resin.... so you looking at 100y of 25oz an 30 gallons of resin or more to skip the core... and it takes 10 times the labor to keep laying in glass and resin in steed of a 1 inch core.... so were stuck with beer cooler boats, but at least they will keep you beer cold

I personally think there is a fair bit of nonsense spoken of in this debate along with all the pertinent points fairly made. Whether you have a monolithic layup or a sandwich type of composite it will not be the hull cost that dictates which. The actual cost of the hull build itself may be as little as 10-15% of the final bill of a fully fitted vessel. Nobody can tell me that a decent cored hull is cheaper to build when using production molding techniques, in fact, it may cost a lot more if it has any sophistication. In any case I believe you would need as much expertise to design a quality monolithic layup as you do a cored one. The weight saving however provides the significant advantages (and reduction of stresses) associated with smaller engines and tanks and weight saving and/or more easily offers the performance outcomes that some desire in their planing boats. The longevity of any boat is about its design and build quality according to purpose (togther with maintenance) whatever material is used.

I agree with Lars when he mentioned that many older solid FRP hulls are very powdery and not strong at all.

I have also seen the need for a builder to contract expertise (with the owners approval and acceptance of the extra expense) to substantially redesign the layup for a large and expensive motoryacht (mega millions) even though the original design was completed by one of the most respected naval architects. This was a monolithic layup hull but it seems that insufficient knowledge and expertise in stress analyisis etc. by the naval architect demonstrated fundamental deficiencies in critical areas that could have caused risk of failure at some future point of time or event. Other non-critical areas were exceptionally 'over-designed' - to use a phrase. The outcomes initially were two plan cabinets full of the details of the new layup and media to be used, a specification bible for the build methods to be employed and the result was a hull being built that weighed several tons less but was infinitely stronger and significantly more impact resistant.

Imitationcell C

Look like an inappropriately low density Imitationcell foam for the vessal size. What did the surveyor say the core density tested at?

Azimut use Divinycell, and I think this post was a bit of a scam.
Unless I see a full picture depicting the bottom with the topside of the boat. Hell you can also manage that if u photoshop a bit