Implications of Bonded Structural Grids in Production Boats

[Dockhead]

Originally Posted by marshmat

Absolutely correct. The "skegs are stronger" argument comes from a fundamental misunderstanding of structural mechanics.
In fact, it is *much* easier to design and analyze the structure of a spade rudder: it is just a cylindrical member, with two points of support and a distributed load. It's an easy first-year undergrad engineering calculation to find all the stresses, the reaction forces on the bearings, etc.
A skeg-hung rudder, by contrast, is a system of coupled bending loads between two members of different materials and different section shapes. That's far more difficult to analyze, and if you try computing a sensitivity analysis on it, you find that the results are wildly different if you assume tiny changes in manufacturing method or material quality. If anyone says "skegs are inherently stronger", then I can be about 95% sure that said person actually knows nothing at all about the structures or the math involved.

Indeed. Not only hard to analyze, but complicated in behavior. Skeg rudders have a nasty tendency to jam after light groundings.

There are a lot of interesting comments about skeg rudders vs. spade rudders by Bob Perry, one of the world' most if not the most prolific yacht designers. Perry, who designed hundreds of boats with skeg rudders, says that skegs are very difficult to make strong at all, and generally don't add any practical strength. I believe he even said somewhere that it's generally the rudder holding up the skeg and not vice versa.


The Kraken people publish a design review of their boat by Bob Perry. The designer of the Kraken is a mate of Perry's. Perry mentions the skeg rudder disapprovingly, and even cites the boat's actual designer as having been against a skeg rudder, but the client insisted. I think that makes a pretty good case that this design feature is intended to sooth irrational prejudices of some yacht buyers, rather than improve the boat in any way.


I admit that I grew up with this "skegs are inherently stronger" prejudice myself, until a friendly engineer was patient enough to explain the reality of it to me.

Originally Posted by marshmat

. . I've even seen a few claim that because a boat is CE-marked and RCD compliant, that the builder obviously knows what they're doing, and wouldn't ever make dangerous mistakes. Then you get something like an Oyster 825 just spontaneously disintegrating.
Paperwork exercises like RCD are no substitute for having actual, qualified engineers perform (and independently review) the design calculations and the manufacturing drawings, and put an actual QC process in place to ensure that mistakes get caught and corrected.
And I think it's been obvious for at least a decade, now, that bonded-in structural liner grids present a lot of challenges for both in-factory QC and for later inspection. The part that frequently gets damaged is just too well hidden, and the failure modes are often what NASA calls "criticality 1" – i.e. the potential for rapid, total loss of crew and vehicle, with no redundancy and no easy way to get early warning of imminent failure.

That sounds like a very solid analysis. Wouldn't an ISO 9001 system provide a solid framework for overcoming these issues? I'm surprised that this doesn't seem to be used anywhere in the yacht building industry.

Note that the horrifying Oyster 825 case resulted in bankruptcy of the company. Bet the new reconstituted Oyster company doesn't do that again.

[boatpoker]

Originally Posted by Dockhead

Indeed. Not only hard to analyze, but complicated in behavior. Skeg rudders have a nasty tendency to jam after light groundings.

There are a lot of interesting comments about skeg rudders vs. spade rudders by Bob Perry, one of the world' most if not the most prolific yacht designers. Perry, who designed hundreds of boats with skeg rudders, says that skegs are very difficult to make strong at all, and generally don't add any practical strength. I believe he even said somewhere that it's generally the rudder holding up the skeg and not vice versa.


The Kraken people publish a design review of their boat by Bob Perry. The designer of the Kraken is a mate of Perry's. Perry mentions the skeg rudder disapprovingly, and even cites the boat's actual designer as having been against a skeg rudder, but the client insisted. I think that makes a pretty good case that this design feature is intended to sooth irrational prejudices of some yacht buyers, rather than improve the boat in any way.


I admit that I grew up with this "skegs are inherently stronger" prejudice myself, until a friendly engineer was patient enough to explain the reality of it to me.



That sounds like a very solid analysis. Wouldn't an ISO 9001 system provide a solid framework for overcoming these issues? I'm surprised that this doesn't seem to be used anywhere in the yacht building industry.

Note that the horrifying Oyster 825 case resulted in bankruptcy of the company. Bet the new reconstituted Oyster company doesn't do that again.

Finite element analysis is time consuming and costly. Even the biggest production boat builders are small potatoes compared to the auto industry and simply cannot afford much of this type of process.

[TeddyDiver]

Originally Posted by danstanford

FWIW, I am certainly not a marine architect nor do I wish to attempt to defend any method of boat construction.
In both this case and Expedition Evans, the boats had hard groundings followed by sailing away and then getting repaired. Is this enough to defend this type of construction? As I understand it, the grid functions as a kind of fuse allowing the boat to flex rather than break.
I haven't hit anything hard yet although I have touched bottom in a shallow channel a few times but just imagine the loads and stresses 10 tons of boat coming to a stop when a lever extending out of the boat stops. Would a more conventional construction just spread the loads and cope with it?

This one on the video wasn't even bad to repair but some boats are build with full liner consisting the grid and about all of the interior as one piece glued together with the hull. Good luck repairing that, professionally done a lot more than a buying a new boat..

[kenbo]

This post has been very thought-provoking and I would like to pose a question to the group. Since it appears that industrially made grids installed in hulls are economically attractive to production builders is there a better way to attach these two parts than the use of methacrylate adhesive along a rather narrow flange? Would widening the flange help? Would a thickened epoxy slurry sufficiently improve the bond? Is the time and $ consuming method of lamination the only answer?

Seems to me that the builder who can economically solve this problem would be able to significantly differentiate itself in the marketplace and still be able to produce the interiors that are selling these boats.

[marshmat]

Originally Posted by Dockhead

Wouldn't an ISO 9001 system provide a solid framework for overcoming these issues? I'm surprised that this doesn't seem to be used anywhere in the yacht building industry.

ISO 9001 is about demonstrating to your customers that you have a systematic, consistent quality management process in place. It can transform an imperfect QA/QC program, one that raises the eyebrows of your large customers, into one where documented compliance with the process is clearly codified.

It cannot, however, catch or overcome inherent latent design flaws in the product, particularly if those flaws aren't explicitly prohibited by the design codes to which the product is built. Nor can it change a company culture that values speed and cost efficiency over quality. Nor can it teach people (engineers, assemblers, managers, or otherwise) how to decide what is or isn't an acceptable design, and what is or isn't an acceptable quality of execution on that design.

And one thing it absolutely can't do is to create a useful QA/QC program where none currently exists.

The boatbuilding industry largely avoids ISO 9001 because proving QA/QC process compliance, in the absence of a customer who demands that proof, is just a paperwork exercise that adds administrative overhead without increasing revenue. Instead, they implement their own internal QA/QC programs, with varying degrees of effectiveness. Some tout their build quality as a selling point, others quietly sweep it under the rug (often literally, as anyone who's found carpet glued to the inside of a hull can testify).

[Cheechako]

One problem I see with grids bonded with Plexus or similar things is engineering numbers dont really mean much. Why? because you have no idea how well the grid fit the hull on installation. Is it really bonded? or how much isn't? Severe damage cannot be readily repaired either. Poor fit is hard to see.
I have seen a 42 ft that hit a rock and suffered $23k damage in 2004 dollars. It was not an extreme grounding at all. Fortunately the debonding was located where it could be repaired.... or maybe other was not found.
I have tested Plexus type product in a marine simulation though and it is very strong. We could not debond it with a big man and a large sledge hammer.
However, the America's Cup boat that broke in half in light air was engineered and built with material that was "very strong" (in theory) also!

[danstanford]

Watching the Evans repair I wondered if, as strong as the bond (hull to grid) is now, what would happen if they hit the same rock at the same speed? Would the hull be unscathed, the keel ripped off, or the hull/grid be broken somewhere to dissipate the energy transmitted.
I suppose perhaps another option would be that the rock would move!

[kenbo]

Originally Posted by danstanford

Watching the Evans repair I wondered if, as strong as the bond (hull to grid) is now, what would happen if they hit the same rock at the same speed? Would the hull be unscathed, the keel ripped off, or the hull/grid be broken somewhere to dissipate the energy transmitted.
I suppose perhaps another option would be that the rock would move!

In the video that Dockhead used to open this thread, the shop doing the repair stated that in subsequent groundings they observed the damage in the remaining Plexus bonded portions of the grid that were not laminated.

[robert sailor]

The average new sailboat buyer is not all that sophisticated when it comes to construction details....they tend to be impressed with CE ratings and the layouts....the manufacturers are very aware of this so it's very unlikely that they will be rushing to improve the current construction methods. By and large these boats are built to a price and there is nothing wrong with that as it allows people to buy a lot of boat for the money. Accountants are busy saving nickels and dimes, certainly not interested in spending more money.

[robert sailor]

Originally Posted by kenbo

In the video that Dockhead used to open this thread, the shop doing the repair stated that in subsequent groundings they observed the damage in the remaining Plexus bonded portions of the grid that were not laminated.

That and the fact that the failures were transmitted outward of where the repairs were made resulting in more difficult and more expensive repairs.

Cast iron keels contribute a great deal to internal damage as well. Years ago when almost all keels were lead ....boats were bouncing off rocks left right and center and few resulted in structural failures, most had some severe dents in the keel but of course the internal structures were all laminated together with fg as the green goo had not yet been developed.

[AKA-None]

Originally Posted by jtsailjt

“Stronger” doesn’t tell the whole tale and it would be interesting to know what sort of boat you consider to be the best fin keel boats.

If you’re talking about even the best high aspect fin, there’s no getting away from the tremendous concentrated forces that the combination of a long moment arm and a small cross section yields. I’m sure you’ve seen these boats with the aft part of the keel rotated right up into the hull and cracking it like an eggshell. Some end up on the bottom shortly afterward but some make it to a nearby boatyard and are repaired at great expense. Ive seen these hauled out and some were very expensive, high performance yachts that many would consider to be “the best.”

More moderate fins with lead ballast bolted on are probably the best for withstanding hard groundings with minimal damage just as long as the keel bolts are inspected to ensure they haven’t developed microscopic cracks that could lead to failure, but very few are inspected because keel bolt failures are infrequent and it’s an expensive process. These keels have some of the groundings energy used to transform the shape of the lead and the remainder is spread over a much larger area than with a more aggressive fin. The damage from even a hard grounding can usually be repaired with some filler and paint.

Encapsulated ballast boats with fins usually have a shape similar to a moderate bolted on lead keel but with the ballast completely encapsulated there is zero chance of bolt failures allowing it to break loose and the boat sinking as a result. However, the damage from a hard grounding can be much more expensive to repair, especially if sea water penetrates into any voids around the ballast, and doubly so if the ballast used is a ferrous metal.

So, for as an owner/skipper responsible for paying the boats yard bills, for a hard grounding I think I’d rather be in a boat with a moderate fin with bolted on ballast, but if I wasn’t going to have to pay for repairs I think I’d choose encapsulated ballast as the safest. Even the best boats with aggressive fins are much more vulnerable to catastrophic damage from a hard grounding than are the other 2 types.

Then there’s another option I can think of and that’s a design where the boat has no keel and instead has ballast encapsulated in the bottom of the hull with a large bronze centerboard that is both ballast and fin but would flip up in a hard grounding, minimizing damage. Ive only seen one of these, a Ted Hood design that was a 49 footer and the owner said it sailed very well.

Southerlies maybe fits this description

[Searles]

Only too true ,have seen many de laminated grids in production boats ,first tell tale is the vsl will not hold forestay tension plus much creaking down below in a descent weaway,all is fixable with time and much money by well trained repairers.⛵️⚓️

[Manateeman]

There is another option, twin keels welded to an aluminum hull built to withstand...oh sorry...the design is limited to old slow manatee beer cans.
Please continue the thread.
The manatee crew.

[boatpoker]

Originally Posted by Manateeman

There is another option, twin keels welded to an aluminum hull built to withstand...oh sorry...the design is limited to old slow manatee beer cans.
Please continue the thread.
The manatee crew.

I've never really understood that... the difference between a fast sailboat and a slow sailboat can be 1/2 knot ..... How does one tell

[Manateeman]

Well my manatee crew have sketched out a simple solution to fin keel issues.
Four airfoil shaped 316 stainless struts.
A thick plate will be bolted to the bottom of the keel to provide an attachment area for the struts. The struts will extend to the turn of the bilge where they will be thru bolted to plates...very much like chainplates and the plates will sandwich the grid flange from the outside. The increase in wetted surface is minimal. The struts, bolts and plates could coated to avoid contact with the sea.
The surface area of the plates will greatly increase the area over which the forces of a collision would be spread.
If the vessel hits anything, the aft end of the keel is no longer a knife edge pushing up and the struts aft are now in compression against a two wide plates.
The forward struts will be pulling against the forward grid. If the vessel falls on her bilge the struts keep the keel from snapping off.
The manatee crew believes the invention will fund the onboard pizza oven.
Happy trails to you.
Mark