Boat flex

Jmc1764

Jmc1764

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I Have been watching a few Utube videos over the holiday break and saw one where a guy reinforced the structure of a fibreglass boat by adding additional cross beams and glassing them onto the hull, The result was and very very rigid boat.
So is this really a good thing ? surely the original design was good enough and must have allowed for a certain amount of flexing ? or is the more rigid the hull the better ?
 
There was a train of thought many years ago that it was a good thing if the boat or ship flexed with the sea but as far a my limited knowledge goes this was disproved and it s now considered that the more rigid the better.
 
It might be more useful to ask whether or not there are any known areas of weakness (or other known issues) with particular designs of boat and if so how owners have overcome them rather than ask the more general question that you have...
 
Just recently there was a discussion on another forum in regards to a Newzeeland Bav 47 that sank in high seas, as was concluded, by the hull flexing, which in turn caused the windows to pop out and the forward hatch to come open. The article referenced was in YM.
 
Jmc1764 said:
I Have been watching a few Utube videos over the holiday break and saw one where a guy reinforced the structure of a fibreglass boat by adding additional cross beams and glassing them onto the hull, The result was and very very rigid boat.
So is this really a good thing ? surely the original design was good enough and must have allowed for a certain amount of flexing ? or is the more rigid the hull the better ?
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I think the answer lies in the design and materials technology used. If the rigidity is designed into the structure with, say, a steel frame, tie rods to link to the rigging then the rigidity is where it matters and the rest of the structure can "hang off" it eg Arcona yachts. In an older design there may be less rigidity and it's achieved by heavy layup, inserted metal or wood pads and tabbing to join bulkheads and floors into the structure. Strengthening such structures is probably good, provided you haven't transferred the forces or flexing to a weaker part of the structure. An example is adding a second inner forestay - Patrick Laine shows how he did this on his Bavaria and it was quite complicated and substantial! And worked well. Although I feel reassured by old fashioned, solid fibreglass (and wood) I can see that the understanding of material strength and the loads imposed is a science rather than an art.
 
Boats have strength and rigidity on two scales. One is in effect, as a large box girder where as a unit it is able to withstand the twisting and bending forces from the shrouds, rudder, rigging, keel, etc. The other is local strength and stiffness where the individual 'bits'' like the topsides or deck might flex and 'pant' under load. It would also include the localized strength to support point loads such as cleats pulling out or windlasses ripping off the fore deck, etc.

So 'reinforcing the structure' is more often likely to mean addressing these localized issues, and usually addresses issues of inadequate stiffness in panels, such as topsides or the hull bottom structure forward of the mast,etc. And by inadequate 'stiffness' it's often the owners dislike of seeing or feeling 'undue' movement, although these may in the longer term lead to cyclic failure. Stiffening the whole global structure would mean there were some pretty fundamental shortcomings with its construction, but not unknown.
 
Flexing beyond the design tolerances will cause fatigue which in turn will affect the integrity of the structure. But, one would have to determine whether there is a need to reinforce the structure, which element needs reinforcement, how and to what extent.
 
I think there needs to be some flexing.

I turned up one day to find a very large dent in my boat's hull, two or three feet wide. This was on gravel hard standing . The cradle leg was in it's centre. The marina came and repositioned the cradle and checked inside and out for damage. The dent came out naturally after a few days .

I wonder if there have been static tests on fibreglass over a long period of time. Nevil Shute 's No Highway springs to mind.
 
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I can't help thinking that this was due to the support not being positioned correctly. Flex like that is the last thing you want when a gale blows up. Wobble wobble crash!

My boats sit happily on a few bits of wood (bilge keel and now a cat), but for a fin, I'd want props against the where the hull's reinforced by bulkheads.
 
Famous story. The No.1 Westerly GK29 was on trials, dried against a wall overnight and the keel finished up in the saloon. Production versions had a beefed-up hull in way of the keel but were still suspect. Mine went aground when its mooring dragged, was beefed up even further during repairs but still suffered keel bolt leaks due to flexing.
 
Stemar said:
I can't help thinking that this was due to the support not being positioned correctly. Flex like that is the last thing you want when a gale blows up. Wobble wobble crash!

My boats sit happily on a few bits of wood (bilge keel and now a cat), but for a fin, I'd want props against the where the hull's reinforced by bulkheads.
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After repositioning the cradle, the Marina said that the reason for the dent in the topsides was due to the ground moving (gravel base). :)
 
Stemar said:
I can't help thinking that this was due to the support not being positioned correctly. Flex like that is the last thing you want when a gale blows up. Wobble wobble crash!

My boats sit happily on a few bits of wood (bilge keel and now a cat), but for a fin, I'd want props against the where the hull's reinforced by bulkheads.
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I have never had any concerns about my fin-keeler in its cradle. Virtually all the weight is taken by the keel and the props are just wedged tight. My hull is very rounded and it would take a massive force to make the props cause the convex surface to invert.
 
My boat seems to flex a little as it's got older, however I seem to flex less as I get older...
On a slightly more serious note, my boat had a completely new, different design keel some years ago, old style fin to a 'modern' fin and large bulb, it came with a lot of additional floor stengthening which seems to have lasted well as there are no cracks or keel movement at all after 20 years
 
Perhaps worth noting that extreme flexibility has been regarded as a Good Thing in some types of boat. Viking long-ships were deliberately made very flexible; the only connection between the ribs and the keel is via the bottom strakes of the planking. Some writers have suggested that the term "dragon ship" as applied to Viking long-ships refers at least in part to the "snaking" movement in a sea. The skipper of the first replica to cross the Atlantic (in the 19th century) remarked on the ship's flexibility and how it helped the ship to ride Atlantic waves.
 
Wood boats, French ones at least, used to have half the stanchions to frames, so above deck was a bit weaker. If you had a knock to the bulwarks the stanchions would break leaving the frames below deck intact. A startling example of this not happening was when a fast dory hit the Lady Hamilton on the port bow. The contact was above WL, but the deck beams provided a pivot point and the frames knocked the planks out below deck. I have seen several cases of overstrong topsides causing damage this way, weakest point will go.
However with GRP.....I hit the Mountamopus buoy, a large steel buoy it was laid over flat in the water. I assumed I was sinking, but a quick inspection showed no problem and I carried on. The deck beams punched the other side of the boat out, but it came back in, the deck was up in the air, but the beams were intact and the hull was the correct shape. All I had was a 2ft x 1ft gel coat and substrate repair the following liftout, plus remake of the deck and top hat frame inside. 1976 hull, mind, over built usually.
 
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Jmc1764 said:
On a different note has anyone noticed that the Artic has gone from google maps ? the south pole and Antartic are there still,
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Maybe some one drove it off to the Antarctic, typing Arctic, brings up a soggy place occasionally covered in white stuff.

In early fibreglass designs the science of long term stiff construction was not well understood. Many went floppy after a few years,
I've sail a 20ft keelboat for some years the class is around 50years old. A lot of them have extra knees at the shroud plates, Also if you wind on the forestay and back stay you can get a considerable bend in the hull.
The boat I sail has additional aluminium angle from stay plate to stay plate across the hull to stop the hull being drawn in and to take the strain from the inner stay being wound on.

Years ago I had an early Lark dinghy that eventually failed with an extremely floppy hull and broken back.
 
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