Hull flex on modern production boats

[Dockhead]

Originally Posted by Polux

They bettered a lot in what regards that when they start using sandwich hulls (up of the line of water) I mean the ones that use them. A sandwich hull is several times more resistant to flexing than a single layer hull.

Exactly.

[Polux]

Originally Posted by Cheechako

As most Bene's now are cored hulls, you wouldn't think flex would be much. ...

Beneteaus and Jeanneaus, at least most of them, don't have cored hulls. They use a counter moule that gives some rigidity but far from the one of a cored hull.

[a64pilot]

Originally Posted by Peregrine1983

Well Sailorboy, since I don’t normally join owners forums for boats I don’t own, it didn’t occur to me to ask this question in the owners forum for this boat. For the record, i was totally smitten with this boat - and my question was sincere. If I do end up convincing the wife to look at these to purchase - I will join an owners forum.

However they can be a real asset, when I was buying a high volume production Boat, it was the owners group that told me exactly what to look for, and what the fix was. I looked, found it which shocked the owner cause he didn’t know about the oil canning. Without the owners group telling me, unlikely I would have found it, and the Surveyor missed it too.
Who goes around pushing hard on the exterior hulls of boats they are looking to purchase?

[Don C L]

I haven't been on any maxis like this since, but I was on a Frers 65 that had a Kevlar honeycomb cored hull that flexed a lot. In fact the floor flexed so much when banging to windward, I watched the stringers in the hull under the sole get pushed up to bang against the steel framework of the cabin sole. The steel "L" beams were winning the battle, and cutting through the stringers. I feel pretty sure the hull wasn't in any danger but it was somewhat interesting to watch. It wasn't my boat though.

[sailorboy1]

Originally Posted by Peregrine1983

Well Sailorboy, since I don’t normally join owners forums for boats I don’t own, it didn’t occur to me to ask this question in the owners forum for this boat. For the record, i was totally smitten with this boat - and my question was sincere. If I do end up convincing the wife to look at these to purchase - I will join an owners forum.

So you aren’t interested it taking the question to a group with the most direct experience? No problem to me.

[Peregrine1983]

Originally Posted by Dockhead

On the contrary, it's the lack of core which makes them bendy. It takes a very great amount of material to make an uncored hull stiff. The Oyster I almost bought (Holman & Pye designed 485) had something like 3 inches of layup in the uncored bottom. Benes of that era are thinly laid up for a reasonably light hull (unlike the Oyster) and, with no core, are naturally very flexible. But the reasonably light hull makes them sail well, and fast.

Interesting. Thanks Dockhead. This is what I was wondering.

[Peregrine1983]

Originally Posted by sailorboy1

So you aren’t interested it taking the question to a group with the most direct experience? No problem to me.

Ok bud. Take er easy.

[Peregrine1983]

I guess my question comes down to - are modern grp boats designed to flex more or less than old ones?

[Dockhead]

Originally Posted by Peregrine1983

I guess my question comes down to - are modern grp boats designed to flex more or less than old ones?

I think you got the answer, at least in bits and pieces.

A few high volume production boats with light single skin hulls are pretty flexible. More expensive boats are usually cored, at least above the waterline, and are therefore stiffer. Older boats are simply laid up heavier, and may be stiff even if they are single skin. But the weight resulting from thick single skin hulls is detrimental to performance and is not much cheaper than coring the hull.

With boats, everything is a tradeoff. Cheap, light, strong/stiff -- choose any two.

[Sailmonkey]

Originally Posted by Dockhead

I think you got the answer, at least in bits and pieces.

A few high volume production boats with light single skin hulls are pretty flexible. More expensive boats are usually cored, at least above the waterline, and are therefore stiffer. Older boats are simply laid up heavier, and may be stiff even if they are single skin. But the weight resulting from thick single skin hulls is detrimental to performance and is not much cheaper than coring the hull.

With boats, everything is a tradeoff. Cheap, light, strong/stiff -- choose any two.

Absolutely correct.

And add to that, flex isn’t a bad thing if the structure has been engineered to account for the flex.

I would also love to have a bene 473. To me their like sailing a big comfortable dinghy.

[jimbunyard]

Originally Posted by Peregrine1983

I guess my question comes down to - are modern grp boats designed to flex more or less than old ones?

It seems to me the phrase 'designed to flex' is a little off the mark.

'Flex' is inherent to some degree in all structures, the key is to limit it so that it doesn't exceed either the load or cyclic capacities under which the structure is designed to operate.

So it sounds a little odd, at least to me, to say that, say, the wings of an airliner are 'designed to flex x amount' rather than saying that 'the strength of the materials from which the wing is made are such that there will be x amount of flex under a given situation'.

I guess it's just a subtle semantic difference though...

As for boats, modern or otherwise, it all depends, ideas on the subject can be very idiosyncratic; for small fiberglass boats, my opinion is that rigidity is the goal to achieve, flex is inevitable, if it can be seen with 'the naked eye' I'd generally consider it a problem and possibly a design flaw or a result of substandard materials or workmanship...

[boatman61]

Wanna see oil canning/flex on boats..?? take a walk around a B'yard where the props have been poorly placed..

[john61ct]

I think that *maybe* flexing is a contributor to seakindly handling.

But more likely it's just a build-to-price / profit point issue, and the above is justification.

Working with your primary material plastic being flexible, I bet getting the overall structure to a stiffness past the minimum required level gets very expensive very quickly.

Not as if the intended market is circumnavigating in them much...

[Polux]

Originally Posted by Dockhead

I think you got the answer, at least in bits and pieces.

A few high volume production boats with light single skin hulls are pretty flexible. More expensive boats are usually cored, at least above the waterline, and are therefore stiffer. Older boats are simply laid up heavier, and may be stiff even if they are single skin. But the weight resulting from thick single skin hulls is detrimental to performance and is not much cheaper than coring the hull.

With boats, everything is a tradeoff. Cheap, light, strong/stiff -- choose any two.

There are some news about stiffness and mass produced boats. I had some inside information regarding Bavarias having improved hugely regarding this, I mean the new C line. They have a new engineer team designing the boat structure that is different from what it used to be. The lead engineer told me he believes that the new C line boats have now the more rigid hull of all main market mass production boats.

I don't know if he is right but things are changing on Bavaria and i have no doubts the new boats are better regarding that. I will have a detailed explanation about all this at Dusseldorf boat show.

Regarding bigger mass production yachts I would say that the Bavaria C65 is something special....in fact it is a revamped Salona 60 and like that one uses a inox steel structure to hold the keel, the shrouds and to distribute efforts by the hull.

[Dockhead]

Originally Posted by Polux

There are some news about stiffness and mass produced boats. I had some inside information regarding Bavarias having improved hugely regarding this, I mean the new C line. They have a new engineer team designing the boat structure that is different from what it used to be. The lead engineer told me he believes that the new C line boats have now the more rigid hull of all main market mass production boats.

I don't know if he is right but things are changing on Bavaria and i have no doubts the new boats are better regarding that. I will have a detailed explanation about all this at Dusseldorf boat show.

Regarding bigger mass production yachts I would say that the Bavaria C65 is something special....in fact it is a revamped Salona 60 and like that one uses a inox steel structure to hold the keel, the shrouds and to distribute efforts by the hull.

That sounds very interesting. Please keep us informed.