Does your boat hull "squash" when on the hard?

skyflyer

skyflyer

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The first time I was actually present when my boat was hauled out of the marina and lowered on to the ground I was alarmed to discover that as the slings were lowered and the weight came back onto the (fin) keel (now on a block) there was a significant degree of flex and "squish" in the hull - 'sag' almost!. I'm not on a cradle so the yard used shores to support the hull for the winter ashore.

However I find myself worrying to what degree a GRP hulled boat (32ft sailboat in this case) is designed (or at least anticipated) to flex in this way.

It is one thing to sit in a cradle and have additional support to the hull, or to be shored as mine was, but what about when drying out alongside or on legs?

A (non-marine) engineer friend suggested it's nothing to worry about as in the first place the boat will have flexed the other way when lifted in slings and secondly there will be times when cresting a steep wave etc when the hull will be unsupported (at least in part) so that this flex is designed and expected and actually occurring a lot of the time?

I know that whenever the boat is on the hard none of the lockers, hatches or doors will close properly nd then once afloat again, everything fits once more!

Interested to know of others experiences - I guess its not an issue for long keelers!
 
why not adjust the cradle or shores so that the boat is in 'normal afloat' trim and the lockers etc close properly ?
 
sarabande said:
why not adjust the cradle or shores so that the boat is in 'normal afloat' trim and the lockers etc close properly ?
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I could but its not really the issue; the point in question is should the hull be allowed to flex in this way or is there potentially damage being done? One could argue that its no different from a small GRP dinghy which flexes all over the place of course!

Pus I think to get the 'correct' afloat trim as you describe would take an inordinate amount of trial and error! At the end of the day you are taking something that is normally supported uniformly across its entire underwater area and supporting it at one point (top of the keel) or maybe five or six (Keel plus cradle). Its never going to be perfect

My engineer mate pointed out that when afloat you have the hull - effectively static, afloat - with the dead weight of the keel pulling down beneath it thus 'elongating' the shape that way, and then when ashore it is the keel that is static with the deadweight of the hull bearing down upon it, effectively compressing the shape so yes there is bound to be distortion from the one extreme to the other.
 
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I saw the same when my Sadler 32 was put on the hard, and that's considered to be a relatively heavily built boat. The amount of flex in the hull was surprising.
 
No observeable flexing of our hull when ashore and all doors and lockers fit as normal. Much depends on boat manufacturer (obviously) and how heavily built; ours is a Nauticat which is not known for lightly built boats.
 
Robih said:
No observeable flexing of our hull when ashore and all doors and lockers fit as normal. Much depends on boat manufacturer (obviously) and how heavily built; ours is a Nauticat which is not known for lightly built boats.
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What sort of keel does she have. My instinct is that this effect is only likely to be seen on a fin keel boat where all the weight is resting on a relatively small contact area.

Bilge keels are to all intents and purposes, huge stiffening strakes and a long keel will spread the load to such greater extent.
 
skyflyer said:
What sort of keel does she have.
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I wouldn't worry about it. GRP is not a particularly stiff material. It doesn't plastically deform to any extent - unlike most metals which deform before breaking. It flexes, then breaks, but you are a long way short of that point. The beams on my trimaran can be seen to flex up and down a few cm when going over a wave, same as the wings on big planes can be seen to bounce around in turbulence. The important point is that the supports are next to a strong point like a bulkhead.
 
Steel built cargo vessels 'hog' or 'sag' according to the disposition of the cargo whilst afloat. GRP I guess will do the same when supported along a short length of keel.
 
As said nothing to worry about really. However a long time ago I saw a little 18ft Uffa Fox design grp fine keel boat where on sitting it on its keel the bottom failed and the keel went up into the hull. Yes it was repaired with epoxy etc. good luck olewill
 
My cat is fine when the chocks fore and aft are under the bulkheads but this is not possible when the chocks have to be moved away from the bulkhead to work on the hull patches covered by the chocks. I then notice that the sole boards become a bit wobbly. The doors still close OK and once the boat is back in the water everything goes back to normal.

I reckon the distortion is perhaps 5mm and you cannot see it by sighting along the bottom of the hulls.

Richard
 
I think a certain amount of flex is normal on many boats. There is a small gap between the hull and the forward end of the keel when the boat is hanging in slings, which closes when it is resting on the keel. It has been like that throughout our ownership, 20 years. We have found that if the keel is chocked slightly more heavily at the aft end, i.e. boat leaning back a bit, the aft keel bolts will press the hull upwards a little. We now check this and ask for it to be corrected before the yard gang leave. In our case there is a secondary problem that occurs when the boat is leaning back, as the deck does not drain through the midships scuppers, another reason to correct it.
 
All hulls without exception will flex when chainging from keel hanging off the boat to boat sitting on the keel. Even if you made the hull of half inch thick steel it would flex. The issue is not does it flex but how much does it flex. And likely an expensive Nauticat with a thick hull lay up will flex less than a high volume cheap American boat with a lighter lay up. But has anything cracked? Have stringers broken away? No? Well dont worry. The ABS scantlings allow for flex.
 
When i worked in ship repairers engineers when lining up prop shaft would first do roughly before boat was launched then exactly when in water , because of hull flex , irrespective of size and construction
 
birdseye said:
All hulls without exception will flex when chainging from keel hanging off the boat to boat sitting on the keel. Even if you made the hull of half inch thick steel it would flex. The issue is not does it flex but how much does it flex. And likely an expensive Nauticat with a thick hull lay up will flex less than a high volume cheap American boat with a lighter lay up. But has anything cracked? Have stringers broken away? No? Well dont worry. The ABS scantlings allow for flex.
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I'd agree with that.
But it's not just a question of the weight of lay up, it's also about design, where are the bulkheads etc.
How do the keel loads get spread into the hull?
Also the rig loads are significant.
I always like to slacken the preload in the rig before craning.

With a wooden boat, we'd be talking about leaks in the next paragraph....
 
Our boat seems to flex very little when moved ashore. There is no distortion evident in cabin and locker doorways and hatchways.

I wonder if a centre cockpit boat tends to be stiffer than one with a large aft cockpit?

Presumably the disposition, thickness and fixing of bulkheads also makes a big difference. Ours has lots of 12mm ply glassed in.
 
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