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Old 17-01-2015, 19:48   #1
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Hull squishing at cradle pad

Went to see my boat today. Man I can't wait until spring! Anyhow... As I walked around I noticed one of the pads on the cradle seemed to be applying so much force on my hill that it was squishing in at the pad. Is this at all normal or is my hull pooched?
I'm trying to attach a photo but am having difficulties.
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Old 17-01-2015, 20:00   #2
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It's. Called "oil canning," and very common on light displacement boats. I had a Seaward 25 that did it on the trailer to a shocking degree, but the hull bounced back to shape as soon as she's in the water, not so much as a tiny crack. So don't worry, she'll be fine.
If she's in a yard you might ask them to adjust the stands to be closer to the bulkheads, but it may not be possible; boats are designed for hull strength in the water, not on stands.

Guess you'll have to find something else to worry about this winter.
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Old 17-01-2015, 22:44   #3
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Re: Hull squishing at cradle pad

The weight of the boat should be taken completely by the keel. The pads are only there to keep the boat upright, should only lightly touch the hull and definitely shouldn't deform the hull at all. The cradle cracked and sagged on our old boat pushing one of the pads hard against the hull. Didn't notice it immediAtely and it had deformed the hull by the time it was discovered. Deformation was permanent and had to be filled with resin and chopped strand and faired.

A lightly built boat on a trailer may be another story.
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Old 18-01-2015, 04:57   #4
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Re: Hull squishing at cradle pad

It helps if you take most of the weight of the boat on the keel by shimming it, and by placing the jack stands at locations where bulkheads are attached to the hull.
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Old 18-01-2015, 05:09   #5
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Re: Hull squishing at cradle pad

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Originally Posted by Hud3 View Post
It helps if you take most of the weight of the boat on the keel by shimming it, and by placing the jack stands at locations where bulkheads are attached to the hull...
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Originally Posted by Hud3 View Post
It helps if you take most of the weight of the boat on the keel by shimming it, and by placing the jack stands at locations where bulkheads are attached to the hull.
Indeed.
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Old 18-01-2015, 05:52   #6
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Re: Hull squishing at cradle pad

You need to talk to the manager of the yard and tell them get their chet together, and fix it before it become serious, if it hasn't already. Check the inside for cracks, where the pad is sitting.
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Old 18-01-2015, 10:42   #7
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Re: Hull squishing at cradle pad

The pads must be located directly under the bulkheads or you will deform your hull. You mentioned cradle vs stands, the boat may not be positioned properly fore and aft on the cradle to catch the bulkheads OR the cradle may not be purpose-built for that particular boat and you may need to have the pads welded in different locations to be positioned under the bulkheads. The hull may bounce back, it may not.
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Old 18-01-2015, 11:24   #8
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Re: Hull squishing at cradle pad

yep ... all supports should be either on the keel or at bulkheads. You can tell where the bulkheads are because when you knock the hull it has a different sound ... even on my alu hull.
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Old 18-01-2015, 11:43   #9
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Re: Hull squishing at cradle pad

This is not an unusual mistake by a boatyard (or just lazy). They may also have not got the boat straight up and down so that it's leaning to one side. Get after them to move the stands (called poppets around here) nearer to a bulkhead and straight over it's keel. I also insist on an extra set of poppets in case one slips out for some reason (in your case I'd want three per side and one in the bow). Finally, check that the chain between the poppets is well secured and that their legs are on level ground.

While you're at it, check that the boat as set at a good angle fore-aft so that it drains properly. It's very common for boats to have ice damage because they spent the winter with the bow a little too low.

I check the poppets three times - at haul, two weeks later for settling and then again after the ground has frozen for frost heaves.
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Old 18-01-2015, 16:48   #10
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Re: Hull squishing at cradle pad

First question is 'What is the hull made of?' With enough point pressure hulls of all materials will yield in different ways.
Certainly the problem requires correction and as the pads used on cradles are often flat, one solution is to have a sub pad made to fit over the flat pad, giving as full a contact as possible over the area of support. Hard closed cell foam may be adequate when spread over at least a square foot and a half area to avoid point loading however as mentioned, extreme pressure on pads is not advised or normally expected to happen.

If the hull is foam core or balsa core then there is likely to be a separation of the laminates when the outer skin pops back out. If it remains 'dented' the laminate may be holding on to the core.

If 'solid' FRP (Fibreglass) then there could be an issue with softening with ageing. As Fibreglass ages, if it is thin especially, it will yield to pressure more readily as it becomes more flexible with age. This is not as great a problem as it first sounds as either stringers or frames (with foam core if required) can be laminated on the inside to stiffen an area, The main caution is to avoid 'hard spots' so a gradually diminishing laminate each side of any stiffening is advised. Forward sections can be vulnerable to 'oil canning' and appropriate stiffening can be introduced in that area if needed. Of course, building up a hull thickness will also be of assistance however that alone would require quite substantial thickness to achieve stiffness and if unsupported by ribs or stringers of the correct design and shape, would still have a large area that may be vulnerable to point loading.
BTW, alloy vessels have a substantially different frame work to steel vessels when building to achieve maximum material strength. The alloy requires more of a grid pattern. This is mentioned as an example of distance between supports required for different hull construction methods.
As an amelioration for the hull popping in. If it is 'solid' glass fibre then as long as it pops out with no surface cracking and no after signs in the vicinity then you may have a lightweight vessel hull that 'recovered' from it's indentation. Any internal de-lamination of the layers is harder to determine and a percussion test, ultrasonic test and if immersed then a moisture test after a year or so may be advisable precautionary measures. All the best for a satisfactory outcome. Regards, John.
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Old 19-01-2015, 09:10   #11
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Re: Hull squishing at cradle pad

Quote:
If 'solid' FRP (Fibreglass) then there could be an issue with softening with ageing. As Fibreglass ages, if it is thin especially, it will yield to pressure more readily as it becomes more flexible with age.
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Old 19-01-2015, 17:28   #12
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Re: Hull squishing at cradle pad

Quote:
Originally Posted by Aha may View Post
Went to see my boat today. Man I can't wait until spring! Anyhow... As I walked around I noticed one of the pads on the cradle seemed to be applying so much force on my hill that it was squishing in at the pad. Is this at all normal or is my hull pooched?
I'm trying to attach a photo but am having difficulties.
Hello Aha may. In addition to the data that I already wrote in response here is a link that has much information, including the softening and flexing of solid GRP. If your vessel is foam core then the outer skin can be drilled after it has or is popped out and resin added in to the void, starting at the bottom through a syringe, sealed in to the bottom hole and used to fill the void until resin appears at the very top hole, drilled at the extreme top of the dent or slightly above, with a channel made down with bent wire, to allow resin to completely fill the void and then come up through the top exit hole. That should repair the damage satisfactorily and renew the bond to the squashed foam core.
https://books.google.co.nz/books?id=...20ages&f=false Regards, John
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Old 23-01-2015, 12:51   #13
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Re: Hull squishing at cradle pad

Probably no bulkhead support in that area.
Those stands should really not be supporting that much weight, have the yard readjust the stands to have the bulk of the weight on the keel.
Provided you have one.



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