Hull impact resistance

Anything to do with hull shapes in general (i.e. not specific designs)
AlexQ23
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Hull impact resistance

Postby AlexQ23 » Sat Mar 17, 2018 3:05 pm

Hull impact resistance: This matter will probably interest all the available brain in this forum! Classification rules don’t really take puncture resistance of a hull in to account. This is especially true, when you design a new light unit and when you are working with rigid hull material like plywood, carbon fibre or other sandwiches. It could be a great opportunity for us to come with an accurate easy to apply test method and why not a formula or at least a lower value to apply to a specific type of hull. Any idea?

Robert Biegler
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Re: Hull impact resistance

Postby Robert Biegler » Thu Oct 11, 2018 8:43 pm

The common test seems to be to take a standardised shape, with standardised weight, and drop it from a variable height. How pointy the shape is matters. I think it was Gerr who told the story of a boat builder who handed around a lightweight kevlar panel and a claw hammer, inviting people to hit the panel as hard as they could, to demonstrate its toughness. Bashing with the blunt end seemed to make little impression. Gerr turned the hammer round and punched the claw end right through. It might be interesting to test both blunt and sharp impactors. I don't know whether it it more informative to find out what weight of object or height of drop is needed to cause any damage visible from the inside, or whether there should be repeated impacts and extent of damage is assessed. The former is probably easier to measure and therefore more repeatable.

The Gougeon Brothers on Boat Building reports that Kevlar on the inside of plywood was more impact resistant than kevlar on the outside. Sven Yrvind makes a similar comment when it comes to one side of a sandwich core being laminated in glass, the other side in carbon. It was better to have the carbon on the inside.

John Perry
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Re: Hull impact resistance

Postby John Perry » Sat Oct 20, 2018 11:04 am

Maybe better to ensure the hull has compartmentation and/or bouyant material sufficient that a puncture does not matter too much.

So, although its a rather academic question, what hull material does have the best specific puncture resistance, i.e. for a given weight of panel, what kind of panel will best resist puncture from a pointy rock? Many people would say steel, but that is heavy.

John Perry
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Re: Hull impact resistance

Postby John Perry » Mon Oct 22, 2018 10:17 am

https://www.facebook.com/BBCArchive/vid ... 454775023/

I just came accross the above link - a fascinating promotional film from the early days of glass fibre boat building. Just look at the clothing worn, both afloat and while laying up glass fibre in the workshop! Relevant here since it does show a comparative impact test to demonstrate the superiority of grp over wood - you can usually prove what you want to prove by choosing suitable test specimens and test procedure.

Robert Biegler
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Re: Hull impact resistance

Postby Robert Biegler » Mon Oct 22, 2018 5:56 pm

John Perry wrote:you can usually prove what you want to prove by choosing suitable test specimens and test procedure.

Yes, that was downright cheeky. That wood seemed to be not plywood, but a plank with the grain parallel to the supports, not going across from one side to the other. Of course it bloody broke. Unidirectional glass with the same fibre orientation would break about as easily.

My initial approach would be to use a fireman's axe on an extended handle, so I can drop it from whatever height the working space allowed, and a sledgehammer of equal weight. Use both the cutting and spiky side of the axe. Let the impact be at both 90 degrees and at 45 degrees to the surface. That gives six conditions. Raise the height of the drop until damage is visible on the inside. In sandwich structures, record both first damage penetrating the outer laminate, and damage penetrating the inner laminate. If there are more fibres in one direction, test with the predominant direction either bridging the supports, or parallel to the supports. That would mean 12 test conditions.

I don't know what effect the spacing between the supports is likely to have. On the one hand, stress increases with distance. On the other hand, ability to flex also increases with distance between supports. I don't know how these balance. If all effects are linear, the relative performance of different materials should not be sensitive to this parameter. I don't know whether all effects are linear. Testing two diferent spacings would increase the number of test conditions to 24, so that could be the number of samples needed for each material.

John Perry wrote:So, although its a rather academic question, what hull material does have the best specific puncture resistance, i.e. for a given weight of panel, what kind of panel will best resist puncture from a pointy rock? Many people would say steel, but that is heavy.

I am pretty sure the Dashews ranked materials in the order steel, aluminium, glass composite, wood, ferrocement. I think that was not based on any testing or systematic data collection, but on how boats fared that they happened to know to have gone aground. But I think a lot of the data points came from Cabo San Lucas, where quickly rising wind put a lot of boats onto the same beach, in the same wind and sea conditions. They did acknowledge, though, that there was a lot of luck involved in whether a boat landed on top of another.


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