| Re: Homebuilding a trimaran with tornado amas...
[Re: Kevin Cook]
#145491 06/13/08 01:01 PM 06/13/08 01:01 PM |
Joined: Oct 2007 Posts: 199 Constanta, Romania isvflorin OP
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Posts: 199 Constanta, Romania | Kevin, I found extremely interesting the fact that less dense construction foam withstands more compression than higher density "marine" foam.
Florin
| | | Re: Homebuilding a trimaran with tornado amas...
[Re: Stewart]
#145493 06/16/08 12:57 PM 06/16/08 12:57 PM |
Joined: Oct 2007 Posts: 199 Constanta, Romania isvflorin OP
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Posts: 199 Constanta, Romania | that's something worth testing - leaving foam test blocks immersed and then weigh them...can that be considered an accurate test ?
Florin
| | | Re: Homebuilding a trimaran with tornado amas...
[Re: isvflorin]
#145494 06/16/08 07:47 PM 06/16/08 07:47 PM |
Joined: Apr 2008 Posts: 34 Central California slosail
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Posts: 34 Central California | About leaving foam samples in water and then weighing them: I think that's a useful test. However, for the results to be meaningful, the blocks would need to be immersed fairly deep in the water rather than just under the surface (perhaps at the keel depth) to match the pressure which would be trying to force water into the foam. The timespan of the test would have to be fairly long also, as the problem of water soaking into the foam probably happens fairly slowly (except for beaded type polystyrene, which soaks up water like a sponge; fortunately, this type is not being considered in this thread).
About strength testing: When used in a sandwich hull, it's not just compressive strength of the foam that matters but also shear strength. I've been messing with some glass and carbon sandwich samples, just to get a feel for how this stuff breaks...and using extruded polystyrene foam (cheap blue stuff), I found that the foam failed very early in shear, usually near the bond with the skin, during 3-point bending tests. PVC foam worked a LOT better. Interestingly, redwood with carbon cloth was considerably stronger (though heavier) than high density PVC foam with the same 2 layers of carbon cloth on each side. | | | Re: Homebuilding a trimaran with tornado amas...
[Re: Stewart]
#145496 06/17/08 10:14 PM 06/17/08 10:14 PM | Anonymous
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Unregistered | Its much more fun this way <img src="http://www.catsailor.com/forums/images/graemlins/smile.gif" alt="" /> | | | Re: Homebuilding a trimaran with tornado amas...
[Re: ]
#145497 06/17/08 10:44 PM 06/17/08 10:44 PM | Anonymous
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Unregistered | There is always a risk of over simplifying answers like this or ending up so complicated that you loose everybody. But here we go.
Everything discussed so far with foams is relevant along with a few other items.
When specifying cores the first port of call is usually Shear strength. As mentioned above the shear strength of the core will usually determine when a sandwich panel will break. As shear strength is basically proportional to density we have to find the minimum acceptable shear strength to be able to build the lightest possible product.
Depending on the use of the vessel the next consideration is usually strain to failure. This is how far you can distort the core without if breaking. A high value here will make the laminate more "robust". The best foams for this use are either SAN (core-cell) or linear PVC
Compressive strength. The advantages of a core with high compressive strength are pretty self evident and a lack of core compressive strength usually materialises on our off the beach cats as dimples.
Temperature form-ability. Anyone who has built their own boat can tell you about the difficulties of forming the foam core to shape. The easiest solution to this is to score one or both sides of the foam so that it will bend to shape. The disadvantage of this system is that these grooves eventually get filled with resin or bog loosing much of the weight saving gained by using a core in the first place (This is particularly the case when infusing). Selecting a core that can be heat formed will allow you to build a lighter boat, however, if you do this paint it white because otherwise it may change shape on a hot day.
You can use blue builder's polystyrene as a core but make sure you have a full understanding of its structural properties. We've used it in 10m ply catamarans to create ply/foam/ply bulkheads and sole and cabin top panels but make sure you have a full understanding of its strengths and weaknesses.
For most off the beach projects 80kg/m PVC is a good start. Beyond that you'll need to engage someone to do the calcs for you. | | | Re: Homebuilding a trimaran with tornado amas...
[Re: slosail]
#145499 06/18/08 01:52 AM 06/18/08 01:52 AM | Anonymous
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Unregistered | Balsa is one of my favourite cores. Its particularly good for decks where its superior compressive strength allows you to bolt straight through it (make sure you drill the bolt holes oversize and seal). It also has good shear strength. We often spec balsa for decks (as mentioned above) and also for bulkheads and other flat parts. | | | Re: Homebuilding a trimaran with tornado amas...
[Re: Kevin Cook]
#145502 06/18/08 11:16 AM 06/18/08 11:16 AM |
Joined: Oct 2007 Posts: 199 Constanta, Romania isvflorin OP
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Posts: 199 Constanta, Romania | So what do you guys think about the core Kjell is using ? Some construction foam 70kg/cum, which showed that can handle compression better than Corecell I believe. If the foam can't handle shear stress as good as "marine" foams, then he might get local delamination during the boat's life.
I assume shear stress in the laminate can be reduced with scoring the foam ? Indeed that will add some resin weight but will it improve shear stress handling ?
Florin
| | | Re: Homebuilding a trimaran with tornado amas...
[Re: Kevin Cook]
#145504 06/18/08 06:14 PM 06/18/08 06:14 PM | Anonymous
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Unregistered | Kevin is correct, build a test panel with your preposed laminate and then bend it till it breaks. If the failure mode is fibre breakage then you're probably ok. If it fails by core breakage or skin delamination off the core, look for a new core.
When doing this sort of home build/design project. There is a lot to be said for paying someone to specifiy your laminates. In my experiance 9/10 home built boats are over built. While this doesn't sound like a problem it does mean you've wasted a lot of money on materials that you didn't actually need. The other 1/10 are underbuilt and we all no the issues with that. Some basic composite engineering will cost you less than the money you save in excess or poorly specified materials and will allow you more confidence to head out to sea. | | | Re: Homebuilding a trimaran with tornado amas...
[Re: ]
#145505 06/19/08 12:11 AM 06/19/08 12:11 AM | Anonymous
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Unregistered | I know a lot of you avoid sailing anarchy, however there is a really good letter from Bruce Niederer on the front page today with some useful and interesting links. | | | Re: Homebuilding a trimaran with tornado amas...
[Re: ncik]
#145508 06/19/08 09:42 PM 06/19/08 09:42 PM | Anonymous
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Unregistered | I model loads on things like beach cats by starting with a simple 3D finite element model. Typically if you design these boats to take into account global loads and build methodology you end up with more than enough panel strength for local loads. Having said that I normally then "double check" by calculating the allowable loads for those panels and making sure they are more than I would "expect to see". For off the beach sailing boats I use 2G accelerations. | | |
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