Manitu's project: Finessa 33

[barnakiel]

@ Minaret:

A wood element is about to get substituted by a low density core (which I assume low density polyurethane is) encapsulated in GRP. (Am I right?)

Is such a layup, in this design and application, equivalent in respective strength (compression?) to a wood block?

I have some (basic) training in lamination and I would expect the original wood solution to be able to withstand higher compression loads than the discussed core+GRP substitute.

Cheers,
b.

[minaret]

Originally Posted by barnakiel

@ Minaret:

A wood element is about to get substituted by a low density core (which I assume low density polyurethane is) encapsulated in GRP. (Am I right?)

Is such a layup, in this design and application, equivalent in respective strength (compression?) to a wood block?

I have some (basic) training in lamination and I would expect the original wood solution to be able to withstand higher compression loads than the discussed core+GRP substitute.

Cheers,
b.

Compression strength of GRP is just below that of steel. Imagine a steel box beam built with 1/2" plate on the top and 3/8" plate on the sides and you will be very close to the compression strengths we are talking about here. Moreover a fairer comparison would be to compare the compressive strength of a rotten wood block, as that is what they will most certainly eventually become when installed in a bilge. I used to actually run numbers on this stuff many years ago, but experience has taught me that when you build fiberglass parts for proper panel stiffness and flexural strength, you are generally speaking dramatically overbuilding for tension, compression, and shear (which is determined by the resin matrix more than anything else). These floors will be much more than strong enough, with a huge margin of error. You could build one floor as described and probably balance the weight of the boat on it with no trouble, in compression. The foam core itself is no slouch in the compressive strength department, and it is not really even needed here, it's essentially just a mold for the box beam at this point.

Airex 5LB #'s-

Compressive strength perpendicular to the plane-160 PSI

Compressive Modulus perpendicular to the plane-9000 PSI

Tensile strength in plane-290 PSI

Tensile Modulus- 7800 PSI

Shear strength- 160 PSI

Shear Modulus-3300 PSI

Shear elongation at break-18%

Obviously these numbers double if you go up to 10 LB density, but as I said I believe the box beam is more than sufficient. The numbers for GRP are easily available and you can do the math, it's not too hard. You will find the numbers on the laminate strength to be quite ridiculous.

[barnakiel]

Thanks for the explanation!

I wrongly imagined a typical sandwich construction while the new elements are de facto going to be relying mostly on heavily 'overbuilt' skins. Hence my misinterpretation of resulting strengths.

I asked because some time back I have read that dynamic load on keelbolts can be tremendous - easily 5 to 10 times the static load (and this represents only the desired working load/strength of the desired BL) - in cases when a heeled boat sails over very choppy water, etc.. The article was devoted to frame design in some modern cruisers/racers vis-a-vis a frameless design of some modern cruisers.

b.

[minaret]

Originally Posted by barnakiel

Thanks for the explanation!

I wrongly imagined a typical sandwich construction while the new elements are de facto going to be relying mostly on heavily 'overbuilt' skins. Hence my misinterpretation of resulting strengths.

I asked because some time back I have read that dynamic load on keelbolts can be tremendous - easily 5 to 10 times the static load (and this represents only the desired working load/strength of the desired BL) - in cases when a heeled boat sails over very choppy water, etc.. The article was devoted to frame design in some modern cruisers/racers vis-a-vis a frameless design of some modern cruisers.

b.

Couldn't agree more, dynamic loads on a keel can be astronomical. That's why I tend to heavily overbuild, it really doesn't take much longer to do. Most of the work is in prep, setup, and cleanup, cutting a few more plies and laminating them takes hardly any time at all.

[manitu]

Tried to do one of the floors yesterday, not completely satisfied. The PU foam was easy to form, but i think the foam soaked up the resin, I had to mix a shitload of it, and it was next to impossible to get the air out. I'll have to either seal the foam with filler , or soak it with polyester before installation.

Making the floors 3 inches wide at the top and 4 inches at the bottom would allso make the laminating easier , I think.

Any suggestions?

.manitu

[minaret]

Originally Posted by manitu

Tried to do one of the floors yesterday, not completely satisfied. The PU foam was easy to form, but i think the foam soaked up the resin, I had to mix a shitload of it, and it was next to impossible to get the air out. I'll have to either seal the foam with filler , or soak it with polyester before installation.

Making the floors 3 inches wide at the top and 4 inches at the bottom would allso make the laminating easier , I think.

Any suggestions?

.manitu

What kind of foam are you using? Airex is closed cell, so resin can only soak in so far. I usually hot glue the part in place, fillet where necessary, resin coat heavily, and while the fillet and resin coat are still wet start to lay up. Start with 2 10 oz. matt layers, it gives the rest of the layup a soft surface to conform to and makes rolling out fillets and radii easier. Obviously if you are doing a sizeable layup this way in any kind of heat you want to catalyze very light and move fast, and you need to know what you are doing. For many it might be easier to bond the piece in place, do all the filleting and let it kick, then sand out the fillets before glassing. Apply fresh resin before and after each ply, roll out all air after every other ply. Roll air on matt plies and squeegee on roving plies. Do the whole layup really rich, then on the final layer of WR or DB, depending on which you used, squeegee all excess resin out of the layup. After squeegeeing finish the laminate with a layer or two of matt, this allows you to grind for prep without grinding on the WR or DB plies that matter. On a thick layup you may want to squeegee at an earlier stage as well. If you are not a very experienced laminator, you may want to consider doing it in three stages. One-bond the parts in place and fillet them all, let it kick and sand out the fillets. Two-apply the first half of your laminate schedule, let it kick, and grind lightly for prep. Three- apply the second half of your layup, let it kick, grind lightly for prep, and then gelcoat. If you do just one beam at a time it will take forever. If you do it in three stages as suggested I would think it would take three days to do, one for each stage. Also remember never to laminate on cold foam, as the resin exotherms it heats up the substrate, causing air trapped in the foam to expand. This causes mysterious air bubbles that keep reappearing and can't be rolled out since they have pressure behind them. If you are working in cold temps make sure you keep a heater in the boat the night before you laminate. Do you have much laminating experience? Good luck!

[s/v Jedi]

I always heat up the part, even plywood, with a heatgun just before applying the resin. Completely eliminates air bubble problems and promotes deeper resin penetration. I always use epoxy bur assume this works as well for polyester or vinylester.

cheers,
Nick.

[manitu]

Hi. We did three floors today, looks great! Forgot to take any pictures, will do so tomorrow. I glued the beam together, made a small batch of filler from polyester, chopped strands and microballons. Mounted the floor with filler and covered it with a couple of chopped strand mats. Then it was sanded lightly when dry. Then the layup. One chopped strand mat, four rowing mats one rowing on top only and finally one rowing and one chopped strand. I think it's like half inch sides, 5/8" top. I'll post more details and photos when i've recovered from today's work.

Thanks again for all help. Heating up the foam and hull did all the difference.

.manitu

[SkiprJohn]

I'm confused about the lay up and the material you are using. You mentioned foam. You want the floors to be absolutely solid. No give, no flex. That's why most the original floors on boats are made of steel or solid glass or solid wood.

If there will be no flex then great.

Where bolts are buried beneath the waterline SS is not your best choice. Monel, bronze or just plain steel might be a better choice.

kind regards,

[minaret]

Originally Posted by manitu

Hi. We did three floors today, looks great! Forgot to take any pictures, will do so tomorrow. I glued the beam together, made a small batch of filler from polyester, chopped strands and microballons. Mounted the floor with filler and covered it with a couple of chopped strand mats. Then it was sanded lightly when dry. Then the layup. One chopped strand mat, four rowing mats one rowing on top only and finally one rowing and one chopped strand. I think it's like half inch sides, 5/8" top. I'll post more details and photos when i've recovered from today's work.

Thanks again for all help. Heating up the foam and hull did all the difference.

.manitu

Way to go! Should be plenty strong like it is. 12 plies alternating WR and matt should be more like 3/8", but that should be plenty for this. 16 plies is pretty standard for 1/2". If you drill limber holes you will get a nice look at how thick your layup was. Then if you feel it needs more you can add a few layers. I like to drill limber holes and install 1" glass tube, leaving it just a little proud and glassing it in with just a matt or two. Then you can gelcoat the limber holes along with everything else, which makes it much easier to see in the bilge and keep it clean because everything is stark white gelcoat. Also leaving them proud a little allows you to fit screens over them if you feel like going that far with it. Split 2" glass tube is even nicer, but you have to put it down before fitting the foam. It's so much nicer because when you do limber holes that way they intake flush to the hull and get every last drop of water, whereas every other option will leave the limber hole high by at least the thickness of the tab and thus leave that little annoying bit of water in the bilge that makes it so hard to keep spotless. Pick your limber locations well too, really good limbering is an art form. It sucks to do a really nice limber hole and then realize it doesn't quite drain but would have if you'd placed it an inch to the right. Nice to see someone dive in and get it done, congrats on your progress so far! Hope to see pics soon...

[minaret]

Originally Posted by SkiprJohn

I'm confused about the lay up and the material you are using. You mentioned foam. You want the floors to be absolutely solid. No give, no flex. That's why most the original floors on boats are made of steel or solid glass or solid wood.

If there will be no flex then great.

Where bolts are buried beneath the waterline SS is not your best choice. Monel, bronze or just plain steel might be a better choice.

kind regards,

Looking at the scale of nobility-

Zinc
Galvanized steel
Aluminum
Mild steel
Wrought iron
Cast iron
Stainless steel
Lead
Tin
Manganese bronze
Naval brass
Inconel
Yellow brass
Aluminum oxide
Red brass
Copper
Silicon bronze
Cupro-nickel
Comp G bronze
Comp M bronze
Inconel (passive)
Monel
304 Stainless
316 stainless, 3% molybdenum
Titanium


You can see from the scale of nobility that for a cast iron keel, as this probably is (it may be pig iron or some other alloy), mild steel or stainless would be about equally good (not considering crevice corrosion) where dissimilar metal issues are concerned. Monel or Silicon Bronze would be the worst choice, because they are on the other side of the scale from cast iron. For all intents and purposes these floors are made of solid glass. They should be thicker than the hull they are transferring load to, and they are a box beam layout. This means the hull will fail before the floor timber. Structurally this is a much sounder solution than wood or metal. Not only is it a stronger more permanent solution than the other options, but the load transferral occurs over a much larger footprint which is tapered off with the tabbing. I have seen keels catastrophically loaded crack at front and back from a grounding, with fractures radiating out around the floor timbers where the sharp edge of the timber point loads under extreme duress. This is exactly what a glass structural part with heavy tapered tabbing is preventing, not only do you have the footprint of the floor timber itself, but also the added width of the tabbing transferring load. Problem will be permanently solved fairly quickly and cheaply, lots of work but not too much expense relatively speaking. Certainly much cheaper than doing the same in Oak, which would cost a fortune just for the wood even if you laminated timbers that size. Have you seen the price of quality sticker-dried white oak in timber sizes? Most places you can't even get it. Laminating and fitting them would also take a long time. Then you still have to worry about rot, which will inevitably occur. Metal and wood is old hat when it comes to boats, technology has moved on. There are faster, cheaper, and better solutions. Unless you are working on a metal or wood boat!

[minaret]

Originally Posted by s/v Jedi

I always heat up the part, even plywood, with a heatgun just before applying the resin. Completely eliminates air bubble problems and promotes deeper resin penetration. I always use epoxy bur assume this works as well for polyester or vinylester.

cheers,
Nick.

Wise, Master Yoda. I once had to saw up and throw away a $500,000.00 part because they wouldn't listen to me on this subject. The foam core was being stored in a shipping container in the parking lot during a 30 degree winter, the mold was in a heated building. Every morning they would pull ice cold foam core out of the container and bag it down, then bag a laminate over it. It was all in solid carbon fiber, which of course you can't see air bubbles or voids in because it's black. Also the mold was only heated on the inside, the outside was exposed to an unheated industrial space. When the part was finished (this was not my laminating team, I was only a consultant on this part), and I sounded it for voids, I immediately found it was trash. Endless voids everywhere, even under vacuum bag with great bag pressures (I know, I checked their bags daily). Threw away the whole deck for an 80' carbon fiber sailboat. Ouch! The part that pissed me off was that the owner ate the loss, when a reputable surveyor, NA, Chemical Engineer, and team of Harvard lawyers were all involved. Nobody wanted to take the blame, and nobody wanted to admit the lowly laminator warned them repeatedly! All because I don't wear khakis with one of those nautical flag belts and deck shoes. No one wants to listen to the resin-spattered guy...
This often a more severe problem with epoxy than poly, because of it's slow cure time and high exotherm temps. With poly by the time you achieve high exotherm temps the layup is already kicked enough that it won't bubble up, generally speaking. Of course there are exceptions, like working in very cold temps, particularly when you are doing a large layup and catalyzing very cool as a result.

[manitu]

Hi all! About to start on today's work. First , I'm off to buy the drills to drill up the holes, then find new bolts and steel plates.

By the way , I'm making the new floors a bit lower than the old ones. The old ones was about 6 inches high , with the nuts at aprox. 3,5 inches. 3 inches wide. The new ones are 4 x 4 inches inside , just above 5 inches outside with the bolts sitting on top

I did not lay every other layer with chopped strand mat , because the rowing mat I'm using has a layer of chopped strands on the backside.

The foam I'm using has a density of 40kg/m3.

Well , then I'm off to work!

.manitu

[SkiprJohn]

Minaret,
I'm concerned that he's using foam.
kind regards,

[minaret]

Originally Posted by SkiprJohn

Minaret,
I'm concerned that he's using foam.
kind regards,

Please read the whole thread and then put forth your arguments as to why you are concerned. I will gladly reply. I thought I explained fairly clearly. Perhaps a more renowned proffessional forum member such as Bob Perry might chime in here at some point. I have built and rebuilt many floor timbers, stringers, frames, transoms, and other structural members this way, sometimes under the direction of an NA, but mostly under my own direction. Never experienced a structural failure on my work that I know of, and we do a ten year warranty on all work. This particular OP seems fairly comfortable with what he's doing, didn't ask many questions and dove right in. I'm guessing he's familiar with the process, he clearly knew what I was talking about right away. I wouldn't lose any sleep over it, he'll be just fine. So far he seems to be thinking it through and executing quite well. In many production boats the methods used are so outdated that it's not hard to rebuild much better than original. This will almost certainly be such a case.
When considering the compressive strength of such a floor, you should remember that the thickness of both walls must be taken into account, as well as the cross section, and the thickness of the hull laminate, which forms the bottom cross section of the box beam. These all work together to form a structure that is actually much stronger than the sum of it's parts. This all has to do with the way that many materials cannot realize their full compressive strength because of the factor of buckling. The laminated box beam construction minimizes buckling and allows the structure to acheive more of it's compressive strength potential. This is part of the same reason that a sandwich construction is stronger than a solid laminate of the same weight, buckling will occur at much higher loads in the sandwich than in the solid laminate.