Soft Deck: How Critical ?

[minaret]

Originally Posted by propellanttech

We did all of the layers in one action. Top glass layer, core, and bottom glass, peel ply breathing material.

We infused from the bottom, through the bagging.

The only concern we had was not being able to see the top glass layer wet out. We had debated to use the top layer, but in the end did use it.

We ended with only one top hole (filled with resin of course, which was dressed and filled with gel coat).

It was a challenge to get the materials to defy gravity when we did this. We used some spray adhesive to tack everything up while we bagged the spot.

I will say, it is much easier to vacuum infuse from the top. But it is possible to do it from the bottom if you don't mind spending the extra time for the different infusion spots.

We actually tested this before we did the repair. I took us some time to figure out the best way to wet out everything from the bottom.

James L

So let me get this right. You removed the inner skin, reefed out the wet core and dried, and ground for prep. Then you used spray adhesive to tack the core in place with no core bond, but you also did an upper skin laminate. Didn't the upper skin laminate throw off your core thickness? Did you have to use thinner core than original as a result? Why layup an upper skin laminate if the original upper skin laminate is still intact? And how did you spray adhesive the core in place with a dry upper skin laminate in the way? Did you leave a void around the core since there was no core bond and rely on the resin infusion to fill the void? This would be very heavy and brittle compared to the usual method. Sounds like you did the resin intake from the bottom and the vacuum outtake from the top, makes sense.

All in all, an interesting method. But it doesn't really address any of the issues I mentioned earlier, like interior masking for dust and resin, and sheer inefficiency of the extra time taken. Plus it requires infusion equipment that most people won't have. It's certainly much better than the usual half assed method of injection, but not a pro repair. I often vacuum bag a core installation, whether it's done from above or below, because I believe it's the only way to insure there are no voids whatsoever. But I bag just the core in core bond, which is lightweight and flexible. Then I do all the laminate by hand. Infusion is a gamble unless it's done in a production setting, great for parts from a mold that has been set up for it, but I don't like to do it for repairs or even one-off parts. Much faster to blast it out old-school.

[SalsaCaliente]

I am early in the process of removing the port side deck skin and replacing the plywood coring. Chain plate problems as you might have guessed. Spent a long time on the phone with an experienced owner who had done the same. Here are his suggestions:

1. Use a Fein multitool (other brands are now available) saw blade - only saw that can make a 90 degree cut and the blade is extremely thin -this is important. Cut the skin along the interface to the toe rail and where the non-skid meets the cabin side. Use the depth control attachment to limit your cutting depth just to the thickness of the fiberglass skin. ( I got the schematics from the manufacturer so I know where the plywood coring is supposed to be)

2. Carefully pry up the fiberglass

3. Chisel out the plywood

4 make templates and cut marine plywood replacement - may have to go to slightly
thinner size

5. epoxy in - weight with concrete blocks (wondering if I should try a vacuum bag?)

6 epoxy filler around gaps

7 epoxy skin back in place - weigh with concrete blocks

8 along the thin cut line sand and fill with matched gelcoat - this is where the fein saw tool pays off - you get a very fine line/gap by using the very thin saw blade

I am considering using carbon fiber tape around the area where the chainplates go through the deck. I don't want plywood close to the entry but it seems like this area also goes through a lot of stress and could be subject to cracking. Feedback on this would be appreciated.

He was happy with the interface between the rebonded skin and the deck. Since the area is mostly non-skid I am considering painting the non-skid with a matching color. The boat is 25 years old so paint is starting to look like a better look than old weathered fiberglass. I also have a number of non-skid areas that could use some repairs for dropped winch handles, etc. and could leave the areas that are non-skid as bare fiberglass.


Appreciate any additional thoughts?

SalsaCaliente

[DeepFrz]

I hope you are also pulling the chain plates and checking them. If possible use a high density closed cell foam instead of plywood. Whatever you use make sure you use an epoxy annulus where ever a bolt goes through the deck. Using thickened epoxy around the chain plates will give more elasticity and higher strength.

[Cheechako]

Sounds good to me, but I would use foam or balsa just so it will contour properly. Or good ply, sawn into squares, is an option. Passport builds (built?) their boats that way so if water intruded into one 3" square, it wouldnt propagate.

[roverhi]

The problem with plywood is it wicks moisture and rot spreads throughout if there is water intrusion. Balsa core tends to encapsulate the water penetration so the rot is less prone to spreading. Foam can have a problem with the core separating from the frp overlay. That makes it effectively the same as a rotted core. That's the reason, besides cost, that most boat manufacturers use balsa core in the decks.

Not sure that getting a thin cut is all that big a deal in cutting the FRP layer off. You need to grind a taper into the the piece and the remaining deck to relaminate it properly. Use Epoxy resin for this. Polyester resin doesn't bond all that well to cured layups.

[durundal]

Originally Posted by SalsaCaliente

I am considering using carbon fiber tape around the area where the chainplates go through the deck. I don't want plywood close to the entry but it seems like this area also goes through a lot of stress and could be subject to cracking. Feedback on this would be appreciated.

Unless you're accounting for everything involved with some analysis you don't want to just add carbon fiber as a cure all, there are several issues with it that need to be addressed:

1) It has a very high modulus of elasticity (~180GPa for unidirectional fibers), higher than any nearby glass by a factor of 3, and slightly more than 316 stainless for most grades of carbon fiber. This means that any stress will be start to be carried by the carbon before any of the other materials, until the carbon stretches slightly and the rest of the assembly can start to take up the load. Even if it doesn't fail initially, this situation is placing the secondary sealing material in the primary load path and it is only a matter of time before it starts to fatigue and crack. An elastic sealant is a better choice as it will stretch happily as the primary load path takes the load.

2) It has very low coefficient of thermal expansion (close to zero, some flavors are even negative). This means that from whatever temperature the carbon was applied at any deviation from that will result in significant bond line stresses (remember that the carbon is very stiff, so it won't tend to stretch). If you apply the material at 50F in the winter and then later on in the summer it heats up to 150F sitting in the hot July sun that is 100F of change in temperature, the carbon will expand very slightly but the stainless chainplate will expand significantly (relatively, small amounts). If this expansion forces stresses that exceed the bond between the two the joint will fail and a nice leak path is now present. It is for this reason that lots of major carbon projects use titanium fittings where needed, it has one of the lower CTEs of commonly available materials. Titanium also has one more big advantage for carbon structures...

3) Carbon is very noble on the galvanic series, and will try to corrode just about anything it is in contact with. (titanium is very corrosion resistant, and in typical environments doesn't corrode even in intimate contact with carbon fiber, hence its other big selling point in carbon structures) Stainless is particularly problematic in its corrosion protection mechanism, in that it requires oxygen to maintain a passive layer on it. In a fitting just bolted on somewhere it can look great for years, but put it into a oxygen deprived environment (a bonded joint) and any leak can cause massive crevice corrosion in a very short time, and without any inspection methods to check for it. So if a tiny bit of water can get in through a crack in the carbon (as could be expected from the issues in 1) and 2)) it will really go to town eating up the stainless in there.

That said, I made carbon chainplates for my F-22 and they were actually really easy (all bonded assembly, no metal other than a pin in a G10 tube), if you're redoing everything it could be worth a look to change over possibly, depending on how your existing chainplates are attached.

Andy

[colemj]

Originally Posted by SalsaCaliente

I am early in the process of removing the port side deck skin and replacing the plywood coring. Chain plate problems as you might have guessed. Spent a long time on the phone with an experienced owner who had done the same. Here are his suggestions:

1. Use a Fein multitool (other brands are now available) saw blade - only saw that can make a 90 degree cut and the blade is extremely thin -this is important. Cut the skin along the interface to the toe rail and where the non-skid meets the cabin side. Use the depth control attachment to limit your cutting depth just to the thickness of the fiberglass skin. ( I got the schematics from the manufacturer so I know where the plywood coring is supposed to be)

2. Carefully pry up the fiberglass

3. Chisel out the plywood

4 make templates and cut marine plywood replacement - may have to go to slightly
thinner size

5. epoxy in - weight with concrete blocks (wondering if I should try a vacuum bag?)

6 epoxy filler around gaps

7 epoxy skin back in place - weigh with concrete blocks

8 along the thin cut line sand and fill with matched gelcoat - this is where the fein saw tool pays off - you get a very fine line/gap by using the very thin saw blade

I am considering using carbon fiber tape around the area where the chainplates go through the deck. I don't want plywood close to the entry but it seems like this area also goes through a lot of stress and could be subject to cracking. Feedback on this would be appreciated.

He was happy with the interface between the rebonded skin and the deck. Since the area is mostly non-skid I am considering painting the non-skid with a matching color. The boat is 25 years old so paint is starting to look like a better look than old weathered fiberglass. I also have a number of non-skid areas that could use some repairs for dropped winch handles, etc. and could leave the areas that are non-skid as bare fiberglass.


Appreciate any additional thoughts?

SalsaCaliente

There is one BIG problem with how your friend fixed his boat that you should not copy. When the top skin is glued back in place, you need to grind the seams out in a large taper and use layers of biaxial glass tape to connect one side of the cut with the other side. To simply put gelcoat in the seam, like you described, will leave a very weak deck skin - particularly in the area of high stress you are repairing. This is probably the most important structural step of the repair.

The taper should be wide - 15:1 is recommended, but 7:1 will be fine. The problem is getting enough width in the place you describe. How wide is the toe rail / nonskid interface? If it is narrower than ~3", you may not have sufficient width to make that repair. However, it is difficult to make the aesthetics good if you cut the nonskid, so you will have to make do with the space you have. Just make sure you taper and glass bond the two sides of the upper skin as well as you can.

Because you will be grinding back the taper, the kerf size of the saw cut is inconsequential. But a Fein tool is an excellent tool for working in the area you have - a circular saw will probably not fit along the toe rail.

Don't fret the depth of the cut - you are removing the plywood anyway, so cutting through it won't matter. Just make sure you don't cut through the lower skin.

As others mention, I would use balsa or foam for the repair instead of plywood. These materials are better and the costs are the same as quality marine ply.

Use solid glass around all fitting penetrations - cut the core back an inch and fill with epoxy thickened with fumed silica. This will prevent any future core damage from water intrusion.

Mark

[Land an]

Why don't we use a "plastic wood" as a deck core on boats when first built or on repairs like these? Something like Trex. If it does get wet in future it cannot rot the core. Still strong etc.

[colemj]

Trex and the like have low tension and shear strength and are extremely heavy. Honeycomb and foam core materials solve the rot, strength and weight issues handily.

Mark

[durundal]

Originally Posted by Land an

Why don't we use a "plastic wood" as a deck core on boats when first built or on repairs like these? Something like Trex. If it does get wet in future it cannot rot the core. Still strong etc.

Plastics can be difficult to bond to (not personally familiar with Trex), the core needs to have a good bond to the skins to transfer the shear stresses between them. Fixing the shear is what gives cored composites their strength, if shear isn't transferred between the skins in a beam in bending you just get something equivalent to a bunch of 2x4s sitting on top of one other rather than a glued 4x8 beam. Plus what was said about the weight and properties.

If you do need a more sturdy core than standard foam or honeycomb but still don't want it to rot there are significantly higher density foams that can do the job (16lb/ft^3 for HD vs. 4-6lb/ft^3 for most cores), or if you really felt the need you could use garolite (cloth phenolic) sheets but they would be very heavy and expensive.

Andy

[Ketchgould]

I just spent two years replacing all core.


It is a huge job. Expensive too.

If you can live with wet core, then do.

Is the deck deforming under stress? Any structural cracks? If so, i'd replace or get a different boat.

Christian

[ArtM]

Originally Posted by Land an

Here is the issue: moisture in deck core as proven by hammer and meter. How critical is the problem? Fix ASAP? Wait until convenient (realizing waiting may make the repair job bigger)? Live with it and fix it whenever I chose?

My parents have a similar problem with a camper they own - the floorboards are rotted, and cannot be replaced because they entire vehicle is built on top of them.

The advice? Enjoy the camper while you can and don't worry about it (until you fall right through the floor).

If it is not a safety issue for you, my suggestion is to ensure that the problem will not spread, and enjoy your vessel. If necessary, you can try to stiffen up the areas with some fiberglass overlay of some sort -- creativity and a friend with a bit of fiberglass knowledge are your allies here.

If this is a sea going vessel and hull integrity is compromised, then you better do something about it - and that "something" could be to retire it to day-sailing use and move on to a more soundly built boat.

[ArtM]

Originally Posted by ArtM

My parents have a similar problem with a camper they own - the floorboards are rotted, and cannot be replaced because they entire vehicle is built on top of them.

The advice? Enjoy the camper while you can and don't worry about it (until you fall right through the floor).

If it is not a safety issue for you, my suggestion is to ensure that the problem will not spread, and enjoy your vessel. If necessary, you can try to stiffen up the areas with some fiberglass overlay of some sort -- creativity and a friend with a bit of fiberglass knowledge are your allies here.

If this is a sea going vessel and hull integrity is compromised, then you better do something about it - and that "something" could be to retire it to day-sailing use and move on to a more soundly built boat.

Why oh why was this not started as a new thread???