Rebuilding Mast Step Area: Douglas Fir vs Marine Plywood

[arjand]

The yard is about to rebuild the mast step area which requires three new transverse floors and two longitudinal stringers that will be glassed over. Yard wants to use very thick marine plywood. Designer does not recommend plywood but douglas fir. Anybody having any experience with this? Any thoughts are highly appreciated. Regards, Arjan

[captain465]

You are the customer.........the yard should respect your desires, especcially if the builder recomends different from what they want to do.
Just be prepared for a cost increase.
Good luck!

[Blue Stocking]

IMHO, and I have worked with both materials, neither stand up to heavy compression loads over time. If it must be wood based, I would try for greenheart, ipea, or the like. Get a wood which grows in a generally humid and wet natural invironment. and is VERY hard.

[atoll]

if it is going to be glassed over,marine ply would be the best option as less prone to rot,whereas fir,once glassed over is more likely to rot.

a better choice for stringers would be a true hardwood like iroko,rather than fir.

[Blue Stocking]

Originally Posted by atoll

if it is going to be glassed over,marine ply would be the best option as less prone to rot,whereas fir,once glassed over is more likely to rot.

a better choice for stringers would be a true hardwood like iroko,rather than fir.

Agree with you, for general use. But have seen many cases where ply cores, even in good BS1088 Thames, and Brunzeel plywood, crushed under mast loads, split the glass, and water got in.

[atoll]

Originally Posted by Blue Stocking

Agree with you, for general use. But have seen many cases where ply cores, even in good BS1088 Thames, and Brunzeel plywood, crushed under mast loads, split the glass, and water got in.

probably best to do the whole job with iroko or equivalent,and cheaper than having to buy a whole sheet of thick marine ply and only use part of it.

also would only need to glass in the stringers,due to the type of material,so would save on labour.

[sailvayu]

The problem with glassing over any solid wood is that it will expand and contract with changes in temperature and humidity. The glass will not change at the same rate and the bond between the wood and the glass will break making a gap water can get into. I would agree with the yard as the plywood will be more stable and not move as much as solid wood. I have even seen thick pieces of solid wood split several layers of glass as it expanded.

[atoll]

looking at the specs for a nordic 40,which is grp.
the best long term solution would probably be to do the whole job in glass,using a filler like microballons as the base,with a s/steel or alloy plate to spread the load on top.

[minaret]

Hi density ridgid urethane foam is the only way to go. I've replaced so many rotten stringers and floors over the years it's ridiculous. One leaking limber hole and it'll rot. Go with ten lb. instead of the usual 5 lb. foam. You can go all the way up to 25 lb. foam, which is denser than cement and is almost impossible to work with 'cause you can't easily cut it. Won't rot, won't saturate, glass bonds to it better than wood, won't delam, it's lighter than wood, etc. etc. I just don't understand why people still are set on using wood in boats, it just rots. Wood is great for cosmetics, but when it comes to structural members....
If you really want to use something similar to ply, which is also much harder to shape than foam, try coosa board. Good ply substitute.

Coosa Composites, LLC - Manufacture of high-density, fiberglass-reinforced polyurethane foam panels

[Seahunter]

Hardwood, glass, stainless. The Douglas fir is actually a Pine tree and named Pseudotsuga the "false spruce". While NOT a hardwood, it is especially good for standing up to high loads when center cut from old timer. I also like minaret's idea too. I've also seen repairs done with hydraulic concrete mixed with chop good to 10000 PSI compression.

[svcambria]

Originally Posted by atoll

looking at the specs for a nordic 40,which is grp.
the best long term solution would probably be to do the whole job in glass,using a filler like microballons as the base,with a s/steel or alloy plate to spread the load on top.

Yes. Did a similar job with a 2" aluminum plate (aircraft grade out of the metal recyclers for $3/pound) and Chockfast Orange (a phenolic epoxy, about $80/gallon, used to set thousand horsepower marine engines):

Chockfast Marine - Industrial Grouts, Adhesives, Repair Products and Coatings

Unperturbed seven years later, will probably last longer than the fiberglass hull (attention future archeologists...)

Michael

[minaret]

Originally Posted by svcambria

Yes. Did a similar job with a 2" aluminum plate (aircraft grade out of the metal recyclers for $3/pound) and Chockfast Orange (a phenolic epoxy, about $80/gallon, used to set thousand horsepower marine engines):

Chockfast Marine - Industrial Grouts, Adhesives, Repair Products and Coatings

Unperturbed seven years later, will probably last longer than the fiberglass hull (attention future archeologists...)

Michael

Chockfast is serious business. I don't see it used much but I've used it a few times and it's amazing. I think we paid a lot more than 80 a gallon for what we were using though. I'd hate to have to remove it ever, you'd be wishing it was cement or something easier to remove like that. Just remember that stuff is forever!

[Seahunter]

Yay Chockfast; I like!

[Cheechako]

Plywood or Fir if glassed over is going to be a problem. Plywood is mostly Hemlock these days which is not nearly as good as genuine Doug Fir, and is terrible in a wet environment and I would think it'll be a problem if not glassed over. A good solid piece of real Douglas Fir, if not glassed over, would likely last pretty well. I've seen old cedar boats up here made with Doug fir frames...as were many fishing boats. I guess if the ply is used only to provide shape to lay heavy glass on..then it's sacrificial! Why not soak fir in copper sulphate fungicide? All in all, in the bilge a good marine moisture resistant wood would be nice though as noted above. It's hard to answer this without more knowledge of what's going to happen. If thickly glassed over, I'd use Western Red Cedar. Cheap, light and wont rot. I still see fence posts up here that are about 50 years old out of untreated red cedar.

[arjand]

Thank you all very much!! Doug Fir is out, now researching the foam. thnx!!!