Wednesday, June 24, 2015

Grumman SP145 - Part 2: Choosing new floor material

I researched a lot of options for new floor material which I will list along with pros and cons and maybe this will be of interest to someone facing the same decision.

When dealing with aluminum boats you should beware galvanic reactions which can occur with dissimilar metals + an electrolyte. Examples would be steel screws into aluminum + salt water, or pressure treated wood (copper + salt) against aluminum.

Plywood - The original manufacturer used plywood, stainless steel screws and carpet. Occasional moisture or splashes will not hurt the plywood but my boat lives outside. Carpet would only make things worse by trapping moisture.

Painted Plywood - Painting is a good option, looks pretty good and lasts for years. The painted plywood the previous owner used held up for over six years with no sign of deterioration.

Epoxy over plywood - A excellent option that is expensive and time consuming but very long lasting.

Pressure treated plywood or decking - Pressure treated wood in contact with aluminum will cause galvanic corrosion because of the copper and salts used to treat the wood. The best advice is to allow the wood to dry well, preferably for some months, before installing. This is because pressure treated lumber has fairly high moisture content when you buy it and that additional moisture will only accelerate the galvanic corrosion. On top of that, you either have to seal the plywood or seal the aluminum so they can never come into contact.

There are new treated plywoods rated for aluminum contact but if you read the fine print that is only under dry conditions. They still contain copper and salts and will cause galvanic corrosion if they get wet.

Marine grade plywood is just pressure treated plywood that costs twice as much. There are other options such as MDO which is very heavy and very expensive.

Composite decking - Seems like an ideal floor material as it doesn't corrode aluminum and often comes with 15-20 year warranty. But it's also very heavy, gets very hot in direct sun, expands and contracts more than wood, and is relatively expensive.

Cedar/redwood/cypress or other rot resistant wood - Unfortunately the same oils that provide rot resistance also tend to corrode aluminum. But the plus side is that nothing beats real wood in appearance.

There are serious pros and cons to every flooring option. In the end I chose Eastern red cedar because it's extremely rot resistant, beautiful, plentiful and inexpensive in my area. I tried to use nothing but red heartwood (white sapwood is not rot resistant). A shot of spray paint over the aluminum will hopefully be enough to prevent corrosion. For fasteners I stuck with stainless steel. I figure if the manufacturer chose stainless who am I to second guess them.

Freshly planed ERC. I wish it stayed this color.