Re: OMC 3.0/Stringer I/O Conversion to Evinrude 225 Outboard project
Help me understand.. most marine grade ply is pine based. In my world, pine is a weak soft wood.
I want strength even if that means extra work to seal it up.
I'm not sure that's ^^^ the real question, is it? If you want to discuss merits of Marine VS interior grade plywood (even if it's cabinet grade) we can, but MOST of the resto's @iboats don't use marine because it's either very hard to find or very expensive. It still needs to be treated w/ either epoxy or polyester resin (if poly then cloth/mat too) for a fiberglass boat rebuild. Tin boats are a bit different, but personally I'd rather use epoxy for them too over other choices.
Like I said before, Marine grade is at the far end of the
best plywood to use scale. IMO, and WOG's I'm fairly sure, interior plywood is at the far other end of the spectrum of plywood choices. There are many choices that would be better, primarily because they use waterproof glue in their laminating process.
As most fiberglass resto's progress, it is usually evident that the question shouldn't have been IF the plywood gets wet, it should have been WHEN the plywood gets wet. They're boats, they do get wet.
Get a piece of ACX plywood & cut a 6"X6" piece out of it. Do the same w/ the interior plywood you bought.
Compare the edges. Are their ANY voids along the edges where the layer didn't quite get 100% coverage w/ it's veneer?
I think there's an edge void along the bottom left curve of the plug, in this pix, you glued up recently, possibly 2:
And 2 more in the edge of the ply plank at the bottom of the pix, just after where it crosses the sawhorse.
Now imagine sampling the 2 types of plywood w/ a 100 6X6 squares. Typically, not 100%, but typically, some exterior grade plywood has fewer voids in the layers of veneer then interior grade. I really like working w/ MDO, it's rated for exterior use (they make wooden concrete forms w/ it & exterior road signage), has 1 extremely smooth face (I buy the good 1 face/side version), and it's nearly void free.
It's not an exact sampling, but if there are edge voids on the plug you made, and on a few of the 6X6 squares we imagined cutting, how many voids are in the layers of plywood you can't see on the rest of the plug you made?
Here's some info on plywood grades:
Plywood Grades and Bonding Types - Woodworking Tips
BTW: this is the standard required for marine plywood:
BS 1088
If you are sealing all the plywood you put into a boat (it sounds like you plan to, and you should) I'm not sure you're getting the added strength benefit you think you're getting using the interior plywood you chose.
From elsewhere online:
In the case of interior grade plywood, Urea Formaldehyde glue (known in the industry as UF) is the most common type of glue used for joining the plies together, while in the case of exterior grade plywood, Phenol formaldehyde (known as phenolic resin) which is a superior quality glue is used.
Strength comparison:
Exterior grade plywood is stronger (has better glue shear strength) when compared with interior grade plywood.
Differences in Cost:
Exterior grade plywood is also known as BWR (Boiling water resistant) plywood and it costs more than the interior grade plywood (which is known as Commercial ply).
Resistance to water:
The phenolic resin used in exterior plywood is actually a plastic resin. Phenol is the substance from which plastics are made. This resin imparts better water-resistance properties to plywood. Quality tests of exterior plywood, often involve submerging the plywood in boiling water for upto 4 hours, and then taking it out and checking its strength and other qualities.
For all practical purposes these types of plywood are considered adequately water-proof and hence can be used outdoors or are often used in making kitchen cabinets and the like.
UF resin on the other hand is not as good as phenolic resins, and hence interior grade plywood can get damaged if exposed to water. This is the reason why its used indoors (kept away from water).
I am NOT telling you to start over, or that you've made the wrong choice (for you, your budget & your boat build) I am asking questions about how & why you chose what you chose, and if we choose to build our boats differently, nothing wrong w/ that.
Same as the question about the plywood plug's shape & how you came up w/ it.
Perhaps you weren't aware of the benefits of Phenolic Resin & waterproofing, perhaps you were. But mostly for those that follow, I thought it relevant info.
