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Thread: Grumman Sport Boat conversion to Duck Boat

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    Default Grumman Sport Boat conversion to Duck Boat

    Phase one is complete...mostly. As most of you likely know, I've been working (for months now) on strippiong and painting my 1969 Grumman Sport Boat for use as, not only a duck boat, but also float hunting, camping, fresh water fishing boat. (Yesterday, we used as a ski boat.)

    This project has turned out to be like the book 100 Years of Solitude (which my girlfriend hated, too her forever to read, so our inside joke now is when anything turns in a quagmire, we call it a hundred years of...in this case...duck boat. I've lost exact tract, but I am well past 200 man-hours of labor in this boat. I started working on this early may and I just finished July 3. In fact, I'm not yet finished as I still have to paint the interior and re-install the oar locks.

    I bought this boat from a young guy in St. Petersburg, FL and towed it the 5,000 miles back here last summer. A previous owner had done a good job of painting the exterior hull blue and had used a good etching primer, so, for the most part, the exterior required little work other than sanding, priming, and painting.

    The interior, however, was in the early stages of peeling apart. A primer gray colored paint with some non slip stuff in it. The bulk of my time was spent stripping paint (required a full gallon of aircraft paint remover) and then stripping down to bare metal using an air compressor and die grinder with rollock abrasive discs. (I went through about 20 discs.)

    With the inside of the hull stripped to bare metal, I treated the entire inside hull for corrosion prevention using Alumaprep 33 and Alodine, which also aids in paint adhesion. I followed that with aircraft zing chromate green primer, which, by the way, is pretty close to a "camo green" color. This part required 1.5 qts of Alodine and a qt of Alumaprep. Currently, the interior is still in the zinc primer green as I ran out of tan, and ran out of time. I will paint the interior as part of "phase two."

    With the interior done, I flipped the boat off the trailer and onto saw horses for the exterior hull. An initial sanding with 220 grit and then sandable, filler type gray primer (automotive stuff.) I followed that with a sanding of 320 and thin coats of Hunter Specialties Marsh Grass Tan. It required 5 cans of gray primer and 8 cans of the Tan to get it done.

    The transom was partially rotted Cedar ont he outside and what appeared to be part of pallet on the inside. Removed those and made two new pieces of transom wood made from 3/8 Marine AB plywood. Two sheets, bonded together with waterproof Gorilla glue, and clamped overnight. The inside piece had some very intricate curves in it, to match the shape of the aluminum, and I spent a good 10 hours sanding and shaping that one piece. Stained everything with Minwax Early American #230 and coated with 6 coats of Minwax Spar Urethane varnish. I sealed all of the bolts and bolt holes with 3M 5200 structural adhesive sealant.

    Actual repairs to the boat consisted of plugging 6-8 holes drilled in the boat by previous owners. Most were associated with a winch and remote anchor lowering/raising system. (Most of which was gone from the boat when I bought it. Only the rusted and seized winch remains.-It now resides at the Anchorage municipal landfill.) Those holes-mostly 1/4-3/8 dia were scab patched with stock aluminum treated with alodine and sealed with 3M 5200. No rivets. I patched them from the inside and filled the remaining space on the outside with bondo. Sanded and feathered, then painted over with primer.

    Phase two will consist of painting the interior tan. Reinstalling the sail thwart (the boat came with some of the original Grumman sail rigging. I recently cut three spruce poles to make mast and boom with and next summer’s project will be to build all of the sailing rig, including a new sail sewn from canvas. But that’s a different thread!) Phase three will be the building of a blind for it.

    Power. So I have three outboards for this boat. A 2 stroke 4hp, a 4 stroke 4 hp, and a 2 stroke 9.9. I figure to use the smaller engines for duck hunting camping etc ad the bigger engine for moose hunting or going in to PWS.

    And now…the obligatory photographs....

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    Inside of hull was covered with peeling gray paint and some non stick stuff. One gal. of aircraft paint remover and dozens of rollock abrasive disks later and it's stripped to bare metal and ready for Alumaprep and Alodine.
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    First bit of Aumaprep and Alodine. This is the bow with the floatation removed. I didn't chromate and pait this because it's hidden from view. I just left it Alodined as a corrosion prevention.

    Dipping small parts (scab patches for some unwanted holes) in the Alumaprep and Alodine.
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    Those boats love a 9.9 2stroke. I spent my entire childhood growing up in them. Dad and my brother both still own one. Do yourself a favor with the oarlocks when you put them back on. Put them on the inside vs the outside unless you intend to do alot of rowing. On the outside like mounted from the factory they grab alot of water when under power by an outboard. The driver is always soaking wet. On the inside you stay MUCH drier. And they still will work for rowing if you dont do alot of rowing. Easier to paddle it with two than row with one anyway. So we seldom ever used a set of oak.

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    There were several hoes drilled in the boat that needed to be patched. Thes once hel a winch to lower an anchor. I scab patched from the inside, sealed with 3M 5200 structural sealant, and filled the remiaing space with bondo for aestheitc reasons. No rivets.

    Half way through sanding and priming the outside of the hull.
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    Some of my bondo handiwork lol.
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    My son staining and varnishing the new transom, and the new transom installed on the boat.
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    End of phase one. Outside pained in Marsh Grass Tan, inside in Aircraft zinc primer green.

    Sportin' the nine nine. (Guess I didn't take any pics of it with the 4 hp on it.)
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    Quote Originally Posted by theultrarider View Post
    Those boats love a 9.9 2stroke. I spent my entire childhood growing up in them. Dad and my brother both still own one. Do yourself a favor with the oarlocks when you put them back on. Put them on the inside vs the outside unless you intend to do alot of rowing. On the outside like mounted from the factory they grab alot of water when under power by an outboard. The driver is always soaking wet. On the inside you stay MUCH drier. And they still will work for rowing if you dont do alot of rowing. Easier to paddle it with two than row with one anyway. So we seldom ever used a set of oak.
    Hmmm. Interesting idea. I hadn't thought of it. The oar locks are off right now, so I can test fit them in there and see how they fit. Seems to me the hull would be too curved for that. Actually, we do row it quite a bit. My son does-I don't. He did Rowing Merit Badge last year at Scout camp, and he's pretty good with the oars. How does putting the locks inside the hull affect the rowing?

    One of my earlier thoughts was to install the oarlocks with wing nuts on the inside and just remove them if necessary and temporarily cover the holes with gorilla tape.

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    The oar locks can be just flipped around and mount on the inside pretty easy. See alot of them in Mn set up that way. Especially those that use them alot on bigger water with motors. The oars dont want to pivot deep into the water with them mounted on the inside as they will hit the gunnwhale. Tough to get a smooth deep bite depending on the oars you are using. If not going far or not fighting a big wind, no big deal. All depends on how you use your boat. Just a thought I would toss out there since you listed your choices of outboards.

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    Quote Originally Posted by theultrarider View Post
    The oar locks can be just flipped around and mount on the inside pretty easy. See alot of them in Mn set up that way. Especially those that use them alot on bigger water with motors. The oars dont want to pivot deep into the water with them mounted on the inside as they will hit the gunnwhale. Tough to get a smooth deep bite depending on the oars you are using. If not going far or not fighting a big wind, no big deal. All depends on how you use your boat. Just a thought I would toss out there since you listed your choices of outboards.
    Well thanks. I hadn't really given much thought to that particular issue. I ran it this weekend with the locks removed, som y only concern was keeping the mounting holes above waterline. I did notice that,, after about 10 mph, I started getting wet anyway.

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    Nice project FL. What coatings, specifically, did you use for the transom? If you have used them before, how long do they tend to last? Would like to know because my last transom job was a complete failure after 3 yrs. (epoxy resin from West Marine over plywood. Resin was fresh, I don't blame West Marine's stuff.) Thanks...

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    Quote Originally Posted by HarryHyde View Post
    Nice project FL. What coatings, specifically, did you use for the transom? If you have used them before, how long do they tend to last? Would like to know because my last transom job was a complete failure after 3 yrs. (epoxy resin from West Marine over plywood. Resin was fresh, I don't blame West Marine's stuff.) Thanks...
    Thanks, Harry. Ummm, I went a sort of different route on this transom than in projects of years past. (I used to work at a marina with my dad, so I've done a fair bit of this stuff...YEARS ago.) In this case, I used simple wood work stuff. I prepped the wood with Minwax brand pre stain cleaner/preparer prior to staining with a standard Minwax stain. After the stain, I varnished with Minwax Spar Urethane varnish, which is an exterior urethane fiinsih very similar to "marine spar urethane" varnish sold at West Marine. On the advice of a friend, who is major woodworking fanatic (I used a lot of the tools and machines in his shop during this part of the project), I thinned the varnish 50/50 with mineral spirits. (Thin using whatever the directions say to use to clean the brushes with.) This thinning allows the varnish to soak/seep/penetrate deeper into the wood. I put on 6 coats on each side, as well as the edges. (It's important to get the edges sealed to keep water out. Same goes for the bolt holes; I pre-drilled all the bolt holes and then made sure I got varnish in there as well.) One thing I learned is that thinning with mineral spirits cause the finisih to be really ugly, however, one last coat of uthinned varnish and it came out perfect, cosmetically speaking. As to the longevity of that work...I'm not a big woodworker myself, other then my 3 years of shop classes in high school, so I don't know for sure, but I'm told this should last a pretty long time (decade or more). If it was goig to be a do-over in 3 years, I would have done it differently.

    My USUAL way of doing transoms like this in the past has been to, again using marine plywood, completely encapsulate the wood in fiberglass cloth and resin to seal it from the elements, then to sand the fiberglass work smooth and fill with bondo or glazing puddy and then to paint a matching color to the boat. That is pretty much a 20 year piece of equipment then. I've built small boat transoms, boat decks (for decks I fiberglass encapsulate PT 1x2's for reinforcing structure and sometimes use plywood as much as 1/2" thick, and even once built a huge livewell for a guys offshore saltwater boat...all the same way-fiberglass encapsulated marine ply. The livewell was still on the guys boat when he died about 10 years later and he was still using it.

    I went this other route on this boat because I wanted the aesthetics of stained/varnished wood.

    As to your project; I can't say what happened. I used towork at West Marine and used their fiberglass stuff a lot and never had issues with it.

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    Thanks FL, printing this out for future use! I think my problem was using cabinet plywood, because I got a piece of it for free, and its extremely strong with its many thin plies. I didnt think about the glue is probably not being waterproof and because epoxy resin will absorb a small amount of moisture, it just delaminated the wood.

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    Quote Originally Posted by HarryHyde View Post
    Thanks FL, printing this out for future use! I think my problem was using cabinet plywood, because I got a piece of it for free, and its extremely strong with its many thin plies. I didnt think about the glue is probably not being waterproof and because epoxy resin will absorb a small amount of moisture, it just delaminated the wood.
    Yeah, several people tried to get me to use cheaper materials (in a lot of places) on that boat. Someone offered to give me CDX construcion plywood. One guy suggested I use roofing cool seal to paint the interior, bla bla bla. You can't get a good finished product with crap materials. I'm sure that cabniet plywood looked very nice, but the glue in marine plywood is waterproof and there are very minimal voids in it to trap and hold water.

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