If you’re not familiar with the Gheenoe or Riverhawk style of boats, check out a more detailed post here. This post is part of a series where I document the modifications I make to my Gheenoe.
One big draw to the Gheenoe line of boats is that they are so customizable. A raised floor is one of the most useful modifications you can make to a Gheenoe or Riverhawk. Sometimes you hear it referred to as a false floor. It is a secondary flat deck above the bottom of the hull. The stock hull mold is contoured and hard to walk around in. A flat, raised floor provides a nice flat surface to use inside the boat.
A few side benefits are that you can delete the middle seat and mount a cooler bow to stern. It provides walking room around the outside instead of having to step over the center seat. You don’t really lose seating though because the cooler is there to sit on. Don’t need a cooler and would prefer more carrying capacity? Easy, leave it out or use a smaller one.
It provides a nice flat surface to mount a grab bar. Bars allow the operator to stand and to move their weight towards the center of the boat improving handling. The bar is also going to serve as a console in my boat. It will house the switch panel, gps and fish finder.
I used ½” standard plywood. In a perfect world I would have used a composite sheet product that would never rot and I would never have to worry about it again for life. But I got the piece of ply for free and it will do well enough for me. If I even had to do it again, I would use ⅜” ply. With fiberglass cloth and epoxy on both sides it doesn’t even deflect to my weight. The plywood floor added 20-30lbs to the total weight.
This is a moderate difficulty project. It is going to take some patience and planning but the work isn’t overly advanced. A checklist is going to be a valuable tool. You don’t have to take it to the level I did, but I have a vision for my boat and that’s what I’m working towards.
The first step is to axe the center seat. Mine had aluminm struts that supported the sides. They have 4 rivets each. My Gheenoe hull was built by Riverhawk, and it was what they were using at the time. Gheenoe licensed their designs to at least two other shops I know of. So, this is not that uncommon. A Gheenoe built hull will have fiberglass struts attached to the seat. EIther way I used a reciprocating saw to cut the seat out about ½” above the bottom. I used an angle grinder with a 40grit disc to remove the rest.
Under the seat seemed thinner than the surrounding, so I laid two thicknesses of 4oz cloth and epoxied them in. Any time you hear me refer to epoxy, I’m talking about two part marine boat building epoxy. Examples would be West System, System Three, Total Boat or US Composites. I have used West System the most but it is just so expensive. I switched to US Composites for this project at nearly half the cost. I’m now using System Three filler products, which are also cheaper than West.
I used a hole saw to open up the drain hole going under the remaining seats. I think it was 1 ¼” but use the one that fits down in the keel the best. This will make sure nothing gets under the floor and clogs the drains up.
Use the grinder to grind out the gelcoat around the perimeter where you will epoxy the floor in. There is a narrower option, and a wider option. I liked the wider option that spanned the entire floor. This grinding is the worst nastiest part. I suit up with long pants, long shirt, respirator (not the cheap N95), and baby powder myself down. Especially arms, neck and face. I would like to wear a painters hood but didn’t have one available.
You need to clear the gelcoat away from anywhere you will be epoxying to. Give yourself the best opportunity for a strong joint. Go all the way around the perimeter and also the cleats that will glue to the seat. The cleats will support the front and back edges of the floor.
Fitting the floor. This will be the hardest part to explain. Lay out a grid on your floor material. One line down the center. One at mid point perpendicular to the first line. Measure the width at the very back edge and very front edge. Transfer these measurements onto your substrate using the center line as your reference point. Divide the widths in half and measure from centerline half each way. Continue measuring back in 12” segments. Measure the width at the mid point of the floor. Then connect the dots.
If you did it right you should be able to see the curve of the hull. Use a reciprocating saw or jig saw to cut it out. Leave a half inch on either side. Better to grind some off than to miss the cut and be short. It’s much easier to take off than add back on.
It is going to take some test fitting. Fit, mark, grind, repeat. I bet I have placed, and pulled my floor three dozen times. It is a real pain in the ass, but if you want it to fit tight and fit right, you need to go slow. Once you get it where you like it, place it in one more time.
You can at this point epoxy the underside of the floor to seal it. I used some spare pieces of 6oz cloth I had laying around. But 4oz is more than enough. Just cut it to fit, leaving an inch strip shy of the outside edge. Mix epoxy and wet the cloth out. Use a fiberglass roller to get good wetting, adhesion and all the air bubbles out. Alternatively you can epoxy the bottom of the floor later, after you have fit any accessories.
Make some marks where the top of the floor meets the seat partitions. You are marking the top of the floor height. Pull the floor back out. Measure the thickness of the floor and make that mark under the line you made for the top of the floor. This line is the bottom of your floor.
With this line, the bottom of the floor, it’s time to start making the cleats. The cleats support the front and back edge of the floor. Start with a ¾” x 1” rip of plywood the full width of the floor where it meets the seat. You need to grind until it fits at that intersection. The top of the cleat needs to be at the bottom of the floor, or very close. It can be a little lower but it cannot be higher. The epoxy will save us from any small holes or mistakes.
Once the cleats are fit, they can be epoxied in. For this kind of glue up, you will need to thicken your epoxy with silica thickener and milled fiberglass. Silica is solely a thickener, it adds no strength, but is as strong as the epoxy would be on it’s own. It is considered a structural filler. The milled fiberglass are tiny fibers of fiberglass and they do add strength.
So, your cleats are fit, place the floor back in. Now is time to fit any accessories that may be tied to the floor. I wanted access under my floor to run wires. I will be wiring my boat, and it will have nav lights and electronics, nav and depth finder. I added a large rectangle deck hatch towards the stern and a 6” round access in the front. These are simple enough. Mark the opening and then cut it out.
A reciprocating or jig saw can be used. A router will give a better cut though. Get the access in and use short screws to mark the holes. Remove everything and drill the holes out to ⅜”. These holes will get filled with thickened epoxy. The goal is to seal the plywood around the screw holes. If you screw the screws right into the plywood it will surely rot at each screw hole. So, we need to isolate the wood around the screws.
I wanted to add a grab bar to my boat, so I made a plywood pad with studs in it to mount the bar. The bar has ¼” plate on the bottom, I drilled it for bolts. The bolts stick up from the pad and I epoxied the heads in. I will make a separate post for the grab bar mounting which you can read here. I wanted to run wires up inside the grab bar so I needed holes in the floor to do it. I clamped the mounting pad in place and used a hole saw to cut the holes.
All the holes in the floor, the deck hatches, and the wire holes i used a router and roundover bit to kill the hard edges. Fiberglass doesn’t like hard edges.
Place the floor back in, you are getting towards the final glue up. You need to make and fit the side struts. You removed them back when the middle seat got axed. They really provide a lot of rigidity to the sides and need to be replaced. There is some leeway in how you arrange them. But, I put the in about the same place as the middle seat. I wanted mine to tie right into the floor, but not everyone does it that way. As long as there is some support for the sides you will be okay.
Use cardboard to make a template. Trace it onto a piece of plywood and then cut ¼” over that line. You want plenty of material to work with because you will need to grind them to fit. They never fit just right. Adjustments will be needed. And both sides will be different. So don’t use the same template to do the other sides. Mine were at least ¼” different side to side.
I wanted a support under the center of the floor. I honestly don’t think ½” ply with cloth and epoxy on both sides needs it. But, I might have a heavy cooler in that spot or other gear and it only takes a few minutes. Under the middle bench there was a fiberglass bridge over the keel. I believe the middle bench is sealed for use as a livewell.
I used a 12” strip of ⅜” plywood. Use a straightedge and go from rear cleat to front cleat. This is the plane of the bottom of the floor. Then check the measurement side to side. Mine was perfect at about 1 ⅞”. Use thickened epoxy to glue it to the hull. Let it stick up for now. When you epoxy the floor down you will spread a little epoxy there and put a weight on it.
As we prepare for the final glue up, lets cut the fiberglass cloth to fit the floor. Remove all the accessories, spread it out and use scissors to fit it. I wanted mine to go up the sides about 2”. Just let it drape over any holes, we will cut them out later. You can make a couple witness marks as reference points when you roll the cloth back out. You want it back in the same spot.
The final glue up is the moment of truth. You have at least a few hours invested at this point. Probably more like 4-6 hours. Make sure you have at least four hours set aside to do the final glue up. And a day is not too much time. Make sure you have scratched up any spots that got new epoxy that will now get epoxy over them. Epoxy can get a waxy coating during the natural curing process that can inhibit good adhesion. Either a scouring pad or sand paper will remove it. Make sure the gel coat behind the side struts is well sanded. Make sure the cleats have gotten a good sanding.
The best thing I have found for doing a glue up like this where you have to spread large amounts of thickened epoxy is an empty caulk tube. You can either buy empty fillable caulk tubes, which you will have to order, or you can buy a couple cheap caulk tubes and spend the half hour to clean them out. Nasty but maybe worth not having to order them.
The fillable tubes are necessary because you have to spread a large amount of epoxy before the chemical reaction happens, it heats up and sets. The larger the amount of epoxy the faster this happens. For this project you want the slowest hardener you can find. Either West System slow or US Composites 4:1. You will need to work fast. It is a good idea to have several containers of resin already measured out. All you will need to do is add the hardener, add the fillers and then fill the tubes. A helper is good to have so that they can be mixing or filling tubes while you apply.
You will want to have several, maybe as many as half a dozen containers that you will pre pump your resin out in. It also might be a good idea to have some smaller cups to put the pre pumped hardener in. Time will really be tight to get all the thick epoxy down before it starts to kick. I was too slow and ended up scraping some out before I put the floor in.
The first step is to wet the joints with epoxy. Be sure to brush epoxy on the bottom, where there is bare wood, the cleats, the center support, and any penetrations like deck accesses. Start laying beads of epoxy where the floor and hull meet. Don’t put any thickened epoxy on the floor it will make too big of a mess. There is a shoulder there on my hull. Don’t go in farther down than contact will be made or you will just be wasting epoxy. Don’t go higher than you need to, you can get the top side after you set the floor and time is of the essence.
Once you feel you have a good amount that will fill the gap, set the floor. Don’t forget to put some on the cleats and on the center support. There are no take backs at this point. What you have is what you get. Kind of wiggle the floor in so it seats everywhere it needs to. You can make some black marks at the edge of the floor before you put it in. This way you will know it’s all the way down. You can put a couple weights on the floor while the epoxy fires. But don’t overdo it, we don’t want the hull contorted.
The floor is in, it’s time to fill and gaps on the edges and make a nice fillet for the fiberglass. When the edges are filled roll out your precut and fit fiberglass cloth. Take your time and adjust it, get all the wrinkles out. Start mixing unthickened epoxy to wet out the cloth. Use a fiberglass roller to get all the bubbles out.
This is the homestretch, the completion of a large project. For me, I need to place the grab bar pad. I have made marks on the floor I can see through the epoxy. Just lay it right on top of the glass. Use thickened epoxy to make a fillet around it. Brush the top with epoxy to seal it.
About the last item to complete is gluing the side struts in. Make some thickened epoxy to glue them in. Make fillets around them to make the structural. Seal them with epoxy.
Let everything cure overnight. Most people just sand and paint. If you want a smooth finish, you could wet everything with epoxy and use one of the fairing fillers. These are very lightweight light density fillers used to fill cosmetic holes. You can spread them over fiberglass to fill the weave and any irregularities. It really makes a quality finished project. After you sand the fairing filler, seal everything with a final coat of epoxy. You should have a finish that is ready to prime and paint.
My next big project is going to be wiring. I’m going to wire my hull to have navigation lights, bilge pump, gps/depth finder, most likely interior led lights, a transom mount underwater light for attracting fish at night and an led bar for the front.