Replacing plywood floor on 18' flat bottom

[Keegan.MW]

Right now my boat has all season carpet over pressure treated plywood. It works well but there are other options. I'm looking to replace it with lighter weight material so hopefully by boat will go that little bit faster. I've been looking into several different materials to use and came across some firewall board used for construction. It's 1/4" thick, very rigid and water proof. Is anyone familiar with this stuff or seen it used for this application? I have an open mind so if anyone has any ideas on using other materials it is also appreciated. Thx!

 

[Woodonglass]

Re: Replacing plywood floor on 18' flat bottom

Is this an aluminum boat? if so then I'd advise you to get the PT plywood out of it ASAP!!!! It will EAT your boat!!!

 

[DeepBlue2010]

Re: Replacing plywood floor on 18' flat bottom

so you want to redo your deck with 1/4" thick ply?!!

This is not enough strenght for a deck. 5/8 or 3/4 exterior grade or marine grade ply is much better fit for this job. How thick is your pressure treated?

 

[kfa4303]

Re: Replacing plywood floor on 18' flat bottom

What type of boat is it (fiberglass or aluminum)? It's generally not a good idea to use non-marine materials in a boat. I don't think 1/4" anything will be strong enough either, unless you're using carbon fiber, or some sort of a composite. For most boats of this size, you'll want to use 1/2"-3/4" plywood, or a material of equivalent strength. If it is an aluminum boat, be sure not to use pressure treated as stated above. It will cause pitting in your hull.

 

[jbcurt00]

Re: Replacing plywood floor on 18' flat bottom

this was the only 1/4" 'board' that popped up via a Firewall board search:

Coroplast

The rest were drywall products

If you can strip 100lbs of 'excess' weight out of the boat, and IMO you can't easily do that, esp not w/ only a change of decking & decking covering, I doubt you'll even notice any speed gains.

If the weight reduction isn't balanced fore & aft, it may even adversely affect your speed, or how long it takes to get up on plane. Additional bow rise at take-off may also increase if the forward weight is reduced significantly vs the stern weight.

What boat

What motor

What prop

What hp is the boat rated for

Has the motor gotten more then just basic maintenance recently

Does the motor's current setup allow it to reach it's optimal rpm range

Pix would be great plus more details on the firewall board you're talking about.

 

[Keegan.MW]

Re: Replacing plywood floor on 18' flat bottom

Correction, it is not pressure treated ply in the boat. I got the boat used and just assumed it would be treated. I was not aware that green plywood would eat aluminum. It's either 1/2 or 5/8 depending if it's swelled up any. And I'm trimming up more than just the floor to save weight. My goal is to save around 100 pounds total when I'm done with everything. For instance i have 3 anchors on the boat now and one of those is coming off. I'm hoping the floor will be 30ish pounds shaved off if I can find the right materials without it being too flimsy. Im not sure of the name of this firewall stuff, I've only seen it on jobsites. But it is different from that coroplast stuff. I'm thinking if I put r-board insulation in between the ribs and put this board over top it should be strong enough and not flex much. For those concerned my boat setup is:

1976 Lowe Line 18' flat bottom (rated for 35hp)
1978 Johnson 35hp seahorse shortshaft with 11pitch prop

I blew a cylinder last fall so I got ahold of a used power head in great condition and just finished putting it all back together about a week ago. Both cylinders are at 125psi and I've only tank tested it but it seems to be running like a top.
I'll work on getting a couple pics.

 

[Woodonglass]

Re: Replacing plywood floor on 18' flat bottom

If you put the pink or blue slab foam down and level it out and then put Sealed 1/2" plywood over that IMHO you should be fine. If you're not a 300# Big Boy like me it shouldn't flex much at all. 4x8 - 1/2" plywood weighs approx. 50# if that helps any.

 

[Keegan.MW]

Re: Replacing plywood floor on 18' flat bottom

 

[Keegan.MW]

Re: Replacing plywood floor on 18' flat bottom

So the front end has plenty of weight with the plywood storage, I'm leaving that alone. Just want to redo the floor.

 

[jbcurt00]

Re: Replacing plywood floor on 18' flat bottom

100lb weigh savings might get done if it's currently got any wet wood or flotation in it that you pull out. But even w/ a 20lb anchor, 100lb savings sounds like a lot. Not saying it can't be done, just that it might be challenging.

Yep, details on the decking you plan to use would be great.

 

[jigngrub]

Re: Replacing plywood floor on 18' flat bottom

If your plywood is old and wet you can get a substantial weight loss just by changing it to new and dry. Sealing your new plywood to keep moisture out will help a lot, also changing over to a painted on finish or marine vinyl instead of carpet will also reduce weight.

If you can find sheet foam the same thickness as your rib height you can drop your plywood deck thickness down to 3/8" if you don't plan to fasten anything to it. If you plan to install pedestal seat bases you'll need a minimum of 5/8" thickness decking.

 

[Keegan.MW]

Re: Replacing plywood floor on 18' flat bottom

If your plywood is old and wet you can get a substantial weight loss just by changing it to new and dry. Sealing your new plywood to keep moisture out will help a lot, also changing over to a painted on finish or marine vinyl instead of carpet will also reduce weight.

If you can find sheet foam the same thickness as your rib height you can drop your plywood deck thickness down to 3/8" if you don't plan to fasten anything to it. If you plan to install pedestal seat bases you'll need a minimum of 5/8" thickness decking.

Ok that's one thing I forgot about... The seat pedestals. I will prob just replace the old decking with fresh 5/8 ply because the old stuff definetely got wet last year when I got rained on while fishing. I take it doesn't dry out well?

 

[jigngrub]

Re: Replacing plywood floor on 18' flat bottom

Ok that's one thing I forgot about... The seat pedestals. I will prob just replace the old decking with fresh 5/8 ply because the old stuff definetely got wet last year when I got rained on while fishing. I take it doesn't dry out well?

We see lots of boats here with wet/rotten plywood and foam, but all of them are usually left uncovered outside and exposed to the elements.

It looks like you keep your boat garaged though, so your plywood probably isn't that wet if at all. The decking will dry out if it's only exposed to occasional weather and kept dry most of the time like I think yours is.

If your decking was installed correctly with the carpet glued to the plywood and then screwed down to the boat you should be able to remove your bottom decking easily by looking for the screws buried in the carpet nap, just back the screws out and lift the decking up and check the back side to see if it's wet. If it's dry, just lay it back down in the boat and put the screws back in. Not a big deal, especially with that nice simple setup you have.

... but! If the decking was screwed down in the boat and then the carpet was installed you'll have to rip up all of your carpet to get to the screws to remove the decking... after you sand all of the old carpet glue off so you can find the screws.

 

[Keegan.MW]

Re: Replacing plywood floor on 18' flat bottom

Thanks everyone for the advice!