changing the layout in my boat...need help

first off let me say thank you in advance to anyone that responds.
i have a 15 foot fiberglass row boat . the setup in it now has 3 fiberglass seats which i added plywood to for more strenth so i could add folding down seats too. what i want to do is replace the wood with fiberglass. now my questions are. what should my core material be if i want it to supprt a large person and a pedistal seat. what wieght fiberglass should i be using in conjunction with this . i will get some pictures later on tonight that will hopeflly shed a little more light on what my plans are for this project

 

Plywood, pound for pound is stronger then steel, so you've found a good core material.

Be careful, a 15' row boat doesn't generally hold all that much as far as number of people and combined weight of people and gear. Over loading small boats is the easiest way to find yourself in trouble fast. In fact, excess weight is the enemy of all small boats.

Before we go any further, can you provide the year make and model of the boat so we can provide reasonably educated answers to your questions? If these are unknown (not that uncommon so don't be alarmed) a few photos would be very helpful.

 

ok well i have a couple photos for you . the boat is around 1950 rowboat. i have no clue who the maker of it is. but its solid as a rock and steardier than most boats out there.i only fish with 2 people in it at all times. never more. basicaly what i want to do is take out the plywood i have in there . its 3/4 inch pressure treated and weighs a ton. i am also worried about weighing down a smaller boat and was thinking fiberglass would be lighter and last longer. but since there is going to be two seats on the boat. what material should i use that will support a seat setup and a large adult. and also i would like to take out the middle of the benches and have almost a walking alley down the middle of the boat. please help if this is at all possible or i'm just beating a dead horse. you can see the photos at this link here http://www.flickr.com/photos/26300232@N02/

 

It's very likely you don't need the plywood "reinforcements" on the seats. The 'glass is probably more then up to the job. It may flex a little under load (or someone's butt) but that's okay.

The two enclosed seats are holding the sides of the boat apart and are structural elements, so cutting the centers out wouldn't be advised.

3/4" ply is over kill for seat lids, you only need 1/2" at the most on those if they are doubling duty as storage compartment lids. Every ounce you can save will be worth it on these little boat, especially if you are rowing or poling.

To make the new wood last longer, apply three coats of epoxy and a light weight cloth (like 4 or 6 ounce). The epoxy will seal out moisture and the cloth will provide abrasion protection. Of course you'll have to paint too.

 

what i would like to do is take out the ply wood and just use fiberglass. i have a 20 hp motor that pushes it around 20 or mph would the fiberglass hold up to an adult seated in a chair at those speeds or should i use 1/4 ply the some fiber for the seats. what wieght fiberglass should i use if i do it this way?thanks agian

 

It's difficult to tell frankly, because I haven't a clue how thick the laminate is in your boat in the locations you mentioned. If you want some "insurance" then yes, a piece of 1/4" ply, embalmed in epoxy and sheath with two layers on 6 or 8 ounce cloth will firm things up, but more importantly, add considerable abrasion resistance to the plywood.

If you're only getting 20 MPH from her now, I'd think she's already built quite heavily and increasing her laminates will not help much.

Your best bet is to test the areas in question, either with weights or an actual volunteer, who will likely be glad to help out (every one loves boat stuff). Check the amount of deflection in the contact areas. Some is acceptable, but if things are "popping" and cracking sounds are heard, you'll probably need some additional reinforcement.