17ft Cobia Monte Carlo Project

[brospano177]

Hello... first I want to start off with, I'm a long time reader of Iboats and first time poster, there is a lot of info on this site. I have been researching the threads to see if i can answer my questions, i have a general understanding as to what i need to do, but need my specific questions answered regarding stringers transom, and fibreglass. Any and all help will be appreciated.

I have recently purchased a 1975 Cobia Monte Carlo as a project boat with a nice trailer for $200. The deck was already ripped out and the stringers were exposed and rotten. I got the boat home and took the cap off, to get to the rotten transom. At this point the cap, stringers, and the transom are removed from the shell.

Question # 1
The stringers have been cut out, do I need to grind the fibreglass down to the bare shell? Its been cut down with a grinder as low as possible. My thought process behind this is to leave what's left as a template for the new stringers.

Question # 2
As of now i am ready to buy the transom materials. From what I understand it is best to use douglas fir 3/4 inch marine ply as the core material. What type of resin/epoxy is best used for the transom? Should I get matting or mesh fiber?

Question # 3
Can you give me detailed instructions on what I need to do to prepare the transom? I know how to shape the transom and cut the wood and that the 3/4 inch ply needs to be sandwiched together. I am unsure what type of material needs to go between the two pieces of ply that are going to be sandwiched together (just resin and fiber or just resin), and how to connect this ply to the boat shell.

Here is the link to the pictures:
http://picasaweb.google.com/brospano/IboatThread#

 

[jonesg]

Re: 17ft Cobia Monte Carlo Project

1. either way, left mine in , you still have to sand the area where new glass attaches stringer to hull, the more you cut down the less old glass to sand.
Bed the new stringers in PL Premium construction adhesive, the idea is to keep the stringer from direectly contacting the glass hull to avoid hard spots.

2.Use what you can afford, most use PT exterior grade ply but you have to let it dry a while. Most of us used polyester because epoxy is rather pricey, once you start with epoxy you have to finish with it because you cannot switch and apply polyester over it on critical joints such as transom/stringers.
Use mat with poly, biax on critical transom/hull/stringer joints.
Use cloth only with epoxy.

3. you can sandwich transom with mat if polyester, use cloth if epoxied.
PL works to bond wood to transom just fine, or use epoxy , I like PL.
Just thru bolt the wood to the glass, use 2x4 backing to clamp it.
its straightfwd and simple.
Some cabosil makes a nice putty to bed the bottom and sides of the transom to the hull, form a filet later, glass don't like 90 deg corners.

My preference is to pre-glass the wood parts, sand and wash down with acetone before installing, its easier on the back. Then all you need do is bed and tab. Look closely at your boat, on mine they originally glassed the stringers in place with biax glass tape. Thats how they made it look so neat.

Which ever way, be sure the edges are sealed, 2 or 3 coats of resin until it stops soaking into the wood. Then install it.

 

[brospano177]

Re: 17ft Cobia Monte Carlo Project

I found the douglas fir 3/4 for about 70 dollars a sheet. I need to dry it out? if so is there a prefered method.

How do I tell what the the hull was originally made with, epoxy or polyester?

As far as the stringers what type of wood should be used?

 

[brospano177]

Re: 17ft Cobia Monte Carlo Project

I was thinking... could I use something like High-density polyethylene wood decking material as the core of the stringers

 

[brospano177]

Re: 17ft Cobia Monte Carlo Project

called the manufacturer and confirmed that the boat is made of poly resin.

I decided not to use any composite for stringers.

I went to some lumber supply yard and talked with them as to what was the best wood to use. the explained to me that the best thing to use is aether marine douglas fir or kem dried pt pine so the poly would stick and not rot the wood because of it being wet

From the Iboats forum I found a poll about using solid or ply wood. There was a very high percentage of people that use ply, so I bought 1/2 for the stringers and 3/4 for the transom all marine douglas fir.

The transom is going to be 2 sheets of 3/4 ply to equal a 1 1/2 inch thick
The stringers are going to be 2 sheets of 1/2 ply to equal 1 inch thick

We measured where the top of the deck used to be and subtracted 3/4 for the deck ply

Then we cut the stringers to length and hight. Next step is to cut the bow curve in them
I plan tracing the hull onto the stringers for this, but I'm not sure how to keep the top of the stringer level to where the deck used to be while tracing the curve. Any ideas would be greatly appreciated.