C-Dory Extension Project

I'm extending a 22-foot 1984 C-Dory by four feet. The short story is that the transom balsa core rotted, apparently due to bad engine installation. After removing the splashwell and starting work on the new transom core I kinda liked the extra cockpit space. Using Freeship, I redesigned the hull, lengthening it and widening the transom; the new transom and splashwell are essentially the same dimensions as the current C-Dory offerings. The wider transom will enable me to mount a remotely operated power-tilt kicker next to the 90HP main using the same steering setup. A naval architect imported my files into real software and gave the design a thumbs up. I'm also working with a marine surveyor who has provided many good suggestions; one of which was to use C-Flex as my hull form where there won't be any plywood core.
At this point, I've installed the plywood cores for the gunwales, sole and transom, and the hull forms on one-foot stations as you can see in the pictures. When the glassing is done, the only wood left will be the aforementioned cores; forms and bracing will be removed.




I'm going to use epoxy for the new construction to ensure that the new boat stays with the old boat. I've also decided to cut the top two 'lapstrake' panels off behind the cabin (you can see them duct taped together at the ends) to stagger the old boat/new boat joint. They were cut apart to allow them to flex to the new width, but it's gonna be a PITA to do a repair - just complicating an already complex job.




The boat will be turned upside down to maximize the use of gravity. The gunwales are gonna be interesting though since they'll be upside down. I was thinking of letting each new coat of epoxy begin to set to hold the glass in place while I brush and/or roll on thickened epoxy to saturate on these gravity-challenged sections.

I'm pretty well sold on using a biaxial stitched fiberglass with mat; either a 1708 or 2408; one layer 45/45, the next 0/90. Which brings me to the first question: I've got samples of each and I was surprised as how well the 2408 conformed to the bends (to be filleted) and stayed put while just pushed into place while dry. I like the idea of using the heavier glass to keep the number of laminations to a minimum. That said, I am new to this and have heard some folks recommend sticking with the lighter-weight 1708. Opinions?




Based on information from several threads here I plan on laying up one layer at a time and coating each with a filler; I think it's referred to as 'bog'. That sounds like a much better surface to sand between laminations. I wonder though about how successive layers bond the previous fiberglass with a somewhat friable filler between; it sounds as though it could cause a weak layup, but I've not done this before.

What is the black coating inside the hull beneath the gelcoat? It blocks light through the hull so that it's not translucent. I'd like to replicate that in the addition.

I'm also pretty much sold on gelcoating the new work as well as the remainder of the hull up to the rubrail. From my research so far, it seems that one can use gelcoat over epoxy if the epoxy if fully cured (several weeks), all amine blush is removed, and if a 'primer' is used between the epoxy and gelcoat. Apparently, a number of gelcoat dealers don't even want to talk about putting their product anywhere near epoxy... I'd like to hear other's experience on that topic?

 

Looks like a great project. Are you going to extend the cabin at all?

I know nothing about gelcoat other than my old boat it's all faded and you can't polish a turd no matter what the Mythbusters say. ;-)

Working with the boat upside down will help a lot I'm sure especially on the bottom and sides. A few tricks I've learned using the biaxial fabrics.

1. The heavier stuff will be a bear to wet out and will soak up a lot of epoxy.
2. When working on your gunnels upside down wet out the fiberglass on some plastic sheeting and let it get really sticky before putting it on.
3. You can use peelply (heavy nylon fabric) between coats to create a nice mechanical bonding surface without sanding.

I think most guys paint boats built with epoxy rather than gelcoat.

Dunno if that helps. Good luck, I'll keep watching.

 

Nope, not planning on extending the cabin; I like the access to the outdoors that the larger cockpit will provide. Someday, I plan on a full cabin to transom canvas enclosure that can be opened up to screened sides and folded up against the cabin when not needed. That would give us some extra elbow room during inclement weather when we're out for a long trip. The little cabin is just fine for us two to sleep, eat and pilot the boat.

On point 1, will I be using more epoxy for the total layup using the 2408 than the 1708? Seems like with less layups there'll be less sanding and waste in the long run. I had planned on pouring and squeegeeing after laying the fabric on a tacky precoating to hold it in place.

On point 3, I like the no-sand idea. I'd imagine that one still has to wash off the amine blush between coats however.

The paint/gelcoat thing is a subject that I've not settled on as yet. So far, I've not found a paint (other than a "bottom paint") that can be used below the waterline on a boat that may be kept in the water. Mine's trailerable now, but that could change. I don't mind the extra effort to make a gelcoat work over epoxy if it'll work; some folks say it does work with the proper prep - others say it can't possibly. And, both of them know more than I do...

 

If you use a peel ply and a decent epoxy there'll be no blush. The peelply becomes saturated in epoxy and when you rip it off the whole top layer comes off with it. It's a virgin epoxy surface ready for more epoxy and fiberglass or paint or whatever. The surface is nicely roughed up ready to go.

 

Why is it you want to use 1708, or any "08" product with epoxy, it sort of defeats one of the great aspects of using epoxy, lighter laminates for the same strength when compared to polyester.

Gel coat and epoxy don't get along all that well, gel coat suppliers say not to use it because they see far too many failures. I've tried to get a good bond, and while there are things you can do that help, I was never able to get a bond I could trust.

A typical hull extension involves cutting the boat in two someplace in front of the transom, adding a mold surface to the outside of the hull in the new desired length, and then glassing it from the inside. There is some glassing and fairing done on the outside, but most of the surface will start off in somewhat good shape. I know you wanted to change the transom slightly, but that's an easy change compared to how you plan to re-glass almost half of a the boat.

 

Why the '08' product? Based on a rudimentary understanding of fiberglassing it seems that a layer of CSM between the heavier woven roving or biaxial stitched is used to ensure that the space between is filled with reinforced epoxy rather than pockets of epoxy. Since the CSM in this case is stitched on it lacks the binder that epoxy won't break down. That's my understanding on the subject so far. One other factor is that I plan on laying up the new fiberglass to the same thickness as it will adjoin on the old boat and want something that builds up reasonably fast. I'd like to hear more of your concern, though since I'm a bit of a newbie.

The lapstrake offset lines on my hull didn't support cutting the boat in two; they sweep in nice arcs from the bow to transom. Adding four feet in the middle just wouldn't have worked. Further, the old transom would have been too narrow to allow both a main and kicker mounted to one side on the transom. The 16' C-Dory hull is shaped to allow that kind of cut and extend behind the cabin and guy in Washington state did a very nice job with his extension.

 

C-Dory was one of my customers, I spent a good deal of time in their plant.

Epoxy doesn't need CSM, this is one of its main benefits, the laminate can be much lighter because you don't have all that CSM sucking up resin and adding weight and not much strength. Because of the added strength, building it to the same thickness as the rest of the hull will sort of be a huge waste of money, this section would be extremely over built compared to the rest of the hull.

In most shops, and if this were done by the manufacturer, it would all be done with either polyester or VE resin. Are they as good or better than epoxy....no…but they hold up fine and its how the rest of the boat was built. If you plan to use epoxy do a little more reading on this site about products and methods to use, it will save you a great deal of time and money.

You can layup more than one layer at a time, it goes much faster that way, and you don’t need to use fairing compound between layups.

The old transom can be cut, widened, raised, etc, easily and still keep the general lines of the original design, this saves a great deal of time and money. The contour lines on the side of the hull can be eliminated in a similar way as to what you already have planned even if you do use the old transom.

 

You beat me to it Ondarvr . . .

 

I have a basic understanding that an epoxy lamination is much stronger than one of polyester. My reasoning for laying up new to the same thickness as old is to aid in fairing between the two. Since I don't have advanced knowledge on the subject, I tend to err toward overbuilding.

Are you suggesting perhaps a stitched biaxial without the mat? I like the way my samples drape over the one-inch lapstrake offsets in the sides of the hull. The 18oz. woven roving I have is waaay too stiff for that kind of bending unless I were to vacuum bag and that's not in the cards with the forms I have built. In regard to the 24 vs. 17 oz. reinforcement I've read that the heavier weights are harder to wet out. I was hoping to use the heaver stuff to keep down the number of layups. Would you recommend going with the lighter weight material for a novice?

My general plan for layup is to epoxy the C-Flex, let it cure, then sand and fair. Successive laminations would begin abeam, crossing the runs of C-Flex, then alternating the orientation. Once the hull exterior is completed, I'd planned on finishing it - gelcoat, painting, whichever - before turning the boat over to laminate the cockpit sole, inside of the hull sides, splashwell and lazarette. I expect that there will be some wait (coupla weeks?) for full epoxy cure before I apply any exterior finish.

I used a fair amount of West System epoxy in my old house restoration projects and have a fair amount of experience with it, but it's pretty pricey compared to alternatives mentioned in this forum. I'm working on that selection as well.

I definitely want to keep the contour lines of the C-Dory; otherwise it won't look like a C-Dory when I'm finished. The old transom is in the landfill and the new transom core is already in place.

I'm up to page 15 of 51 in this forum so far and am soaking up as much as I can. What I'm learning is filling in gaps left in all my books on the subject.

 

If the older part of the hull is poly resin id be making vinylester glass panels and making it look like the rest of the hull !!!
Grind and glass them all together with good long over laps and bouble bias glass . Use vinylester resin for the outer parts and epoxy glass them to you timber frames etc etc. If you use epoxy all through its going to be exspensive . Can make you panels to shape on a bench with a smooth side outermost .

Because the hull has shape on the topside panels wax the last part of the existing hull and lay up the panelover the top of peel ply on the inside to a reasonable thinkness and demould and simply slide it aft then Marry it to the end of the exisiting hull and tear the peel ply off and glass join it . Think simplisty !!

 

@Tunnels

Since I've got the hull character already cut into the C-Flex forms it seems like laying up directly on the boat would be much easier. I'd have to remake the same shape in forms on a bench to make panels. Then the process of gluing the panels onto the existing boat and hoping to adhere them to the plywood cores? Don't know what you're referring to in regard to "timber frames".

I guarantee you have more experience than I do, but I can't see reinventing the process after all the work I've put into the hull forms shown in the pictures.

My epoxy budget estimate is between $750 and $1K. Cheaper is always nice, but those numbers are within my budget.

 

It looks like a worthwhile project. I like it!
Are you gonna post this on the C-Brats forum?

 

It's been on there for a while. Current boat name on that site is Prairieboy.

 

Thank's. I'll look for it.
The new sheer line is pretty good, you've kept it from looking cartoonish.

 

Thanks. For my first design attempts, the sheer line was just an extension of the original. In those, the transom was over 36" above the sole and was definitely cartoon-like. It doesn't take much sweep to provide a graceful sheer line.