Cobalt GT500 Build

[rustnrot]

Note: The Cobalt GT500, built by Cobalt Boat Co. is virtually identical to the Donzi Corsican in every way including most if not all the hardware on the hull itself. However, in my opinion, it appears to be a heavier layup than a Donzi as the sides seem 2-3 times stiffer when the top deck is off.

Our plans at present include a new 351 efi engine and surface drive. This boat has terrible gel coat cracks so it must all come off. Eventually it will be painted, not gelled. Not sure of the condition of the stringers yet. As you can see there is a "waterline" on the inside of the hull so the inside of the boat was under water for some time which means the stringers need to be looked at carefully.

We started the build this weekend by pulling the deck and engine/drive combo out. Ran out of time before we got to the tank. I gotta say, this is the first tank I have seen foamed in where the foam was still viable (i.e. not watersoaked).

As this build progresses I will post a couple of pics with each entry, but you can always see all the pics I took here:

http://s104.photobucket.com/albums/m178/rustnrot/Cobalt%20GT500/

[HOWARD O]

Wow, that's some project. Amazing how similar it really is to the Corsican. Aft of the engine hatches, the gelcoat looks shiny, almost as if it's coated in fresh resin or something. Is that just a camera illusion or just what is that?

[rustnrot]

There is no shiny gelcoat. It is ALL cracked. Looks like those pictures in National Geographic during the dry season after a big wet season.

What I think you are seeing is where the green is worn off the white.

[Conquistador_del_mar]

Wow! You are tackling a huge project. Any idea what your projected completion date might be? Are you going to change colors? Surface drive sounds like you are after speed. Bill

[rustnrot]

More for the uniqueness than the speed. If we keep on it, this should be done in a year.

Presently thinking of a yellow color where the green is but painted more like a Donzi than a Cobalt, i.e. yellow all the way to the chine.

[Conquistador_del_mar]

Are you planning to add the windshield? It sure seems like it would be easy to get it to around 80MPH with a good stroked engine making 400+HP - nudge - ;) Bill

[rustnrot]

It came with the windshield. We will be putting a new one on it.

Brand new engine will be one of the last Pleasure Craft Marine 351 efi Ford engines built. It appears that the H-M exhaust manifolds will fit -- polished of course!

[rustnrot]

Amazing what alot of pressure washing and clorox bleach can do!

[Ghost]

WOW. Nice work.

[sweet 16 1966]

Tom,
Looks good so far. I really like the engine. You have been busy!

[rustnrot]

Took a little hiatus from this project got back on it yesterday. Thought you all would like to see how they attach the stringers to the transom. If you look closely, the stringer plywood is still there butted up against the transom. However, since there was no fiberglass barrier between the rotten transom plywood and the stringer plywood, the stringers are rotten. Looks like it goes about halfway up. We will replace the entire stringers. I'll probably use laminated veneer lumber (plywood where all veneer grains go front to back) like I have done in the past. While I would rather use treated boatply, it is not available in the long lengths. The extra thickness of the LVL is also an advantage as the surface drive does not push against the transom but only the stringers.

From what I understand Donzi's were constructed the same way as above although I have not personally verified.

[rustnrot]

Correction: Only the bulkheads and gussets are plywood, the stringers are 3/4" pine complete with "joining fingers" as shown...

[rustnrot]

Cored bottom.

None of the Donzi Classics were ever cored on the bottom were they??

[mphatc]

WOW . you are really into this deep! sadly it looks like Cobalt took some short cuts from the way my Corsican is built . .My stringers are mahoghany . . and sealed where they met the transom . . the small side bulkheads were plywood, and I did replace the rear ones, and added two more between the originals.

What have you chosen to do with the fuel tank? Same size and location?


Mario L.

68 Magnum 27
69 Corsican

[rustnrot]

First the easy; a single fuel tank will be mounted just forward of the engine under the cockpit, that moves weight significantly back. The hull seems to have significant rocker. Not at all like a 16 (which I compared to) that is ruler flat (not counting the little hook on the outside a 16 hull has).

Regarding the coring of the bottom...did your Corsican (or any Donzi for that matter) ever core the bottom?

I know it looks bad, but once all the Old Crap is removed going back is rather straightforward. If I had a mold for an 18 hull I'd rather layup a new and put the Cobalt deck on it.

[rustnrot]

Balsa-cored bottom and what a mess it is. Take your pick of rotten balsa or water soaked balsa it's ALL gotta come out and it ain't easy, 90% has to be air-chiseled out.

At this point we are talking, on the hull bottom alone, of removing close to 50% of what fiberglass was there originally. Not only the stringers and inside layers of glass and core but once that is "done", the outside has 100% crappy cracked gel coat that has to be removed as well.

In other words about a fraction of an inch thick of a fiberglass shell is all we will have salvaged from this. We may wait on this project for a crapped-out Donzi 18 hull to come along and use it instead. At least it won't (or shouldn't--I wouldn't buy one that was) be cored and if we find one with reasonably shaped gel coat that would reduce alot of prep work for painting also. I'd be all over that "$500 Donzi 16 splash" on craigslist if it was an 18!

Too bad Python Boats in Atlanta is no longer there. They made 18 splashes. In fact I saw the molds just a few years ago when Jim Clark? the owner was still alive; I would be asking how much $$ for a hull bottom. On the Cobalt or Corsican it is the deck that is so cool, after all.

I'm not so sure I couldn't build a high performance wood hull for the deck at this point and it would be easier. I wonder how the Cobalt deck would look grafted to a Chris Craft XK18 hull?

[olredalert]

------Im a bit out of touch but maybe it would be possible to get a new hull from north of the border. It would be worth getting in touch with Lenny and check on that possibility............Bill S

[Conquistador_del_mar]

Is the outer shell so thin that it really has no stength? If it is strong enough to hold its form, then it would not be too bad rebuilding it. What a bummer that the hull needs so much work. I can certainly see buying another hull that needs very little work to make it sound. Is the Donzi 18 hull identical? Sometimes, there were subtle changes made to molds to circumvent copycat laws. Good luck no matter what. I have seen 18 Donzi hulls go for as little as $2500. Bill

[rustnrot]

This post was more of a desperation call. We will keep at it, it just will be some ugly work. I am going to borrow a demolition hammer, you know a rotary hammer drill that has a hammer-only function and then get a 4 inch chisel bit to fit in the end of it to pry between the outer and inner core layer. That should help.

Actually balsa-core bottoms are still considered state-of-the-art as it is supposed to make a lighter weight more rigid structure. Not good when they get wet though as you can see. But nobody designed this, or any boat for that matter, to last 40 years. As you all know, many of the Donzi's had a balsa core deck; at least I do not see any evidence of this in the Cobalt (so far).

The remainder of the hull is remarkably stiff and should provide a good basis once it is cleaned up. I will not go back with a core so will have to beef up the laminate to compensate. I've laid a balsa core before and while doable it is a rather picky affair, especially when you do not want to vacuum bag. Off the cuff, I'll probably go with some layers of 1200 biax to strengthen the hull and then stringers on top of that held with layers of 1700 biax.

All epoxy resin will be used if for no other reason than the boat is being built at a residence and I simply cannot have the styrene stink. However, I also appreciate the superior strength and adhesion of epoxy especially for these secondary bonds to old polyester.

[mphatc]

Rustnrot

Years ago I worked for Gordon Douglas Boat Company near Deep Creek Lake in western MD . . they built the "Flying Scot" sailboat, a rugged well built 19' centerboard sloop. They had a balsa core deck and hull . .every once in while we had hull that came back used for a recore.

We removed the core by cutting through the covering layer of fiberglass with a carbide tipped blade on a skill saw! Try this . . .
Set the blade to cut only 3/16" deep and cut a tic tac toe pattern . . . and everything should come part easily.


I did the same thing on a Capri 25 fin keel sailboat that had grounded and fractured the keel bed. . We bought a new outer skin section from Catalina Yachts and I cut the old damaged section out of the boat, saving the inner structure. This boat had an Airex Foam core . . .

FWIW, I still have the same Makita skill saw to this day . . and I can't tell you how many sheets of 1/8" thick aluminum it has cut!

Good luck!

Mario L.

[rustnrot]

I may try the skil saw trick. I did cut it some with a thin kerf grinder but probably need to do more as I was trying to peel off foot square sections..

While prying I did go thru the outer hull. And it is actually a good thing I did. The outer hull is 5/16" thick of pure mat fibers. I would have expected some reinforcing fabric of some kind there but there is none. All of the reinforcing fabric was the woven roving that was on top of the core. The reason I say breaking thru the hull was a good thing is I never would have expected such a layup for the outer hull and now realize even more reinforcing needs to be done on the inside.

Also, the outer hull 5/16" of matt fibers is quite wet. I guess the glass will dry out?? Even more disturbing is I cannot seem to discern the layers of mat, could it be that the outer hull is all chopper gun? <shudder>.

[mphatc]

Rustnrot,

call me at my shop on Sunday . .I may have a solution for you!
603.228.2888 If I don't answer, I will call you back.

Mario L.

[Conquistador_del_mar]

We removed the core by cutting through the covering layer of fiberglass with a carbide tipped blade on a skill saw! Try this . . .
Set the blade to cut only 3/16" deep and cut a tic tac toe pattern . . . and everything should come part easily.

Mario L.

That is exactly how I cut out the top deck skin on my Magnum project, but it was bad enough that it came loose with very little persuation so I did not have to resort to small square pattern cuts. Bill

[rustnrot]

Chipping away...a rotary hammer set to hammer-only helps but still a bitch of a job. Cut into 1/2 ft squares with a narrow kerf grinder helps. This stuff is so juicy and wet it is disgusting. Amazing also how hard to chip alot of it away!

Never mind the hole where I put a crow bar thru the bottom, that is an easy fix.

Mario, I did not get a chance to call yet but will, thanks..

[zelatore]

I've got a quick question about the balsa core. Is it sheet stock? Like balsa plywood? Or is it end-grain stuff?

A lot of respectable yacht builders are using Baltek end-grain in their layups. Back when I sold Cruisers Yachts they spent a good deal of time telling us just how it was the greatest thing going and was better than a solid layup. They insisted that due to the end-grain construction you wouldn't get water migration for any distance.

Not that they'd be biased or anything.

But when a boat like a Nordhavn uses the same process, you tend to think it's a pretty solid idea.

[Just Say N20]

Tiara also uses end grain balsa coring, with the same "no water migration" claims. In their traditionally laid up hulls, it is use only above the waterline. In the bagged boats it was used everywhere.

[rustnrot]

It is end grain balsa, and like i said in an earlier post still considered state-of-the-art today. But I can assure you it is still ALL wet, "squeezably' wet as you push on it with your thumb and can wring water out of it. So wet you get splashed by old bilge water as you remove it.

I'm not sure I ever bought into the 'ol "endgrain keeps water from wicking all over the place" rule. However, in all fairness this is a 40 year old hull and also had water in it for some time in its past. Note where I have circled "ancient shoreline" in this pic. Most of this "bathtub ring" came off when I washed the inside of the hull but it is still visible. Like I said who knows how many years this hull was full of water...

[rustnrot]

Progress continues. While Holyfield may have won the previous 3 rounds, I won today's. Precutting the core into even smaller pieces made the chipping easier.

That and pushing on the chipping hammer with my feet :superman:

While I'm certainly not gonna claim any kind of "mission accomplished" yet I am hopeful this core will all come out and leave a viable shell to build on. I may leave a piece of the original core in the very tip of the bow. It is mostly dry and represents 10% at the most of the total core. And the drier the balsa is the harder to chip out.

[Ghost]

Glad to hear it is coming along better.


...had water in it for some time in its past. Note where I have circled "ancient shoreline" in this pic. Most of this "bathtub ring" came off when I washed the inside of the hull but it is still visible.

(By the way, the "ancient shoreline" was hilarious. And perfectly descriptive to boot.)

[rustnrot]

Not even sure I can believe it, but the core removal is coming along nicely.

Take a look at the stringer and you can see they made no effort to fill any voids at the bottom of the pine boards.

Also, the bottom is nearly 1" thick in places. The balsa is 1/2" thick and the inner glass is 3/8" on top of that! The outer matt is 3/16". I am no expert but I cannot figure out why they did not put at least one layer of reinforcing cloth (woven roving) on the outside prior to laying in the core.

[GONZO]

To ALL

I have a1970 Cobalt XV 200 for sale

It is a truely a one of a kind and the only one ever made

351 ford v drive

Wayne
913 208 8276
wsie273425@aol.com

[Donziweasel]

Pics?

[olredalert]

-------After reading this particular thread you may want to check for a balsa cored hull!!!.........Bill S

[rustnrot]

I have about 90% of the inner hull removed. I still have some at the bow -- still wet. I will keep going as long as I encounter wet balsa but hoping I can leave a little bit at the very front.

I would venture a guess the Cobalt 16 above also has a balsa core...after 40 years I would find it hard to believe it was completely unrotten or dry...

[rustnrot]

Make that 100% out. Wet and/or rotted all the way to the tip 'o the bow!

To give you an idea of the effort to remove this core I had my wife video this youtube:

http://www.youtube.com/watch?v=9rbGY4ntmPM

[CHACHI]

Looks real good.

I can see the hull was strong enough to support you after the coring was removed, was there much deflection as you moved across it?

Ken

[Just Say N20]

I can't imagine how much you must itch! :boggled: :nilly:

[Ghost]

It's amazing how little it looks in the pictures, and then how big it looks in the video, with you in it. Must seem like acres cutting all that stuff out.

[rustnrot]

The deflection is there but not bad. That's a Good Thing and bodes well for blocking the hull properly.

It did seem like acres. At first I could only do a very few square feet in a day but near the end got better at it and finally knew how to optimize the tools I had. Still took over a week 'cuz after 2-3 hours a day I was pooped.

This week will see pressure washing (don't wanna grind any dirt etc in), then sanding the inside then pressure washing again and getting the hull in the garage on level floor and blocked.

[Morgan's Cloud]

Never mind the hole where I put a crow bar thru the bottom, that is an easy fix.

..

That's incredibly funny in a strange kind of 'true' way :kingme:

When I saw that I knew this project was in good hands !

It reminds me of when I had to de-core (balsa) the fuel tank hatch on the Magnum. Once you've got the 'heavy stuff' out you quickly learn a 'light touch' on the grinder to remove the rest of the stuff stuck to the glass.

[rustnrot]

Since there seems to be some interest in this post again, I thought I would update it.

The Economy Happened to the owner and he is not in the mood to go further with it at this time. So the empty hull sits under a protected area. A new windshield was bought however.

And it has some hardware but of course it is off the hull. I know he'd sell it... The PCM engine was sold.

[Tall Cotton]

I'd been stalking this boat since it was for sale in Houston a few years back and found out that it was for sale again only three hours away. After some humming and hawing for a month, I went ahead and pulled the trigger. Thanks to Rustnrot for doing the hard work for me and the previous owner for graciously giving it up for considerably less than he had in it. It's gonna sit for a while before I get to it, but I will. I'll keep the updates coming as they happen. Hope I'm not in over my head...

[silverghost]

Good Luck finishing this Cobalt GT 500 project.
It would be great to see it on the water once again.

Please keep us posted on your restoration progess with forum updates .

There is still a Cobalt GT 500 project sitting in a boatyard storage lot in Sommers Point NJ~~~ so I am told.

And yes~~~
It too has extensive hull bottom core rot issues also .
At least 1/3 of the aft bottom is wet and rotted like this project above.
Possibly more ?
Last asking price I heard was $1500.
This project Cobalt GT 500 is very rough indeed.
It's Not for the faint of heart, or wallet.

[Tall Cotton]

After sitting on a trailer not made for it with a ugly blue tarp over it for two months in my back yard, I capitalized on my time off over the holidays and got started. I blocked off the huge hole in the transom with treated plywood and plugged some of the holes in the hull with sealant. Then I spent two days measuring, cutting, bolting, fussing and cussing putting together bunks for it to rest on. I then put it in the water for the first time in maybe 20-30 years and floated it over to the lift where it awaits my next move.

[Just Say N20]

Nice job, and congratulations. When done, you will have an appreciation for the boat that few "normal" boaters can't even imagine.

Please keep us updated when you begin the project.

[Tall Cotton]

Oh I'm "feeling" it alright. Can you say ibuprofen cocktail?

[Tall Cotton]

I've heard that only nine of the GT500s were built - twice now. Can anybody confirm or elaborate on that?

[silverghost]

I have no idea of the actual production numbers~~~
But it was only a very small number I believe .

There is a famous Don Aronow story of Don, & some friends, confronting the Cobalt folks at I believe the Miami boatshow, accusing them of ripping-off a Donzi design with the Cobalt "splash clone copy.
I believe that he had already already sold Donzi Marine.
Cobalt actually splashed several Donzi boat designs.
Donzi Marine soon took legal action to stop Cobalt from building these rip-off "splash' clone copy boats.

[mphatc]

One might try contacting Holman Moody . . the Cobalt GT500 was a full blown accurate copy of the Donzi Corsican, right down to the gage cluster, Morse control and H&M / Volvo engine package. They may still have folks on staff that will have some recollection. Way back 10 years ago before H&M moved their facilities Lee was able to recall much of this.

Note that H&M Marine built marine engines for numerous boat builders, and I have no idea how many went into other Cobalt boats.

a good read:
http://mycobalt.net/2009/05/30/ultra-hot-1969-cobalt-xv-200/

Mario L.

Donzi Corsican C18 C32

[olredalert]

----The show when Don confronted the guys from Cobalt was the Chicago Boat show.......Bill S

[Craig]

Tall Cotton - Keep the photos coming. Love to watch the progress on the old Cobalt!

[Craig]

Tall Cotton - Just checking in to see if any progress has been made on the GT-500. It's been since February and I had completely forgot I even wrote the previous comment until I was reading and checking out the photos. Then it all came back to me. I turned 50 in July so that probably explains it! ;)

[Tall Cotton]

The process is moving slowly but forward. I managed to get a free '97 Vortec engine that I may use for power. It's apart now in the garage awaiting rebuild. I'm going to look/start a thread in the performance section to see what I may need to do to make this work. Good start though, dimpled rods, 906 heads but two bolt mains...
Also looking for outdrives. Tampa Craigslist has tons of Alphas for cheap but that limits me to 300hp which may be more than enough for this little thing.

[Tall Cotton]

Tall Cotton - Just checking in to see if any progress has been made on the GT-500. It's been since February and I had completely forgot I even wrote the previous comment until I was reading and checking out the photos. Then it all came back to me. I turned 50 in July so that probably explains it! ;)

I just hit the magic number this week. CRAFT syndrome starting to kick in...

[Tall Cotton]

Well, on one of my few days off I began sorting through the hardware seeing what I have and what may be salvageable.
I'm in need of a little help here.
The emblems mounted on the rear of the boat are all but shot. I can probably refurbish the frames and the flag emblem is recognizable but I have no reference for the Cobalt emblem. Anybody out there have a Cobalt XV 200 or GT 500 that they can post a picture of this emblem so I can try to reproduce one?

[mattyboy]

Tall

isn't the script C surrounded by the laurel leaves? like at the top of this page? I didn't know the emblems were metal not decals pretty cool

check the link posted a few posts back it has a pic of the emblems

[olredalert]

----The metal emblems are cool! Its a shame they did them in pot metal, but anything else probably would have been viewed as too expensive.........Bill S

[Tall Cotton]

Yeah, I did download the brochure image and the picture of the V-drive XV-200 for sale. The brochure gives a good design image but I can't quite get the colors right from the photo. I was looking for a good picture of the emblem itself if there was any out there to be had.

[~CobaltGT500~]

Hi Guys,

i get a gt500 and i would like to restore it.
and now i need further Information about the manufacture-Year, the Color-Code from the Blue and white. The hullnumber is 5001003
is here anybody with further Information about the boats?

http://up.picr.de/17653571fy.jpg
http://up.picr.de/17653590ic.jpg

thanks and best regards from germany

dustin