Structural advise needed to correct poor boat construction

I am repairing a boat that is no longer made because they did not hold up. I am hoping that poor workmanship caused the failures rather than an uncorrectable design flaw. I was considering some structural additions and am looking for someone with boat building experience to help/advise me.

 

I think pretty much everybody here has some boat building experience.

What kind of boat is it? FRP? Alu? Steel? Concrete? Inflatable? You posted in the Fiberglass forum, so I'm guessing it's FRP, right?

What about power? Sail? Steam? Pedal-powered? Electric? hamster-powered paddle wheels? Or maybe do you hook a team of dogs up to it and have them paddle you around? That would be kind of cool. Maybe you could even use dolphins- they swim faster than dogs.

How long? eight feet? two hundred feet?

While a lot of people here have experience with a lot of things, nobody knows it all. If you clarify what you are doing, someone may be able to help. And, if by chance, your boat is an inflatable dog-powered 100 foot submarine with a flight deck, then I'm your guy. I know a lot more about inflatable dog-powered submarines with flight decks than any of the so-called "engineers". They're actually pretty clueless when it comes to such things.

 

My boat

It is a 23 foot Shallowcraft, 8' wide tunnel hull with a 200HP Yamaha outboard. The hull looks real similar to a Dargel Scout, like a flat bottom boat in front with a small V slapped on to it. The visible damage was a longitudinal crack about 24" long in one of the body lines (?) in the front part of the hull.

I talked to a shop that had repaired boats like this that had cracked from side to side like the front part of the hull was breaking off. With this, I removed the sole (1" Nidacore) from over the damaged area of the boat to investigate

No cracking was evident but the stringers were broken loose from the hull where it curves up. After removing what little tabbing material attached the stringers to the hull, the sides of the stringers pushed out, I think from too much foam poured inside of it. It looks like this and poor tabbing may be the reason for failure in this area.

Using Epoxy, my plan was to first reattach the stringers, add several layers of 17oz biaxial to the sides and then go over all this from one side to the other with several more layers of 17oz. Then I was thinking of adding some longitudinal stringers in the front part of the boat only.

 

Hey sbklf

Before using the epoxy might want to test your resins to ensure they're going to stick. Epoxy doesn't necessarily stick well to poly or vinylester resins so a test should be in order. If it sticks well then Ok. If not you might have to look for a more compatible resin.

You'll want to get back to good glass on the stringer which might mean grinding the crack back a little bit. Wipe everything down with acetone before you start to apply any new resins. Make up a filler with your resin and get it in as far between the stringer and the hull as you can so you're essentially bedding the stringer in the filler. This should help prevent any hard spots between the hull and the stringer. That's usually where cracks start.

Create some fillets (1" radius) along the stringer/hull line with your filler to give the glass some radius to follow. Greatly improves the strength of the joint. Right angled sharp corners & fiberglass are much weaker so avoid them unless absolutely necessary.

Retab the stringers back to the hull with some biaxial tape, 6" minimum. Probably 3-4 layers with a 2" overlap for strength. Work wet on wet for better adhesion. You can follow up with the 1708 for strength. More than one layer of 1708 might be overkill but then again it's up to you if you feel you need more.

Rick

 

Rick, I chose Epoxy for its bonding and strength characteristics. I read that Epoxys bond strength is better than Poly and Vinylester resin. I remember some exceptions that it wont bond to, I think polyurethane plastic was one, but figured I wouldnt have to worry about it in this application. I have applied a small amount of it and it seems to have bonded very well. Where/what conditions have you seen it fail to bond? Thanks, Kevin

 

Hi Kevin

I think the biggest problem I've seen with older boats is after they're junked. Previous owners who've tried to fix problems with stringers & decks etc. have used god knows what as resin to patch up an already bad situation. Usually the problem area gets covered over and nobody ever sees it again till the boat passes hands and someone decides to do a fix-er-upper.

Everything I've read and been told appears to suggest that boats built with poly or vinylester resins don't repair well with epoxy. But then again, not all epoxies are created equal and not everybody uses them properly. If you've done a test and it appears to have bonded well after proper surface prep you're probably going to be ok. I tested with a 3" x 12" strip of 10 oz glass on the hull, half epoxied. I let it cure and tried to rip it off with vise grips. Half of it stayed and half didn't so it wasn't what you'd call a 100% successful test.

A planing hull, like your boat, is going to be exposed to extremes of shock loading and hull flexing. This will test the qualities of any material you use. The only way to know for sure how well your repair is performing is have a look at it after you've put some hours on it. If you're sealing it all up tight you'll never have that opportunity. The next owner may or may not let you know how well you did.

I'd say go for it with the materials you have. Bed your stringers as best you can and make some good fillets to distribute load. Stagger your glass tape and get the largest contact area you can for your glass & epoxy.

Good luck.

Rick

 

Hi Knottybuoyz, being a boat builder for many years in epoxy composites I can definetly say that epoxy resin will stick to just about anything that is preped properly eg: well sanded back to glass and solvent wipped.

Polyester and to some extent vinylester resin don't stick to epoxy, so if not 100% sure of original layup epoxy is the go.

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Dave

 

Atta boy Dave

There's a definitive answer from someone who definitely knows what he's doing. Last repair job I did was on a buddy's transom and we used a polyester boatyard resin because the epoxy wouldn't stick but then again, neither of us knew what we were doing, hence the test.

I was checking out your photos. That boat is "HUGE"!!! Did Andy give you more storage space for your pics? If not, I've got unlimited space, just e-mail them to me and I'll post them for you.

Rick

 

No space required,did a bit of research and found out how to compress my pics. maybe I'll delete and resubmit some of my biggies, but i'll wait and see how I go.
Can't wait to get out and start building again, had an op a cupla weeks ago and have to take it easy,

Thanks for your help

Dave

 

The hull should have stringers that run the full length. This hull design is comparable to a flat bottom boat not a tunnel. It takes a beating in rough water, meant for smooth water. I have never seen anything that epoxy will not stick to,except wet wood. The only reason I prefer poly is cost. A properly prepared surface is the most important thing. Grind with 24 grit. You are after a mechanical bond more than a chemical bond in a repair. I think this boat has been abused to crack like that. I would add 2 layers of 1710 glass from side to side over the stringers and try to extend the stringers to the front. That is your weak spot. Also some side supports to tie the stringers together.