Heres the pics of the boat cracks i need to repair so hopefully it will give you guys an idea of what i need to do and hopefully youll all give me some helpful pointers.

Gary

Thanx Ash. Im a welder by trade and i tried to gas weld it with a "0" tip and aluminum rod but the heat was still too much for it and i dont have a tig welder so i guess its the plate and Sikaflex sealant.

Gary

Plate. 

the Al will burn with a regular torch.

You might want to put a thin plate on the inside and one on the outside.  Sort of sanwich the two with solid layer of sealant inside so that no water gets in between and could freeze in the off season.  Also add a slight taper to the leading edge of the outside, so it doesn't easily catch on brush or a rock.  a heavy coat of paint on the outside would do it.

Gary, see how nice these guys on BSB are?  I thought you question "an idea of what i need to " would have garnered comments like:

 "get a new boat"

"get a buddy with a new boat"

"donate to Good will charity"

etc.

we expect pics of the repair, good luck, fishing season is coming.

BEAR

Ive been thinking about putting a patch on the inside as well but do they have rivets that long that will go through 3 pieces of metal with sealant in the centre? Are they normal rivets that i use in a regular work shop type rivet gun?

Gary

Gary is the boat riveted or welded? If it is riveted why not replace the entire panel, if you are going to have to plate it why not the whole thing? Just a thought.

John

Its rivetted John. I would like to replace it but it would probably cost quite a bit. The pics show two cracks one on each side of the boat below where my outboard would sit so id end up replacing the whole boat.

Gary

One more thing! Do i have to use rivets or can i bolt the plates on with rust proof bolts?

Gary

It is not the rust you need to worry about.  The dissimiliar metals react, sort of make a little battery.  The aluminum under the bolt head will just disolve.  It turns to white chauk dusty and disappears.  You will end up with just holes and the bolt will fall clean out.  It would take some years and a heavy paint would slow it down, but it will disolve.

I repaired my 12 foot Al boat last year (new transom) and found aluminum bolts nuts and waskers at Home Depo.  remember they don't torque down like steel, gentle.

BEAR

Bear.... Can i apply the Sikaflex when its cold out or does it need warmer weather to cure. I dont have a garage with doors to get my boat indoors unless i hook up a heat source of some kind.

Gary

Gary, do you think the cracks are from long term stress? If so, you might want to view your proposed patching effort with the idea of reinforcing a larger stern area to slow down further cracking. There are hundreds of clam diggers/harvesters in coastal Maine and most of them use 12'-14' aluminum skiffs. They most all run them with 30-40 yahamas with around 200+ work trips a year. When these boats start to wear they show signs of fatigue stress cracks around the stern area and bottom wear just under the bow from constant beaching on mud flats. Once the aluminum in the stern, which takes all the stress from the motor, becomes stressed, work hardened, to the point of fatigue cracks then the whole integrety of the stern is questionable.

You ought to check the corner bracing real carefully as well as the angle bracing from the botton up onto the stern for signs of crack development. There are liquids that can be applied to metal that will accentuate cracks that are not easily discerned.

The point I want to make is that you said the cracks were just under where the motor mounts. If years of motor vibration and running force stress has created fatigue in those two spots then there is probably more fatigued metal ready to crack/fracture. You just might want to consider what you need to do to reinforce the whole stern area beyond the two cracked areas you have described. Work hardened aluminum has no structural integrety and could let go at the worst possible time. ie. choppy water, squall making up, trying to out run it to shore and all of a sudden the motor is attached to the stern very loosely.

Someone else want to jump in on this line of thinking?

Disregard all after Gary, if you are just going to use it for a rowboat.

Hi Sam, Its my fishing boat that i use with a 15hp outboard. The cracks happened last summer from my boat bouncing on the trailer rollers that werent properly adjusted and one of the rollers fell off and the forks that held the roller in place went through the side with the bigger crack.

Gary

OK Gary...gotcha....punched holes, not fatigue cracks. Got a little slop jowled didn't I

The boatbuilders around here use 3M-5200 to attach fittings to the fiberglass boats they build. I've heard that Sikaflex is similar to 3M5200. Some adhesives do not cure properly in the presence of aluminum. You might check on this for the Sikaflex you plan to use.

 Don't be afraid to make the patch 2"-3" wider than the cracked area as a CYA measure and provede a good "toothed" surface for good adhesion strength.

I've used a lot of silicon sealant and 3M-5200 and I would NEVER use silicone adhesive in your repairs.

Are you using pop-rivets? The metal "nail" that compresses the rivet might be a ferrous metal and through electrolysis, weaken the pop-rivet over time because of  exposure to water. Just a thought.

My son learned to weld aluminum patching up a sears 10' aluminum jon-boat with a mig-welder. Lots of bublegum on that one. He welded together an aluminum heat exchanger using .055 irrigation pipe and sch 40 aluminu fittings. We had the devel's own time stopping all the little pisser and dripper leaks and never stopped it 100%. So I think the patch with the sealant is the way to go  on a thin skinned boat hull.

Sam. If you look at the pic with the small crack youll see another patch there and its got a rubbery sealant in it. That was there when i bought the boat and it looks like a boat shop did the repairs judging by the rivets they used. I talked to a marina here and they said theyll sell me some rivets that they use. I plan on using a patch bigger than the crack but if you look at the top pic the crack runs right to the edge of the rib. You can see i drilled small holes to stop the crack spreading. Also, should i cut the cracked parts out so i have a square hole?

Gary 

Gary, regarding Sealants.  Most are RTV, meaning "Room Temperature Vulcanizing".  room temp or ambient is usually 75-90 degrees F.  Lower temps usually increase the cure time expodentially, and at 40* it could take a month or more.  And you would not get the proper bond to the materials.  I ofen put stuff in my refrig or freezer to delay curing where I have to do some mechanical things between coats.  Cold generally stops the reaction.

You MUST read the labels.

If it requires warm; WAIT till spring.  You will not be happy with a poor repair and you will just have a larger patch to make next time.

BEAR

I wouldn't remove any metal. That will contribute to anchoring the whole patch in place.

What is the process for afixing solid aluminum rivets? Two person job with someone on the head side with a dimpled rod of steel to hold the rivet in place while you peen it over on the other side? There is a special tool to hold against the head of a copper rivet when fastening the hull planks in a wood ship. Kind of like a tapered crowbar with a dimple in the end that just fits the head of a rivet. It soaks up the energy from the peening blows.