Fixing a Marinette Hull? Then ... 

bullet Check the hull first before you decide to weld.
bullet Aluminum hull percussion sounding or ultra-sounding can cost  $20.00 per foot to find flaws, ask a surveyor who knows this stuff.
bullet If it feels deeply pitted, it's too far gone, see below.
bullet A small patch is usually just fine.
bullet Alloy : Plate and Sheet Aluminum 5086-H34 or 5086-H116 (GATW or TIG welding). Don't switch to a "better" alloy and for heaven's sake don't use non-marine aluminum. A specific welding alloy is recommended for H5086-H34 or H5086-H116, use it. Aluminum Roamers, I'm told are usually the same metal. (Yes, ABS American Bureau of Shipping rules say mixing marine aluminum allows is OK with the right filler metal (weld wire)). 
bullet Hint, A wrecked Marinette has aluminum for patches.
bullet But Marinette hulls can always be fixed. 
bullet Thickness : 
bullet Bottom plating - 28' and 32' 5/32 inches 37' and up 3/16 inches
bullet Sides and Top  - 1/8 (source Aluminum Cruisers Brochures, circa 1977)

bullet Repair Welding is GATW with a ceramic backing strip. Hire only expert welders for repair. Expect an aluminum welder to make at least $35-$100 hour depending on location and demand.  Fixing a hull with a few plates should not be an exorbitant amount. A prepped boat can be welded quickly.
bullet Hire an expert welding shop with expert aluminum welders. 
bullet An expert won't weld in the wind.
bullet Will use a ceramic backing strip.
bullet Will know which welding alloy to use. 
bullet Will know when to Mig or Tig weld.
bullet Will not have a "sooty" weld.
bullet Clean the areas which will be welded. Get all the tar out and thoroughly clean it as a welder would.  Be ready to weld. Do not use grinders on aluminum, they damage the surface with heat. Sandblasting is not generally required to weld aluminum. Do not use steel or copper brushes to clean aluminum, stainless brushes are ok to break the oxide layer. (Aluminum melts at 650 C, but the oxide layer melts at 2000C +. A clean weld area is essential, and that means that even finger print grease is not permitted. 
bullet Do not braze marine aluminum that will be immersed, it's too large a galvanic (voltage) difference. Aluminum brazing (like Duraweld) is an above the waterline patch. It's better to use epoxy than this stuff. In fact I'd just avoid brazing marine aluminum, unless you boat in distilled water. The brazing metals are high in zinc and will corrode. (ABS does not state that brazing is permissible for hull construction.)
bullet Brazing is using a lower melting metal to attach two higher melting metals together. It's not necessarily weaker than welding.
bullet Brazing may be ok for many above the water uses where salt is not an issue. 
bullet Check the welds. 
bullet Good welding and repair makes that part of the boat "new".
bullet Expect that welder will know how to check the quality of the welds. Hull fixes should cost less than you'd think. 
bullet After it's done right; clean it, rough it, and paint it.
bullet Remember, It's usually cheaper to fix it right, than to try some weird epoxy fix. A large epoxy patch may fall off as the hull flexes  (you can use it for fairing), but don't put the life of the boat on it.
bullet Marinette Hull anode is a aluminum alloy called Sealloy. It's common here to put zinc and Sealloy zincs on the same hull. I am trying to get the exact metal composition of the Sealloy.
bullet You can hammer and fair out a dent in a Marinette. It'll stiffen the aluminum in that spot a little, so I'd replace that area of aluminum after it's first 10-20 dents. After the first 10 dents, consider replacing the captain also.
bullet Non-Marinette boaters : Inspect the hull of the boat you own often, because too many companies put out cheap aluminum hulls that end by breaking up.  One of my friends lost two small Jon boats and had it replaced twice by the company. They considered it normal business. BTW: He had warning cracks that he ignored, and was happy with his 3rd boat. Go figure...
bullet A rivet on an aluminum boat hull is a bad idea. (But you should never mix rivets and welds on the same boat, the fastening systems are too different.)
bullet Tongue and groove boat construction on an aluminum boat hull is a bad idea. 
bullet Ultra thin hulls on an aluminum boat hull is a bad idea. 
bullet A real hull is welded and is nearly bulletproof. 




Aluminum 5086 H-34




Aluminum Balance 
Chromium 0.05 - 0.25 
Copper 0.1 max 
Iron 0.5 max 
Magnesium 3.5 - 4.5 
Manganese 0.2 - 0.7 
Remainder Each 0.05 max 
Remainder Total 0.15 max 
Silicon 0.4 max 
Titanium 0.15 max 
Zinc 0.25 max 

Machinability in the strain hardened tempers
H34 is relatively good. Use of lubricants is advised when cutting with tools. 
This alloy is readily weldable by conventional 
methods for Aluminum. Use of 
electric arc welding (GTAW) in particular 
produces excellent results. Differing chemistries of Marine Aluminum should not be welded together. Most commonly welded with 5356. Must be clean and oxide free to weld.

Form Sheet 
Condition H34 
Temperature 68 
Tensile Strength 47 
Yield Strength 37 
Elongation 10 

Sources ATSM and SAE




Fixing Stainless ? Then ...

bullet You'll replace the stainless shaft and the rudders before the aluminum hull.
bullet Not all stainless is equal, so use only series 300 stainless clamps (316). (Beware " automotive all stainless" clamps, they rust.) Marine stainless is 300 series only.
bullet Double clamp all hoses.
bullet Buy the better clamps.
bullet Stainless shafts
bullet Stainless welds just fine, but in my opinion brazing should be an above the waterline fix. Rudders on the Marinettes are probably brazed (with a nickel alloy) to their shafts. I just don't care for brazing in the marine environment. 
bullet Stainless can be filled and fixed, and shafts can be remade, (unless they look like Swiss cheese). 
bullet Oxygen starvation or electrolysis corrodes immersed stainless steel. That's why the area around the cutlass bearings and seals goes first. (Enclosing stainless under water will chew it up).
bullet Check shaft isolation from the engine.
bullet A bronze prop and a stainless shaft just need to be zinced correctly to be together. 
bullet A shaft zinc too close to the prop will affect performance. 




Fixing Steel ? Then ...


bullet Know that steel can almost always be fixed. You can fill holes with  welding and grind the steel down. 
bullet Rudders that have huge pits can be weld filled or have plates welded. 
bullet Steel is the easiest metal to weld, and I'm told it's OK for brazing in marine applications (when properly zinced). 




Off Site help 




    Walke Point - a discussion of welding aluminum




    Aufhauser An aluminum brazing company explains marine alloys and welding. (FYI: Marinettes are 5086-H34 or 5086-H116 alloys.)