Glen-L.com

I have had an old rivited 17 footer with a 6' beam and 36" max freeboard for some time with a rotted transom.
Instead of replacing the rotted plywood I have decided to spend the money and replace the plywood with a heliarced panel.
This is an outboard powered by an old 1978 3 cylinder evinrude 70 HP.
Since this is a trailered boat and will probably never spend more than a few days in water at a time is it necessary to spend the money on more expensive aluminum alloys?
I have available locally 3003 5052 and 6061.
My intentions are to use .125 plate reinforced with 14 gauge square tube to around 2" and weld it to the existing aluminum transom skin. Every thr-bolt will have its own tube front to back.
A new floor made from .125 aluminum will be welded to the new transom plate at the rear and revited to the existing cross members eliminating much of the rot prone plywood floor.
I will be paying a professional shop $95. per hour to weld so I want it simple and easily fabbed.
Anyone have alternative ideas on this job? Will .125 and 14 gauge be strong enough?

oleman,
the primary consideration of stiffening a hull built of thinner material is to insure the new joints don't allow any flex. If they can flex most often the result is to crack/break/bend/distort or otherwise tear up the original boat. Thicker stock can be a good solution for many modifications as long as that new material doesn't get welded to the thinner material where the thin stuff can move.

A composite transom made of 5052 sheet and 6061 extrusions built up to a 2" thickness should hold a 70 vertically. Does t his boat have a tray along the inside of the transom? Many lighter boats have/had/were originally equipped with a sheet of aluminum joined to the transom at near right angle that kept water that came in over the stern, or spray, from getting into the bilges. This 'slop tray'/splash well/motor wash guard was a critical transom stiffening member as it formed a huge (thin) T with the transom.

If you're replacing the stock sheet metal and plywood type of transom with an all welded built up panel, I'd want to make sure there was a stiffening element that stopped the engine from flexing the entire transom aft as it pushed the boat. The primary force of the engine at the top of the transom is aft- not forward. The forward thrust happens lower at the bottom of the outboards' mount plates.

Welding a deck to the transom at deck level will act as a T there, at that level, but the T that will keep the transom from folding/bending/kinking back is the slop tray/wash pan at about 5" to 7" below the top of the engine's clamps at the cast mount which hooks over the stern/transom mount.

I can't say if the scantlings you've suggested are strong enough without pictures or drawings. Nice shop rate though, I wish we could charge that, and we're in Alaska! [where things are supposed to be more expensive.]

Cheers,

_________________
Kevin Morin

$95/hr,,,,,,wow!!! I don't know where you live but here in the panhandle of FL you could buy a brand new all welded, constructed out of .125 5052 boat for not a lot more than you are going to end up with in your remodeling job. I have done several of those type of repairs on boats like yours and I am not a big fan of doing so, in fact i won't do them anymore. I equate it to painting an old truck and not touching the motor that has 300k miles on it. May look better but you will still have some issues. In your case it will be old and stretched rivets that if not already leaking soon will. Obviously I don't know your boat and not aware of all the issues but don't be surprised if the shop ends up with 15-20 hours in the job. Then throw in the material cost and it could end up being a very expensive fix.

Kevin has told you all the things you need to be aware of. Only thing I might would do differently is go with .090 on the piece for the floor simply for weight reasons. As for the tray concerns, make sure the shop does install some type of transom bracing to compensate for the removal of this tray. Kevin's advice on welding the new floor to the transom is very important. I have seen several "home jobs" where this was not done and soon the transom cracks next to the bottom weld.

This boat is a Lone Star 17' made from .090 aluminum
This boat has a sizable trough in front of the transom that I thought was a splash guard and a battery and steering cable holder. I have no intention of modifing it since it is an integral part of the hull.
I thought I would just replace the current 1.75" plywood reinforcement with an an aluminum equivilent. With minimum welding to the boat hull except to attach the new floor material to the new transom material.
The current revited outer transom that is revited to the bottom and sides will not be modified.
I am not married to this boat although it is a classic, so I have no intentions of spending much money on it.

I would just replace the wood....it lasted how long??

_________________
Boat building can best be defined as an endless series of
tragedies obscured by the occasional miracle, followed by a good bottle of beer.

Don't Dream Your Life, Live Your Dream

oleman,
if it were my skiff...... I'd do an epoxy saturated plywood sandwich and follow upspirate's advice. Welding on these boats except to repair corner cracks or casting repairs isn't the most productive way to prolong the boat's life. It can be done, and many welders have done countless versions of a repair- but they're not originally welded boats so those repairs sure have a high cost for such a mixed history of problem solving......

I'd seriously consider the plywood/epoxy sandwich with good bedding and replace the rivets/bolts/fasteners and maybe add some 5200 bedding to the entire panel (?), and go fishing. Drill out to the next size each of the original holes in the transom where there is likely some white chalky looking corrosion, clean the inside of the transom with hydroflouric acid (Aluma-Brite or Zep-a-Lume, brand names for etching solutions) then rinse and let dry, then 5200 and clamp the new epoxy ply panel on, when dry cover with the new inner transom aluminum sheet -repeat the gluing and re-attach the tray and knees.

New knees are often good investments as the rivets/bolts and other mechanical fasteners will have corroded in many places so replacing these bench built items is worth the time. I've welded up plenty of these instead of trying to find a press brake to form them.

IMO; you do well to "divorce the classic" and save the proceeds from her sale for a welded alloy boat- build her yourself from Glen-L's plans!

Cheers,

_________________
Kevin Morin

Since the origional plywood has only been replaced twice since 1965 construction, last time in 1992, this forum has convinced me that I would be fixing something that is not broken if I went through with a welded aluminum addition.
I actually have been working on a glue-up of multiple pieces of 8 layer 1/2" plywood for some time it just needs to be completed and epoxy saturated. This plywood is good quality but not marine grade, it is only exterior, I did not think marine was required because it would almost be an epoxy piece when completed.
I am going to take it to a shop that has an 18" table sander to get it flat, square and straight edged. Although it is not twisted or warped to the eye. I want the finished part to be like a table top.
I assume it will be better to round off all the corners prier to epoxy soaking.
Also skin the inner side with aluminum for protection. Any thickness required. Think .0625 is thick enough.
This will be a PITA because the boat is in a storage yard and the engine has to be removed. We do have a 50HP tractor and a 3-point boom pole to lift the engine with but that requires moving the tractor 40 miles.
Couldn't two good sized people lift the engine and place in a truck bed. I have never had the engine off the boat before. I forgot we also have an engine hoist, if we can get it high enough!
Opinions appreciated?
By the way, I have an old marine 3-71 DD and paragon HJ in very good condition for my REAL boat project. 1700# and 112 HP. Sort of build a hull around an engine.
Thinking displacement steel hull so the very heavy engine would not be a serious problem
Guess I am a sucker for the obsolete.

I have lifted a 70hp off a transom by myself when I was younger and dumber, would not attempt it now even with another person,(I'm 6'6" 280lb former college ball player) If one person stumbles or can't carry his load both will end up getting hurt. I have used an automobile cherry picker style engine hoist to lift my 150hps off my catamaran. Even if yours is the shortarm lift style it should raise up enough to do the job. If not you can always put it on blocks and get it high enough. If nothing else throw a strap around a good limb and use a come-a-long and lift it. Anything other than manually trying lift it off.
Good luck!! and be safe

I have a 18' MonArk, built by Brunswick that is very similar to yours in construction. I did a lot of research on doing just exactly what you are thinking and I would have to advise against it. There is a sealing strip (Goodrich rubber) between all the riveted joints and if you burn that out your rivets will be loose and your boat will leak.

The aluminum is tempered which is why they get away with it being so thin. There are few supporting members in the hull so it flex's a lot. The rivets allow for that occur. If you weld on the tempered aluminum and inadvertently harden it, it will crack.

My solution was to use Super Ply instead of Marine ply because it is made with a waterproof resin glue and has the fewest voids of any ply on the market. I made two layers of ply and sandwiched in a piece of multi-directional weave carbon fiber. I used West Systems Epoxy as the adhesive. I finished it up with a two coats of Epoxy. When I dropped it in I used 5200 around where the motor mount bolts were to help maintain some level of intgrity at those entry points.

I use a Jack Plate rather than mounting directly to the transom and I spread the weight by using 1/4x2xnnn spreeder bars. Mine are 30" long buy that varies from boat to boat hence the "nnn".

The boat was max weight rated for the average 300lb 115 hp motor. This hull happens to be weight rated rather than Max HP rated. Not sure why though. I have run up to 225 (100lbs heavier) hp motors on it and the transom handles it with no problem whatsoever. My original plan was to make it ready for a 4 stroke motor.

_________________
Steve Schefer
Santa Rosa, Ca.

New Years Resoluiton - Never leave something for someone else to do when I should be doing it myself.