I/O to outboard conversion on 18' Aluminum

I'm well into this project boat i have began on. I bought this 80's Springbok 18 foot aluminum boat, the hull originally was installed with a 3.0L engine, the previous owner i purchased it off had begun converting it to an outboard but skipped many crucial steps and sold me the boat and trailer without a motor.

The outdrive hole was originally covered with an aluminum plate using steel blind rivets...
There were never any knee braces added and he simply used lag screws to attach angled aluminum from the transom to the wooden floor...


Thus far, i have completely stripped the boat, rebucked or replaced any rivets that had some seepage from a leak test and the boat now doesn't leak a drop. I've filled the inside of the hull as well as dropped into the local river to test for leaks and it seems as if I've gotten them all taken care of. I used Gflex 655 with colloidal silica to epoxy a full aluminum sheet onto the existing transom to take care of filling any miscellaneous holes and covering the outdrive hole, and then secured with solid rivets. I also went ahead and cut the bow open as i plan to make this into a fishing boat better suited for the fishing i do, opening the possibility of a bow jigging deck. I plan to use some angled aluminum bent with the radius of the bow to reinforce the bow and hide the sharp edges of the gunwale.


I've created an inner transom sheet with aluminum that will allow the wooden transom to be sandwiched between. I then created some tabs and tig welded them to the sheet so this sheet can be riveted to the sides of the hull. I have just begun attaching some angled aluminum to the sheet so that i'm able to rivet a splashwell (yet to be designed) to this sheet and then to some other supports that will become a rear deck. I only would like it large enough for a storage area of my batteries and perhaps a place to mount some fishing equipment.


I need to fabricate a knee brace, I've got some 0.100 aluminum that i was planning on bending into the knee brace shape so it can be riveted to the hull floor and bolted through the transom for a proper transfer of forces.

I've also contemplated adding two other braces, connecting the stringers to the transom for extra strength and support.

My reason for posting is that i've got some questions on how to go about fabricating this main knee brace and splashwell/rear deck...
- what's the best way to bend and form the knee brace to the required shape?
- has anyone else built a splashwell and rear deck in their aluminum boat?
- would adding some angled braces running from side to side be best to then add decking on top of, being able to tie the front edge of my splashwell into? This would have supports beneath it attaching to my stringers and subfloor as well

 

I've designed this file to plan out how i would go about bracing my transom to the hull, one main knee brace riveted down the middle and bolted to the transom in the center. I created two spots where i can attach additional braces that would connect from the stringers to the transom. I'm very open to comments and suggestions. The main knee brace will require using a brake to bend the aluminum sheet to its proper sheet, the two additional braces i would use some angled aluminum welded to some flat bar where it can be riveted to the stringers and then bolted through the transom.
This is the full picture, in the center you can see how the knee brace will conform to the hull shape and then go up and be bolted through the transom. On the sides is where i would attach braces to connect from the stringers and bolted through the transom.


My main knee brace, gotta take some final measurements and create a cardboard template to ensure it mounts flush to the floor and to the transom. I'll rivet it to the hull and possibly add some backing plates on the outside so that it grabs securely and is strong. Then bolt through the transom with all the goods to seal around the bolt holes.



My stringer knee braces, these should be much simpler to fabricate... Going to use angled aluminum welded to some flat bar so it has a larger contact area against both my stringers and the transom. These may not be necessary but I'd rather have the peace of mind knowing it is much more structurally sound than required.

 

Nice work Michael.

Unfortunately wood and aluminium do not mix.
As soon as the wood gets wet - which it will - it will start the process of poultice corrosion 'inside' the composite transom and then eventually fail - most likely without warning too.

I would not use wood for your core.

 

I respect your reply but just as boat builders have been doing for numerous years, I’m going with wood. Properly sealed with the OTF and all my holes pre drilled 1/4” larger and filled with sealer and redrilled. Along with 4200 at every bolt and nut head. I don’t expect this boat to outlast me by any means, but doing it the good old fashioned way is what seems best. I was never asking about the wooden transom in regards to my original post, it was mainly about the structural pieces.

 

Re your previous project that was described in this post below (re the rebuild of the transom on your fibreglass Crestliner) , did it all work out well in the end?
Rebuilding Transom/Stringers https://www.boatdesign.net/threads/rebuilding-transom-stringers.66148/

What size outboard engine do you intend to use with this boat?
Will the engine be hanging on the transom (and bolted to it), or will you be mounting it on a pod / bracket?

Be aware of the consequences of stuffing the bow into a wave - with the old design most of the water would be shed from the foredeck if this happened, but it sounds like you are now building a 'bow rider' and the recessed deck will hold a lot of water if you take a wave over the bow (look at the videos on Youtube of bow riders stuffing their bows under in notorious places like Haulover Inlet).

Ad Hoc offered a good suggestion above........

Can you expand a bit on your thoughts re the design of the splash well?
I am guessing that the floor of the well will be just above the blue brackets shown in Post #2 above?

Re the fore and aft length of the splash well, bear in mind the length required when the engine is tilted up fully, so that the housing does not bash the forward end of the splash well.

Re your main knee brace, what are the approx dimensions of this?
I am thinking that a 'belt & braces' approach (excuse the pun) would be good, and try to make it as big as you can, within the size constraints of the space available?

Re your stringer knee braces, if you do use them, then the design as shown does not look very efficient with such a shallow angle of the bit (what is this bit?) connecting the stringer plate to the transom plate - if you will be welding here, then maybe you could weld in a full gusset plate connecting the plates riveted / bolted to the stringer and the transom?

 

- I apologize but i am unsure of what you mean by 'bit'.
- Upon further thinking yesterday and as i was replying to your above question with splashwell, welding a gusset would make the most sense onto the inner transom plate as i could then create additional knee braces that can attach to the stringers with rivets and be riveted onto the gusset. I've got plenty of solid rivets i intend to use for these bracing components to avoid any vibration weakening of pop rivets and to ensure a strong connection.

Thank you for your reply, i greatly appreciate the info and input. I'm always willing to hear what people have to say as i dont know what i dont know, but i am enjoying the process of building and learning.

 

Sorry, I was trying to think of a way to describe it - the 'bit' is the skinny section joining the plates against the transom and the stringer in the sketch below.
But it sounds like you are going to have a solid gusset plate instead.

 

My apologies, that was going to be a piece of aluminum angle, I like the gusset idea a lot which will help add better braces and make a more secure transom.