I/O to Outboard Conversion Question

Hi All,

Got myself a project boat - an '84 Wellcraft 248 Sportsman. Now, I grant you that it is not a wooden boat, per se, but I will need to do some woodworking to get the boat I ultimately want.

As originally configured, it came with an I/O powered by a 350 V8. Well the idiot that had it previously didn't bother to remove all the drain plugs on the block - not a good idea in New England. So I trashed both the block and the outdrive as I prefer O/Bs anyway. So I now have a Johnson 200 V6, as well as a smaller Johnson 25hp as a kicker/trolling motor/emergency backup. The 200 will be mounted on a Gil bracket.

Anyway, my question relates to repairing the hole in the transom left after removing the outdrive. I plan to bolt a piece of 1/4" thick stainless steel plate I just happen to have laying around to the inside of the transom, and then to fill the hole with layers of 3/4" ply/fiberglass cloth and epoxy resin. I even plan to do a three stage stepped plug - the first step (against the stainless plate) will be about 18" square. The next step will be about 20" square - fitting into a ledge I'll rout flush to the first layer. The third layer will be about 22" square fitting into a second ledge.

To take the stress of the outboard pushing on the reconfigured transom, I know that I need to add a pair of knees extending up from the floor stringers along the inside of the transom. My question is: what is the best way to make the connection between the stringer and the knee? The existing stringers are 3x6's, so the knees will be white oak 3x4's or 3x5's. I plan to use a fairly large stainless steel lag screw angled through the knee to pull it tight to the top of the stringer, but I can see that some form of gusset along both sides of the joint would add substantially to the strength. Marine plywood, or more stainless steel plate, would be the obvious choices, but I was wondering if anyone had other suggestions - either for material(s) used or even an entirely different way of doing this.

Thanx,

Brian
Taxachusetts

Brian,
I am not very familiar with this boat but it sounds like you may have some additional problems. By mounting 200 hp and a 25 hp off of a transom that was not designed for it you will need to reinforce the entire transom. That's a lot of weight hanging off of the end. I would also add a couple of stout knees horizontally at or near the deck to help transfer some of the load to the sides. You are also changing the center of gravity by hanging the 200 off of a Gil bracket which may affect the performance/safety of the vessel. As for the verticals from the stringers up the transom, they aren't going to help much without a stout gusset (Knee) to distribute the load. Remember that the transom is now going to see a large shear load even under way. The force of the O/B pushing the hull is minor compared to the weight it now has to support.

Hi Jim,

Hmmm, horizontal knees to the sides is something I had not thought of. I should be able to incorporate them into the new flooring so they won't even be seen.

In terms of the weight, even hanging the 200 off the 24" Gil bracket I have is not going to be a problem. The engine/outdrive I pulled weighed over 900# ! The 200 is barely 450 and the 25 is maybe 90#. I'll be foaming in floatation/insulation under parts of the rear deck and probably add some additional ballast forward if it's needed to balance her.

Brian

Brian,
Keep in mind that 450 lbs suspended 24" away from the transom translates into approx 900 lbs+ of load trying to pull the transom down like a tailgate on a pick-up truck. And that is just static load and doesn't account for additional stresses like bouncing down the road on a trailer or lumpy seas. Again I do not know if your transom was designed for this type of loading so just be certain you add additional strength as needed. It would not bode well to have the transom fall off on the way to launching or even worse, after she is in the water.
Keep us posted.

Jim,

Thank you for the concern and the warnings. I am well aware of the moment arm I'll be creating and have been planning on creating an "overly" sturdy reinforcement system for the transom. I had initially thought to put several dry storage lockers in the floor for fishing gear, but in retrospect, leaving the floor as intact as possible should serve as the horizontal knees. I'll still need to have access for bilge clean out and the bilge pump of course, but that doesn't need to be huge.

The boat is an '84 Wellcraft 248 Sportsman. That model came from the factory in several configurations: 3 versions of I/O's and 1 version for O/B's with two (possibly three) different motors. In cruising various online forums and talking to a friend that's had a Wellcraft with which he used to teach Noah his sea craft, I can find no difference in the transoms on these various versions. The transom is quite massive - a full three and a half inches thick. I have found several similar conversions documented online, but only photos of the completed boat and without specs on how it was done. From what I've read, these older Wellcrafts were massively over engineered, so I think it will work out OK.

I'll be sure to document what I do in case someone wants to duplicate it; or not, if I end up sinking the thing in Boston harbor by either driving the motor through the transom or if it rips off the boat leaving a massive hole!

Cheers,

Brian

Sounds like you have things well in hand. Post some pics. I would like to see how it comes together. I do get concerned when people start making major modifications to a boat because so many of them haven't got a clue. I believe the stupidest thing I ever saw was a fellow that bolted a piece of 2 x 4 across a kayak and hung a small outboard off the port side and a gas tank tied off on deck. Needless to say it didn't work as he thought it would.