252G outboard to inboard conversion

[rjqn]

I have a 1989 Grady white Sailfish 252G that I am considering converting from twin outboards to a single SBC Inboard. I believe I have most of the major parts I would need to complete the conversion. I have an SBC engine, A direct drive BW gear, a 18/19 prop, and various other assorted parts.
My plan is to mount the engine as far aft as possible and have the prop shaft exit the hull at the extreme bottom of the transom in the area of the drain plug hole. The center of this hole is about 1.5" above the bottom of the hull. If the prop shaft extends about 12" past the transom between 1/2 and 2/3 of the prop should be below the bottom of the hull.
There is an extension on the transom on which the outboards mount which is about 27" long and the bottom is about 6" below the waterline. I plan to use this to mount the prop shaft strut and rudder.
I have been a mechanic and fabricator my entire 47 years and as for whether or not physically it can be done I have no doubt it can.
I have however a few questions in my mind about the project.

1) Has anyone out there accomplished this or a similar project.
2) With this amount of the prop below the hull would backing be a problem.
3) Would a SBC (300 HP) with a direct drive gear be enough power to turn a
18/19 prop and push a 6000 lb boat

I would appreciate any input and if anyone is interested I would post pics tracking the projects progress. As I have access to any equipment and machinery I would need to complete the project, I figure it should take 1 to 2 months to complete

 

[Grog]

If you don't plan on planing with the boat there probably isn't a problem but you don't need a SB for that. Why not go I/O instead?

 

[seasick]

Sounds nuts to me
I can't imagine good performance with half of the prop out of the flow of water. It seems that you will have a very big eggbeater. If you are set on doing away with the OBs, go I/O as suggested..

 

[Tuna Man]

You certainly have some unusual ideas. I have hard of people converting I/O boats to outboards, but never outboards to an inboard.

Many things to consider, what will the angle of propulsion be? Will the stringers handle the engine mounts or will you have to add supports to handle the load? Is the engine already marinized? Are you going to use fresh water cooling or not? Where is the best location for the additioonal thru-hull fittings needed for the additional engine plumbing? Will the prop 'see' enough clean water if partially obstructed by the transom? Will the transom handle having a relatively large diameter shaft pass through the bottom of it? Assuming 'BW' stands for Borg Warner I assume this is a transmission? If so, is it availalble with the gears you may require to meet your needs?

Most Grady Sailfish boats that are I/O powered (I am referring to roughly 1985 till 1992) came with a single big block (454) or twin four cylinder engines (140 CI I think, each). With either setup I suspect I believe they topped out in the 30 knot range, probably cruised at around 22 knots. With what you propose a single small bock Chevy, I suspect you would be underpowered if fitted to an I/O. If you could pull off using the engine with a straight shaft (inboard) I suspect you would need a prop with about 11" of pitch. I also suspect the boat would barely get up on plane.

Just for comparison purposes, I have a friend that owned a boat that was a foot shorter and had twin small block Ford inboards (basically mounted where the fishbxes are in your boat). The boat was no speed demon. If I recall we cruised around 22 knots and I could not telll you what it topped out at. I can tell you the tall motor covers dd warm you up on cold fall days when striper fishing.

 

[gw204]

I like the idea, but I would do it a bit differently. Get rid of the outboard bracket entirely and slide the engine, trans and running gear forward to get it all under the boat. Cut the hull and build a prop pocket to lessen the downward angle of the shaft.

Or you could just buy an '81 or '82 Grady 256 Wahoo.

Do a search on Boat Trader. I remember seeing a '90 or later Sailfish that had been converted to a single screw diesel inboard. It might still be for sale.

 

[BobP]

If you go to Home and keep clicking on Home, you will go thru all the photos until you see a wahoo with a prop pocket original single screw IB.

 

[Tuna Man]

Oh yeah, welcome to the site rjgn.

 

[fishie1]

If you go to Home and keep clicking on Home, you will go thru all the photos until you see a wahoo with a prop pocket original single screw IB.

Lemme see if I can help - Is this the one?
http://www.greatgrady.com/gallery/grady ... e=1&cat=30

If so, you can probably PM the owner for advice etc . . .

 

[BobP]

That's the one, view showing the pocket , prop, and rudder from the stern view. The guy has a different idea with no pocket though. He may not have known Grady actually sold an inboard not just I/Os in those years.

With respect to the pocket or tunnnel, seems to be straightforward, Strip existing bulkheads, build pocket over foam mold on topside with hull as is, then cut out existing hull and transom in respective areas from bottom side, add more glass from bottom side, install new bulkheads to mate with tunnel shape. Beef up stringers for motor. Need motor box. Can get rid of FW tank to gain more fuel capacity.

With the fuel tanks removed, the entire middle of the boat's cockpit nearly 4 feet wide from bridge to transom, is all open. Not a cut in gelcoat needed for full access.

Nice playground for DIY crowd !