Outboard to inboard Grady White 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 or 7000 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

 

[jonesg]

Re: Outboard to inboard Grady White conversion

Why not take a close look at some I/O Grady's,
most people go the other way,
they ditch the I/O and bolt a bracket on the transom.
It would be nice to get some motor mount measurements.
The grady's have their own owner website, its a good boat and theres lots of grady fans.

 

[proshadetree]

Re: Outboard to inboard Grady White conversion

Let us know if you go with the conversion.Would like to see a swap like that.

 

[b.gagnon]

Re: Outboard to inboard Grady White conversion

I have officially heard it all!!!
I thought that when Oops decided to lengthen an old Sea Ray that it could never be topped! Well congratulations you win???
Seriously, I think it would be a shame to butcher such a good boat. The engineering alone would be a huge job! I have no doubt that it could be done, but an easier route would be to strap an I/O in. At least they made some of them like that. Even then it would be a ton of work. On the other hand you could be filthy rich, with plenty of time to play with it. If this is the case I totally envy you! I would love to do something like this!

 

[Renny_D]

Re: Outboard to inboard Grady White conversion

Might it not make more sense to do it as a vdrive. The big issue that comes to mind is that the prop will be too near the surface and unless you're going to run an Arnesson drive you're going to be cavitating alot. You are going to loose a fair amount in interior space on your boat. I think it is doable but an I/O would be a better option.

Thanks
Renny

 

[zach103]

Re: Outboard to inboard Grady White conversion

Why do you wanna go to an i/o. sounds like it will be a cool project and deff interesting if you go thru with it..

 

[rjqn]

Re: Outboard to inboard Grady White conversion

Thanks for your input jonesg.
I want to stay away from I/O and stick with straight inboard. Reason being inboards are, in my opinion, easier and cheaper to maintain. I can rebuild a SBC for $1000.00 using good parts and a BW gear is a lot les trouble prone than an outdrive.
Thanks for the input on the Grady Forum site. I found it and will post my question there also

 

[rjqn]

Re: Outboard to inboard Grady White conversion

b.Gagnon
Why would you consider this butchering the boat?
If a modification to a Boat, car, truck, motorcycle, whatever is done with professionalism I do not consider it butchering. Only if the modification is done in a sloppy haphazard manner should it be considered as such.
As far as the engineering being a huge job, I may be missing something and if so please fill me in, but it looks fairly straightforward to me but, like I stated in my post, I have been a mechanic and fabricator my entire life. I have access to everything from a screwdriver to CNC lathes and milling machines.
I bought the boat on a 3 axle loadmaster trailer 3 years ago cheap, 10K, from someone in financial trouble. but got tired of hauling it to the coast and back. I hate to see the boat just sitting in the driveway, which it has been for 2 years. I have a 37? Silverton at the coast but the cost of fuel being what it is I don?t go out on it very often. I figure I need something to fish on a little more fuel efficiently and I know I can run a SBC more fuel efficiently than twin outboards.

 

[seven up]

Re: Outboard to inboard Grady White conversion

Enjoy

 

[rjqn]

Re: Outboard to inboard Grady White conversion

Seven up
Thanks for thr reply. The PROP would be about 12" from the transom. There is a large aluminum casting that is attached to the transom, this is where the outboards mount. This casting extends about 27" aft. I would mount the shaft strut on this braket. I think you misread the post. My only question about the prop is with only 1/2 to 2/3 below the hull, would backing be a problem. Do I need to get the entire prop below the hull.

 

[Ned L]

Re: Outboard to inboard Grady White conversion

It sounds like you are planning on ending up with basically a surface piercing prop when you are done. I believe the lateral forces on the shaft will be MUCH different than on a traditional inboard installation that a typical strut is designed for. Also, I think the prop shape will need to be very different (check out some surface piecing designs). Reverse will be very limited as the prop wash will be close to 50% against the transom by what you describe. I suppose it could be an interesting project i fyou want to try. As someone else suggested, I'd go with a V drive if you want an inboard.

 

[zach103]

Re: Outboard to inboard Grady White conversion

why don't you just make it so that its under the hull?

 

[jonesg]

Re: Outboard to inboard Grady White conversion

why don't you just make it so that its under the hull?

I wonder what the draft would be , and getting it on a trailer....

 

[zach103]

Re: Outboard to inboard Grady White conversion

don't a lot of the bigger boats have the props under it. . im not sure the affects i just remember seeing that . thats why i suggsted it. . yah the trailer might be a problem.. but if he had one with the rollers could he adjust the height of the rollers to make it fit.. im just throwing stuff out there lol

 

[oops!]

Re: Outboard to inboard Grady White conversion

congrats on offically becoming a boating nutball (i mean that in a good way)

i was gonna convert the ray into a inboard......but the hulls are totally different ....so i could not

the concept was discussed here for a while with some of our top guys.....(mechs).......but it was not to be.....so i stuck with i/o

hit search.....i think it was called i/o to inboard and i posted it in the mech forum

cheers.....and a big thumbs up to ya
oops

 

[wca_tim]

Re: Outboard to inboard Grady White conversion

I see a huge problem... with a single smallblock, that boat is going to be a dog, add a surface drive that you can't trim, and even if you spend the big bucks foir a well designed surfacing prop, you're going to take forever to get on plane.

Add that with the propshaft surfacing, and mounted directly through the holl with no play, you're going to need a lot more than the brackets that are there now to keep things from going al to pieces. I'm pretty sure that surfacing prop set-ups are usually in some way decoupled a little from the boat - otherwise all of the vibration from the blades only working on one side is transmitted throuhg the hull, etc...

I'm no expert by any stretch, but I would lay odds that you'll be very dissappointed if you do what you're thinking about. even if you can get on plane in a country mile, yea, steering is going to suk. If you're dead set against outboard(s), an io would be at least a more reasonable choice.

 

[salty87]

Re: Outboard to inboard Grady White conversion

what's the power comparison between the twin o/b's and the small block? what about weight?...don't forget exhaust mani's and risers.

the prop half out of the water isn't going to work. as tim mentioned, steering is also going to suffer. ideally, you'd want the rudder behind the prop on an inboard. you're putting the rudder in front of the shaft? could be affected by the bow lifting on plane...depends on the details.

have you tried to make room for a more basic inboard design? would clear up room for fishing in the back. also give you some table and/or storage options up behind the cockpit from the doghouse. put a door in your transom and a platform outta your o/b brackets. :^)

i have no clue about the difference in hull designs. i know i have tracking skegs under my inboard, amidships. 3 i think, about 14-20"

 

[rjqn]

Re: Outboard to inboard Grady White conversion

Salty87
Thanks for your input. I really like some of your Ideas.
Power wise the SBC conservatively puts out 300hp. Probably closer to 350, It?s not stock I built it, but I will figure the conversion based on the lower number. The outboards are 200hp each. With the outboards the boat maxes out at about 40 to 45 mph on flat water on the gps. I don?t believe 50 or 100 hp is going to make the boat a slug I have seen this boat with a single 200 hp outboard and I think it was available from the builder as such. It may not do 45 mph, but how often can you do 45 on the big water anyway.
Weight wise
**The outboards weigh 400 lb each and the bracket they mount to has to weigh 150 lbs based on it?s size and thickness. That would make a total of 950 lbs.
**A small block weighs about 550 lbs. The gear is about 125. Add another 150 for the exhaust manifolds, risers, heat exchanger, prop shaft and prop and you get a total of 825 lbs. Even if I am off by 75 lbs weight wise I am equal to or less than the outboard propulsion.
I think you are the one that finally changed my mind about engine placement. There is a large space amidships between the helm and passenger seats, which is raised up about 9? from the cockpit deck. Forward if this the deck drops down to the cuddy. This area presently houses a live well and under that the fresh water tank. Measuring that area, I believe a small block would fit in there and I would lose absolutely no deck space. Then I would be able to configure it with a conventional inboard type design. Now my only concern is balance. I would be removing 900 lbs from the stern and relocating it amidships????

 

[colobiker]

Re: Outboard to inboard Grady White conversion

hey all,

I have been on Iboats for about 3 weeks. looking for a cheap boat.
found 1957 Magnolia runabout with no motor..havent seen it or purchased it yet

I do not intend to hijack this thread, but I am watching closely to what everyone is saying about this grady white conversion. to me it sounds like alot of work.. but loads of fun. (but then I am twisted and sick, I have built cars and MC's in the past)

 

[seven up]

Re: Outboard to inboard Grady White conversion

Enjoy