Converting I/O to straight Inboard

[legoman67]

Well, I was talking to a guy today about our 29' Carter Safari restoration. Which we are in the process of putting the new transom in. He was saying that he has 2 units off a older 37' fairliner that are inboard transmissions, but were mounted through the transom, instead of out the keel. He said the full drive system including engines was only about 7 feet long off the transom.

Our boat is currently designed for twin volvo penta 280 drives, which we have and our in decent shape. The boat also has a large back deck, that is divided into 3 compartments lengthwise, the engine/bilge, then a large storage compartment, then the fuel tank.

I was thinking that it may not be that hard to rip up the storage compartment and convert to this system?

Has anyone done this sort of conversion? Any input?

We are convering the T holes already with the new transom, so it would just be a simple case of fibreglassing them over on the outside, instead of cutting out. The engine mounts of course would have to be redesigned, but that may not be that hard either.

Here is an older picture before we ripped out the old engines showing the layout.

 

[Don S]

Re: Converting I/O to straight Inboard

This is more of a boat building/hull problem than a engine/drive problem.
Moving over to the Boat Restoration and Building forum.
You are going to have some serious boat balance, shaft log installation, stringer, rudder problems.

 

[legoman67]

Re: Converting I/O to straight Inboard

thanks don, i was unsure where to put this one.

 

[tashasdaddy]

Re: Converting I/O to straight Inboard

you already have a good drive system, that you know will work. don't fix what is not broken. i don't see the benefit for the cost, and work.

 

[Mark42]

Re: Converting I/O to straight Inboard

That is a totally different drive system. The boat may or may not perform well after the conversion. There is a lot of engineering that goes into those designs, so its not just a simple bolt in.

I suggest you keep what you have.

 

[erikgreen]

Re: Converting I/O to straight Inboard

Usually, boat hulls are designed for their drive systems.

Due to the nature of outboard and I/O drives it's sometimes possible to switch between them.

Going from an I/O to an inboard is harder, but still possible. It's not a common modification because it's typically more expensive than fixing an existing drive system, and because inboard boats' design is fairly far, structure and weight wise, from an I/O.

What you're talking about isn't really a full conversion though, it's just hanging a short prop shaft off the back of the boat. Assuming the engine weighs the same or similar to the I/O engine, your balance and trim should be fairly simple to fix.

There's actually a high end drive system for go-fast boats called an arneson drive or surface piercing drive that has an arrangement like this, with straight shafts going back from the transom. That's about the only similarity though.

There are some big problems. First, the prop shaft has to be very short so flex isn't a problem... even a 1 inch shaft of hard steel will flex enough to be a problem without support. You can support the shaft possibly by running an extra strut back from the hull, but that's more work and design. Also keep in mind that the shaft can't go straight back, it has to go at a downward angle to get the prop just below the level of the keel, and going down at a sharp angle will cut power... you'd be pushing water down instead of back. If you extend your shaft so it pushes more back than down, you get more whip problems. There's a good reason most inboard boats have their engines amidships and start the shaft downward from there.

Second, how will you steer? Inboard boats have a rudder and steering gear attached to it, and for any decent maneuverabillity it has to be aft of the prop, so the wash pushes against the rudder. So you'd have to extend a set of struts out beyond the prop to hold the rudder and allow it to turn.

Or, you'd have to arrange for the whole engine/transmission/shaft to turn hydraulically, or else put a universal joint or gimbal in the shaft so you can turn the shaft hydraulically.

If you wanted to go the simplest/cheapest route you could install a short shaft pointing straight back, put a rudder on either side hanging off the transom attached to your steering gear, and drive. It would steer very poorly though, and look like Frankenstein's monster.

To sum up: Yes, you could do it. But to put it in automobile terms, which most people have a better handle on than boats, you have a front wheel drive sedan with a bad engine and some drive train problems, and someone is trying to sell you a cheap engine and transmission from a rear wheel drive pickup. Yes, you could make it fit and work, but it would end up being much more expensive than just fixing the front wheel drive, you'd have to replace the front wheel drive transmission, alter the steering linkage, suspension, body (to fit it all), probably alter the rear axle/suspension, modify the passenger compartment to put in a hump that forms the tunnel for the drive shaft, and modify the driver's seat arrangement to allow the new shift lever to operate. Then try to make it all look good.

I suggest you just fix the I/O

Erik

 

[salty87]

Re: Converting I/O to straight Inboard

^it would be one of a kind though


if you really wanted to go inboard, a v drive might help you accomplish the goal but as EG said there's still a buttload of other stuff to consider.

either way, when you're done you're gonna have an odd-ball boat.

 

[legoman67]

Re: Converting I/O to straight Inboard

there is nothing wrong with the outdrives, yet.. just might be nice to get those money pits off the back, but yeah its alot of work, and it may not be worth it.

Probably just stick to what it was designed for