Outboard to Sterndrive conversion.

[70sangersean]

Anybody convert a tunnel hull boat with an outboard to I/O. looking at a 21 cougar mtr outboard less motor. Dont know if they made any cougar mtr with I/O just OB,jet and some vdrives are all I've heard of. Is it doable or a waste of time/money ?
Thanks for any input.

 

[AzGeo]

Well since no one else will bite; I think you need to first consider how tall the transom is at the center pod area . Does that hull have enough "hull transom height" to install a "transom piece" and not have the propshaft 2' under the boat's keel . (considering 13.5" is a normal X measurement)

An I/O install in an air trapping hull should normally be "a little higher" than it would be for a conventional V-bottom boat . So you would want to mount the transom piece and the motor higher than the "run of the mill boat" set up .

IF, this hull has enough "transom height", does it also have enough space for the steering, motor mount, and trim hoses, at the top and bottom ?

I've seen I/O boats with the transom piece mounted across the "cap rail", and bolted to both the hull and deck, to achieve their desired "X" dimension .

Consider the distances between the crankshaft center line (X dimension to keel) and the propshaft centerline . Looking at having the propshaft centerline close the the keel height of the center pod .

Also consider that "2" shorter, lower units are available, to help raise the propshaft height .

Not knowing the exact measurements of that hull, I cannot give you a "yes or no", but measuring the drive and transom area will tell you what you need to know .

Have you seen other boats of this brand and size with I/O's ? What are their "X" dimensions ?

 

[azhouseboater]

Also I believe the transom thickness has to be an exact thickness and the inside parallel to the outside for the IO where it's not as important for the outboard. Or you could use something like the Volvo to Mercruiser adapter plate if the transom thickness is close.
http://www.mercruiserparts.com/827691a1-template-kit-tran

But then also the stringers are probably designed differently. For an outboard the stringer doesn't have to help support a 1000 lb engine. But it does have to support more torque coming from the top of the transom so the outboard tunnel I had there were fiberglassed triangle supports going up into the transom that would be in the way of the outdrive and motor in an IO.

You would likely also need about 200 HP more to go the same speed I think. My 21' Eliminator Sport Tunnel with a 235 Outboard did 70 probably back 20 years ago. The 20' Eliminator IO I'm trying to work on probably will need 425 HP or more with a 454 just to hit 70.

And most importantly the tunnel was very sensitive to weight. Sitting still you only have 1/2 or 1/3 of the hull of a v-hull to support static weight. Mine sat very low in the water with the outboard hanging off the back but granted it does put all the weight off the back of the boat. The sterndrive would 800 or so more pounds but further forward. The jet and vdrive put the weight even more forward.

Once up on plane the tunnels trap air and help lift it up which speeds you up. But ad a few more people and you drop top end speed faster than you do with a v-hull. Again granted you have more top end speed to lose.

Also consider you loose a few feet of space in the cockpit of the boat.

All that said I have seen my same hull with an IO in it. But I do know that I loved the ride of my outboard and could blow most I/O away at the start and at the top end.

Also check out the performance articles on cougars site.
http://www.cougarboats.com/39.html

Looks like they had a 22 MTR with a 300 Mercury Outboard with a top speed of 91 and one with a 500 HP Mercruiser with a top speed of 87.

 

[70sangersean]

Have you seen other boats of this brand and size with I/O's ?

I've searched the web, but have not read about or seen a 21 cougar mtr with an I/O. They are very similar to the eliminator daytona and liberator tunnel hulls. I've seen pics of daytonas with I/Os and you can still get a new 21 daytona or liberator with an I/O. I briefly spoke with Bob Leach of elimnator and was told the hull is the same. Also was told the price of a new bare hull was around 45k. Way out of my budget. There is one for sale on here for 55k that runs about 130mph. I would be money ahead to buy that boat but dont have the money up front. I'm looking for a project I can do as time and money allow. Figured an outboard would be less money to convert than a jet due to transom work that would be involved.

Also I believe the transom thickness has to be an exact thickness and the inside parallel to the outside for the IO where it's not as important for the outboard

I never thought about the transom being different or an issue other than the braces that go from the stringer to the transom. From what I've read OBs seem to be faster with less power. I don't really have a legit reason for wanting I/O other than the look of OBs don't appeal to me, jets take even more power to run fast, and I/Os are what I'm use to.
I'm still researching. A lot of things I need to find out, such as if the stringers and transom are the same. I would say 90% of these tunnel hulls in 21' are OBs and jets so you hardly see any for sale. Maybe thats saying something, but i have read about and seen video of some I/O and v-drive 21 tunnel hulls that haul ass and are stable at 100+ mph.
Thanks for the replies/info and if anyone else has some info they could share i would greatly appreciate it.