Seeking advice on replacing Inboards with outboards

Discussion in 'Powerboats' started by celser40, Dec 28, 2007.

  1. celser40
    Joined: Dec 2007
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    Location: Texas

    celser40 New Member

    I have a 1989 Century 330 that blew both motors last year after running over a net in the gulf. I want to remove the Merc 260's and v-drives, shafts, rudders and install outboard motors. I will probably use 225 or 250 C\R merc optimax's as I have one on my 232 Hurricane. One of the problems I see is that there is an 11 inch offset between my transom and the back of the boat. I feel that I can cut the back of the boat off below the swim platform, reverse it and build buckets for the outboards. This would retain the rail above the outboards and the back of the swim platform. The outboards would have to be mounted directly to the transom or I could use jack plates.

    My questions are these:

    1. Will this make my boat handle bad (Changing the weight distribution)
    2. Will this drastically improve fuel economy (Currently 2gal per mile)
    3. Will a transom handle the weight (Mine is at least 16in thick)
    or will I have to fabricate a bracket that mounts to the old engine mounts
    to support the weight
    4. should I use a transom bracket
    5. Am I crazy to try this..............
  2. Kay9
    Joined: Oct 2006
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    Location: Central Coast Oregon US.

    Kay9 1600T Master

    1, Most likely
    2. I doubt it.
    3. Not without being reinforced I would think.
    4. Not sure what you mean.
    5. I think it would be cheaper and safer to buy a new boat, but its your boat and you can certainly try.

    Its a buyers market right now.

  3. celser40
    Joined: Dec 2007
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    Location: Texas

    celser40 New Member

    Well, anyone tried this before, I read that its popular in South America to convert a boat like this. I like the style of the boat but cant afford to run it much with its current consumption. Another reason is I haul 150 gals of fuel, figuring 2 gals per mile X2 (in and out) gives me a range of 35 miles + trolling. I estimate that the 225's would get about 6 miles per gal. If I just got 2 it would make my range 150 miles. Is this realistic. the boat weights about 13,000lbs loaded. My Hurricane with 1 225 gets about 12mpg (GPS) and weights 6,000. The hull design of the Century is a deep V w\trim tabs. The only thing that I have been warned about has been slow speed handling but I can put in a bow thruster if I have to. 225 Optimax are heavy and I plan to install a fuel tank and generator in the empty hole to compensate for the weight distribution. So again I ask, any one seen this, or have any advice other than buying a different boat ?
  4. Frosty

    Frosty Previous Member

    I would do it. I think it would handle better. Any way of reducing weight is an improvment.

    Your transom is how thick 16inches??? please carify.

    Your only concern will be bolting to the back . You boat was never designed to hold two big motors on the back but it should'nt be too much of a problem.

    There is another thread almost identical to this here somewhere?
  5. celser40
    Joined: Dec 2007
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    Location: Texas

    celser40 New Member

    I measured the thickness thru the exhaust holes on the top of the transom and then again thru the drain plug. These boats were built very solid, very thick. I put a thru hull transducer (raymarine) thru the bottom and it measured 2.5 inches. Its a very heavy boat. I know that I will have to build a aluminum plate for the mounting on both side's of the transom and could build an aluminum support from the engine craddle's to help prevent flex. Since the transom is so thick I would assume that it has some foam or plywood in it. I can not tell and will probably drill a core out to see what its made of. The boat ran about 30 @ wot with the 350's (260hp) on it. I am hoping that my MPG will improve along with running a lower RPM crusing. With th old setup, all passengers have to go forward to get on plane but once there the tabs kept it out but only if you maintained WOT, not a good situation in rough water. But how she handled.........
  6. hmattos
    Joined: Jun 2004
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    Location: Devon UK

    hmattos Senior Member

    I presume that the Century 330 is a 33 foot boat?
    We have fitted twin Evinrude 150s to large RIBs and the handling is superb. see
    You have the advantage of tilt and trim to get the boat on the plane, and ahead and astern motors for slow speed handling, plus real vectored thrust as you turn the motors.

    I too would do this swop to reduce boat weight, but do not expect too much fuel saving as the outboards will drink a good deal.
    If you have the chance I would go for the Evinrude ETEC outboards to save weight and fuel, or Honda or Suzuki four strokes for quietness - the Mercury Optimaxes here in Europe have a reputation for poor fuel consumption, short life and they sound like coffee grinders!

    Good luck
  7. celser40
    Joined: Dec 2007
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    celser40 New Member

    Yes, its a 33 ft, I have an Optimax already and yes its loud but runs good. If this gets bad fuel economy, the others must get real good MPG. I am taking the boat to a body shop in Feb to start the refit but wanted some advice before I go through with it. I could deal with the loss of this hull, and could re-use the motors and electronics on another boat, but have to try this one last effort to make this boat useable. The inboard propulsion system struts, rudders, props are so low and fragile that I cannot see repairing this with the poor performance even if I use EFI 350's. As I have read in many threads, Outboards are the solution to hanging these fragile parts in harns way. By the way, I trailer my boat........ Outboards sound like the solution to me, but will the boat still pop out on plane and be stable. Is 225 EFi too big, too small, once I restore the ballance by adding A Fuel tank and generator, will I restore the handling of the boat. I figure that the props will be roughly in the same position but set back about 2 feet. Swim platform extends off the back of the boat and the rear of the boat will have to be re-enginered but I will be basically be bringing the motors into the rear of the boat. As I stated, I will probably build a Jack plate type mount that I can manually adjust Horizontally and vertically so I can dial In the new set-up. But, and I know I keep going over this.......... Will I be better off replacing the inboards knowing that boats like these are basically worthless and I will never get anything close to a return of my investment, or should I take the risk and try to improve what I have knowing it mite make the boat worthless as a boat. What I need is someone who has done this conversion to mentor me on the problems\solutions he faced and what he ended up with. Its hard to think I have to give up my Ocean RV for a center council
  8. AroMarine
    Joined: Jul 2005
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    Location: Atlantic City NJ

    AroMarine Junior Member

    The thickness of your transom will probably not be enough on its own you will probably need knee braces from at least as close to the top engine mounting bolts to the stringers and possibly one horizontally across the transom. Also if you already have problems getting the bow down taking the weight of the engines and running gear from the inside of the transom and then putting out past the transom is qoing to make this problem worse not better. When doing big change repowers remember you are re engineering and redesigning the boat. Lots of work.
  9. Willallison
    Joined: Oct 2001
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    Location: Australia

    Willallison Senior Member

    There's quite a bit involved in replacing inboards with outboards. Whether or not you transom is capable of taking the extra forces is something that can only be determined by an experienced builder / designer / or perhaps surveyor who can physically check the boat out. The money would be well spent.
    You will almost certainly have to do some shifting of on-board weights. Preferably ones that don't change (fuel and water tanks should be a last resort...) you'll be pulling 1 1/2 tons out of the middle of the boat and adding 1/2 a ton to the back. This has consequences, though none that can't ususally be overcome.
    The lower weight and higher technology of the Optimax's will improve fuel economy somewhat - but you can foget about getting 6 mpg. I haven't run any numbers, but I'd be surprised if you can better that 1.5. Take a look at some tests of similar sized and weight boats, powered by similar o/b's and you'll get a pretty good idea.
    The lower weight will also effect stability - particularly at rest. Chances are the the boat will float higher to the point that the chines will be out of the water at rest. This will make the boat tender - with a tendency to flop noisily from chine to chine.
    You might alos want to consider the loss of onboard hot water. It may not be an important consideration for you (but it may be a deal-breaker for your wife!)
    Personally I'd go for a bracket rather than hacking into your transom. It will be cheaper, easier, the boat will be quieter underway, and you'll be less prone to flooding over the transom.

    ps. I got your email. Sorry it took me so long to get to you - been away for Xmas...
  10. TnD
    Joined: Aug 2007
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    Location: Victoria. British Columbia, Canada

    TnD Junior Member

    I'm not a boat designer...but I have been involved in conversions like your talking about on boats in the 20ft range, both fiberglass and aluminum.
    I was working at an aluminum boat manufacturer at the time and all the conversions involved manufacturing an aluminum pod that was either welded to the existing hull, or in the case of glass boats, was bolted through the existing transom with heavy backing plates and liberal use of Sicaflex.
    As to how strong the bolted method was...we once removed a pod that had been added to a glass boat several years before, and once the bots were out, we still had a ***** of a time getting the pod off. The Sicaflex holds so well that we were lifting the entire back of the boat off the trailer while trying to break the bond.
    The size of the pod varied depending on application. in some cases it was a full width extension, in others it was about two feet wide for just a single outboard. Since you are generally reducing weight going to the outboard, the tendency is for the stern to sit high. This is balanced by the addition of weight farther back.
  11. TnD
    Joined: Aug 2007
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    Location: Victoria. British Columbia, Canada

    TnD Junior Member

    In all the cases I was involved with the customers were very happy with the results and no handling issues were reported. I don't have any efficiency data for you unfortunately, and I think it would be highly subjective in any case.
  12. charmc
    Joined: Jan 2007
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    Location: FL, USA

    charmc Senior Member

    There are several manufacturers of pods/boxes/hull extensions with experience in mounting, weight shift, transom reinforcement, and related issues. A similar conversion was discussed in some depth in this forum last year:

  13. CDK
    Joined: Aug 2007
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    Location: Adriatic sea

    CDK retired engineer

    Wonderful idea, don't let people talk you out of it.
    Of course you need some metal to distribute the pressure over a larger area of the transom, but anyone who even considers such a job must have at least some idea about engineering.
    The center of gravity shifts a bit, yes, but that's why big outboards have hydraulic trim.
    I converted a 26 ft Draco from sterndrives to jets, was dissatisfied and am now converting to tunnel drives of my own design.
    Blood, sweat and tears, but it's educational. And hopefully: rewarding.
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