jackshaft drive considerations

Discussion in 'Boat Design' started by johndezman, Aug 26, 2002.

  1. johndezman
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    Location: south coast of lake erie

    johndezman Junior Member

    i'm planning to acquire an old (1977-1979) 30 sea ray overnighter. the most commonly available units have straight inboards either small block ford or chevy or big block chevy.

    i am not a big fan of straight inboards, and would entertain a drive conversion to either outdrives or surface drives of some type.

    my initial reaction would be to leave the existing engines in place and jackshaft to the transom. where can i obtain design info to plan such a project?
     
  2. gonzo
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    gonzo Senior Member

    You will have several design and structural modifications necessary to make a change like that. Inboard motor boats have their center of displacement much more centered than I/O's. The stern on an inboard does not have the reinforcement needed for the I/O unit. They require 1 1/2 to 1 3/4 thickness to be installed. Also you have to measure to make sure there is enough headroom for the I/O with its steering mechanism. You should also consider that an I/O installation is much more complicated and has a lot of mainteinance as compared to a straight inboard. For example it will require a haulout to change the impeller and the gear oil every 100 hours. These units have aluminum housings so corrosion is always a major concern.
    Surface drives are much more expensive and usually used in high performance boats only. The cavitating props only work well at high speeds. They also require major structural modifications of the stern. I would gues that you would spend about $10k to $12K on materials if new. The labor depends on the shop rates in your area.
     
  3. johndezman
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    johndezman Junior Member

    the boat in question, was available (when new) with either straight inboard, v drive or i/o power. from my research thus far with the i/o and v drive the mass of the fuel tanks and engines were swapped. i agree that depletion of fuel load will affect trim. this could be dealt with by use of a water bag like the wakeboarder's currently use to add displacement to the back of the boat.

    the boat will be used in fresh water (great lakes) where we have a limited season and water pump impeller replacement ( and other maintenance ) can be done during normal spring commissioning.

    i have two volvo 280 drives in great working order, currently installed on another hull so that expense is none.

    removing the fuel tanks in order to gain access to the inside transom shouldn't be that big of a deal, most of the aft cockpit flooring is removable hatches.

    my concern is the actual details of the jackshaft itself. i know a perfectly straight alignment would be ideal but i need to know the limit of misalignment for a "reasonable" installation. we've all seen pickup trucks raised way up with very severe driveshaft angles and they survive for some period of time. also what hardware is available/necessary to install a yoke on the input of the drive itself - is it necessary to provide additional bearing support to the shaft, etc? i guess i should limit my original question to just the mechanical items necessary/involved with a shaft installation.
     
  4. gonzo
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    gonzo Senior Member

    Jackshafts are custumarily installed with u-joints to allow for alignment. They will give you at least twentyfive degrees to adjust, which should be more than enough. The shaft goes into a coupler at the flywheel and if you put u-joints will need two intermediate supports for the center section. Usually the engine lines up OK, with less complication. The coupler has a rubber hub that helps with some misalignment. The other thing to check is the gear ration in your drives. The prop pitch can be used to adjust too.
     
  5. johndezman
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    johndezman Junior Member

    since i recall something on the order of a 10* angle on the straight inboard and probably a 5-10* transom angle( net 0-5*) there should be plenty of latitude for alignment in the vertical plane.

    the engines are on something like ~34" centers as i recall, so the spacing of the drives on the transom could be "spread" a little and still be within the 25* alignment.

    is the 25* a "cumulative" type of spec ie that you add the # of degree's in each plane or is it 25* in each plane?

    what/where would be a source for the yoke that would attach to the input shaft of the i/o and also the flange for the output of the transmission? what range of options are ther as far as spline details to match up with the input shaft?

    i guess i'm surprised that mercruiser or volvo wouldn't have info available!
     
  6. gonzo
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    gonzo Senior Member

    The center spacing should be enough. I can find out for you on the availability of parts. I am on vacation and won't be back to my shop until Monday. We may have some used or rebuilt parts.
     
  7. johndezman
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    johndezman Junior Member

    great, i appreciate any help.

    i can certainly wait as this is still conceptual at this time.
     
  8. FRANKIEFRANKIE
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    Location: FLORIDA

    FRANKIEFRANKIE Junior Member

    Trimmable Surface Drives

    Go to www.pulsedrive.net and check with them they will do the engineering for you with the purchase of the 5 year warranty drive.
     
  9. johndezman
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    Location: south coast of lake erie

    johndezman Junior Member

    the pulse drive looks very intriguing but as stated in a reply above, i have a couple drives i already own.

    i'm not trying to double or triple the investment in a 25 year old hull, just make it run better with stuff i already own and a little sweat equity.
     
  10. Timm
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    Location: Crystal River, FL USA

    Timm Senior Member

    25 degrees sounds awfully high to me. Most of the U-joint installations I have seen were limited to just a few degrees. Also, the angles of the two joints must be equal or you will have some bad vibrations. You may want to use CV joints like the Aquadrive or PYI Python. These allow more angle and the two angles can be different. These also allow you to soft mount the engines for less vibration. Do you have a special problem like draft restriction that makes I/O's better? Most people consider inboards to be the better drive system in boats this size, thus their better resale value.
     

  11. johndezman
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    Location: south coast of lake erie

    johndezman Junior Member

    as i recall (from reading magazine articles of the time) the straight inboard was 8 or so mph slower (25%) than an identically powered i/o. the article i recall reading had identical 30' weekenders with twin 233 mercruiser engines (351 ford 2bbl's) with straight inboard and i/o - straight =32mph; i/o = 41 mph.

    this kind of efficiency gain is attractive to me (and i have never owned a boat that only did 32 flat out - that's just to slow!) also with fresh water use there is no disadvantage to the i/o.

    surfacing drives, while technically interesting, would definately be cost prohibitive. in addition, i have never seen direct performance comparisons (i/o Vs. surface drives same power this size boat) to see a proof of their efficiency advantage (if any). if you have any please forward.

    lake erie's western basin has a multitude of places to go and see and when i acquire this craft, i want to get out and use it and stretch its legs!
     
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