Newbie with a few (repower) sterndrive questions

Discussion in 'DIY Marinizing' started by mbjeeper, Jul 30, 2006.

  1. mbjeeper
    Joined: Jul 2006
    Posts: 7
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Moundsville, WV

    mbjeeper Junior Member

    I'm new here, and it seems like a really good source of information. Can't believe I didn't find it a long time ago.
    I have had outboard boats before, (still do) but I've never owned an I/O, so I can't say that I know much about them. This is about to change.
    For various reasons I am going to convert a 14' fiberglass runabout to an inboard with a stern drive.
    I have spoken to local boat mechanics about what I want to do, and asked them why it wouldn't work, and after discussing a few potential problems, they seem to think it would work fine.
    I am going to power the boat with a 50 HP Volkswagen air cooled 4 cylinder. I am going to mount the engine right behind the front seats and do away with the rear seating, and run a drive shaft between the engine and the sterndrive.
    I am confident in my abilities to reinforce the transom, mount the stern drive, fashion a suitable mounting for the engine, take care of steering and electrical issues, balance the boat, provide ample cooling and ventilation for the engine, etc., but I would like to get some reccomendations on what makes and models of stern drive unit would be suitable.
    I need something light, because the boat is so small, but it will need to withstand up to 60 HP. I would like to find a durable unit, obviously, and something that parts are still available for. I want to steer clear of electric shifts, because I was warned of their high expense to repair.
    I know that there was a 36 hp VW marine engine in the 1950s with a stern drive, (and a Volksliner boat), and I would love to have either one, but they are made out of Unobtanium, and from what I've been able to learn, the VW engine/drive combo would probably put too much weight too far back for this particular boat.
    Also, what direction do the sterndrives rotate? (as viewed from the input shaft) That is something else to take into consideration, but it isn't a deal killer, because I could always fashion a hub to drive the driveshaft from either end of the VW motor.
    I will not be buying a new or rebuilt sterndrive unit, but will probably have to look around for a junked boat that has something I can rebuild and use, so if you could help me out on what kind of boats the drives could be found on, that would help, too. (as an aside, how would a jet drive unit work for something like this-- I have no experience with these.)
    I have the USCG pamphlet on home boatbuilding and I think it shouldn't be much of a problem to stay within the regulations.
    Any information about similar conversions or input on types of sterndrives would be greatly appreciated.
    Thanks,
    Greg
    mbjeeper@aol.com
     
  2. mbjeeper
    Joined: Jul 2006
    Posts: 7
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Moundsville, WV

    mbjeeper Junior Member

    Ah, there was another thing I forgot to ask. What RPM level are typical stern drive units limited to? A VW is not a particularly high revving engine, but I can't govern it down too much from max (which is also cruising) RPMs because it will affect the speed of rotation of the engine cooling fan.
    Thanks,
    Greg
     
  3. marshmat
    Joined: Apr 2005
    Posts: 4,127
    Likes: 148, Points: 63, Legacy Rep: 2043
    Location: Ontario

    marshmat Senior Member

    Welcome aboard, Greg.

    I take it you're talking about marinizing the engine of a VW Beetle or similar?
    Sterndrives are pretty rare under 120 hp or so; remember, the purpose of a sterndrive is to provide an outboard's great steering and trim ability, with more motor than can be safely hung off the swim step. Up to around 250 hp the Mercruiser Alpha is the drive of choice these days. You might have a hard time finding a prop to suit your engine, with this drive. There would be a lot of losses from turning so much drive hardware with a small engine, and the Alpha ain't exactly light. I'm not aware of any smaller sterndrives that are readily available these days.

    In jet, Hamilton's HJ212, http://www.hamjet.co.nz/index.cfm/Model_Range/HJ_Waterjets one of the smallest non-PWC jet pumps in common use, can handle 50 hp below about 2,500 rpm without overloading the motor. If your engine's more high strung you'd need either a gearbox or a smaller pump. The 212 weighs 77 kg dry. The Aggressor Junior pump is also an option; again, though, it's meant for more power. You might look at converting a PWC's pump, but they aren't as efficient. Not all hulls are suitable for jets, check with the pump maker.

    You might consider a straight-shaft or V drive, instead of a sterndrive. It'll weigh a lot less and gives you more freedom to get the centre of gravity right, and the drivetrain losses will be smaller. As far as trimmable/vectorable drives go, though, it's pretty much outboard-only below 120 hp these days.
     
  4. mbjeeper
    Joined: Jul 2006
    Posts: 7
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Moundsville, WV

    mbjeeper Junior Member

    Thank you for the reply.
    I do intend to to marinize a 1600cc VW beetle/bus engine. It will turn at about 4400RPM max, which in a VW engine is the cruising RPM also.
    I am not locked into a sterndrive. My limited knowledge in this area had sort of led me into thinking this the best option. I am certainly open to any better ideas.
    Initially I was thinking about a straight shaft to the prop, but there were a couple of things that I was having trouble figuring (wish I'd found this forum sooner). I don't know what sort of transmission would be suitable, and how could I deal with the thrust from the prop that would try to drive the shaft through the transmission? Too, there would be the issue of trimming and steering.
    After considering these problems, I had decided that a stern drive might be the way to go. Like I said, I am still open to, and would certainly appreciate, any other ideas. I know almost nothing at all about a V drive. What advantage over a straight shaft would the V drive have?
    As for a PWC pump, I had figured, in my limited knowledge, that they would be too small to propel my boat, and hadn't really looked into them. I will have to do so.
    I just want to be able to make a VW powered boat that will handle decently and plane correctly. How I do it is still very much open.
    There were older sterndrives that went on smaller craft in the late '60s/early '70s that were made for lower HP applications. Would one of these be suitable?
    Again, I am welcome to any other ideas or pointers that would help me plan this project. Vdrive, stern drive, straight shaft, jet drive, whatever will work best.
    Thanks again,
    Greg
    Moundsville, WV
    mbjeeper@aol.com
     
  5. marshmat
    Joined: Apr 2005
    Posts: 4,127
    Likes: 148, Points: 63, Legacy Rep: 2043
    Location: Ontario

    marshmat Senior Member

    The axial thrust in a straight-shaft installation is taken care of by a thrust bearing; sometimes this is part of the transmission itself, sometimes it's a separate and relatively inexpensive piece. A V drive is related to the straight-shaft; but the engine is mounted output-shaft-forward and a V-shaped gearbox connects it to the prop. Advantages: CG is farther aft, and shaft angle is not as steep as with straight shaft.
    Modern PWCs can be had with over 130 hp, and may weigh 300 kg or more. There's some pretty beefy hardware to be had in the dead-PWC parts piles. Especially since people crash them, or burn out motors, a lot.
    Most sterndrives have a reduction in the lower end, something on the order of 1.3:1 to 1.8:1 or so. Almost all can handle 5,000 rpm or more on the input, continuously. With 50hp though, you probably wouldn't be able to turn any current sterndrive that fast. There may be used ones that suit you, but the question is, can you find parts when they break.
     

  6. mbjeeper
    Joined: Jul 2006
    Posts: 7
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Moundsville, WV

    mbjeeper Junior Member

    OK. I need to find out more about suitable transmissions for a straight shaft installation. I don't mean to be asking such basic questions, but I am really new when it comes to boats.
    I am confident that if I can arrive at the correct way to do this I will have no problem with the fabrication end, but I lack enough information to make a good decision on which way to go.
    Most of the old classic runabouts ran an inboard with a straight shaft. Unfortunately there are no boats like that in my area (that I know of) to take a look at.
    Is there a good book(s) or even a web page that would give me a better idea of how to perform such an installation?
    I am in no hurry to get started, so I have plenty of time to plan this out and gather parts.
    Any reccommended reading or other input would be appreciated.
    Thanks again,
    Greg
    Moundsville, WV
    ??Hickman Sea Sled (wooden)
    '59 Owens Cutter 14'
    '67 Sea Sprite 14'
     
Loading...
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.