Homebuilt Personal Watercraft

Discussion in 'Boat Design' started by preventec47, Jul 31, 2016.

  1. preventec47
    Joined: Jul 2016
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    preventec47 Junior Member

    To me this sounds like an argument against the whole world of PWC and
    the millions of owners of them dont seem to be getting bored doing figure 8's.
    I think you might be a "proper" old timer boat guy having trouble reconciling with
    the world of PWC. Honestly...same here. I dont have a cell phone or a TV.

    There are of course skiffs, and dingys and tenders all of which have a bunch of utility
    I agree but I just want to be transported across the water at
    an exciting pace and that is what I think a tiny little thing with a non anemic
    motor could do. Did you see the youtube video of the CrabSkiff race?
    Do you think any of those guys were bored ?
     
    Last edited: Aug 7, 2016
  2. Mr Efficiency
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    Mr Efficiency Senior Member

    PAR laid out the perils and practical difficulties of adopting air-cooled industrial engines to boating applications. Dinky mini boats are near useless in anything but flat water, not least because you can't stand up and bend the knees to cross wakes etc, without re-locating your kidneys.
     
  3. upchurchmr
    Joined: Feb 2011
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    upchurchmr Senior Member

    Negative, negative, negative.

    Let the guy play.

    What has he done to you?

    Everybody starts from a different place.
     
  4. Mr Efficiency
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    Mr Efficiency Senior Member

    No, they are impractical and it would be far better to have a small runabout with an outboard.
     
  5. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    I wasn't trying to be negative, but just pointing out some concerns.

    I've personally built just as he's suggesting, admittedly many decades ago. There's two general approaches for those racing skiffs, which are; leave the un-marinized engine fully exposed, eliminating two of the mentioned issues, but the others still remain. Heat will be a big factor in one of those skiffs, but given they only run fairly short races, I guess something you can live with for short durations. About these skiffs, they steer very poorly, pound unmercifully and I hope you like getting wet, because you will.

    The other option is a true inboard setup, which was what was popular when I was a kid.

    [​IMG]

    This is the typical setup and once was quite common to see getting slapped together in a garage. Again, these puppies can't steer worth a damn, but are fun as hell for a short hop around the local puddle, in smooth water. This one is 12' long, but there are smaller and larger versions of similar.

    The evolution of these puppies, moved to more deadrise and outboard power for vectored thrust steering. These two features dramatically improved handling and sea kindliness. They could take on rougher water and carve a corner with the best of them. Even current designs (I have one for a 12' class racer) show more evolved hull forms, effective strake and chine placement, plus more deadrise. Of course, these are outboard or jet powered for ease of installation, better handling and economy concerns. It would be possible to retrofit an inboard into one of these, given the limitations of the straight shaft setup.

    Most of the time the problem with the design of these straight shaft setups is length. You need a lot of length to get the engine/trans in front of the driver, with a shallow enough shaft angle to make it practical. When the drive system as a whole is 6' long, it's tough to stuff this into a 10' and have room for a driver.


    No one is married or in love with PWC, in fact you'll find they aren't much liked around here, but the design decisions employed in them are valid, for the reasons previously stated, as well as others. Simply put, no one wants you to give up on your idea, but just pointing out it's a whole lot easier to use a jet or an outboard to produce the same thing. Given you can buy a well used PWC cheap, fix it's issues and splash it again, likely for a lot less, there is a practical consideration, which is what you're understandably hearing.
     
  6. Mr Efficiency
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    Mr Efficiency Senior Member

    Yep, the mini-boat with the inboard will be so cramped as to be not much chop at all.
     
  7. PAR
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    PAR Yacht Designer/Builder

    The image shown above is the very old style, with shallow, warped bottom hull forms. These are fast, but beat the crap out of you and handle like crap.

    This is one of my designs and more typical of the modern interpretation of these old class racers. This will get up and scoot, with a 10 HP outboard and fly with a 25 HP outboard. This one is shown with a long shaft, which I consider a safety issue, but a short shaft cutout is also indicated. An inboard could be fitted, but with this deadrise, you'd be hard pressed to get a reasonable shaft angle. Maybe a tunnel hull would be a better choice.
     

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  8. kerosene
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    kerosene Senior Member

    How is (good) advise a bad thing - even if it was negative?
    Knowing the weaknesses If intended plan is a good thing for the thread starter. If he still thinks it makes sense then he can go ahead but much better to do it from an informed position.
     
  9. preventec47
    Joined: Jul 2016
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    preventec47 Junior Member

    I want all to know how much I appreciate every comment and suggestion.
    In our lifetimes we all evolve linearly regarding the subject matter
    and cannot hardly imagine ourselves going back to the beginning when we have
    progressed to some more advanced stages. I recognize that many of
    you guys are well into the advanced boat/yacht progression and have trouble
    relating to a primitive and basic mission statement. Has anyone ever known
    a proud father to give his five year old kid an $800 lever action 22 rimfire rifle?
    I have. What a shame because the kid now cannot grow through the normal
    progression of rubber band guns, water guns, air soft guns, BB guns and then pellet
    guns before he ever gets a real firearm. Fellas, I concede I am at the
    rubber band gun stage of boats.

    1st let me remind again that I have been "given" a Kohler 25hp VTwin
    engine so while it may be easier and simpler ( I agree) to use an outboard or jet drive, it cannot be cheaper.

    2nd much like autos and especially airplanes... both of which I have extensive
    experience... we need to define a mission statement for this boat choosing
    among priorities and tradeoffs in function, practicality, stability/ease of use,
    utility, ease of building, fun factor(speed?), cost etc.

    Perhaps it will help to start with what I dont want to do first.
    I dont care to haul passengers, payloads, or operate in rough water
    ( at least at high speed) or stay dry. Further it has been my lifetime
    experience that the perception of speed is inversely proportional to the
    size of the vehicle. ie. I dont think a big boat at 30 mph thrills. However
    a dinky water scooter at 30mph just might. At this point I dont see the
    need to hit the 60/70mph of the advanced PWC crowd. 25, 30 or 35mph
    might be thrilling enough until I get tired of the thrills.

    What I do want to do is "scoot" around conveniently and ECONOMICALLY solo ( easy to launch
    and retrieve the boat.. must be small and light) at an "exciting thrill seeking pace"
    which for a first time non-numbed boater doesnt have to be near an Evil Knieval risk-your-life everytime suicidal activity.

    I ask that you not superimpose your mission statement over mine.
    I want to use my free motor in an offroad dirtbike like water borne vehicle. Not a pickup
    truck or a van or a highway commuter etc.

    While it is semi irrelevant because I already have a free motor and the choice
    has been made, for those of you who question the capability or longevity of
    V-twin industrial lawn mower type engines.... I invite you to view any of the
    dozens of youtube videos of the mud motored jon boats and skiffs etc. Wow.

    I still cant get over the fact that no one found any of the 1960's magazine photos
    "interesting" enough to comment on. Every single one excited me. But then again...
    I am a rubber-band-gun entry level boater at this point and that might explain it.
     
  10. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    The reason no one has commented on those design photos (except me) is those designs suck pretty bad. They usually appeared in Popular Mechanics or similar magazines, suggesting you can build a fun fast boat in a weekend on the cheap. These boats were often outdated at the time of the publication's interest, poorly designed, short lived things, that didn't do any of the things they said (especially the cheap and easy part) in the cleverly written article said they might.

    Your thoughts it will be cheaper to use the lawn mower engine is false. By the time you make a shaft, weld on a coupler, buy a cutlass bearing, possibly a strut, a prop and a transmission, plus the controls, you'll be way over the cost of a used outboard that needs a new set of plugs and carb rebuild.

    This said, go ahead and use the lawn mower engine. Now, you just want a puddle scooter. You are correct in that a 10' boat with a 15 HP engine can scoot you along at seemingly breakneck speeds, though in actually, you'll only be doing 20 MPH or so. With your butt a few inches off the water, the sides of the boat measured in inches, the perception of speed is real. 12' will likely be about right and a well designed, modern (taped seam) build, will be in the 150 - 200 pound range. If you pick one of the Glen-L or similar designs, these will likely be a half a century or older (most are early 60's or prior) designs (300 - 400 pound range) and though they show an inboard setup, the hull designs use shapes that we've since learned aren't very good, particularly if setup as an inboard.

    I'd recommend the Comet FRN transmission, which is 1:1 in forward, but around 2.5:1 in reverse. My understanding is these can be run full up in reverse, so you can have a reasonable reduction on the shaft. These run about $250 new, but can be had cheaper on Ebay. You'll need a 3/4" shaft and a coupler, plus a stuffing box, maybe a strut and its bearing, plus a prop. I'll let you price these up, but just remember what BOAT really stands for. You'll need about 8 to 10 sheets of plywood, depending on the design, 3 - 4 gallons of epoxy, several yards of fabric and maybe 100' linear feet of solid wood stock (SYP, Douglas fir, etc.). Steering can be done with cable and pulleys, though you will need a rudder port (sort of a vertical stuffing box), a rudder and it's shaft and a quadrant arm. You can probably sort through figuring out how to shift it and working the throttle.

    Again, this is just the "Cliff's Notes" version on this puppy.
     
  11. Mr Efficiency
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    Mr Efficiency Senior Member

    Sell the 25hp Kohler engine, and save yourself the grief !
     
  12. preventec47
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    preventec47 Junior Member

    Hmmm . Let's see here... far better ? ? ?
    Cheaper ? ummm no. My inboard motor is free.
    Practical ? ummm... can I launch and retrieve a runabout without a trailer ?
    Can I get 25 miles per gallon at 30 mph in a small runabout ? ( my target )
    Can I achieve 35 mph with my 25 hp Kohler motor ?

    Mr Efficiency - I hope you dont mind my having a small serving of
    chocolate ice cream today even though it would be "far better" and
    less impractical to eat hi fiber green vegetables.
     
  13. Mr Efficiency
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    Mr Efficiency Senior Member

    OK, I just think it a bad idea to build a boat around an engine not designed as a boat engine, regardless of how cheap. You will pay one way or the other.
     
  14. upchurchmr
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    upchurchmr Senior Member

    Preventec47

    I sincerely hope you succeed, what ever the actual cost.

    You have my admiration, I kept forcing my self to not respond badly to Mr. whatever.
     

  15. Mr Efficiency
    Joined: Oct 2010
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    Mr Efficiency Senior Member

    The actual cost could be more than $'s. Buggering around with petrol engines in confined spaces that have not been engineered for the purpose is not that clever.
     
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