Considering jet drive, concerned I'll screw it up

Discussion in 'Jet Drives' started by nbehlman, Aug 15, 2011.

  1. nbehlman
    Joined: Aug 2011
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    Location: CT

    nbehlman Junior Member

    I plan to build a small (~14ft) wooden boat in the style of the early chris craft and hacker craft racing runabouts. I would like to be able to drag the boat up onto the beach if necessary, and I would like the boat to be easy to trailer and launch. I have ruled out inboard prop drive based on these requirements. An outboard would be the easiest path, but I am worried about the aesthetics of an outboard on a classic style runabout. I believe an inboard would be much more pleasing to the eye.

    I have thus come to consider jet drive propulsion. My initial idea is to buy a jet ski and modify it for my boat. Is this a terrible idea? Will i pay for it in fuel economy?

    I have read around a bit on the forums and it seems that there is a lot more to consider in sizing and picking a jet drive vs an outboard. It seems to me that most problems with an outboard setup can be alleviated with a different prop, but it is not so easy with a jet. If I just go out and find any old jet ski and use it for my boat, will it really be that bad? In general I think it is pretty easy to get something 85% right, and most of the time I can live with that 85%. It seems that the cost of getting your project to near perfection is immense. How much of a penalty in efficiency will I pay if I put a jet ski motor in a 14ft boat? Is it really worth the effort and cash to size and buy a brand new jet drive when I can go buy a used jet ski for $400 and run with it?
     
  2. nbehlman
    Joined: Aug 2011
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    nbehlman Junior Member

    Here are a couple more details about the kind of hull I'd be working with:

    Length: 14ft
    Beam: 4.8ft
    Avg deadrise: 3.5 deg
    Weight: unsure, but definitely less than 650 lb
     
  3. Submarine Tom

    Submarine Tom Previous Member

    Have you considered an inboard/outboard?

    -Tom
     
  4. mohawk
    Joined: Jun 2011
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    mohawk Junior Member

    It should be OK with a 130 hp+ jet ski set up , But try keep the weight under 300kg or you might be dissapointed.
    Id look for a Hamilton / kodiak single stage 7.5 inch unit and chuck a light 2ltr engine in it. Subarus are great and the 7.5 inch units are bullet proof.
    Something like this wont dissapoint
    http://www.nzjetboating.com/yabbse/index.php?topic=18635.0
     
  5. nbehlman
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    nbehlman Junior Member

    I have not considered the inboard/outboard because I have never seen one on a boat this small. According to the savitzky method, the hull form I'm working on will achieve 25 mph with an effective horsepower of 11.5. So with a propulsive efficiency around 50%, I would need a 25 hp motor. That seems rather small for an inboard/outboard to me, but I could be wrong.
     
  6. nbehlman
    Joined: Aug 2011
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    nbehlman Junior Member

    130 hp seems like a lot of power for a 14ft boat. I am really not interested in going any faster than 30 mph, which is about where jets become favorable in terms of efficiency.

    My rough estimate for the weight of the hull is 330 lb (150 kg). There is 13ft 8in Wm. D. Jackson design ("thunderbolt") out there that is supposed to weigh 250 lb (114 kg) so my estimate may be slightly conservative.

    I saw this on craigslist, and I am rather interested.

    http://southcoast.craigslist.org/boa/2530695611.html

    62 hp might be reasonable based on my savitsky calc and considering you need to throw more hp at a jet anyway. The guy says it will do 44-48 mph, so it might get me going around 30 mph in my boat.
     
  7. mohawk
    Joined: Jun 2011
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    mohawk Junior Member

    That ski should be fine for what you want, but keep the unit in good condition and the over all weight as low as possible. The ski pumps cant cope with too much weight or wear , but are very cheap and easy too maintain. You will also be surprised how heavy the ski is once youved stripped it.
    We use everything from the ski, even the handle bars.
    Heres another link that may help, this is jeffs 3rd jetski powered boat. http://www.nzjetboating.com/yabbse/index.php?topic=21586.0
     
  8. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    Jets do very will at high speed, moderately well when in the low 30's and truly suck with low speed operation, such as what you have in mind.

    The hull form you've got will take the jet ski drive easily enough, but it'll be a high reving machine, pushing a lot more boat then it was intended to, at much lower speeds, so efficiency will be lower then more conventional arrangements.

    Depending on the model your looking to reproduce (assumed warped bottom, utility or small runabout), you may be able to hide an outboard in a well when it's down. I've done this on several of my designs. On the smaller ones, the engine well cover will have to be raised when the engine is in the up position, but this is a fairly small price to pay for an absolutely shoal hull. Jets are quite shoal, but don't like their inlets dragged up a shell and rock strung beach. I don't repair many jets, as there are plenty of shops and marinas around to charge 80 to 100 bucks an hour for this type of work, but the ones I have really don't like eating debris. You could go with the smaller inlet on a 2 stage unit, but now you'll be getting into some serious money. Maybe you could look into tunnel hull configurations and use a more conventional arrangement.
     
  9. speedboats
    Joined: Jun 2006
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    speedboats Senior Member

    As Mohawk suggested the jet ski type setup would be fine in your intended application. Depending on the overall weight and use of the vessel a larger diameter pump may be better, and at least with the use of either a boxer or vee configured motor it would sound like it's ment to. A used 700 series Hamilton or Kodiak pump shouldn't cost moon beams.

    Most of the full size jets will suck shells and small rocks with minimal if any at all internal damage. Jet ski pumps must be able to deal with shells and sand else everytime you drag your ski up the beach you'd wreck it and have to get it serviced. Some guys charge like bulls for maintenance on jets simply because they can. The internals of any pump are increadibly simple, and apart form jet-ski's, most jets utilise automotive engines meaning that any auto mechanic can service it at their standard rate.

    The Chris Craft and Hacker Craft boats are nice vessels, I'd be keen to see progress...
     
    1 person likes this.
  10. Frosty

    Frosty Previous Member

    To every motion is an equal and opposite, so if your boat weighs in at the same as a jet ski then it will perform the same.

    As its very unlikely it is'nt, then it wont.
     

  11. speedboats
    Joined: Jun 2006
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    Location: New Zealand

    speedboats Senior Member

    Jet skis are quite heavy for their length, some even up to 12' long. However your boat probably has more lifting area than a narrow jetski meaning getting the boat to lift onto plane shouldn't be to much of an issue, perhaps it won't quite do the 60+mph the 'ski will do, but do you really expect it to?

    A jetski package into a 14' boat has been done successfully plenty, it does work well, but is it what you really want in your boat?
     
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