Aluminium jetboat - A couple of questions.

Discussion in 'Boatbuilding' started by Lurvio, Mar 21, 2012.

  1. Lurvio
    Joined: Jul 2009
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    Lurvio Mad scientist

    Hello

    Time has come to start a new project. I've been toying with this idea for about a year and I think the hull design is ready for being built. The boat will be a little over 3 meters LOA, 1,5 meter beam, 15 deg continous deadrise with a delta pad (1,5 x 0,3 m). The plating thickness is as follows:

    Delta - 8mm
    Bottom, transom and nose - 6mm
    Sides, hull extension - 4mm
    Sheer and deck(thread plate) - 3mm

    The boat is going to be used on inland lakes, rivers and anywhere it will fit in. I'm sure it'll meet a rock somewhere, hopefully in a slow speed. I'm thinking about a Subaru boxer coupled to a PWC pump unit for power.

    The hull is modeled in Delftship and I will use the coordinates from plate developments to create cutting files. I have a quote for water cut aluminium that i think is reasonable, 115 kg of 5754 alu, about 1500€ (incl. wat). Now there is a couple of things I'd like help/opinions with.

    1. Hull shape
    Is there something wrong with the hull form? It is based on a few boats I've seen built on forums and in action in Youtube, but is otherwise just something that looks right to me.

    2. Internal structure
    I'm contemplating on adding a keel bar and/or ring frames but are they really needed with the plating I have? I need some sort of engine beds anyway.

    3. Engine cooling
    I don't want to use raw water so I have two options, a heat exchanger is a sure bet. But I am also thinking about keel cooling. The water temperatures around here peak at +25 celcius during the summer and I think a 80 degree thermostat should be fine on the engine. How much surface area would I need with this setup? Is a keel cooling even feasible in this sort of boat?

    [​IMG]

    Heres for starters, I'm sure I'll have more questions coming later and thanks in advance for anyone taking the time to help.

    Lurvio
     
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  2. BayouDude
    Joined: Feb 2005
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    BayouDude Junior Member

    I am toying with a similar idea as well. I have a kawasaki jet ski with a busted hull but the engine is still running. I am thinking of doing a 14-16' bay boat style rig for fishing the saltwater marshes in my area. I figure you could sit ontop of the engine the same way it was in the jet ski and have the fuel tank housed in front of the console and double as passenger seating. IMO you will need internal framing. I would place three or four transverse webs with longitudinals running every 6-8 inches apart.
     
  3. Lurvio
    Joined: Jul 2009
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    Lurvio Mad scientist

    BayouDude
    There is going to be two seats side by side in front of the engine, just like this boat (click for the source):
    [​IMG]

    I've been looking for information on keel cooling and the rules of thumb seem to suggest an area of about two square meters for the engine power(100-120hp) I'm looking at. I have about 3,5 square meters of bottom area (including delta) so I'd need to weld baffles and a top plate for over half of that area. :confused:


    Lurvio
     
  4. messabout
    Joined: Jan 2006
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    messabout Senior Member

    That is a pretty small boat for use with the Subaru engine, pump, battery, fuel tank, heat exchanger,two passengers, and the weight of the boat itself. Adding another meter of boat length would seem prudent.
     
  5. Lurvio
    Joined: Jul 2009
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    Lurvio Mad scientist

    Yes, the weight is to be kept at a minimum. At this point the weights are.

    * Bare hull ~130 kg
    * Subary EJ20 130 kg (turbo version with 220 hp is just under 150 kg)

    With this weight the chine is just touching water (if floating level).

    * Jet pump (guessing under 30 kg)
    * Cooling 10-30kg (keel cooling should be under 20 kg with coolant if in a manageable size)
    * Battery 5-10 kg
    * Fuel 50 kg max.
    * Seats, electronics etc. 30-50 kg

    About 400 kg ready to go. Add the driver and possibly a passenger and were in 500-550 kg range. About 25 cm draft. I have a couple of options, I can add boyancy by boxing the hull extension if needed. I already added 5 cm of height to the sides.

    I don't want to add lenght because it takes away from the whole idea of the boat.

    cheers
    Lurvio
     
  6. RivrLivn
    Joined: Nov 2007
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    RivrLivn Junior Member

    For what your wanting to build you need to talk to the New Zealand guys.
    They build exactly what you're thinking of. Tons of great info on there webboard here: http://www.nzjetboating.com/yabbse/index.php

    Sounds like a great project.
     
  7. Lurvio
    Joined: Jul 2009
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    Lurvio Mad scientist

    RivrLivn
    Already did. I've been following that forum for a couple of years, they'r the ones that got me interested in the boat type. :)

    cheers
    Lurvio
     
  8. Lurvio
    Joined: Jul 2009
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    Lurvio Mad scientist

    Just a quick update on the project. The hull model has had some changes due to feedback.

    [​IMG]

    The bow end is a little shorter (has larger nose plate). I just bought an engine, a Subaru EJ18 still attached to a 1990 Subaru Legacy. I am also considering building the jet pump myself as jet skis are in ridiculous prises around here and I have a good set of tools at my disposal.


    cheers
    Lurvio
     
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  9. goldhunter_2
    Joined: Apr 2010
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    Location: USA

    goldhunter_2 Junior Member

    I built a couple of boats in the past using some of the features your asking about .

    1) "jets" there is a formula but basically you lose about 33% power compared to a prop it doesn't matter if it is a outboard jet or a inboard like form a jet ski as mentioned.

    also a jetski pump is small (some smaller then others) you may find it hard to locate a impeller with enough pitch for your engine. Just a thought but you mite consider rigging up a dual jet drive powered by the single engine.

    2) "keel cooler" are lots of extra fab work but that work good. I used some rectangular aluminum 1x3 tube that doubled as skags on a 16' mud boat with a SBC400. the aluminum hull it self will absorb the heat and cool fluids you could double plate the bottom of the hull with spacer bars in between and use that open space between for keel coolers

    3) The "rings" (traverse sections) as you called them , yes even with aluminum hulls unless you go super heavy you will need at least some but with all the hard chines you have in the hull design it will be pretty stiff limiting the number of stringers required. The aluminum alloy and thickness you choose will also effect these stuff

    hope that helps,
    Robert
     
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  10. Lurvio
    Joined: Jul 2009
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    Lurvio Mad scientist

    goldhunter
    Good points. The jet might have more losses, but the kind of places this kind of boats get put into, there is not much options in propultion. The impeller selection is not a problem, if I make the unit myself. :)

    I forgot to mention that the keel cooling idea is scrapped for this boat. I'll use heat exchanger.

    I might get away with one bulkhead/firewall and two stringers. I might add a couple of transverse frames under the floor, but we'll see. I don't want to make the hull too stiff as i think I'd like to have the hull bottom dented rather than my skull in case i hit something hard.


    cheers
    Lurvio
     
  11. goldhunter_2
    Joined: Apr 2010
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    goldhunter_2 Junior Member

    To stiff is not a worry but to flexible is essipally the higher HP and speed you expect out of the hull (in my opinion). I have beat several hulls and trust me they go in the air with you:D I only gave this lite throttle this time so I could stop on top the dike this time instead of crossing just for her to video but stiff is not a bad thing .

    ******CLICK ON THE PICTURE TO START THE VIDEO*****
    [​IMG]


    I mite put four stringers personally , the transverse section will keep the hull form twisting and all your longitudinal hard chines weather welded or bent will strengthen the hull.

    In may be cheaper/easier just to find and use one of the newer model 215hp turbo seadoos with the larger jetpumps

    don't get me wrong I am partial to jet pumps systems but there disadvantages are worth being aware of , floating weeds, ropes, trash are all a pain in the you know what when they get caught in the impeller.

    one other options just to mention it even though you seem happy with the jet idea would be a tunnel hull with a raised jack plate like this one pictured below. this boat would run 3 people gear and a dog in 1" of water

    [​IMG]

    [​IMG]

    notice the vent tube for the tunnel in this picture , this boat has four stringers under the flooring
    [​IMG]

    you said keel coolers are out now but figured I post picture anyway this was the sbc400 that I used keel coolers for
    [​IMG]
     
  12. Lurvio
    Joined: Jul 2009
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    Lurvio Mad scientist

    goldhunter
    Thank you for the comments. :)

    The project is slowly taking shape. I have cut the bellhousing off the transmission and I have an option of using the car clutch if needed. I'll probably need to make a mockup to determine engine placement, jet intake shape and cockpit layout before I can order the plates.

    Hull data at this point.

    Delta 8mm (extends under the jet)
    Bottom 4mm
    All the rest 3mm

    Hull weight 93 kg (full deck plate and no internal structures) Should be close to 100 kg all welded. Subaru EJ18 weights about 110 kg in stock form and I may be able to lose some kg's off that.

    I might add another bulkhead forward to make a flotation chamber there.


    cheers
    Lurvio
     
  13. goldhunter_2
    Joined: Apr 2010
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    goldhunter_2 Junior Member

    Mocking up stuff for fitting always helps me out sometimes even gives me a few new ideas ..lol

    you can save a few pounds of weight by breaking as many chines as possible form one sheet instead of cutting and welding smaller plates.

    Flotation chamber is a good thing!!! you never know when you mite need it and it make recovery so much easy if you push it a little to far one afternoon.

    The cars clutch........ I will not say I haven't seen them used before because I have but my personal opinion is to get rid of it will save you weight and removes something you don't need. if your thinking vessel motion control at idea you can handle that with a Reverse gate on the jet setup on a manual lever with neutral position.

    one other suggestion I mite add is I would not run the alum sheets under the jetpump and block accesses or option to replace high impact area. instead I would recommend using a stock jet ski ride plate that can be blot on plate to cover jetpump , you can do this by machining a heavier 5/8" or 1" plate to allow the ride plate to sit inside and drip and tap mount bolt holes into it, your hulls plate can be welded to this same frame to make a water tight seal

    good luck and happy building
     
  14. goldhunter_2
    Joined: Apr 2010
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    goldhunter_2 Junior Member

    oh one other thing you can make you bow rounded instead of flat if you want even with aluminum
     

  15. Lurvio
    Joined: Jul 2009
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    Location: Mid of Finland

    Lurvio Mad scientist

    Hello

    Hull assembly started.
    [​IMG]

    edit: Here's the latest drawing
    [​IMG]


    cheers
    Lurvio
     
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