School me on jets. Looking for something that may not exist

Discussion in 'Jet Drives' started by curtis73, Oct 3, 2021.

  1. curtis73
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    curtis73 Junior Member

    I picked up a project in the form of a tiny boat. 4' wide, 8' long, tri-hull, glass. Bare hull weight is probably around 85 lbs, guessing from lugging it around on my back. Primary use will be fishing on a river that is 2-3' deep where I put in and spots of 6-10'. Some spots are plenty deep, others have jagged rocks sticking up. I have a 3.5hp outboard for it which I could cage the prop, but in my experience a cage does nothing when a rock whacks the gearcase and cracks it. Suffice it to say, there are two types of people on this river... those who have a jet, and those who haven't switched to a jet yet, but they will.

    The plan is to be able to plane where it's safe/deep enough, and to have enough oomph to make it back up river without measuring the time it takes with the moon phase. The plan is not "hold my beer, let's see how fast it goes." I figure this thing will plane at 10 mph, so I'm shooting for 15-ish.

    The knee-jerk reaction is to snag a 500cc jet ski and cut the hull around the drive and graft it into the bottom of my boat, but 500cc is definitely "hold my beer" territory and also has significant weight. I'm thinking more along the lines of maybe a 150cc in the 12-15 hp range which should put about 7-10 hp to the water.

    But where would one find a waterjet to match that? The name of the game is doing it as right as I can so I'm not wasting weight or efficiency. Can I swap impellers in a jet ski housing at get what I need? I considered something like a Mokai jet, but they're expensive and also polystyrene which doesn't really hold epoxy as well as I'd like. They also started life at 5-6 hp and might be outside their capacity with 15 hp.

    Pic of the boat for reference

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    [​IMG]
     
  2. Mr Efficiency
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    Mr Efficiency Senior Member

    The easier solution would be an outboard jet perhaps ? Bolt in on and away you go, subject to transom height being OK. Whether you would get one small enough, I don't know. Might be a market niche for a small jet outboard if you can't ! The problem with outboard jets is they suck power without giving a lot of thrust, especially at low speeds, so a tiny one would be going nowhere fast.
     
  3. ondarvr
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    ondarvr Senior Member

    With an inboard jet, where do you plan to sit?

    The smallest (20 hp I think) outboard jet will work. You'll still need to raise the transom up significantly, plus reinforce it.

    The outboard jet won't be cheap.
     
  4. curtis73
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    curtis73 Junior Member

    Centrifugal pumps on outboard jets are supremely inefficient. It takes about a 20hp powerhead to put 15hp to the water. Imagine a 20hp motor hanging off that tiny boat originally rated for electric only. That kind of weight would just be so prohibitive, and honestly sink it. The 3.5hp that is on it is something like 36 lbs and even that is pushing it a bit. It's an 8' boat that is only about 14" deep. Also, the smallest jet outboard I've found is a 25 hp. There may be conversions for something like an OMC 15, but then you're looking at still hanging nearly 100 lbs off the transom to get 10 hp to the water.

    I figured something like a GY6 150cc makes an easy 12-14hp at only 30 lbs, and a jet (especially if it's plastic) won't weigh much.
     
  5. curtis73
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    curtis73 Junior Member

    Edited to not sound like a jerk :)

    Yeah... we can ditch the outboard jet idea. I appreciate your desire to help me keep it simple, but I would rather use my engineering talents to DIY. The outboard is about three times heavier (and 10 times more expensive) than this boat (or my wallet) will handle. Keep in mind that it's the size of a sheet of plywood and only 14" deep. It's tiny. My shortcoming is that my engineering talents don't extend to building a pump from scratch that will have any kind of efficiency.

    As far as sitting, several options. Something like a GY6 engine is very slight in it's profile, so I could rip out that back "seat" and make a doghouse that goes a little further forward to house it. I can also build a doghouse in the middle and straddle it like a jet ski or motorcycle. Controls already sorted mostly. Nozzle/steering will come from a tiller with a throttle. Nozzle bucket (F/R) would come from tiller up/down/middle.
     
  6. curtis73
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    curtis73 Junior Member

    Re: Seating. Option 1 shown in red for something like GY6. Option 2 in green was the original idea if I sourced a jet-ski hull/propulsion... until I realized that even the smallest 500cc donor would add at least 130 lbs and probably be double the speed I want. I really appreciate the responses, folks. Keep it coming.

    upload_2021-10-3_20-9-32.png
     
  7. Mr Efficiency
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    Mr Efficiency Senior Member

    a 6hp outboard with one person up, would plane that to the low teens mph ? I really think you should stick to outboard, and work on ways to protect it better.
     
  8. Mr Efficiency
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    Mr Efficiency Senior Member

    I'd be inclined to bolt on a hull extension about 2 feet long, with a little tunnel in the middle to allow the water to rise to the motor that is set back there and raised, get a stainless prop if they make them that size, and put a couple of well raked skegs on both sides of the extension so that they bottom out before the outboard does. Stainless props will run with a portion out of the water without letting go. Ideally more than three blades, even less inclined to let go, but maybe those props can't be got for small engines.
     
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  9. curtis73
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    curtis73 Junior Member

    It would do low teens, but on this river, you would make it about 100 yards before the lower unit was missing. Unfortunately, on this river, a propeller outboard is disposable. You might get 10 days out of it or you might get 5 minutes. Guys have come up with all sorts of ways of welding up impressive ways of protecting the prop, but it means one of two things will eventually happen; 1) you hit something hard enough that it breaks the lower unit housing, or you hit something hard enough that it damages or removes the transom. Low teens MPH with a lower unit in the water is not an option. Having a lower unit in the water below the hull means you can maybe turn 1500-2000 rpms. Any faster and you're going to crunch something.

    On this river, your only hope of seeing something in the cloudy water is if there is enough flow that you can see the water disturbance around the rock. I really can't stress enough how much having a lower unit in the water just isn't a thing for this river.

    Just humor me. I don't want simple, I want useful. So, how do I go about finding out how to size a jet for an engine, and what might be some sources for said jets?
     
  10. Mr Efficiency
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    Mr Efficiency Senior Member

    I don't think that hull is going to stand up to hitting rocks, so it is all a bit academic what might happen to a drive leg, I can only suggest what I did, raise the motor with the extension, raise the motor further by use of a prop that will run high without letting go, and place skegs that will bottom out before the hull proper or the outboard leg does. Riverboat.jpg
     
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  11. Mr Efficiency
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    Mr Efficiency Senior Member

    If there is that little water available, that a tiny boat hull will bottom out, it has to be a marginal proposition for anything
     
  12. Ad Hoc
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    Ad Hoc Naval Architect

    Sounds like a neat simple idea to me.
    But can you not get a lesser size/power jet-ski to suit?.....if you're not after breakneck speeds... just find a smaller unit.
     
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  13. ondarvr
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    ondarvr Senior Member

    This is a little background for you.

    I've lived on a river that required a jet pump to get around for 20+ years, prior to that I lived very close to the river. My friend ran an outboard jet business out of one my out-buildings. When I didn't live on a river I lived on a lake or an island in saltwater right on the water.

    As a young kid we made all sorts of boat and motor combo's, it helped that my best friend's dad owned a marina about a block away. We put some very large motors on fiberglass boats under 10' long.


    When people put inboard jets in boats that size it turns them into poorly performing jets skis. The motor and jet pump take up most of the usable space, so you're mostly just sitting there like on any other jet ski. The problem is the hull and motor combo aren't designed for each other, so neither one will perform very well.

    Small fishing boats aren't good candidates for inboards because it leaves very little room left to actually fish.

    My current jet boat boat was an inboard when I bought it 20 years ago. At 15.5 feet it had no room to move around in. I gutted it and made in into an outboard jet, it was a huge improvement.

    Yes outboard jets are extremely inefficient, but they give you the space needed in a small boat.

    I like smaller boats, as kid I wanted to race boats, if sprint boats had been around when I was young, that's what I'd be racing now. We do sponsor one though.
     
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  14. bajansailor
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    bajansailor Marine Surveyor

    How about becoming the first of a third type - somebody who uses simple oars on the boat?
    This is a genuine suggestion - it would also reduce the risk of you mashing up the poor boat when you hit a rock just below the surface at some high speed.
    If you hit it at rowing speed, the odds are that you will bounce off it.
    And rowing is good for the soul, and for helping to keep fit :)

    But if you have to travel a few miles up or down the river to your fishing ground, then maybe it is not a realistic proposal.

    Edit - another (crazy) thought - ok, so you have your wee dinghy already, but if you could pick up a ubiquitous old Yamaha Waverunner with a 'small' 700 cc 2 stroke engine in it for not-a-lot of $$'s, then would it be feasible to convert it into a trimaran for use as a fishing boat on your river?
    You could design the outriggers such that at rest with no crew on board they are floating clear of the water (like a racing sailing trimaran might do), and when you climb on board they are just kissing the surface. They should be able to plane easily, and to run over most submerged rocks if there is a few inches (at least) of water over them.
     
    Last edited: Oct 3, 2021

  15. Barry
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    Barry Senior Member

    The common outboard jets are axial flow not centrifugal, though the shape lends itself to assume that it is centrifugal
     
    Last edited: Oct 4, 2021
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