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Thread: small jet drive

  1. #1
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    Default small jet drive

    I spoke with someone Sunday who was thinking of putting one of these in as an inboard for a small sailboat, anybody try something like this or have an idea from the engineering data how efficient this would be compared to the same engine driving a regular prop? Or comparing the useful thrust you would get out of this compared to a similar weight outboard? - the engine/pump weight is 50 lbs and the piping would not add that much so it's lighter than all but the smallest outboards.

    http://www.pacerpumps.com/files/gx_pb.pdf

  2. #2
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    Default Re: small jet drive

    not very efficient.....personal opinion.....(unprintable)
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    Default Re: small jet drive

    A zillion real good reasons why nobody else is doing it. Your friend can believe 'em or spend the dosh to find out himself....
    "The enemies of reason have a certain blind look."
    Doctor Jacquin to Lieutenant D'Hubert, in Ridley Scott's first major film _The Duellists_.

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    Default Re: small jet drive

    I think there is more to this than meets the eye.
    The pump is plastic and I don't think it was designed to discharge underwater. It may tolerate it, I just don't know. Also, if it's a centrifical pump the impeller has to turn at a certain minimum to get suction. Would that setup be able to operate slow enough so the boat doesn't become dicey to operate?
    Would the plastic guts of the pump be able to handle any debris that got in?
    If you want to replace an outboard with something else that something else has to be able to do what the outboard does. If it doesn't, modifications have to be made till it does. Outboards are flexible. They can drive the boat slow or fast. If they are watercooled they pump their own water. They discharge their exhaust in the water so it isn't a problem. The newer ones are fairly quiet. And you don't have to cut any holes in the hull to get them to work. Trying to get that plastic pump to be as accomodating could become a long term hobby for whoever tries it.
    If time is of no importance go for it.

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    Default Re: small jet drive

    A lesser problem (but real) would be getting that Honda engine to be marine-safe.

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    Default Re: small jet drive

    It won't work, period.

  7. #7
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    Default Re: small jet drive

    He said he wanted to tinker and was familiar with these pumps from the construction industry. I was curious from usable-thrust vs weight vs fuel-consumption if it was workable.

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    Default Re: small jet drive

    Try it out.
    Borrow a random boat, set a pump in it with the suction hose over the bow and the other over the stern.

    Gentlemen, place your bets.

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    Default Re: small jet drive

    Reverse gets interesting.
    Even when they work, they don't work well. I blame engineers.
    The only thing engineers have done to the toaster in the last 80 years is make it disposable. I think it applies to a lot of things

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    Default Re: small jet drive

    The max pump capacity is rated at 280 gals/minute. Assuming a set up that would work (whether it is or isn't this particular pump) what level of thrust would that provide? Is there an equivalent hp ob motor size, for example?
    "Now and then we had a hope that if we lived and were good, God would permit us to be pirates." ~ Mark Twain


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    Default Re: small jet drive

    Quote Originally Posted by rbgarr View Post
    The max pump capacity is rated at 280 gals/minute. Assuming a set up that would work (whether it is or isn't this particular pump) what level of thrust would that provide? Is there an equivalent hp ob motor size, for example?
    You'll need a proper pump specialist - there are lots of different ways of specifying a pump's capacity - so a single figure will tell you almost exactly nothing.
    Even when they work, they don't work well. I blame engineers.
    The only thing engineers have done to the toaster in the last 80 years is make it disposable. I think it applies to a lot of things

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    Default Re: small jet drive

    I have done it and it does work...not well, but it does...some points... I did it because two strokes were banned on inland waters where I sailed a 14' dinghy called a Vaurien...bit like an Enterprise...my 2.5 hp outboard was redundant over night...I used a pump and motor unit designed to pull water from a well, with I think a 3.5 hp 4-stroke and impeller type pump... It really whacked the water out of a 4cm dia.1.75"hose with a head of ten metres...The obvious disadvantages were it wasn't any noisier than a garden rotovator, but still noisy, especially on a becalmed twilight lake, and the positioning, even under the centre board thwart was inconvenient, It got in the way, and when you could sail in light airs you don't need that in the bottom of the boat.

    It was self priming, and economical, the debris problem no worse than any engine ...mine came with a plastic basket over the intake to deal with emptying cesspits and coping with those girly things...not a problem on the lake, but the main problem was I never really sussed steering by paddle at the jet end, like wetbikes/jetskis have, so I used the tiller and kept the jet at about 5cm below the waterline with a simple through hull fitting just off centre at the base of the transom and didn't try to make it steerable or have reverse..... but in the end when 3 -4 hp fourstroke outboards came down to a sensible price I stuck it back in the shed and use it as intended round the house ...draining the pool emptying the septic tank etc It was whacky, not really suitable honestly, but don't let anyone tell you it won't / can't work. I have no idea about thrust etc, but it pushed the dinghy along with two people and a dog and camping gear better than an electric trolling motor,... I would guess two or three knots... better than rowing too I often thought I would try and do it properly one day as it was all a bit jerry-rigged. But again, it did work...I'll try and dig some photos out, but it was about fifteen years ago....

  13. #13
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    Default Re: small jet drive

    Good info. "Efficiency" is a relative term.

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    Default Re: small jet drive

    The link has curves so somebody with the background might have an idea of the efficiency. I think it would be fairly easy to plumb as described to me with almost a straight pipe out the back through the skeg, use the tiller for steering. The inlet would be a tee running transverse with a 90 on each end angled forward to inlet boxes. If it pushed a 16' 800# sailboat at 6 knots I'd call that a success, if it was only 3 knots I'd say it was a failure.

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    Default Re: small jet drive

    if it's a sailboat I assume the only reason you need auxiliary power is because you are becalmed or as was the case on the long narrow reservoir where I sailed the dinghy, (150kms, x 1km max but loads of inlets, think centipede) tacking is a pain and you cover no ground in hours....if you are looking for 6 or 7 knots all the time buy a motorboat, or at least a Hobie cat or similar ( that gives the adrenalin rush!) I only ever used the pump set-up and the outboards before and after to get out of a hole... back to work next day, that sort of thing....on the subject of alternative propulsion....

    I tell you what won't work, though we had so much fun trying ... was trying to use a brush cutter mounted with an old johnson prop someone had lying around.I think we'd just seen that 007 movie where they are tooling around Bangkok or somewhere with what look like car engines on the end of a pole. we'd been cleaning up the mooring/ camping banks and thought we'd give it a go... I think John Barleycorn had something to do with it and we laughed so much it hurt . also we ripped a sizeable lump out of the rear deck of the punt I remember...Don't try this at home , kids Though never say 'it won't work' till it doesn't... The principle is good after all....

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    Default Re: small jet drive

    I'll put my 47 lb. 4HP four-stroke Yamaha up against that thing any day of the week.

    It moves my 20' 2" Caledonia Yawl (OK, so I have trouble following plans) effortlessly at 5/8th throttle (max hull speed) and with its remote tank, it can do it all day long without refueling. It also has clamps that can be loosened so it's pretty easy to mount or remove. At hull speed it is very quiet. I couldn't find the transom on my boat so I mounted it midship.


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    Default Re: small jet drive

    What I was looking for was somebody that knew how to calculate between the pump curves and drag of a small sailboat and figure how fast it would go. Opinions are great but I'm really just looking for data.

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    Default Re: small jet drive

    I'm really just looking for data.
    And you asked here?

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    Default Re: small jet drive

    I know it absolutely ridiculous some of the crap that comes up here, meanwhile the slotted vs phillips thread is up to 2310 views and 89 replies, by golly, now there's a worthwhile subject that everyone can sink their teeth into, and no lack of expert opinionators.

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    Default Re: small jet drive

    Hi Phil. Are you thinking of puting this in you sailboat?
    I will look and see if I can find my old physics books and try to calculate the thrust.

    Check out this link: http://www.eng-tips.com/viewthread.c...=217997&page=1

    I think outboards with a jet drive instead of prop require about 30% more HP for the same performance. Dont know if the pumps you linked would be as efficient as outboards with jet drives. They probably wouldnt be as they were not designed for boat propulsion.
    Last edited by KMacDonald; 04-22-2009 at 08:56 PM.

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    Wink Re: small jet drive

    Quote Originally Posted by Philip Maynard View Post
    I know it absolutely ridiculous some of the crap that comes up here, meanwhile the slotted vs phillips thread is up to 2310 views and 89 replies, by golly, now there's a worthwhile subject that everyone can sink their teeth into, and no lack of expert opinionators.
    Not sure if the opinionators have any teeth left.

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    Default Re: small jet drive

    Quote Originally Posted by Philip Maynard View Post
    I know it absolutely ridiculous some of the crap that comes up here . . . . . .
    Phillip, you have been remarkably patient with some of the replies.

    Some have been reasonable. Thanks to them, even if I don't agree with all they say - eg one says it would have to be below the waterline to prime, but the pump spec says it will vacuum lift to 25ft.

    As for inefficiency - I have just been researching outboard fuel usage and have seen a rule of thumb that each 10hp means 1 gallon per hour. some fuel usage figures make me believe better results are obtained but still - it looks like all engines use a lot of fuel.

    I often see jet skis idling along near their launching point and wonder what sort of performance figures they are achieving then.

    For small sailing boats it would be great to have some sort of drive that has no underwater appendages, but how well would jet-drives work? They seem to be designed for going real fast without caring too much about fuel.

    I also know that about 20 years ago I saw an ad in the british Practical Boat Owner magazine, for a jet-drive system that claimed to be suitable for small displacement launches. I cannot find if such a thing still exists.

    Good luck with your and your friends experiment.

    cheers buchie

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    Default Re: small jet drive

    Just want to say I appreciate experimentation and wish you guys the best of luck. You will, however, have to eventually measure the success of the project against what is already available and that is what I have provided for you in my little 4HP four-stroke Yamaha. That it can quietly run all day long pushing a 20' boat with plenty of power in reserve is not an opinion. It's just the way it is.
    Last edited by kenjamin; 04-23-2009 at 09:19 AM.

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    Default Re: small jet drive

    Just as an interesting ethnographic note, I think "pump boats" are used somewhere in the east, but I can't recall where exactly -- Phillipines, Indian Ocean, Indonesia, lots of places....? They are used as power alternatives, using stuff available: various waterpumps, various engines, not a "commercial" set up as I recall. -- Wade

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    Default Re: small jet drive

    This is the wrong type of pump. Marine Jet Drives are low pressure high volume pumps. The impeller is what's called a tube-axial style.. water enters the pump in the same direction it will be ejected.
    Jet ski pumps are axial, but the lower units of jet drive outboards are centrifugal type pumps.

    Here is a link to a discussion about calculating pump thrust. http://www.eng-tips.com/viewthread.c...=217997&page=1 Maybe you'll have better luck than I did trying to figure out what they are talking about.

    There have also been several discussions on the Boatdesign.net/propulsion Forum about this.

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    Default Re: small jet drive

    I once wondered if I could power a small skiff with a big bilge pump, but I gave up when I couldn't figure out the thrust. I didn't have high enough hopes to buy the pump and try it out either.

    It's an interesting thought, but looking at real-life implementations, pump jets belong on large, fast boats. It's the same with planes, large and fast planes use jets, small and slow planes uses propellers.
    1947 Nordic Folkboat "Nina"

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    Default Re: small jet drive

    Information from "The old outboard book" by Peter Hunn indicates that many companies had a go at producing jet powered small outboards during the sixties. Names such as Aqua Jet, Lancaster Guppy, Mid Jet, Outboard Jet, and Starling Jet. Almost all had Tecumseh 2 cycle powerheads, except Starling Jet which had a Clinton powerhead. Most were below 10 hp.
    Typical claimed thrust figures were; Mid Jet 65lbs. @ 4200 rpm 5 hp, Outboard jet 100lbs 5.5 hp., Starling Jet 82 lbs @6800 rpm 5 hp.
    All except Outboard Jet exited the pumped water below the surface.

    Someone interested in pursuing this might be able to to find a pump off one of these old outboards. At least it would be the proper type pump to start with. A place to to try may be the "webvertize section" for free ads at www.aomci.org
    Good luck, Phil

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    Default Re: small jet drive

    Uncle Floyd had one of those "outboard jet drives" that he got for the 12' aluminum fishing boat. I don't remember how big it was, 5-10 horse; it was impressive because it worked, but it was not as good as its "weeds? no problem!" claim, and was replaced the next season by the tiniest at the time (10? 12? hp) Merc outboard, which was much easier to clean the weeds from.

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    Default Re: small jet drive

    Well I really appreciate getting some more info from you guys. KMcDonald -yes some basic physics would be great, That link is very intersting - not surprising somebody's already looked at this. I can sorta just follow them enough to get into trouble with the calculations. They are talking about a 65 gpm pump where it looks like these are 2x to 4x larger, so where it wasn't looking feasible there maybe it's better with these pumps. This is not going in my boat, I was approached by a fellow that wanted to build Monk's Cerlew (which I have also built) and he was curious about my engine setup and he also mentioned the jet drive idea and I thought its an intersting idea if the fundamental physics worked. When I looked at close to 300 gallons/min, if you can move that volume of water that fast, could you move a boat with it (it also would make me very careful that I do not break anything on the outlet side or you will sink your boat faster than you ever thought possible short of a gas explosion). But if a 5 hp outboard is moving 3x that much water anyway, then that's why this thing would only go 3 knots. Those were the kinds of things I was curious about. Kenjamin, if you want to Compare it with a honda 4Hp 4 stroke, lets actually have the real comparison by looking at the data first (and then disagree about the result). And comparing them to outboards, these pumps are pretty cheap and if it can be plumbed in plastic those costs are moderate. If you like to tinker, dollars are tight and as an aux in a sailboat fuel costs are nil anyway, possibly that's where this could be an option. If I had to guess I'd say it's not really going to have enough thrust but I'd like to know that from numbers as opposed to just having an opinion or hunch about it.

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    Default Re: small jet drive

    The efficiency of jet propulsion (just the efficiency of the jet part, assuming the device that produces the jet is 100% efficient) is

    eta jet = 2/(1+ Vjet / Vvehicle)

    If the jet velocity Vjet is the same as the vehicle velocity Vvehicile the efficiency is 2/(1+1) = 1 = 100%. If Vjet is 2 times Vvehicle the efficiency is 2/(1+2) = 2/3 = 67%. The faster the jet relative to the speed of the vehicle, the less efficient the jet propulsion.

    The "specific thrust" = F/mdot = the force generated per mass flow rate through the jet is

    F/mdot = Vjet - Vvehicle

    So this is the problem: If you make the jet velocity close to the vehicle velocity to get high efficiency, you get low specific thrust.

    Now, if your vehicle is going very fast, this can work. Suppose you have Vjet twice Vvehicle for 67% efficiency and your vehicle is going 500 miles per hour.

    F/mdot = 1000 - 500 = 500 mph

    But if your vehicle is going slow it doesn't. For 67% efficiency at 10 miles per hour

    F/mdot = 20 -10 = 10 mph.

    So to get efficient propulsion in a low speed vehicle you need to move A LOT of fluid A LITTLE FASTER than the vehicle is moving. Jets are better at moving a little fluid very fast. Props are much better at moving a lot of fluid fairly slow.

    So jets are used for high speed vehicles: high speed planes in the air and jet skis on the water. Props are better for low speed planes and low speed boats.

    Most people with sailboats or displacement-hull powerboats are aware that the typical outboard is too much high speed - low flow for their boats. They need a bigger, slower prop that will move move water more slowly. A jet drive on a low-speed boat is like the small, fast prop of an outboard, only worse.
    I will beg you for advice, your reply will be concise, and I will listen very nicely and then go out and do exactly what I want! (Apologies to Lerner and Lowe.)

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    Default Re: small jet drive

    Quote Originally Posted by alkorn View Post
    The efficiency of jet propulsion (just the efficiency of the jet part, assuming the device that produces the jet is 100% efficient) is

    eta jet = 2/(1+ Vjet / Vvehicle).....

    Most people with sailboats or displacement-hull powerboats are aware that the typical outboard is too much high speed - low flow for their boats. They need a bigger, slower prop that will move move water more slowly. A jet drive on a low-speed boat is like the small, fast prop of an outboard, only worse.
    Ahh - thank you - a nice neat coherent explanation.

    and the "Why" of Kort Nozzles.
    Even when they work, they don't work well. I blame engineers.
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    Default Re: small jet drive

    I'm no engineer, by any stretch of the imagination, but it would seem that the intake would produce considerable drag for a sailboat.
    That is, unless you could close it off while under sail, but that means more moving parts.
    It's my perception these jet drives work best for primary propulsion and not as well for auxiliary power.

    Maybe you could design an impeller type engine to drop down thru a motor well.

    And speaking of alternatives, the electric drive styles would seem worthy of consideration.http://www.kraeutler.at/motor_details_en/items/32.html

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    Default Re: small jet drive

    Philip,

    Hear ya about the cost of outboards. I agonized quite a bit between the great 2HP Honda 27 lb. air-cooled four-stroke at $850 and what I eventually bought, the Yamaha 47 lb. water-cooled 4HP fourstroke at almost $1400 with remote tank. At 60 lbs. the Honda 5HP was just too heavy for my aging back. What sold me on the Yamaha was the remote tank which allows for a tremendous range without having to refuel at sea. Now that I've used the Yamaha, I've come to appreciate that it powers my Caledonia Yawl hull at max hull speed at about 5/8th throttle and very quietly for an outboard motor. I think my wife's sewing machine is louder. I've heard from others that at max throttle the Honda 2HP is quite loud and irritating. The other factor was the amount of reserve power available with the 4HP so that no matter what the conditions (even towing a small boat) the motor is up to the task.

    I'm definitely not against experimentation as you can see from my curved mast on the port side of my boat. You can also get a glimpse of my waterproof Lexan port for my outboard midship port side.



    I've also experimented with an oar for a mizzen mast which by the way was a failure. It just had too much flex.



    The experiment did, however, give me the needed information to glue up a stiffer oar that will work (with any luck at all).

    So go for it, dude. Without experimentation we are just stuck in the present with no eyes toward the future. And for the the people who say all the good ideas have already been thought of, please have them call the U.S. patent office and tell them their services will no longer be needed.
    Last edited by kenjamin; 04-24-2009 at 10:27 AM.

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    Default Re: small jet drive

    Well I'm no expert here but the efficiency calcs don't make sense. If the boat was going 5 mph and the jet was 100% efficient - that is it was leaving the nozzle at 5 mph, then it would in fact be at a standstill relative to the water around it - in other words it would not push at all against the surrounding water. That calculation makes more sense as the 100% effiecency is really effiency of slippage or something like that but there is something completly missing here. Airplane wings lift by pushing air downward (after you get through all the BS about lower pressure on the upper side, etc, what they are really doing is pushing air down), props push a boat forward by pushing water backwards, so there is more to it than this, it seems like a formula that is being misapplied of is just a little part of a much larger caculation. Jets are always pushing against something, it's not pure displacing mass, action = reaction, (unless you are a rocket in space).

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    Default Re: small jet drive

    Phil, search google for thrust calculations for water pumps or jets. Hopefully enough info is given in the specs you posted to fill in the blanks. Convert to horsepower. I would guess 2 HP would be plenty to get the job done.

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    Default Re: small jet drive

    I can explain a little better what I mean. If one of the original jet boat manufacturers recommended that the pumps discharge above the water, then it's operating as (what I'll call) a pure jet. If it was discharging below the water, then the pump will see the surrounding water that the jet is pushing against as increased backpressure, and that will slow down the flow and the thrust from simply moving mass will go down but offsetting that - what the pump see's as increased head would be thrust gained from the moving jet of water pushing against the surrounding water. So what is the net gain or loss? I dunno. Think of a firehose at a river, would the backpressure on the nozzle increase or decrease if the volunteer fireman submerged the nozzle in the river? it's not really a fair question because those pumps are probably so much more powerful, their flow may not reduce much. So it's not a simple calculation - that's where those curves come in.

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    Default Re: small jet drive

    The jet wont be very far underwater so the resulting back pressure will be minimal . (about 1/2 psi per foot of depth)

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    Default Re: small jet drive

    Last edited by KMacDonald; 04-25-2009 at 09:19 PM.

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    Default Re: small jet drive

    Don't the high-speed catamarans and jet boats that run rivers all discharge above the waterline? The one's I've seen do. I believe it is all about throwing weight backwards so the boat goes forwards. But as was very articulately explained by alkorn, this is efficient for lighter boats going fast, but innefficient for heavier boats going slowly. I don't think you'll ever see an oil tanker equipped with a jet drive.

    I suspect that as soon as you talk about underwater discharge, all you really have is a prop of some sort inside a tube. Then the question is, why the tube?

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    Default Re: small jet drive

    I can see the advantage of the tube. You don't get slashed to bits when to touch it. However, you are still faced with the 30% loss of efficiency that the outboards with jet drive lower units experience. And for a homemade, off-the-shelf bits and pieces unit, you are probably looking at worst efficiency. Still, if you can find all the right pieces and slap something together, it could be an interesting experiment. We tend to learn more from our failures than from our successes.

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    Default Re: small jet drive

    KMacDonald, Interesting, As far as backpressure I was not thinking of the depth below the water, I had exactly the misconception described in the link. To bad nobody knows how to actually do calculations using the pump curves. You would have about 3' of 2" straight pipe coming off the discharge and a 2" tee on the top with again about 3' legs each with another 90 degree bend (in addition to the 90 added by the tee). Kenjamin, The Caledonia yawl is beautiful, easily driven and hard to gracefully put an outboard in. It would be ironic if it turned out that a jet drive had much greater utility, not because the pump was quieter, smoother or more fuel efficient than a honda 4HP - it would be none of those, but the jet drive could be a better overall choice if you don't have to bisect your boat with a wet well for the sake of the honda. But that's just my opinion and at this point I have no idea if the pump is feasable. I'm not actually going to do much more with this thread, it's not a project I'm actively working on, it was just kind of a what-if idea.

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    Default Re: small jet drive

    I have an old aircooled 5 hp clinton jet... Its been a while since I fired it up... but the great "bubbler" drove my 8500# full keel h28 an amazing 3 knots! Hardly enough to win against a flood tide in the SF bay but a good light push out of the marina. It weighs in at about 30 lbs and gets good fuel consumption.

    Not mentioned in this thread here but it also make for a good outboard for young kids and summer lake play in a dink... no prop - no fouling - run it aground and simple starting... whats not to like?

    It is cool though! I have sent it into a small motor shop for a rebuild... maybe I will get 3.5 knots when it returns... or look to sell it for the a very fair price.

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