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Homebuilding of Jet Drive Mod to 10hp Outboard

03/14/2012 4:48 AM

I have a Tomatsu 9.8Hp outboard I intend to convert to an outboard jet for the purpose of pushing an aluminium canoe(also to be constructed) up braided rivers to access hunting grounds.

I have a mig, and some 2mm steel shelving, and a half baked idea of bolting on a housing over the existing prop rather than constructing from scratch a replacement leg.

Any suggestions to firstly: the feasibility?, and secondly technical aspects I may ignore at my own risk?

Obviously I have only what funds my wife wouldn't miss.

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#1

Re: Homebuilding of Jet Drive Mod to 10hp Outboard

03/14/2012 12:23 PM

Don't try to made a "jet drive". Just build yourself a rugged guard that still allow water to flow into the prop. Some heavy, solid shield under the prop to keep it out of the ground and some HEAVY 6-8mm screen to block weeds and debris from fouling the prop's inlet path.

Just a thought.

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#3
In reply to #1

Re: Homebuilding of Jet Drive Mod to 10hp Outboard

03/15/2012 1:14 AM

Kort Shroud is used on tugboats. Lighter versions are made for many sizes of outboards.

Ducted propeller shroud would be their name in general terms. (Ducted Fan?)

Generally increases drag a bit, but improved thrust more. Very good for slow speed maneuvering. Think of it as as a ring mounted around the prop. Really works, low cost, you can make your own.

Jet drives need large engine and give about 25 less thrust than a prop for same power used.

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#2

Re: Homebuilding of Jet Drive Mod to 10hp Outboard

03/14/2012 10:44 PM

Jet drives work with an impeller and not a prop. It's technically impossible to make a jet type drive with a prop. Too many reasons to list but you should Google to get an idea as to the principles involved. You're better off as Lyn suggested. All you need to do is protect the propeller.

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#4

Re: Homebuilding of Jet Drive Mod to 10hp Outboard

03/15/2012 3:21 AM

Ok I have googled Kort shroud-excellent suggestion, I see a version called Rice shroud is more hydrodynamically efficient, HOWEVER.....

The reason for the jet drive idea is that most of the river travel will be in 100mm or so of water over large stones. I have just measured the prop diameter at 240mm. Appoximately 70mm of intake height will be lost due to canoe depth as well.

I envision a wide mouth intake like a manta ray, angled and grated to slide over stones-perhaps with a large outlet like a kort shroud instead of a nozzle? I have noted that the exhaust and coolant intake are within a small drop down pod after the prop, they may not need rerouting at all.

Lyn, rereading yr post-is that what you meant?

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#6
In reply to #4

Re: Homebuilding of Jet Drive Mod to 10hp Outboard

03/15/2012 8:56 AM

More or less, but I had assumed that you might have a weed fouling problem, which you may not have. So, you may not need the screen in front of the prop at all.

The fully shrouded prop will give you much better directional control than my idea which was sort of a half shroud that protected the bottom of the prop but left the top half completely open.

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#7
In reply to #4

Re: Homebuilding of Jet Drive Mod to 10hp Outboard

03/15/2012 9:20 AM

Re: "most of the river travel will be in 100mm or so of water over large stones" (and) "Obviously I have only what funds my wife wouldn't miss"(OP)

Since you can't afford extensive casting/machining/sheetmetal fab (from a well-outfitted shop) ... me-thinks an entirely different approach might prevail ... àla "Rocket City Rednecks" if you will.

Wouldn't 4 out-rigged dirt-bike wheels (independently sprung of course) propel a "canoe" through 4 inches of water (with large stones underneath)? Using the shrouded outboard for the deeper areas, of course.....

Just thinkin' out loud ... (I see lightweight 'pontoons' to hold 'em above the waterline when not being used, adding stability while propelling-along)....

Or : (?) How long of a river-ride are you referring-to? Doesn't fording-by-walking/wading still work these days? Like yourself, I dream (all-the-time) about gizmos and gadgets that would make life simpler/sweeter. But, realistically ... an honest-to-goodness jet-powered canoe using ... (toss the 9.8) start with perhaps here ... talk about 'redneck'...

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#5

Re: Homebuilding of Jet Drive Mod to 10hp Outboard

03/15/2012 8:49 AM


Having worked for Boeing Marine Systems on the PHM i strongly suggest that you just apply a shroud to the unit. The amount of engineering and design/production cost is greater than you think.Ron

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#8

Re: Homebuilding of Jet Drive Mod to 10hp Outboard

03/15/2012 9:45 AM

I know that this is what you are trying to avoid

but 100mm of water (about 4 inches) is not very much. What about portage? This sounds like a lot of extra weight to carry for limited benefit.

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#9

Re: Homebuilding of Jet Drive Mod to 10hp Outboard

03/15/2012 10:05 AM

If you haven't fabricated the canoe yet, maybe consider building a hovercraft instead. It won't care how deep the water is, or even if there is water. i don't see how the outboard motor will help you with a hovercraft, however.

Good luck.

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#10

Re: Homebuilding of Jet Drive Mod to 10hp Outboard

03/15/2012 8:12 PM

seebackwaterinc.com for an impressive video of a "mud motor" and what it can do. There are other names. Do a search on shallow water motors. The props are barely below the hull, and well protected. They should be fairly simple to make for a talented person like yourself. You can also buy them form various sources. People started making them years ago, at home, for just such conditions as yours.

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#11
In reply to #10

Re: Homebuilding of Jet Drive Mod to 10hp Outboard

03/16/2012 10:05 AM

Are they called tunnel hull(wiki says no), or something like that? I have heard about a boat being built with a half pipe shaped indentation built upwards into the center of a flat bottom boat. I think that the idea was that when the boat got moving, the water would be sucked up into the tunnel, where a large part of the propeller was mounted.

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#12
In reply to #11

Re: Homebuilding of Jet Drive Mod to 10hp Outboard

03/16/2012 10:55 AM

Review any source you trust regarding a 'Canoe'.

Wikipedia is usually acceptable.

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#13

Re: Homebuilding of Jet Drive Mod to 10hp Outboard

03/19/2012 3:17 AM

I know it sounds like very little water, but the idea is to go right up to the headwaters of rivers to where the deer are. There would be times where the canoe would be scraping the bottom, making necessary a portion of speed.

As far as portage is concerned, our braided river beds are that shallow for many km at a time, so far I have been walking the distances, up to 30km a day one way, leaving little energy for actual hunting. Motorcycles can sometimes make it however are not allowed in national forests.

I have done a lot of reading about mud motors, they all seem to run the risk of a stone outcrop hitting the side of the prop, losing the prop so far from home could mean life or death for me and the risk of that design is too high.

Considering a tunnel hull design, Wouldn't the action of pulling water up the hull also pull the hull down deeper into the water, also effectively making the canoe weigh more?

Now would also be a good time to mention over the last 3 months I have fabricated a glass over ply canoe 4.5m long by 0.75 wide with an entirely flat bottom and med chines. I have built a jet pump using a 7hp stationary engine and a 3" centrifugal pump to power it. It is not quite fast enough for me to use, travelling at a med jogging pace on flat water. Hence the wish to use a 10hp outboard and to make a nicer looking alloy canoe.

Over the next couple of weeks I will fabricate a prop surround that has a leading tail and a grated intake, and test run that with the prop 2/3rds out of the water on the glass canoe. Started marking out yesterday. All your comments and advice have been appreciated, I will take some pics to illustrate if anyone is interested?

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#14
In reply to #13

Re: Homebuilding of Jet Drive Mod to 10hp Outboard

03/19/2012 8:56 AM

Oy. A homemade canoe.

While these are usually beautiful, they are usually heavy as well. Have you finished it? What is the weight (or anticipated weight)?

Here in the US, there are areas of the forest lands that don't allow ANY motor vehicles. You mentioned no motorcycles, so the rules in NZ must be a little different.

Yes, We would be interested in watching your progress. I will remain subscribed here, just post within this thread and anyone subscribed will see it.

Good luck and good hunting!

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#15
In reply to #13

Re: Homebuilding of Jet Drive Mod to 10hp Outboard

03/19/2012 10:00 AM

With the prop 2/3 out of the water, how are you handling engine cooling water? The pump intake is normally on the foot above the centerline of the prop shaft.

I am interested in seeing the progress of your project, please post pictures on this thread.

Good luck and Good hunting. Wish you could share some of those venison steaks.

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#16

Re: Homebuilding of Jet Drive Mod to 10hp Outboard

03/20/2012 4:33 AM

Doorman, my canoe isn't that pretty, it was designed for function rather than aesthetics. It weighs 38kg without the motor and pump assembly(i made them removable for transport). They weigh 25kg. A bit of tidying and a kg more epoxy to finish. My steering mech got banged off on its test run so I have had to cut up a ball valve for steering-still to fit.

This is the canoe and the depth of water i need to run in. The front is to the right.

This is the 7hp and 3" pump in the engine bay

This is at WOT and traveling upstream at a quick walking pace in very easy river flow-not fast enough. ( I had no steering so had to use paddle to fend off the bank)

This is the leg of the outboard i want to modify. Approx 30kg of weight. Note the pod mesh that is the water intake, I'm hoping that the throw of the water from the bottom third will get water in there.

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#17
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Re: Homebuilding of Jet Drive Mod to 10hp Outboard

03/20/2012 2:44 PM

Take a look at www.Mokai.com to see what is being done on a larger budget.

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#20
In reply to #17

Re: Homebuilding of Jet Drive Mod to 10hp Outboard

04/10/2012 1:27 PM

Thanks for the reference. I added it to my wish list.

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#18

Re: Homebuilding of Jet Drive Mod to 10hp Outboard

03/21/2012 12:38 AM

Hi Not Too Smart, yeah I found one locally and crawled all over it. They sit low in the water but I'd give a lot for that propulsion unit. Note!! that is powered by a 10hp. I could have copied the design (crudely), but i wanted higher sides to stay dry and load carrying capacity.

Before making this canoe I hadn't used fibreglass at all, and building it out of 4mm ply and wiring the flimsy panels together-I had quite a few(all) mates telling me it wouldn't go and would break up. Now I can stand in the middle with either far end on beer crates no problem.

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#19
In reply to #18

Re: Homebuilding of Jet Drive Mod to 10hp Outboard

04/10/2012 7:02 AM

Have a look at this video on EWE Tube "jet boat home build" If you want to access hunting grounds and carry gear and Meat home you'd be far better off with one of these wee beasts?

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#21

Re: Homebuilding of Jet Drive Mod to 10hp Outboard

06/04/2012 6:43 AM

How's the manta ray ducted prop thingy working out?

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#22
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Re: Homebuilding of Jet Drive Mod to 10hp Outboard

06/30/2012 11:50 PM

Problems, too much weight up high, the shroud seems to work alright, my first model had the attack angle wrong, too much water went up, then it seemed to bog down before reaching singing speed. Had to work out a counterbalance and damper to keep it from slamming the mount when it hit the bottom, now I've got to make a stronger intake cos the stones are crushing the port. Orig was 3mm plate now its 5mm. I did take it hunting and a couple of additional probs turned up, one when I go fast(and it can near fly) it balances on the pointed rear end and wobbles badly(its amazing how your fear increases the colder the water gets), and the tohatsu 9.8hp is quite tall and tries to twist the canoe over if you give it too much throttle whilst going slow.

The wide mouth port theory works well, and is practical, I'm quite happy with how efficient the prop, worn and abused as it is, is working, the expected cavitation problems at idle and slow are not enough to affect operation. Tuning the intake size to speed seems to be the tricky part, I'm not quite prepared to create the adjustable gate it seems to require.

With all the quakes in chch nz projects been put on hold, hence no followup for a while, house unlivable.

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#23
In reply to #22

Re: Homebuilding of Jet Drive Mod to 10hp Outboard

07/02/2012 3:51 AM

Sounds like excellent progress there mate.

Do you reckon you could flick some pics up for us to look at?

Instead of adjusting the gate size could you put in a waste gate arrangement? ie dump instead of throttle?

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Doorman (3); kiwi (5); Lapin (3); lyn (2); ndt-tom (1); Not too Smart (2); ronclarke (1); ronwagn (2); TerraMan (1); Turbo soup (1); Wal (2)

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