newbie

[plainsmoken]

Hey guys new to the site planning on building an boat soon i got to questions has anybody ever used a jetski motor or a diesel cause i was thinking it would be high torque at low rpm on the diesel

[Papee]

Jet ski motor pose a problem with cooling. It's been tried but didn't fair well. Diesels make their power at different rpms needed for an airboat. Snowmobile engines are more suited for airboats than the two you mentioned being mostly air cooled or having a closed water cooling system.

[plainsmoken]

I have a 1448 flat bottom any recommendations on a set up i have a 25hp vertical shaft im trying to trade for a side shaft

[Papee]

If you're just looking to build a pusher type setup a 25 hp motor will work fine. A 48 in bottom is a little narrow for a planing boat. With a narrow bottom like that it takes more horse power to get them on plane. To get more horsepower you need to go with a bigger motor or reduction drive. Doing this brings up two problems, you have more weight up high making the boat unstable and you may not have the clearence needed to run the right prop because of the width of the boat.

Im not sure what your plans are, I'm just trying to give you all the info you need. If you're building something to get you around in low water the setup you are going for will work fine.

[plainsmoken]

Yeah i fish an duck hunt the river alot an i just want to get up the skinny water an not have to worry about hitting a prop or clogging a jet

[plainsmoken]

Also i was thinking if i ran a vertical shaft belt drive an kept the motor an weight low would i lose alot of power

[dr.peck76]

Take a look at my build on the belt drive. It isn't as pretty a a circle-s bit it is functional. You won't lose much power I'm this arrangement but you are adding weight. In a low hp build you want to keep everything as light as possible as anyone here will attest to. More parts=more weight. Same idea follows the diesel engine. They usually weigh more than a gas engine of the same hp rating although as you mentioned they do make more torque at low rpm. You may be able to spin a larger prop with a diesel .

[plainsmoken]

Dr peck i was think of useing about the same idea as yours with the vertical shaft motor i work in a lawnmower shop an they are some mower use the side shaft with 2 vbelt pulley to the mower deck an they dont seem to slip . The reason i ask about a diesel is a friend has a 3cyl kubota an it weighs 125 pounds an from what ive read some are putting reduction drives on gas cause they spin to fast i was just thinking if the diesel could be enough torque gain vs weight it might work plus if he decide to let me experiment with it maybe some knowledge for others to go by

[dr.peck76]

You can call me Dale,
I think prop selection is the key here. With custom builds you can adjust just about everything to work. With a v-pulley set up your reduction ratios are endless as well, so that should help you. So going back to prop selection, you need to know the peak hp of your motor and at what rpm then build a reduction drive with a ration to match your max recommended prop speed in rpm. It is essential to have the right diameter and pitch in relation to the HP you are making. I'm not sure if you have thought about this, but the faster you spin a prop the more load it puts on the engine and it is very possible to 1) overspeed the prop if you don't have a large enough one govern the engine to a max rpm or 2)have to big of a prop and not be able to spin it fast enough to have it work efficiently.

For example the max rpm of my prop is 3000 rpm, my engine makes max hp at 9000 rpm therefore I built a 3:1 reduction ratio. BUT I selected a prop that was a little big and my engine doesn't have enough power to turn the prop to max. I'm close at about 2700 prop rpm which still leaves a little on the table in terms of performance but I'm happy with the way it works so I'm not intending on changing anything.

Long story short, choose a prop rated for the HP you are making and you can adjust your drive ratio to suit. Also if you do decide to go with v-belts, bear in mind they do steal HP through friction, leaving less available for work at the prop. You can speak to the manufactures or research their catalogs for how much horsepower loss to expect with specific pulley diameters and centerline distances.

Oh, and I think Kabota diesels are great, very reliable and that weight isn't too much for an airboat. IMO. Have fun

[Papee]

Very good explanation Dr Peck. Sometimes my brain, well most times, is not able to communicate with my fingers to get things in print.

Plain,
Some people build with what they have on hand or can afford on a tight budget. I was and still am one of these people. I still run a direct drive because I just can't afford the reduction drives. I either have to make things or do without. Over my 45 years of experimenting with these boats I've tried many things and most have worked with any injuries. Lol

My biggest problem with the Jon boat was the hull would rub through at the bottom. Right on the corner where the transom meets the bottom I was forever patching and fixing the hull because it wore through. This area is very important to how a boat runs.

Keep in mind as mentioned earlier not to oversize your engine for this boat. If you put in and engine or engine/reduction combination on that is too big you will not have the room to run the prop it requires.

Im not sure how wide that boat is at the top but I'm guessing around 58". With your cage and prop spacing you would be down to about a 48 inch prop so you need to keep your engine and reduction in the range that makes that prop useful.

[plainsmoken]

Thanks for everything guys i got home today an measured my boat its actually 36 in bottom what hp you think should i should go with im probably not gonna be running it on dry land an i usually hunt by myself or 1 other im guessing bout 600 pds for 2 people an gear max

[Papee]

I would not use that hull for an airboat at all. It is too narrow to be safe with any weight higher than the gunnels.

[plainsmoken]

I believe u are right sir i guess its back to the jet jon or cutting it down the middle an making it wider lol

[dr.peck76]

I really wanted to do a jet Jon a while back. A friend convinced me to build the airboat. I say split it and make it wider. You could add a 4 foot sheet to it and put all the power you could find into it. ( insert diabolical laugh here)

[plainsmoken]

Lol i like the way you think i do have a 350 chevy that doesn't have a home. Just need a tig welder now