15 foot Aluminum Boat with Z482 Kubota

I have been lurking this board for years. A lot of knowledge here. I have a project I am in the process of getting together. I have a 15 foot Grumman aluminum boat. I also have a low hour z482 Kubota I bought at a great deal. This is a 13.5 hp (3600 RPM) engine. My plan is to keel cool it inside the live well, and dry stack the exhaust.

I have bought a HBW 50 Gear for the little Kubota. Having a hard time finding a bell housing, looks like I will have to buy a new one from a Beta 14. This is a 2:1 gear.

I like my plan so far, but am trying to figure out how I want to drive the boat. The boat will be used for trout fishing in Arkansas. The river can get shallow at times, and with rocks, but the rocks can be maneuvered around.

I have considered a jet drive, but not sure about the power/efficiency loss. I see them going up the river and they are wound out and not moving very well. I have also considered a surface drive out of the bottom of the transom. I just am not sure how well this would perform. I figure this would be a low speed boat, but with the surface drive would it porpoise? Would it draft fairly shallow?

From what i understand, a surface drive doesn't have bow rise. My only concern with that is how would it do going upstream?

Are there any other Feasible drive options for a boat of this type? If i make the surface drive, I am considering the tunnel rudder system with a fixed shaft.

And last but not least. I know I Could go buy a 15 hp outboard CHEAPER. I already have 2 boats with outboards and want to run the diesel. I know it probably doesn't make sense but I am going to do it any way ha!

 

For a jet drive you will need an overdrive, not a reduction. They run at higher RPMs. The power you have is low, so not ideal for a jet. Tunnels can work OK if properly designed and with a hull that has the proper shape for it. However, they are rather bad in reverse. Maybe a longtail setup may work best and be cheaper.

 

Thanks for the reply I really appreciate it. I see you said the power is low for the jet. This makes sense but I do see a lot of low power motors on the White River with a jet prop, usually 10-15 hp. They sound horrible haha. You always know when one is coming.


I am trying to understand the "longtail" concept. I have considered the tunnel rudder, but also understand it may not work. I have also considered a single rudder behind my prop going out of the transom. I assume this would be a surface drive? I understand you have to have a designated prop for this, one that doesn't cavitate.

The question i have for this is how many degrees would I want to run the shaft? And how deep would the rudder need to be?

I am kind of sold on the idea of a fixed shaft with some sort of rudder system. I don't want to get into CV joints and hydraulics. what I am looking for is Simple and rugged.

I also see a lot of boat with jacking plates on the river. I guess what I need to figure out is where I need to position the prop. I have owned only outboards, I/O, never anything else, so all this is new to me, but it is Interesting and FUN!

 

Surface drive is a misnomer. It is usually referred to a setup that uses a surface piercing propeller, also known as a ventilated propeller. Ventilating a propeller allows it to run with a higher load without cavitation. Google longtail motor and you will see thousands of examples. You can run it with a submerged propeller or a surface piercing one.

 

I posted a picture of the tiny motor. I can literally curl it one handed. I've read about the longtail some. how far out of the transom would it need to be? I am a welder so putting a tunnel on the boat would be very easy for me if that would help it . Would this benefit the boat?And with the long tail, how would the boat operate? What degree shaft and how deep do they run? would it still draft fairly shallow water? Would the boat plane?

 

not other advice ?

 

That statement is not correct, 3600 rpms would be perfect for a small, low-to- medium-speed jet. Your notion about observing jet boats with 10 to 15 hp is also questionable, what engines do you refer to? The smallest commercial units for outboard conversion I have come across have been for 25 hp (with some effort adaptable for 18 hp).

That said, the waterjet is not a good option when the weight/power ratio exceeds about 15 kg/hp (more for a slender hull with low hump drag), and your all up weight will certainly be more than 200 kg. My guess is that you would rather come up with something like 400-500 kg with everything onboard for a weekend's fishing trip. That would boil down to a speed of abt 13 knots with the Grumman hull.

Instead a prop on a straight shaft would be fine, the difficulty is to get it right regarding ventilation. With the 2:1 gearing you have, you'd need something like prop dia of 11,5" to 12" and pitch 11". I suggest you take a look at the "LM drive", from Lancing Marine, UK. If you scale down with the prop diameter ratio as a scaling factor, you would have a chance to come fairly close to a working solution. But-----be aware that there will be some final fiddling to find the sweet spot. Please note that the prop must have some trailing edge cup to function in a near-surface operation!

Good luck and keep us informed on your progress!

 

My experience is mainly with jetskis and they run at about twice that rpm. Are you getting the same performance (power losses) at lower RPMs?

 

Thanks for your reply! Very informative. The jets I see on the river are Suzuki engines, 4 strokes. I don't know how they have jet props on these, but the jets are extremely popular on the white river. I could be mistaken about the horsepower, but they look like lower horsepower engines.

If i can get 13 knots i would be pretty happy, Would like a little more, but you really cant run that hard on the river I am building the boat for you have to watch out for the shoals and it has a hydroelectric dam so it is consistently changing depths.

That sounds like a large prop! But keep in mind I don't know a lot about prop pitch or size.

Currently I am still trying to source a bell housing for the z482. I have a contact from beta marine getting a price on one, it looks like that is the route I will have to go unless there is a way to run the HBW50 divorced. Does anyone know if these transmissions can be ran like that? Meaning a shaft to the transmission?

I know I am thinking to far ahead but I have a hull idea. I was thinking of getting a section of aluminum pipe, say 2 feet long. The diameter would depend on the prop diameter. if the pipe was say, 12" diameter pipe, I would cut out a 12" section of the bottom of the hull for and then a Portion of the transom. Then I would weld the pipe in the cutout section of the boat. i would run my driveshaft through the pipe. (I would have to weld a cap on the end of the pipe where the driveshaft comes through. Then, i would cut the bottom portion of the pipe in half(the part sticking out until i got to where the prop was. I could then leave the pipe around the prop, as a rock guard. This would also provide a small tunnel. So if the pipe was 24" total, I would cut out 20",and leave 4" for the prop guard. Is this making any sense? The pipe would be expensive, i would use schedule 80. But i think it would run shallow and the pipe would protect the prop!

Dumb idea? The hard part would be know at what angle to run the shaft. Once you welded it there would be no changing it without major work

 

You can run them separate, with a jack-shaft connecting them. However, that will require a frame to keep the parts aligned. Most setups like that will have U-joints to prevent vibration. You could find a belhousing that fits either the transmission or the gearbox and make an adaptor plate. It is much easier.

 

I am very much leaning toward the bell housing/damper plate solution. The only advantage of running the transmission divorced is that I could put the engine dead level and have more flexibility on the output shaft. However, I am not sure the HBW 50 would like the driveshaft, especially with the soft motor mounts on the engine. I would think this would vibrate and put stress on the input shaft of the transmission.

 

Forget about the pipe idea, you will get an absolutely lousy propeller performance from that setup. Any kind of tunnel in this case must be adapted to the shape of the incoming flow. Besides, you should check how the Grumman is built, IIRC it is quite thin gauge, riveted with some sealing gunk in the seams. If that's the case, then it's not suited for welding (but I may be mistaken here).

 

Thanks for reply! I wasn't sure about prop performance, thats why I posted it. I will have to do some welding to make this an inboard. I have no trouble there as I have been a welder 21 years. I am certain i can weld on the boat. I just want the boat to perform well to get a return on my time and investment. At worst case I could weld ears to the pipe and rivet them to the hull, then seal it. But hey, if I knew what would work I wouldn't be on here asking questions. I am wanting it to perform well.

When I first started this project, i really considered making it a hydraulic drive. This would be fairly easy to make work, but the loss of efficiency would be a killer. plus it would be pretty loud. But no holes in hull! You could even run dual props!