7.0L EFI or 7.3L PSD for jet drive boat

I have often thought of away of using a belt to bump the out put speed of the motor. We'll say a 2:1 overdrive I know Harley's can put some big #hp through their belt drive system as well as supper chargers I read some where that that a high hp motor(I can't remember the figure, i think it was 2-3000hp) but the supper charger was consuming 600hp to make the high hp's. that's a serious load on a belt. Plus there is the belt driven air boats big hp's on them as well, but the problem is with single engine to single drive I would have to go up or down or side to side with the motor. All is not the greatest unless I get a pump that comes of the hull with no downward angled thrust line. I seen one but can't remember what pump it was. Twin pumps would be no good for my hull but would be sweet especially seeing AT's on eBay for 1200$. I'm still stewing on how the boat is going to be driven, but it will be powered by the diesel. I emailed DBD back to get more info then what they sent me last time, plus I emailed saro but they look like mega bucks.

 

Everything that I have seen for gear drives on the net here have been for under drive units not over drives. It kinda sucks when everyone is goin one way and I'm trying to go against the opposite. I like the simplicity of the jet but I keep hitting a wall at every turn.

 

This is the most common setup, almost like an outboard:

https://www.youtube.com/watch?v=-hS06haIad4
https://www.youtube.com/watch?v=niRFAL3-MGo

Have you considered building a custom drive from parts? I don't know if thats good advice or not.

Basically, you'd need a u-joint assembly at the transom, housed in a rubber flex boot, and some thrust bearings, and then some control arms and etc.

With 3 control arms, 2 would control the left to right movement to steer, and the 3rd would control the up down motion, but you'd want a spring or something on it, so it the prop shaft hits ice or rocks, it can bounce up instead of getting wrecked.

I think you'd want the shaft made with a cutlass bearing style tube, and then a fin on the bottom, extending lower than the prop, to help protect from damage.

Then you'd need a shaft from the engine to the transom, and I think you'll need an adapter that will bolt on the engine and be able to bolt a bearing pack on.
I think this could be done fairly easily, and without major expense. And, I think it could perform very well.

I don't know for sure, this is all just an idea, maybe someone else will have a better idea how feasible this would be.

 

I've wondered the same thing, for other reasons. I want to build a pickup truck, with an oversized diesel engine. But, I don't want to have to spin it at 2000 RPM down the highway like the original smaller engine. 1400-1500 RPM would be fine.

If you do a bit of reading in some offroad truck forums, you might find examples of some guys using different transmissions and transfer cases in different ways to play with gearing.
A belt or chain drive would be simple, but likely not as reliable as a gear drive system.

http://www.pirate4x4.com/forum/jeep-hardcore-tech/1298377-transfer-case-used-overdrive.html

http://www.cumminsforum.com/forum/98-5-02-powertrain/346733-transfer-case-overdrive.html

 

I have also wondered if by going to a 2:1 overdrive yes you are doubling output speed but are you effectively halving your torque and horse power output? Something like this might be where the high torque of a diesel would be good.

 

Yes, if you had a speed doubling gear, you would change the TQ curve totally.
Instead of say 250 HP @ 3000 RPM, and 500 ft lbs @ 1800 RPM, it would be 250 HP @ 6000 RPM, and 250 ft lbs @ 3600 RPM.

BUT, you'll lose a bit of power in the gears, plus, add a failure point.
How much money roughly do you want to spend on this?
I was kind of thinking you wanted to do this for a fun project using as many lower cost and locally available parts as possible.

If you decide to go the engine-through hull - U joint - prop mud motor type design;

A lot of the bigger outdrives look ( at a quick glance) like they have engines that run at 5000 - 6000 RPM wide open, and use a 1.5 - 2 :1 gear reduction, so the prop would be spinning around 3000 - 4000 RPM.
Using a shaft from the cummins, without any trans, you might be able to use a very similar prop, probably pitched a little higher considering it's a small boat.

I still think that would be the ticket, to putting together something relatively simple and inexpensive. Bearing mount on engine, shaft to bottom of transom, thrust bearings, U joint inside rubber boot, Shaft with prop, held in position using 3 arms, and some type of bearing around the shaft near the prop.
Since the bearing and boot would be behind the transom and out of the water while cruising, I'd opt for a really big SOB so it isn't a weak point.
Something the size of a 1 ton truck or something.

Maybe you could find someone who knows how to fab something like that up, it would be much cheaper than a jet drive.

 

My problem with fabing stuff now is my location. If I still lived southern Ontario where I grew up, it would be a piece of cake. But now I have to work with what is at hand or fly everything in. Still there are a number of broken down vehicles
and heavy equipment that parts can and will be scavenged from, but specialty stuff hard to come by. I have the skills and equipment to do it, I grew up farming and a number of my friends are machinists. I can still call south and have things built but then I have to ship parts there have it and then have it shipped back. Pain in the arse but still can be done. And with the money I guess it will take what it takes. I was looking at new outboards and a 115 exec here cost's almost 12000$ now taxes and delivery. I think I can have a much better setup than an outboard with that kind of money.

 

Once some of the snow melts I will be able to look around through the scrap pile a little better to see what is available for scavenging.

 

By the way no matter what it cost's it's going to be a fun project

 

Shipping boats here is bloody expensive, because the rates are taken by the m3 so they take the full height and then the full length and charge the shyt out of you. That's why I'm sticking with this old scow of a boat. But with some love it will be cool.

 

Well you could still get some parts fabbed up somewhere else and do the final assembly there.... I mean, it sounds like you're going to bend over for shipping charges any way this goes.

I would ditch the jet drive idea though, nothing against jet drives, I just think it doesn't fit the type of boat. Who knows what issues you could end up with cutting a bunch of fiberglass apart.

I'm pretty sure those boats are filled with foam in the floor, in fact I watched a guy hoist one up in the air, and they drilled holes and added a solvent or something to get all the foam to melt and ooze out.

It's a boat worth a few grand at best, and if it was me, I'd be looking for a cheap way to power it.
Outboard motor is always an option.
But if you want to utilize a cummins diesel, I still think the most simple way to do it is just by fabbing up an outdrive with a u joint or cv joint or something. Similar to a surface piercing propeller drive.
Minimal modifications to the boat, simple construction, low cost.

Sure, it might not work as great as an arneson drive, but it's an old boat and this is an experiment anyways, why spend a big pile of dough?

 

I am going to have to check into the double floor that makes a big difference for mounting motor beds they will definitely have to be to the hull not the soul if mine has one, that was probably acetone that the guys used to melt the foam out. I am starting to lean towards the idea of a surface drive. Whether it be custom or bought still remains the question but I think it will have better performance on my hull then a jet. It's a little more complicated then a jet but might be better.

 

How is a surface drive more complicated than a jet?
Maybe we are not on the same page?

Here is how I envisioned it:

The engine is sitting in the boat, it has a bell housing adapter, not sure exactly what it's called, but looks a lot like an industrial clutch. Basically 6" housing that bolts to the engine, and then bearings and a shaft drive out the back.
(Because you can't just connect a driveline u-joint right to the flywheel, that I know of. )
From there, there would be a drive shaft from that housing u-joint, to the bottom of the transom, where there would be a small shaft tube / bearings seal assembly, with another u-joint.
On the outside of the the assembly, there would be another u-joint, and that would be the long shaft ( maybe 4 or 5 ft? ) with the prop on the end.

In front of the propeller, there would be a steel tube around the shaft, with cutlass bearing inside, and it would have a fin welded on the bottom, and some type of mount on top to bolt the control arms to.

There would be 3 control arms, 1 on each side of the boat lower down, which would push/pull to steer, and the 3rd would be in the center up high, and would move the prop shaft in and out of the water, and have a spring or shear pin in it, so if you hit rocks or a log, it would allow the prop shaft to raise up instead of breaking.

Maybe you can get from the engine all the way through the transom with a solid shaft and no u-joints. Just the 1 u-joint for the prop shaft.

Maybe CV joints would be better.

 

I guess saying more complicated isn't the right word. Maybe more parts or more involved. Lets look at everything
Jet drive- engine, drive hub, jet, steering cable, reverse cable, throttle cable.

In keeping everything on the same playing field.

SD- engine, drive hub, transmission, shafts, support bearings, thru hull seals, u joints, steering and linkage etc. etc. etc. you know what I mean its not that it is more complicated actually I think in reality it is less but with the jet being its own nice little self contained unit, that's what makes it so appealing.

 

What about all the seals and bearings etc for the jet drive? Sure it's self contained, but it's going to be expensive, and just cause it comes in a tidy package doesn't mean it will be more reliable.