Spinning a transmission backwards to get more RPM out back??

So I've been trying to figure out what to do with this Benz 5cyl Turbo diesel, and my old man came up with the idea of putting in the 24' V-king hull we've got sitting out back. My grandfather stripped it down to the hull and stringers before he passed, and it's been sitting there waiting for us to figure out what to do with it.

Dads idea is to put it in near the front/center of the hull and kind of build a center console around it, and run a driveshaft with a carrier bearing to the outdrive. It would weigh a bit more than the old V8 that came out of it, and should have enough power to push it along rather well.

My big concern with it is that he wants to put a jet drive on it. We have a lot of soft, grassy, muddy shallow water around us and the boat was originally a jet drive. So all the holes are already where they need to be.

Next concern is the RPM a jet needs to turn. So I came up with the idea of turning the OEM 3 speed backwards so the bell would go to the outdrive, and the "driveshaft" would go to the back of the motor with a PTO style u joint/connection to the engine/flywheel.

Can the transmission be turned backwards. and still survive and function? I was thinking of using the transmission to increase the RPM the jet pump sees without having to run 5k rpm on the motor. I was thinking if we could get a 25-30mph cruise on 1.5-2 gph with the diesel loafing along it would be a success.

So could it be done?

Does one of those "jet" impellers really need to turn more than 2 to 3K rpm? A simple gearbox (not transmission) would do what you are asking it to.

I don't see why not! Sounds awesome. You could also maybe try a 4x4 truck transfer case backwards, that's effectively a chain drive gear reduction (or increase).

A manual box, yes, a slushbox, no.

rcutclif wrote: I don't see why not! Sounds awesome. You could also maybe try a 4x4 truck transfer case backwards, that's effectively a chain drive gear reduction (or increase).

Many of these use gears instead of chains...at least the really good ones do. Usually high range is 1:1 and low range is around 2:1, some have 3 speeds with an ultra-low crawler range like the Toyota FJ-series.

Here are the RPMs and Horsepower requirments for different size impellers.

Just use the original setup with the new engine. No reason to over think it.

A Samurai transfer case is 1.409:1 in high and 2.268:1 in low. I would think it would do the trick. It has flanged inputs and outputs weighs about 40# and is tiny.

How many rpms will the diesel turn? Also how much rpm is the outdrive rated for? I would pay carful consideration to gearing.. those outdrives probably dont need too many rpms considering all the bbc jet boats ive seen around here. Ive been in hot boats since ive been alive but never messed with jet boats, daddy always said they were too slow

Edit I looked up a couple things, max rpm for the motor is 4000-4350 rpm and the jet drives are rated 4000-5000 rpm. Id just mount up the motor and go, forget overdriving it.

In reply to chiodos:

Daddy was wrong.

https://www.youtube.com/watch?v=q2PBYXqeweM

In reply to Toyman01:

Trust me I know of those but those still arent that fast, just quick... im talking over 100mph in a boat

yeah, you aren't building a race boat and those aren't race jets. Put the engine midship or toward the stearn. You don't want to put it forward. Bad things happen. The RPM available is appropriate for most commercially available jets. You can always get the impeller reworked if you need to get it tuned out.

Just an FYI most diesel jets are 1:1, gas still runs a reduction.

In reply to chiodos:

The boat pictured will run 140 mph. Gas turbines are wonderful things.

Most jet pumps start to cavatate pretty severely over 6K. The Berkeley jets are happy in the 4-5K range.

I doubt a old MB diesel will survive long in the 4K range so I'd be tempted to reduce it some. Impeller efficiency falls off quickly at lower RPMS and speeds. That's why most commercial jet drives are on fast cats and jet skis. They also don't like heavy loads. If you want a slow cruiser, you may be better off with a prop.

Hamilton has a pretty good website. You might check it out. http://www.hamiltonjet.com/index.html

Wow wasn't expecting this kind of response to this oddball question, but hey.. this is GRM! It's why I came here instead of the boat forum's I frequent.

Honestly, I would like to see cruising RPM's on the engine in the 1700 range, hence the overdrive. The Sammy transfer case might do the trick. I don't expect it to see lots of load, and I'm not expecting a speed demon out of it. For those that asked, the OM617 is rated @ something like a 5700 rpm redline, and 180ish hp, and 280 torques.

Face it, after buying a boat, one of the biggest costs is fuel. 2-3gph is GOOD on a new engine with enough power to push a 24' boat. So ~9.00hr plus wear tear/maint is what a typical outboard runs you if you're running for any distance. Around here you've got to go ~30 miles to get 60' or better worth of depth, (extremely jealous of So. FL's 2 miles off shore and fishing for dauphin, etc) depending on where you run out of. If you're estuary, and flats drifting you're on the throttle 20-30 min depending on where you want to go. If you want to hit the middle grounds, you're running for 5-6 hours. (if we're still in the area this summer, I might just have to sign on with a friend for one of those 3-4 day trips) The idea is to have something that theoretically should be easier on the equipment, makes use of one of the "most reliable engines on the planet" and is big enough for my dad to take the 4 grandkids he's gotten in the last 2.5 years fishing, in the area he knows from when he was a kid. (family tradition fishing this area) Occasionally I'll probably use it just to take it out to the grouper grounds and bring home some fresh grouper. If it turns out quieter than a traditional 2 stroke outboard, GREAT! Fingers crossed that it would be more efficient on that size vessel as well. Especially if I get the pump to see 3-4k rpm, and only be turning my target 1700rpm.

Yes more research is needed to find out where the motor is at it most efficient rpm, and adjust what I can to get the most rpm out of the pump with the motor turning those rpms. (sounds kind of like prop'ing an outboard, the more I think about it) But it is an exercise in bench boating at the moment.

Not sure of the weight/power/etc but if you want a bulletproof transfercase to use, the NP205 is what you want. You can get a divorced case as well so it has input yokes and output yokes. 2:1 and 1:1 ratios.

Run it backwards to double RPM obviously. Everything is 32 spline shaft outputs if you get a Chevy application IIRC. There are many different iterations between Chevy and Ford. Spline inputs, married/divorced cases, input side bolt pattern could be 6 bolt or Texas pattern IIRC, maybe one more?

No idea about boat stuff but I know a thing or two about transfer cases.

If you want to get real crazy you can get an NP203 case, unbolt the 2:1 gear reduction off the front of it(the gear reduction is separate from the 4x4/2wd section which is chain driven on the 203), and with an adapter and shaft, you can have selectable 4:1, 2:1, and 1:1 speeds. Not sure how a doubler box would survive being run backwards. I imagine a boat would be less stressful than bouncing a 5000lb pig off of rocks?

http://www.offroaddesign.com/catalog/doubler.htm

http://www.northwestfab.com/NP203NP205_c_77.html

You can build the adapters yourself if you have machining capabilities obviously to save some money, I think both companies above sell the shafts separately.

Bonus is depending on how you want it set up(no idea about boat stuff again), but running a doubler setup backwards would give you potentially 1x/2x/4x speed at the very end, but with the case having an additional output(normally would be for the front driveshaft in the vehicle) spinning at either 2x or 1x speed.

edit: Completely spaced on the reverse rotation requirement. If that is needed look into 4x4 "portal boxes" such as found on the HMMWV. They are rotation reversing boxes that go to the wheels also gaining ground clearance.

I would definatly -not- put the engine any further forwards than the hull was designed for. Unless you really want to hit a wave wrong, bury the bow, and turn it into a submarine.

mad_machine wrote: I would definatly -not- put the engine any further forwards than the hull was designed for. Unless you really want to hit a wave wrong, bury the bow, and turn it into a submarine.

What he said! Plus, the oil burner weighs more which makes the center of balance issue worse.

Offshore with a diesel 24' jet boat with grandkids? You sir have larger cahones than I. I was expecting it to be a river rat or something. 24' and 180hp jet drive is a tall order...expecially if its overdriven (doesnt that reduce torque too?) Good luch though man ive been day dreaming about something kinda similar but with something like a sr20 in my 1949 alumacraft..but with a prop..lol

something else to remember about powerboats (and all boats in generally) they "rear steer". When you turn the wheel or move the tiller, the stern (rear) of the boat moves in an arc around the centre of gravity and/or resistance (depending on hull shape) and many speed boats actually "Slide" the stern through the turn. Moving the weight of the engine forwards could actually make the boat track straight like it is on rails and make it a real bear to turn

chiodos wrote: Offshore with a diesel 24' jet boat with grandkids? You sir have larger cahones than I. I was expecting it to be a river rat or something. 24' and 180hp jet drive is a tall order...expecially if its overdriven (doesnt that reduce torque too?) Good luch though man ive been day dreaming about something kinda similar but with something like a sr20 in my 1949 alumacraft..but with a prop..lol

I said I might take it out to the grouper grounds, which at the beginning of the month would have been about 6 miles, or still in state waters. but as we've warmed up they've steadily moved out. A buddy found keeper Gag around the 40' mark this last weekend, which put him about 35-40miles out. I spent the weekend with my nieces and nephew.

Dad would never allow the grandkids to go out that far, too much of a chance for something to go wrong and if it did go wrong, it's too much time exposed in the water for our liking. I was 20 before I got to go out that far. (paid for my own charter trip) I'm sure @ closer to 15-16 my grandfather would have taken me out there, but he was gone by the time I hit that age mark. I can say I that by 13 I knew the Suwanee sound inshore area better than most adults that fish that same area. Heck by the time I hit 13, once we had stopped for bait grandpa was pretty much along for the ride.

One concern I would have about turning the transmission backwards is that the thrust bearings may be in the wrong spots, making it wear out a bit faster.