Manual automotive transmission usage?

Read through hundreds of posts here, and have seen innuendos that automotive manual transmissions are either fine or will self destruct if used. I think that usage would be the determining factor, so I will post my intended usage. I know that a true marine transmission would be preferable, but I am a 15 hour drive from the coast, so shops with stacks of rebuild-able transmissions out back are pretty rare. My situation is I am building a 32 foot sailboat, 12 000lb design displacement, and 16 000 lb max, and want an auxiliary engine for when the winds are not being cooperative but I still need to be at work Monday morning. I have a Mercedes 240D, with manual transmission, taken off the road due to excessive body rust (I used this motor for veggie oil experiments) as a donor vehicle. I would like to use the motor (Bowman sells appropriate marine manifold) and transmission. I know that the reduction in reverse is excessive, but a prop matched to 2nd gear (2.30) and 2000rrpm would do for reverse (3.66) and momentary 3600rpm while backing out of a slip, as this is well below the red line of the engine. As a displacement boat, it's hull speed is in the 7 knot range, and I read that 1 hp per 1000lbs displacement was normal, so this is overkill power wise anyways. I intend to buy a big marine alternator to charge house batteries as part of the install, as well as the required thrust bearing/stuffing box and prop.Existing clutch would be retained, with a hand lever.
Now the actual questions, are the stories of self destruction due to high speed boats speed shifting, and would I need a shaft brake to prevent this? I intend to select the gear for the intended use (trolling/cruise or reverse) then leave it there, not run through the gears. Any other reasons not to use a standard automotive tranny? Another (unrelated) question, what is the normal way to change oil in a marine engine, is the engine always mounted with clearance underneath it, or is a pump used to suck the old oil out through some sort of fitting? I ask, because the motor/tranny now become part of the ballast weight calculations, so lower is better.

 

Building a thrust bearing and buying all the parts for it will cost you about the same as a Velvet Drive in fair condition. If you are looking for a hobby and like to design and build things, I would say "go for it". If you are looking to save money and have something reliable, a used marine transmission is your best choice.

 

Because your intentions are definitely light duty, I don't expect any problems with the Mercedes gearbox. A hand operated clutch is a bit awkward, but not impossible.

I would use a short universal shaft between the gearbox and the thrust bearing block to reduce noise, vibration and avoid alignment issues.

Marine engines normally have insufficient clearance to change oil using the drain plug. Remove the oil pan from your 240D and modify the dipstick tube so it extends as low as possible in the oil. You can then easily change oil through the tube with a pump and a rubber hose that fits over the end of the tube.
This works much better than a thin nylon hose inserted in the tube.

 

i have owned these conversions and they do the job. u can make thrust setup for a few dollars. the clutch is better with a foot pedal so u can change gears by hand. i don't remember having any drama going from fwd to rev. it would be a good setup in a sailboat because u can select a higher gear for motor sailing.

 

if u find u need a shaft brake u can make one using a drum or disc brake out of your wrecked merc.

 

Gonzo's message is probably right on. I'm not a boat expert (nothing beyond tooling around in fishing boats with outboards) but i've nutted around with cars alot. It's not about the torque from engine to propeller but either about forces pushing against the transmission case or linear thrust forces coming up the shaft. If you can figure out a way to mitigate and absorb those forces however it could work. It's possible you might damage a transmission or two. The cost of junkyard backups may be cheap enough to make this feasible if you don't mind the swapping though.

 

With a U-shaft and a thrust bearing these forces do not exist.

 

a thrust bearing can be a simple angle steel bracket with a plummer block bearing . a cut down car driveshaft to the gearbox. mount the engine on original rubber mounts out of the car. how can that cost more than a velvet drive which also needs an oil cooler and a flywheel housing to suit.

 

The plummer block bearing is not a thrust bearing as they are designed for radial loads. The systems with the locking collar will provide some axial load ability but it is
only as much as the set screws screwed through the collar and onto the shaft can hold.
Of course you would get some additional thrust capacity from the collar interference but then you will mark the shaft, something you would not want to do.
Question, a velvet drive needs an oil cooler, then would not a manual automotive transmission?

 

Cost of marine transmission

I have been researching the cost of a marine transmission, and the lowest cost I have been able to find is a Hurth in Texas for $500, plus $100 shipping. This unit is used and untested, so I might need a rebuild kit for it. I would also need a bell housing adapter and flex plate to mate it with the motor. Stuffing box and Cutlass bearing are the same in either case. The costs that I see for using the existing transmission is to cut off the existing driveshaft and machine it to match the bearing, and the cost of the thrust bearing itself. I have been reading bearing manufacturer sites and am trying to figure out the appropriate bearing. I am thinking that a shoulder machined on the end of the prop shaft will transmit forward thrust to the bearing, and a clamp collar clamped on the forward side to handle reverse thrust. Anyone else have a proven bearing number, preferably mounted in a plummer block or flange?

 

Velvet Drive transmissions must be really cheap where you are because I built my thrust bearing assembly for a lot less than $200.....

PDW

 

But if you had charged your machining time at commercial rates, what would that have added up to do ya reckon ?

 

Hi Dennis, just noticed your location
Try Alec at Outlaw marine, Red Deer.
Something came to mind about a conversation that I had with him several years ago about the fact that sometimes he took the transmission off complete new engines before he put them into jet boats, he might have a take off on the shelf. a long shot to be sure

 

you can buy a taper thrust bearing in a plummer block. these things have been done with good results for years and no a manual does not need an oil cooler . how do you propose to circulate the oil if u did fit one. you people are trying to turn something simple into a major engineering feat. the op is after a cheap auxillary motor using what he already has.

 

what machining. all u need is a taperlock thrust bearing. and basic welding skills to make the bracket.