3208N cat with 72c tranny and merc 2 tr leg

I have a 25´ deep v planing hull vessel. It currently weighs about 5500 lbs. with twin 165hp 250 cu in. inline 6 engines and prehistoric alpha 1 drives. The engines will soon need replacing as they are raw water cooled and I use them in salt water.

I´m considering the idea getting rid of the gassers and installing a single Cat 3208NA 210 hp engine with a Mercruiser II-TR commercial stern drive. These are big stern drives that can swing a 20 inch prop and have a 1.5:1 ratio. Unlike most modern stern drives, they have no provision for exhaust or raw water pickup and they contain no cone or dog clutch shifting mechanisms. The shorter TRS drives are used by offshore racing boats with big block chevy engines that output more than 500 hp.

A special version of the Velvet Drive 72C 1:1 transmission is used to supply F/N/R control for the drive. It has the same internals as any 72C but it has a special tailstock in order for it to mate to the TR/TRS input shaft. There is an outfit in Florida called BAM Marine that modifies the transmission clutch packs and gears for high performance racing use for engines that produce 1200 ft/lbs of torque. I´ll try the standard version of the transmission first.

I´m guessing that the torque output of the 210hp 3208 @ 2800 RPM will not immediately destroy the drive if I am nice to it and go easy when getting up on plane with a boat that weighs 7000 lbs. I´m not so sure the drive would survive behind a 320 hp 3208T and I imagine that a 375 hp 3208TA is completely out of the question.

I´ve hunted around and it appears that an CAT SAE #3 to B/W 72 series bellhousing and a damper plate are available from Barr Marine. So it would appear that it is possible to mate the engine to the transmission.

I´m wondering if the torsional damper on the 3208 is effective enough to damp out the power pulses so as not to destroy the gear sets in the transmission and stern drive?

Is there anyone out there that has a 72C behind a 210hp Cat 3208? What sort of transmission reliability are you experiencing?

 

There was a charter boat in Queensland OZ that had this set up 20+years ago.
The torque of the 210 3208 ( say 600ftlbs )is more then the 2 drive was designed for so they got no w/tee but it did last with a light boat and careful throttle arm
Only problem you have now is getting 2 drive parts is very difficult
They also used the 73 trany as Merc did supply this years ago on some packages although I guess one of BAMs will be fine.

If you could find III drive Merc with a club foot that will be a much better bet but the lower will also be difficult to obtain
I had a customer that had III drives with the club foot and they worked fine connected to some Ford Sabres ( until you hit something and the diesel doesnt want to stop, that is)

If you have to obtain all this stuff it might be a better idea to look at a Konrad drive.

PS TR and TRS has thru prop exhaust, a II SSM does not
The II was generally only raced on small blocks as the III drive existed at the same time.
It was sold as a package on upto a 460hp if my memory serves me?

 

Thanks for the feedback.

According to some engine performance data published by Caterpillar, the 210hp 3208NA engine full load power curve indicates a peak torque of 489 ft/lbs. @ 1500 RPM which then tapers off to 394 ft/lbs. @ 2800 RPM (max RPM). If I understand correctly, the full load power curve does not exactly correspond with propellor demand. Propellor demand is lower at all points along the full load power curve except at WOT where the demand curve intersects the full load curve.

All that being said, I think one can say that the torque curve of a 330hp 454 cu. in. gas engine is more or less equivalent to the 3208NA diesel engine with each engine producing about 400 ft/lbs. when operating @ 2800 RPM. So as long as I go easy on the throttle when getting up on plane, the drive should stay together.

As I have said before, I doubt the drive would stand up to the torque of the 320 HP or 375 HP 3208 engine.

As for replacement parts, I've made some inquiries and it seems that except for the drive casings and shafts which are no longer manufactured and a few other parts, most parts for the Mercruiser II drives are still available. There always seems to be a good assortment of parts available in the used market from offshore racing boats that are being dismantled or upgraded. If I really get stuck, I could go with a Konrad drive as has been suggested. I won't be able to go to my nearest 'stealer' and expect to find them in stock but I will still be able to obtain them.

I have seen a few 390 gas HP clubfoot drives available for sale at much higher prices than the II-TR and TRS drives.