Transmission Options...

Having spent several years working in the Oil Fields of the Gulf of Mexico Twin Disc is my natural selection. 90% of all the 100-250 ft supply and crew boats in the Gulf run Twin Disc transmissions. In my humble opinion these transmissions are simply the best gear that you can put behind an engine in a boat. Borg Warner's drives would not be considered based on service record and durability at these horse power levels. If you would like to get a feel for marine transmission dependability check out the message boards on various marine related websites. I think you will discover that the Borg Warner boards are positively full of people trying to find solutions to their problems and the Twin Disc boards are typically fairly quiet, aside form the guys that never bothered to check the oil level or condition. There are plenty of happy Borg Warner equipped boat owner's that would argue this point with me....oh well, this is my opinion about marine gears.

 My current boat runs a Twin Disc 5011A and the transmission has performed flawlessly. The only thing left to do is determine which model to install. Of course Twin Disc offers several variations on each model and the determining the correct model for the application is the only question that I need to answer. The first requirement of the gear be that it is "down angled" or to be one of the "A" models. Simply put all this means is the transmission out put flange is angled down on a 7^o or 10^o angle, depending in the model. The A model transmissions are extremely compact and the down angle flange keeps the engine installation angle low, or in some cases level. Very beneficial on steep shaft angle installations. The down angle also allows for a shorter, more compact engine installation. Based on my project requiring the extremely short drive line, this feature is mandatory.

Above images are the Twin Disc 5061A Model. Note the short length and down angled flange. Courtesy Twin Disc Marine.

 Transmission's that feature the compact, down angle design are relatively new on the scene...15-20 years or so. It's my guess that if these transmissions had been readily available at the time Bertram was designing and building the 25's in the late 60's and early 70's the V-drive V8 boats may have very well come from the factory as in-line inboards like the 31's.  This modern design saves lots of space and allows the engine to be further aft than other transmission options.

NOT SO FAST....

What about ZF? As with most projects this one is maturing, with that noted the ZF A series are starting to display some advantages over the Twin Disc selections. Basically for my application there are 3 main advantages with the ZF. First, the 220A is about 3" shorter than the TD MG5050A. Second, the ZF is lighter....by about 70lbs. Third, They cost less! about $500 bucks less than the TD 5050A. The horsepower capacity is excellent and I "may" end up with the ZF 220A for the above mentioned reasons. Also, the ZF 301A is offered as a 10 degree down angle in higher horsepowers than the TwinDsic. As general rule the ZF's are much lighter than the Twin Disc and that's a big plus on my project.

FINAL GEAR OPTION....

A used gear. As much as I would like to avoid "used" in my drive line this has to be cost evaluated. Marine transmissions are just too expensive not to consider a running take out, or a re-man unit. So with this on the table let's set a couple of ground rules. First, the gear needs to have an excellent reputation. Second, there needs to be a bunch of them out there. Keep in mind the gear still needs to fit my requirements of small, down angled and the required 1:1 to 1:20 ratio needs to have been an option when the gear was produced.

GEAR CONCLUSIONS...

The chart below represents the acceptable marine transmission options for this project. Basically any of these gears could be used. Developing a "shopping list" when watching the used markets makes finding a bargain much easier.

Gear Notes...

The 4 cylinder engines and the Cummins 6B series can all use a fairly small marine gear. Twindisc 502A, 5010A and 5011A are all suitable options. The CAT 3116, 3208 and the V6 Detroits need bigger, beefer, heavier and much more expensive gears. The cost for the heavier gears can easily add 3K to the project.

PROPS, SHAFTS AND PERFORMANCE EVALUATIONS...

Calculating the propeller, shaft and running gear size is well beyond my technical expertise. What to do in this event, find an authority. Initially I drove my local prop shop guys nuts with questions concerning various drive line options. This is until I found the boatdiesel.com website. This site is completely packed with basically everything you need to know to establish a very a very well educated guess on your boats requirements. The cost is $25 a year for a Level 1 membership and is the best money you could spend on a project of this magnitude.

Boatdiesel.com is actually a good place to start designing a re-power or new build project because the various software calculators provided on the site allow you to play around with boat performance based on different engines, gear ratios and prop sizes. The prop pocket size for the Bertram is actually in part a derivative of the calculations that I performed on their software. The pocket on the Bertram is designed to facilitate an 18" or 19" prop, provide a 2" - 2 1/2"tip clearance and be positioned at a 13^o  shaft angle. (now changed to 15 ^o shaft angle) The strut design is also a product of these parameters. I cannot over emphasize the amount of help this site has been during the design phase of this project; http://www.boatdiesel.com . The site is moderated by Tony Athens and as I mentioned before he is an authority on small boat diesel installations...but don't mention to him that your considering an in-line for a Bertram 25 or you will get an argument. He's a V-Drive advocate on this particular hull. I guess he really cannot mess around with someone else's money and end up with a poor performing boat....that he was paid to re-power. Prior to this article a viable, proven prop pocket design and/or mold for a Bertram 25 did not exist. If I were going to spend money for someone else to design and build a complete re-power I would want them to be on the conservative side.

So much for power option design decisions.

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