Hi,
I just replaced the transmission cable on my '84 C30. I have the M25 engine and the former Hurth HM50 transmission. This was a factory installation. I went through the same grief you are facing trying to get a definitive answer to the length question. The best answer I found was for a length of 17 feet. After installation I believe that I could have done with a 16 footer but the longer one works very, very smoothly. I bought my cable from Defender and paid about $36.00.
It took three tries for me to successfully change the cable with lots of research into how to do it. The good people at edson have a publication on their site that suggests that you should loosen the steering cables sufficiently for you to "lift the chain from the sprocket and move it forward. Nope, that wasn't enough. The next call to Edson got me to the secret for success. Yes, loosen the long eye bolts that maintain tension on the cable as far as you,can. But once loosened, you must slip one of the cables from the pulley like quadrant. It was a little scary but it worked like a charm.
After removing the compass, and the adapter plate below it, carefully disconnect both throttle and shift cables (without dropping the pins- don't ask me how I know) and remove the control assembly. A note here is to consider replacing the plastic bushings on the shafts of both shift and throttle. Edson will send you these free if you ask them. It makes a big difference. My throttle always tended to slip over time at speed and this took care of that little issue. The new bushings provide better adjustment.
Next, remove the plate the restricts the movement of the steering brake pads by taking out the two screws that hold the in place. Don't drop the screws. You should then loosen the steering brake knob to permit the brake pads to move freely (sort of). At this point you should be able to lift the chain over the sprocket and move it just forward of the sprocket. You will need to fashion a couple of tools from coat hanger like wire to allow you to separate the hanging portions of the chain from the sprocket itself before you can lift the chain.
On my boat, I have additional cables for a plotter unit which further reduced the space available. I was advised that on my boat I should try to move both control cables to the port side of the pedestal before removing them. Remove the acorn nut from the bracket (about 8 or 10" below the wheel shaft.) This will be easier to put back than you would think. Now another tool or a length of heavy string should be tied to the clevis on the shift cable (port side). Now holding the shift cable with one hand, force the throttle cable down so that you will be able to pass the assembly underneath the steering shaft to the port side of the pedestal. The worst part is now over.
Disconnect the shift cable from the transmission and tie a line to it that you will later use to pull the new cable back down through the hull. Back at the pedestal pull the shift/throttle cable pair past the steering shaft. Note: I had to twist the pair so the the mounting screw was facing toward the center of the pedestal, not the outer surface. You will indeed be able to slide this past the shaft and up. You,may have to turn the steering shaft a little to help it move. It will feel like catching your first fish when the package sticks up out of the pedestal. Do note the relative placement of the adjustable clevis's on the end of the shaft. Take a picture. Same with the connections at the transmission and throttle.
Remove the locking pin (long cotter pin) that holds the cables into bracket. Don't drop the pin. Note how the old cable fits into the bracket. Take a picture if necessary before you remove it. At this point your shift cable and/or your throttle cable should be able to be slowly pulled up through the pedestal. Plan on a few trips below and back up to clear kinks and bends. Make sure the retrieval lines are making it through the hull as you remove them. Once the offending cable(s) are free, resist that temptation to fling them into the ocean as some poor shark may choke on it.
Putting things back together is just reversing the process. Take your time, think about each step before and as you do it. Initially pulling the cable(s) down through the pedestal will take some time and is a teensy bit frustrating but you will get there. If you are replacing both cables, (my boat would have required an 18' throttle cable) be certain that you don't switch them inadvertently. Very embarrassing. The last tricky part after getting the cables assembled to the bracket is to orient the cables and slide the assembly down past the steering shaft. Once accomplished, pull the throttle cable up and use both cable ends to move the cable bracket mounting screw through its mounting hole and secure it. Having a third hand helps on this one but it's not as bad as one would expect. Refer to the pictures you took. If the new looks like the old, you've done it right.
Once the pedestal is back together make the connections to the transmission and the throttle. Consider replacing the bolts holding the cable brackets with stainless. I think mine were 10-24 about 3/4" long. Use lock or star washers. Before replacing the compass and adapter hold the assembly and operate the shifter lever and the throttle lever. The shifter lever should move almost freely and you should feel the detent in the transmission as the mechanism passes through neutral. If all is well, button it back up. If not, look for unexpected kinks in the cable. The new cable is probably better able to deal with a smaller radius than the original. My cable claims it would tolerate a 4" radius. Try for bigger.
My transmission cable had been slowly deteriorating over the past dozen years. The new one works very nicely and I am very glad to have done it. Come layup time I'm going to dive back in and replace the steering cables as I found a couple of meat hooks that shouldn't be there.
This job isn't that tough but it is a little scary if you've never done it.
Cheers,
