The Yanmar 3GM30F originally came with a Hurth/Kanzaki transmissions KM3P 2.61:1 transmission, rated for a slightly larger engine than the 27hp supplied, recent trend from local distributor is to supply replacement with the KM2P, lower rated transmission. Both transmissions have a large reinforced aluminum front transmission cover/keyhole adapter; with a fixed bladed propeller and treated with a healthy respect for avoiding high speed shift changes, these transmissions have held up reasonably well.
However, neither transmissions respond well long-term with folding/feathering propellers which load up suddenly; end result is distortion of bearing recess in the front cover, the press-fit bearing race spins, causing premature cover bearing recess wear, generating heat, bearing misalignment, excess input shaft end play and a real nasty howling sound. In severe cases, the transmission locked up into a pile of scrap metal.
We replaced 3 of these KM3P transmissions in one year on three different boats of course, no reason to create panic here, most have gone many years without failure. For the weekend warrior, used with a little common sense, they seem to do just fine. For the guy planning on doing some serious cruising to out of the way places, if you have an older KM2/3P, you might consider looking at the MER conversion using the ZF transmission, price with trans, keyhole adapter and drive plate is more durable and cost less than Yanmar replacement which does not come with a drive plate. If you are planning on using any type of folding or feathering propeller, you might consider cutting your shaft about 1-1/4" shorter and installing a R&D flex coupling sold by PYI to put between your transmission and propeller shaft coupling to reduce shaft loads and dampen engine vibration to the shaft, positive side effect is reduced vibration to the hull, I.e. reduced boat butt.
Concerned about excess movement of the Yanmar engine on the soft mounts, excess maintenance of the packing gland due to high side loads, and wanting some protection from transmission damage from underwater impact from floating debris, a recent strike took off a blade, bent the shaft and distorted the transmission front cover; at our last haul out, we installed a R&D flex coupling and are glad we did, the vibration to the hull has dropped dramatically, the packing gland seems to experience less side loading and our bilge is dry. The flex coupling absorbs considerable thrust loading of the propeller to the transmission, is designed to shear in the event of underwater propeller strike, this should make the transmission happy for years to come.
I had a customer trying to install some expensive soft mounts in his boat last year, complained of extreme vibration from the old Bushings Inc. mounts, admittedly they are stiff. Unfortunately, he had modified the mount, won’t say how, before asking for advice. I explained his boats packing gland was hard mounted to a flange in the hull, no rubber packing gland hose, I explained to his dismay, the engine would move too much on these new mounts, resulting in transmission, shaft or stern tube failure, thankfully he took my advice.
I receive a copy of a photo from a Yanmar dealer, which showed a 3GM30F which had been installed in an aluminum sailboat, with a similar fixed packing gland as mentioned above. The boat yard installed the new engine, not taking into consideration the fixed packing gland; the end result, the transmission output shaft coupling sheared free from the transmission and the hull at the stern tube cracked 8" causing a significant leak. Operating with out a clue, the boat yard called Yanmar demanded a replacement transmission and reparation for hull damage. Looks like the yard will have to eat this one… I have a new idea to market, a cross between a chia-pet and a pet-rock, chia-rock, wonder if these guys will bite.
Long of the short, there are many factors to consider when dealing with engines and the forces surrounding their operation. There are a lot of cool ideas out there to help you get the most out of your boat regarding vibration and impact damage reduction; some that come to mind are the Python drive or AquaDrive CV joint/thrust load bearing arrangement and drive dampeners, these are some of the things that will make your driveline last longer and smooth out the ride.
Best to all,