Twin Disc MG 360-New transmission for Aeolus

Note: The post below tells the story of choosing and installing the Twin Disc MG360. I've unfortunately learned the hard way that this transmission will corrode badly if you install it like it is. My Hurth never showed a speck of rust after 20 years, but Twin Disc for some indefensible reason has built this transmission out of metals that corrode aggressively. I've been told after reporting this problem to Twin Disc that they recommend painting the transmission and shifting mechanisms before install. BE WARNED. See my later post for pictures of the corrosion.

Among the more significant decisions any sailor can make is how they propel their boat. Sails and rigging are huge, obviously, and yet the engine/transmission might in fact be higher on the list of anxieties given their importance at key times. Here in the Northwest, we are blessed with the finest cruising grounds in North America, and plenty of winds here and there. And yet, in our summer months we are often calm, and even in winter, we can go days with less than 10 knots. Because we have such a cornucopia of islands, each not far from the other, and because our currents are tremendous, regularly exceeding 5-10 knots, we are unable to rely upon sails alone to travel safely.

Twin Disc MG 360 right out of the box. Note separate oil fill.

Of course, one can travel by sails alone anywhere given somewhat unusual patience, but my point is that motoring is a very common method of navigating tidal passages and simply getting about when the wind is either absent, or contrary, to your destination.

On Aeolus, I put about 150-200 hours a year on my diesel and transmission. Recently, to my chagrin, I've noticed that our Hurth HBW 100 has begun to slip going into gear. This is a common problem, especially, I hear, on newer Hurth's which evidently aren't as well made. My Hurth has about 1200 hours on it, and is original to the boat in 1988. I have changed the oil every year, and kept it in proper adjustment and done everything else to insure long life. I treat my machines with every possible kindness and they show me the same loyalty, usually.

Well this old Hurth has finally given up the ghost. It hesitates to engage when you put it into gear, and you have to increase the RPM's to get it to click into gear. Facing the prospect of rebuilding the gearbox, which I would enjoy, and the timeline of our trip around Vancouver Island this summer, I decided I could not afford the time or risk of rebuilding it. Plus, the cost of parts for the Hurth rebuild is essentially the same as a new tranny.

2:1 ratio for this beast

And so my research started. Yikes. There is a fair amount of information available about Hurth transmissions, and plenty of diatribes about recent troubles with their quality (seemingly as a result of Chinese parts?), and yet not much information about actually swapping out a Hurth for another, or changing brands to a Twin Disc.

My research finally led me to believe that a Twin Disc would be a better choice over a new Hurth. Although there are happy Hurth customers still today, I read too many horror stories of people seemingly doing the right things and still replacing Hurth boxes every 100 hours. Whereas, with the Twin Disc boxes, I could not find a single, not a single, story about premature troubles or quality concerns. In fact, I found several people who changed to Twin Disc and have been reporting happy travels ever since. I have a friend up in BC who owns a Gulf 32 and who is both an old salt and quite a mechanic, and he has also replaced his Hurth with a Twin Disc and reports that his BC commercial fisherman friends are all going Twin Disc and report nothing but good things about their quality. Satisfied that I had exhausted all my research options, I chose Twin Disc.

Then the question became which model. Twin Disc recently bought a company called Techno Drive, which I believe was an Italian firm. This Italian firm makes the smaller sailboat trannies that now bear the Twin Disc name. Their smallest model is a MG340, which is rated just at/below my needs with a 32 HP motor at 2800 RPM. Their next model up is a MG 360, which is a bit over-rated in that it can handle 39 HP in continuous duty mode and 60 HP in lighter applications. Wisdom says that you should go with the largest transmission that will fit your application, and given our full keel heavy craft and heavy cruising, I went with the larger model.

I ordered her from Mill Log Marine here in Kent, WA. They had to bring it down from their BC shop but it only took a couple days. I went today to pick it up and finally have my hands on her. Mill Log was a pleasure to work with and their Kent shop folks were both friendly and informed. It is an actual machine shop, so the guys in the back know their transmissions inside and out.

Some significant details about this Twin Disc MG 360 is that it was about $1600 before tax and shipping, and this was about what Hurth wants for the ZF 15 with similar ratings. Twin Disc is not more expensive, as I was lead to believe. Compared to the original Hurth HBW 100 in Aeolus, the Twin Disc MG 360 is physically a bit different. It is about .5" higher and will require me to raise my motor mount nuts a bit. No problem. It is about 1.5" longer, and this, combined with adding a flexible coupling (which Twin Disc specifies, and I have flexible mounts, though lacking the flexible coupling has never seemed to cause me any trouble) will require me to remove my shaft and have it cut down and re keyed. Finally, of significance, the shift arm is on the port side instead of the starboard side.

Perhaps most significantly, as it took me some time to determine, is that the hole pattern on the bow side of the Twin Disc is identical to the hole pattern on a Hurth ZF10 or HBW 100. This means it will mate up exactly with the bell housing existing on my Universal 5432, which is an SAE 5.

I've read some confusion about what sort of oil these trannies take, and let me state definitively that they use standard ATF fluid. The box is labeled clearly ATF, even though the manual says SAE 30. And the Mill Log mechanic made clear that these boxes, unlike most others, use ATF. So there you go. The mechanic said that they would expect to get 25,000 to 50,000 hours out of one of these smaller transmissions. I told him I would be thrilled to get 5,000!

I notice that the Twin Disc has a separate oil fill hole on top, unlike the Hurth, and has an oil drain hole at bottom, which I plan to find an oil drain hose to attach to in order to facilitate easy oil changes like with my diesel.

So my next step is to install the darn thing, which I will do when I haul out later this spring in April. At that time I will be removing and replacing my engine mounts (which are only about 4 years old but they are cheap to replace, and how often does one lift their engine?), having the shaft cut down, putting in a flexible coupling and replacing my cutless bearing (which is also about 5 years old but again, I'm pulling the shaft and bearings are cheap).

I will of course post pictures and such when I do the install, so that anyone else looking to swap transmissions will have the benefit of my troubles and experience.