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In this message from late 2019 I mentioned that I was considering emptying my fuel tank during winter layover:

https://c34.org/bbs/index.php/topic,10409.msg80510.html#msg80510

I was unhappy with the quality of my diesel fuel, and with the frequency with which I had to replace my costly spin-on Racor filters. I tried a number of things to improve the fuel (I use Biobor and Star Tron Enzyme at the recommended dosages), and now I think I've finally found the answer.

After emptying my fuel tank for the past for four winters, I am happy to report a dramatic improvement in fuel quality. My current Racor filter is now over 25 months old (and destined to last even longer), already about double the age of my prior filters that plugged after about 13 months. I was getting a little nervous for waiting so long this time, so last week I disassembled it and had planned to replace it. However, the filter was pristine (see picture) so I'm leaving it in for another year..

Some history from my engine log documents the improvement in fuel quality:

• My first Racor filter plugged on 7/19/2017. I aborted a planned overnight cruise, sailed home, and limped back into the slip at low RPM. The filter was 13 months old. I replaced it and the secondary fuel filter.
• Replaced Racor filter proactively on 8/18/2018 (13 months old). Upon inspection, I could see that it would have clogged if I waited much longer, since the pleats were filled with gray-black stuff.
• Racor filter plugged on 9/28/2019. I aborted a planned overnight cruise, sailed home, and sailed back into the slip. The filter was 13 months old.  I replaced it and the secondary fuel filter, as described here.

Up to this point, I had dutifully topped off my fuel tank every fall, following the traditional logic that eliminating headspace would minimize breathing and condensation and help preserve the fuel through the winter. For 2019 winterization, I decided to try the opposite, and ran my tank down as far as possible during the season, then drained the remaining diesel from the tank into jerry cans for storage at my home.

• For spring commissioning in 2020, I siphoned the fuel back into the tank, filtering through a Mr. Funnel fuel filter (which claims to remove water along with particulate contaminants). I operated the entire 2020 season without a fuel filter incident.
• At the end of the 2020, 2021, and 2022 seasons, I again ran the fuel down to 5-10 gallons at the end of the season, and drained the tank contents into jerry cans for storage at my home. Upon subsequent spring commissioning, I again siphoned the fuel back into the tank through a Mr. Funnel filter at the beginning of each year.
• I had replaced the Racor filter (and secondary fuel filter) proactively on 4/24/2021 as part of spring commissioning. The Racor bowl remained pristine for the entire 2021 and 2022 seasons and so far this year. The filter that I show in the photo below is the one that I installed in April 2021 and will remain in place.

After 3 years of a control experiment (topped off fuel tank for winter), and 4 years of testing the alternative (drained fuel tank for winter), I conclude that the storage of the fuel in sealed containers during the off-season is the reason for my dramatic improvement in fuel quality.

Quick comment: I did some searching. There are rebuild kits available for ZF10M/HBW100. There are various kits depending on how comprehensive the rebuild needs to be. So it does appear to be an option for you.

Like I said before, if you are experiencing slipping, have a close look at your transmission fluid to make sure it is the right stuff. My transmission was slipping during my first year of ownership, which I couldn't understand since it had just been rebuilt a year ago. I changed the fluid annually with the proper DexronIII fluid, and it gradually got better over a few years. Now, 7 years later, it works perfectly. I don't know what PO had put into the transmission, but in retrospect I suspect it may have been synthetic ATF, which is known to cause slippage. Presumably it took a few refills to fully eliminate any adsorbed traces of it from the interior surfaces. If you are having slipping problems and suspect you have synthetic ATF, I'd suggest a few refills with the correct Dexron fluid, with a few hours of use in between each refill. Much cheaper and easier than a rebuild or replacement.

I'm going to sound like a broken record here, but you've outlined the reasons why you would be much, much, MUCH better off rebuilding your current transmission. The PO of my boat had it rebuilt about a year before I bought it (would have been sometime around 2015), and I am so glad he did. From what I recall of the repair records, it looks like it was removed without having to remove the engine.

I know that the original Hurth transmission has been discontinued, but not sure whether the rebuild parts have been discontinued. Hopefully a good transmission shop would know of alternate parts if needed.

Just a question: You didn't put synthetic ATF into it, did you? If so, and you're having severe slipping, you might have an easier fix available to you.

Maybe the MkII boats have a different strainer design and/or location, but I used to remove the hose from the seacock and stick it in a bucket of antifreeze.

In more recent years, I just remove the top from the strainer and put the antifreeze in there to suck it through the engine. I'll open a couple of gallon bottles ahead of time to have them ready to continuously pour.

Here's the one I have. I assume you can find a better price elsewhere. On my boat, it fits in the extremely limited space under the aft berth next to the stuffing box, so it's very low-profile:

https://www.westmarine.com/forespar-raw-water-strainer-180641.html

The problem with the "B" engine bracket is that the stud "ears" (one with a clean hole and the other with a threaded hole) is made out of an aluminum alloy. If you tighten the bolt and there is space between - you can crack or break of both of the ears.  Any space must be filled with a washer/shim. -- so be careful tightening !!!
If you want to check your bracket just try putting a magnet on it!!!

Thanks for pointing this out. I want to be sure I understand you. Is the part I highlighted below (#17) the one that B-engine owners should check out with a magnet?

It's interesting, I've never attempted to tighten the studs before. I move my alternator 2-3 times a season to keep my V-belt taut, but I've never loosened/tightened the pivot studs, since I figured the rotation of the alternator is only a couple of degrees. Maybe it's a good thing I've never tried to tighten it. It almost looks like the studs are not meant to be too tight in anticipation that V-belt adjustments would require some limited rotation. Maybe just snug enough to compress the lock washers.

Here's something really odd: The two adjacent studs are different - #11 is metric and #14 is 3/8". WTF is the reason for that?

I would be careful going bigger than 75 amp until you do a bigger upgrade due to the origina brackets that have been known to fail. 

I'm almost sure his MkII has an improved bracket vs  the older motors.

Thanks guys. I'm going to bring the alternator over to a local shop today, but I have strong suspicion it is beyond a rebuild.

It's worth a try. If rebuild it possible, it will buy you time for a more carefully thought out upgrade/replacement.

I've never had an alternator problem, so not speaking from experience. But in reading others' stories here and elsewhere, I'd suggest looking into a rebuild of your existing alternator. Every time I read about someone replacing something, whether alternator, transmission, engine, windlass, water heater, etc., the shiny new replacement is just different enough that you invariably go down a rabbit hole of new fittings, belts, connectors, shafts, stuffing boxes, adjustment brackets, etc.

Maybe it's too fried to fix, but I'd at least have it diagnosed for possible rebuild.

I'm 800 miles away from the boat right now, so this is off the top of my head and I haven't verified that all this is accurate:

Trace the heater hoses that carry coolant to/from the HX. If they're old, cracks can propagate under the bottom of the hose clamps, and slow leaks can travel along the bottom of the hose. If the leak is on the front of the engine, it can travel along the bottom of the hose down to the HX in back. Also check your coolant pump. If the bearing fails you can get leakage from that, and it can also travel along the bottom of the heater hose to the HX. Also check to make sure your end cap gaskets on the HX are not leaking. Finally, make sure all your petcocks are closed - the drain one on the HX as well as the air bleed near your thermostat.

Aquamet 22 + Flexofold 3-blade here too for the past 4 years. I love the combination, and the FF prop has allowed me to sail in light wind conditions when I used to have to motor. (Chesapeake summers) I've passed a lot of boats up to 45' in light air. I can often sail on genoa alone at 6-6.5 kts if the wind is 7 kts or higher. My hydrodynamic drag is dramatically reduced.

Regarding shaft whip, I just found this thread where Maine Sail posts a number of useful comments. The link goes to one of his comments, but you should read the entire thread if you have time. It describes what I was trying to say:

https://forums.sailboatowners.com/threads/prop-shaft-runout.103188/#post-581542:~:text=It%20is%20also%20important%20that%20the%20exposed%20space%20between%20the%20prop%20and%20strut%20does%20not%20exceed%20the%20diameter%20of%20the%20shaft%20it%27s%20self..

I agree with Noah, the gap between the prop and strut is a little larger than ideal, but may be a non-issue.

I will check the gap as you mentioned, and I think the PYI also has a spec for the same thing.  I was wondering about the space between P bracket and prop.  When I read about that the first time, the gap looked suspiciously the same as the coupler...  I check the cutlass bearing every year and there is no play.  But... the engine has low hours on it.

To clarify, the type of "wobble" I'm talking about could occur even with a well-aligned and unworn cutlass bearing. You will not detect it by manually moving the shaft - only under dynamic load while in gear. Our boats have the shaft constrained at only two points near the ends - at the strut and the packing gland - with a pretty long shaft run between them. That length can lead the shaft to dynamically flex at certain RPMs due to dynamic torsional loads, kind of like a guitar string vibrates between two frets. This problem is made worse when there is too much shaft between the strut and the prop. In such cases, the slightest balancing issue with your prop, or even a hydrodynamic interaction between your prop blades and the strut or little keel-skeg over the prop, can provide the vibrational driving force that causes dynamic shaft whip between the constrained ends of the shaft.

Maine Sail had written about this on a website somewhere, but I can't find it. I specifically remember him mentioning that Catalina's drive train design  was particularly susceptible to this (due to the long shaft length between the two constraints), and mentioning that bronze shafts were more susceptible than SS alloys like Aquamet (since bronze is softer).

This may be a total non-issue for you, so don't get overly concerned. Just keep it in mind if the problem gets worse.

One thing that I noticed is that your belt is a little loose for my taste. I see some wobbling on the left and right sides. I don't like that and I try to have enough tension that it's taut all the way around when running. In my experience, that wobbling can be enough to create a little vibration. That's just me, it's possible that I over-tighten mine, but I like it that way. Replacement belts are $5, so they're cheap to replace if they wear faster.

Also, you can see in the video with the transmission engaged that the stuffing box and shaft move ever so slightly.  Not a vibration, more a swaying motion.  No idea if that's an issue...

That swaying could be a sign of a few different problems. You shaft could be slightly bent or wobbling (especially since it's bronze), your coupler might need re-facing to ensure it's perpendicular, and/or your engine could be out of alignment. The easiest thing to do first is to manually rotate the shaft by turning the coupler. When doing so, is the resistance uniform throughout all 360 degrees of rotation? If so, that's generally a good sign. Another option is to detach your coupler and verify its alignment with a feeler gauge (and also check its alignment through all 360 degrees of rotation). This is something that you should do with the boat in the water, since the boat can flex on the hard.

I notice you have that white insert between the transmission and coupler flange. I have no personal experience with those, so don't know if they're meant to flex and make perfect alignment less critical. But I know from my polymer science background that anything that's elastomeric also has resonant frequencies that might cause bad things to happen at certain RPMs, so they're not always an improvement.

More importantly, if that white insert was added without shortening the shaft to compensate, your prop has been pushed back another inch or two from the strut/cutlass bearing. I believe that the guideline is to have no more than one shaft diameter of distance (1" in your case) between the front of the prop and the back of the strut, otherwise you may be susceptible to shaft wobble (especially with a bronze shaft). I think Maine Sail has written a lot about these issues, so go search around https://marinehowto.com .

I don't mean to make you nervous, everything is probably OK, but if you wanted some ideas I'm offering them. Feel free to ignore them if you prefer.

Folklore?

I referred to it as "folklore" in emphasis quotes because I've heard it many places, but couldn't cite exactly where. I was not implying that it is inaccurate. Literally, it was lore coming from the folks.

There is ONE instance of that on here...

Maybe only one case here (I wasn't counting), but C34.org does not have a monopoly on information, and the C34MkII is not the only boat that uses the 35 horsepower M35B. I've heard of other cases from different sources and on different boats, and you can find them if you choose to look. My statement was factually correct, and I stand by it. I acknowledged that there are other pumps that work in many cases, but people should get all the facts. CatalinaDirect gives the same warning about the Oberdorfer pump. I may even purchase one sometime, but not just yet. Note that Pentair has discontinued the Sherwood G908.

Depco is good but sometimes "They ain't all that."

The same can be said of all free internet advice. And unlike some of the advice, Depco will happily sell you Oberdorfer, Sherwood, or other brands, as well as DIY rebuild kits or even do rebuilds for you (sometimes for less than buying the rebuild kit, if like my you only need a partial rebuild). So they have less incentive than some to give biased advice.

What's a crankshaft coupler and where is it?

I was referring to the issue described in Service Bulletin #235, and the failures ("horror stories") that led up to that critical fix. I could not look up the proper terminology because I was having online access issues.

Was this issue fixed in the MkII?

Yes.