Jim, I don't know "why" but this is from the C36 Forum, they know their stuff:
Pos M25-xpb
The PO repowered his 1987 C36 with a M25-XPB. It is a terrible application for a marine auxiliary. The damn thing will not idle smoothly below 1150 RPM, and is really hard on damper plates and engine mounts. Check out the C310 Owners' website, since Butler & Co used that engine in those boats, and the vibration at idle is a well known problem.
He had ordered a Yanmar, but upon further investigation the yard told him it would require modification of the engine cowl/companionway ladder to get it in there. He cancelled that and went with the Universal/Westerbeke engine.
I owned this boat, in what now seems a previous life, and its M25-XP purred along at an 850 RPM idle, smoothly ticking over. The XPB will "run" below 1000 RPM (the published idle is 1000-1200), but acts like it wants to crawl out into the cockpit with you.
The XPB was not entirely a "drop in" replacement for the XP and the PO reported some issue with the engine mounts. I'm not on the boat, and can't recall what had to be done to get the new engine in there and aligned. The heat exchanger had to be shifted to starboard in its brackets to fit, resulting in its end cap rubbing up against the throttle and engine kill cable sheaths. The heat exchange zinc wound up nearly inaccessible (directly above the engine mount) and can only be changed with liberal application of the f-word.
Westerbeke used the deservedly much-malinged backwards facing Sherwood raw water pump, the impeller in which would be a real PITA to change anywhere but at the dock. One of its hoses must be removed, spilling saltwater onto the lower engine pully, when changing the belt because some rocket scientist figured it wasn't important to employ the Oberdoffer off to the side of the engine as in the M25-XP.
I was in a 3-way partnership in a 2005 310 before reacquiring my 36. With under 700 engine hours, we had already gone through two damper plates and an engine mount. That engine exhibited the exact same reluctance to idle as does the XPB in my boat.
Not being a bit fan of shifting the transmission at 1200 RPM, I reduce the throttle to around 950, grit my teeth while the engine rumbles around, shift and quickly reapply power to 1150 to get it to run smoothly. Makes for a jerky approach to the dock. The old XP could stay at 850 and shift back and forth effortlessly as needed to get into the slip.
As much as I have misgivings about rebuilt diesels, that might be a consideration, depending on what's wrong with the existing engine. Doubt one would save a lot of money and, to my way of thinking, sailboats are about beauty and relaxation, absent the worry of a rebuild going TU.
Had it been my call, I would certainly have tried to get a Yanmar (or something?) in there, even if it required downsizing the HP to fit. I have no experience with the Beta or other offerings, but would only go with the XPB if there was no alternative.
Can't remember the exact figure, but recall the yard bill being around $13k, including new transmission, dripless shaft seal and flexible coupling in 2008.
Hope this is helpful.
Jack
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Jack Heaston
C36 Mk I #692
Silent Passage