Got a Yanmar 1GM10 in Kazbek with the original non self draining cockpit sole.
The engine is basically inside the cabin with the bell housing and gearbox under the cockpit. I have had to do some work on the engine beds and I can categorically state there is NO WAY that it could have been mounted any further aft on the standard mounts.
(Its on some aftermarket ones now as the standard ones had totally failed and I decided I was willing to trade vibration at idle for shielded mounts proof against disintegrating if a trace of fuel or oil gets on them which would also give me more stable shaft alignment.)
The amount of engine box in the cabin is quite large, but as my galley and chart table are by the companionway, its a usefull seat and worksurface.
To improve the clearance between the air filter and the corner of the cockpit sole with the cabin rear bulkhead at the companionway, its both possible and easy to mount the air filter baseplate upsidedown (Its held by two bolts to the intake, with the hole off centre in the baseplate) which buys you several inches of vertical clearance but you then have to be carefull where you point the intake on the filter housing so you can still get at the two latches.
Access is otherwise acceptable and I can get to any external part of the engine with a spanner or socket though if one has anything major to do from the bell housing on back, one is best off pulling the engine. (goes up the companionway easily enough on a halyard with a tackle fore and aft to control it's swing). I've even got enough space to get a paint tray under it to catch any oil and small parts or tools while doing maintenance
If the tight access is really a major problem, one could remote the oil filter and the primary (on engine) fuel filter though Yanmar might not be too happy from a warrenty point of view
Idle and dead slow are somewhat similar to sitting next to a car with a tricked out sound system at the lights: Thump ... Thump ... Thump ... with the backstays and shrouds rattling but one only runs at idle for a short time when manouvering in a marina or if you need engine backup for a harbour entrance. At sensible throttle settings the noise and vibration is acceptable even with no sound insulation. There are a couple of resonances, but one just doesn't run at those engine speeds, a touch faster or slower and everything is fine.
I've got 300 hours on it this season since a major rebuild (it was over 20 years old) with no significant trouble and on flat water, I get 6 knots flat out and 5.5 knots thottled back a little to a speed it can keep up all day
There are three things that a 1GM10 owner needs to keep an eye on besides what's in the manual. The exhaust elbows corrode through internally and dump water into the exhaust port which *will* wreck your cylinder head if left. When laying up, remove the exhaust hose and cooling hoze from the elbow when laying up, preferably with the engine still warm, look up the elbow with a mirror to check for excessive corrosion (a torch down the spigot for the water hose may help spot pinholes, but *any* flaky rust isn't to be trusted), give it some good anti corrosion spray and tape a plastic bag over the openings with a fresh silica gel sachet inside. Remove the air filter. inspect, spray the housing and base plate, and re-fit then bag the intake the same way. Use the hand crank and the decompression lever to leave the piston at TDC on the power stroke so both valves are closed (a good policy if leaving the engine for a couple of weeks even if not laying up). The elbow is a wear item and is *much* cheaper than a top end rebuild. If in continuous commission, check the elbow at least twice a year and if suspect, for heavens sake pull it and inspect it on the bench. If you have low compression or excessive oil consumption, the chances are the head is rotted out due to the elbow having previously failed and nothing having been done about it.
The water pump also tends to suffer from shaft seal problems, especially in silty water. There is a massive weep hole so as long as you keep it clean, ther is no real risk of water in the oil, but there is a steel oil line running accross the front of the engine under the pump and any drips of salt water *will* eat this pipe and if left will result in total loss of engine oil when you least want it! I got a copper one made up using the steel one as a pattern so any future drips will do much less damage. Apparently the water seal cant handle the chamber pressure if there is too much clearance on the rotor ends and grooves the shaft. As there isn't a replaceable wear plate, that means a new pump though a new rotor and replacing both shaft seals with a large diameter narrow fibre washer under the water seal to moove the lip along the shaft a touch clear of the wear groove will buy you a season or two.
The remaining trouble is corrosion from water weeps from the thermostat gasket and Zinc anode gasket. Keep any salt crystals washed off and wiped dry, replace the gaskets, using a good brand of gasket dressing if there are more than traces or if you've opened them and use a good anti corrosion spray or brush on (WD40 isn't good enough and is a waste of time - you dont have an iceing problem)
Keep on top of that lot and with the standard maintenance in the manual you'll have no trouble in the spring or during the season and the engine will probably out last you - its basically pretty bulletproof..