Questions about M25XP
The 1987 Catalina 30 MKII I just purchased has a Universal M25XP engine with a 2-blade fixed prop. I have a some questions for those of you with a similar configuration: "Alan Gomes"
I have a 1986 Catalina 30 (original before "Mark" designations) with a M-25 engine.  I understand the engines are the same except for piston size. "George F. Goodall"

(Q1) At what RPM is this engine supposed to idle? With the throttle lever all the way back mine drops down to about 750 or so, but seems to idle more smoothly at about 900. Is this in the ballpark?
(A1) I get about the same results as you at idle. 800-900 can be even smoother without hurting the engine or transmission.
(Q2) What is the max RPM that you achieve? Running flat out I can hit about 2600. (I did not run it at this RPM for any length of time, only for a brief test.)
(A2) I get about the same max rpm.  The book says that the engine governer is set at the factory for 3000 rpm.   However, remember that the rpm gage is NOT reading the engine RPM.  Since diesels don't have ignition pulses to count, the RPM is taken off the alternator.  My rpm gage is generic with switch settings to approximate the engine rpm.  Alternator pulse output will vary with the pulley ratio and type of alternator (# of windings, etc) and belt slippage.  I.E. Don't expect the rpm gage to be an exact match of real engine rpm.
(Q3) What is a good cruising RPM? I've been running it between 1900 and 2100. Temperature seems to hover between 160 and 170 deg. at this RPM, which seems reasonable to me. I noticed that the temp began to climb somewhat when I was running it at 2600, but did not do so for long enough to see how high it would go. It was still under 180 before I  throttled back.)
(A3) The big boys tell me that good cruise rpm is between 80-90 % of max. This vintage M-25 with hot water heater in the cockpit lazerette is known to have a slightly under designed cooling system.  If you have a 100% efficient cooling system, the eng temp will stay at 160 (or thermostat setting) regardless of engine rpm.  There are several upgrades and things to check.  First check for obstructions in sea water cooling, grass in filter, broken impellor blades down stream from pump, and pencil zinc pieces plugging heat exchanger core holes. (The only way to do this is to remove
h.e. and shake.  If it rattles, you have got them.)  Have radiator shop boil out H.E.  Make sure impellor is fairly new and flexible with all blades still attached.  Best bet is to upgrade from 2" to 3" h.e.  Replumb hot water heater from bypass system around heat exchanger to attached direct to thermostat bypass connectors.  This will help eliminate air locks on the fresh water side of the cooling system.

(Q4) What kind of speed might I expect to make with this engine? At 2100 RPM in flat water I'm around 6 kts or a bit shy of that. Adding any headwind or chop drops me to the lower to mid-5's.
(A4) Sounds about right.  Big fat boat will slow down considerably in chop. Thats why Catalina went from 11 hp engine std with 21 hp (M-25) optional to 21 hp std.

(Q5) What are your experiences with prop walk? Compared to my 27 footer the kick to port seems more substantial. My technique at this point has been to goose the throttle to get the boat moving at sufficient speed in reverse to overcome the prop walk. This means that at first I'm actually going somewhat the wrong way but I find that as the boat gains speed the rudder takes over and the boat begins to go where I want it. I tried using less throttle but found that it did not correct itself as readily.
(A5) I use the same technique except after I goose the engine in reverse, I go to neutral shift.  Rudder immediately becomes more effective.