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I would recommend a 2 blade prop and start as the mechanic recommended with the existing prop. After a season evaluate and maybe replace.
'Evening Light' came with a 3-blade that cost about 1/2 knot when sailing. Seeing as I was sailing most of thee time , I replaced it with a "14RH10" 2-blade (both props would give a 0 to 5 Kts in 20 seconds in flat water performance.)
Fair Winds,
Leo
Fair Winds,
Leo MacDonald
Founding Fleet Capt., NE Fleet
Past Commodore, Member No. 223
A 'Cape Dory Board' supporting member ~1999 to ~2015
This is an interesting discussion. I have a 25D with the 1 GM and original two blade prop. I have been having trouble with the engine for the last two years. Starts fine but rpms now max out at 2700 under load. This enough to get the boat to almost hull speed in calm conditions. I have been advised to have the head pulled with a view to examining the valves, etc. I am contemplating doing this and at the same time making the switch in prop that John Vigor recommends. Together, based on a close reading of the archieved posts on this mb, this would represent a real increase in effective power at a modest expense compared to engine replacement.
Any comments or suggestions on this strategy are most welcome.
Dick, I had the same, but worse problems when I first purchased Seraph. Engine wouldn't reach 3k and blew black smoke in the attempt. Transom was forever with a black smudge. I read all the issues and thought maybe I needed an engine rebuild or at the very least the inj rebuilt or some other repair process.
I then remembered the previous owner's statement about being able to sail 2 minutes after leaving the slip. Hum, maybe the engine never got up to temp time after time. That would surely lead to a build up of carbon through out the exhaust and probably the valves.
So I went out and motored sailed for several hours with the engine turning over at about 1.5k rpms and was very careful to stop sailing and motor for at least 1/2 back to the slip. Well, lo and behold after doing this several times I can now reach 3.6k rpms with absolutely no black smoke and at 3.1k I'm OVER hull speed at 6.1kts! And I haven't had any black smudges on my transom in that last several years. I also make sure I keep fuel stabilizer in my fuel and don't let it get too old. Diesel starts to degrade after 30 days! How old is your fuel?
I bought a Campbell Sailor last June when I was in the yard for my annual haulout. It has been one of the most frustrating boating experiences ever. It has been a very long 6 months sorting this out. The origional prop sold to me and sized by West by North Enterprises was a 14x9. This prop was so over pitched for the engine that it would only make at max throttle 2000 rpm and the engine smoked profusly at rpm's over 1500. I called them and they had me almost believing that something was wrong with my engine. Finally they sent me a 14x7 which I swapped out and the max rpm only increased to 2400 rpm still with a smoking problem. After $240 of mechanics time to verify my engine's health (It is just fine) and $150 to my local prop shop to depitch this to 14x5, the performance is now acceptable. She makes max govenered rpm of 3000 and cruises very nicely at 2400 making about 6kt. She stops well when I hit reverse and the vibration is significantly reduced. The new engine mounts have a lot to do with that also.
My conclusions... its an ok prop but they have no clue how to size one. When pressed Norm at W by N Ent. said "we take the michigan wheel program and subtract a couple of inches of pitch and compare that with our data base". Sure seems flakey to me.
If done again I would stick to a prop supplier who actually knows something about sizing one.
My experience with the 25D and the 1 GM is the same as yours. I think if she is running right you do well with a 12 x 9 two-blade, achieving hull speed rather easily but still able to get near maximum engine rpm when you need too. I have to run about 1/2 on both ends of the sail so she is warmed up.
I've come to the same conclusion about Campbell and their US distributor when it comes to sizing. Their recommendation to me is out of wack with everything I've been reading and all the recommendations received from this board.
My personal decision is to stick with Michigan Propeller.
, I replaced it with a "14RH10" 2-blade (both props would give a 0 to 5 Kts in 20 seconds in flat water performance.) 
