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I have a Mk.II 27" with an Atomic 4 that I believe is over-propped as I can only get about 1800 rpm. The prop is a two bladed bronze of about 12" diameter but unknown pitch. What would be a suitable maximum rpm from the motor and what diameter and pitch would I need to achieve that? I would prefer to modify my two blade for cost reasons but would also be open to other suggestions. Any help with this would be greatly appreciated.
Les.
Contagious Mk2
Comox
Les.
Contagious ; ;Mk2
Comox
Les. ; ; ; ; ; ; ; ;
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My MKlll uses a 11 inch prop with a 5 inch pitch.... moves the boat well and it does not lug the engine.
"IRIS"
1975 C&C 27 MKlll
Hull #453
Kittery Point, Maine
On my Mark 3 with the direct drive atomic 4, I went from an 12 dia 5 pitch folding Michigan "blue dot" to an 11 dia 5 pitch Martec folding. This is the older style Martec with "blunt ends"; the newer eleptic does not have the same area near the outer ends so a direct comparison is hard. I am very happy with the smaller prop, the engine does not lug and I get 5.5+ Kt cruising with 7 kt possible if pushed.
See the "black arts" section for some more info on props as this is a common question.
Bob Wooden
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The standard prop that came with the 1975 Mark III with the atomic four is a 7/8 dia. shaft 11 inch width and a 7 inch pitch. This would be C&C's choice for the best prop at the time. I believe folding or feathering props were optional.
With this set up and a healthy atomic four engine you should be able to make hull speed at 1/2 throttle (on flat water). I'm guessing the RPM would be about 2,000. The atomic four is a relatively low RPM engine.
For more help you could tell us what speed you are getting at 1800 RPM (GPS if possible as knot meters vary boat to boat)? Does the engine rev freely in neutral? How about reverse? Do you plan on racing? or motoring for long distances? Status of the stuffing box? cutlass bearing condition?
John on Lake Erie
Thanks for the input guys. In regards to your questions John. speed at 1800 rpm (wide open) would be about 5.7 kn. The motor revs freely in neutral. In reverse the motor seems to rev more, though having said that I've not checked that with the rev counter (hand held on engine shaft), I had asumed that the prop did not grab the water as eficiently in reverse but will check that. Pesently I am not considering racing (but then two or more boats going in the same direction ----) but I do like to sail as best I can. On the west coast here we seem to motor about half of the time in the summer anyway thats the reason I would like to optimise the prop. The stuffing box was removed, bead blasted and reinstalled with new teflon packing in 05 the cutlass bearing was also changed out at that time along with the 7/8 prop shaft. The current plan is to lift the boat in early Aug. and check to see what exactly I've got for a prop. and armed with as much infomation as I can get make a decision at that time.
Les.
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