C&C 27 Association – Props

The following is information submitted to the Forum on prop choices. Negative opinions have been provided along with positive ones to give a sense of the range of possibilities. If you have a match that isn't covered (or that is, and you'd like to reinforce comments here), please let us . All props are 2-blade, right-handed unless otherwise noted.

Most shafts are 7/8" but there are reportedly some 1" out there. The taper is probably SAE (aka, 'Standard'), but you should ask a prop shop to check. These boats are old enough that a previous owner may have slipped something different in there.

Many people don't like the Martec and Michigan Wheel non-geared folding props because they have little reverse thrust, but non-geared props are relatively inexpensive when new (from Martec – MW has dropped their line), cheap when used and have a very small frontal area compared to geared props, hence slightly less drag. They do get worn with use, but can be refurbished at prices better than new. Martec operates its own refurbishing service; in addition many prop shops can do the required work.

Prop walk - All props "walk" to one side, particularly in reverse when starting from stationary (the stern of a 27 with a standard engine and prop setup swings to port when put in reverse). The way to avoid this is to get some way on the boat and some flow over the rudder so it is effective in counteracting the swing. (3-blade props have the reputation of being less prone to walk, not because they are inherently less prone to it, but because even at low revs, their far greater blade area and consequent thrust gets the boat moving more quickly and the rudder working more effectively, before the stern starts to walk.)

Instead of putting the engine in gear and easing on the throttle, shift into reverse and give the engine a shot of power; in other words, rather than accelerating from idle to 1,000 rpm, run up to 1,500 or so, but just until the boat begins to move. At that point, drop back to a point midway between 1,500 and idle. That little shot of power will give you enough motion to allow your rudder to bite, and if you give your rudder a touch of starboard helm to counteract residual walk, you'll exit your slip under control and straight as a die.

Do something similar when stopping - rather than easing into reverse, shift and give the engine a good shot of power. That burst of reverse thrust will stop the boat reliably and quickly enough that there is no time for prop-walk.

"After I saw someone handling a boat this way, I took out a C&C 34 (with a Martec folder, a worst-case combo) and practised around a plastic buoy (so I could run into it with impunity). In short order I could put that boat anywhere I wanted. It wasn't difficult - in fact it was fun - and since then, I've had no qualms about putting Towser or any other boat anywhere." - David Weatherston

Vibration in props – First, the vibration may not be in the prop; misalignment of the engine, insufficiently tight coupling bolts, a badly worn or very slightly bent shaft, or a worn cutless bearing may be the root cause. Overtime, a Mk V, had a persistent problem that was solved by going to a Martec. Subsequent investigation showed that the limit stops on the original Gori had worn, so the prop was opening into an uneven state, unbalancing it. Sometimes it would smoothe out after running for some time, sometimes not. Owner Dave Moores related, "My mechanic showed me where the limit-stops on the Gori get flattened over time and allow the blades to go over-centre. Unless the wear is exactly symmetrical each blade takes a slightly different angle and that causes the vibration."

Mark I & II

Boat Name Engine Prop Comments
Hyperion Yanmar 2GM20 Max-Prop 13" x 13" (28 deg) "I usually run the Yanmar at 2800 RPM as this is where it seems most comfortable and the boat will go all day into the wind and swell at 5 1/2 knots." – Ralph Ainslie
Penny Whistle Yanmar 2GM 13 HP 11x9, 3-blade "I can rarely achieve more than 4 to 4.5 knots. This seems slow." – Carl Armstrong

Mark III & IV

Boat Name Engine Prop Comments
Cygnus Atomic 4 Indigo 10x7.4 3-blade "My reasons for going from the original two-blade were exactly the same as Santiva's. I am happy with the prop, but if I rev the engine, I am pretty sure the prop is cavitating. I have heard of other people with a similar problem. I asked the manufacturer about it, but they never answered." – Allan Wong
Kat's Paw I Atomic 4 Gori 2-blade folding, dia & pitch n/a Fine in forward but no reverse thrust, lots of prop walk. Owner and others (skilled boat-handlers) hate it.
Iris Atomic 4 Martec folding 11x5 "This came with the boat and delivers plenty of drive. The 5-inch pitch allows the engine to turn over at a comfortable rpm when cruising at 5 1/2 knots." – David Masury
Rhapsody Atomic 4 Michigan Blue Dot folding 12x5 "The builder's file shows that there was a 12x5 folding prop instead of the standard when built. I always thought that this prop was a bit large, as the A4 seemed to lug, unable to come up to decent RPMs (no tachometer, but I've spent a lot of time around old gas engines)." - Bob Wooden
Santiva Atomic 4 Indigo 10x7.4 3-blade "Prop-walk from the original fixed two-blader led me to the fixed three-blader from Indigo, designed specifically for Atomic 4's. Prop-walk now is negligible, reversing brings the boat quickly to a stop and while a folding prop would have yielded a fraction more speed (at significantly higher cost) I'm a cruiser." – Ken Pole
Towser Atomic 4
Flex-o-fold folding 12x7
"Great prop with lots of thrust in forward or reverse." – David Weatherston
  Yanmar 2YM15 Flex-o-fold folding 14x12 A replacement diesel required only new blades on the original hub. Note that this is the maximum diameter that can be mounted – a longer blade would contact the rudder when folded.

Mark V

Boat Name Engine Prop Comments
August Westerbeke 12B 2 diesel through a 2.05:1 reduction. 12"x7" fixed 2-blade "Can't make hull speed in still water at 3000 rpm (rated maximum)." – Lou Potente
Distant Thunder Yanmar 1GM10 Flex-o-fold folding 13 x 10 RH "Works great in both directions." – Jim Wente
Dragonfly Yanmar 1GM10 with 2.62:1 gearbox Gori 13x9 RH (original fixed-blade was 12x9) "It seems about right. Will rev to 3600 giving boat speed about 5.5 knots or so. Would continue to accelerate if I let it (which I don't!). Am told not all these boats may have same gear ratio, and that matters a lot with prop selection." – Rod Millar
Heatwave Yanmar 1GM10 Gori 13x8 "The engine cannot achieve 3600 RPM in gear, so I think this prop is too big, or else the boat is overloaded with gear." – Bob England
Overtime Yanmar 1GM10 Martec 12x12 "Drives boat really well." – Dave Moores
Underdog Yanmar 1GM10, 8 HP Gori 14x9 "Seems a little big but it only cost $100 so I went with it. The boat will only do about 5 knots at about 2500 - 3000 rpm. The old fixed prop was better at 12RH9 for speed which pushed the boat along at about 5.5 knots." – Cameron Paine (Later, he reported that a new cutless bearing added 3/4 knot to boatspeed.)
Mk I