Prop, cruising RPMs and cruising speed

[rigspelt]

We can cruise under engine at 6 knots without too much difficulty. Do not have a rpm display, so can't provide specifics...We are still experimenting, but have noticed the gains in speed drop off over 6.5 to 6.8 knots, when I have gunned the engine in flat seas. It is quite easy to get to those speeds, but much harder to get anything more. That's what I find, but I'm no expert. (1974 C&C 27 Mk II)

Ralph, I am interested in your experience with the same boat we have, and I know others are interested in sharing experiences with props/cruising rpms/Atomic 4, so I started this new thread here. Hope that's OK with everyone.

I'm getting used to our new to us 1974 C&C 27 Mk II with A4 and original two-bladed prop. Have not had much flat water to experiment, and have not got around to optimizing the carb at idle and cruising speed, but I did install a tach. On the one run where I made some observations, I easily got to 6 knots at 1800 RPM but the stern exhaust tip sat just at or under the sea surface. Came up some when I sent the crew forward, and came up more when I eased the RPMs to 1400 at speed to 5-5.5. Temperature seemed to drop too when I did that, though that might have nothing to do with the exhaust tip dipping below the sea surface. Still experimenting.

The Mark II's have a shorter stern aft of the rudder than the later Marks, and a heavier stock rudder, so might be more prone to cockpit weight pressing the stern down. At cruising speed, a sailboat squats and the stern wave forms, bringing the sea surface up. In another post, I understand Don investigated and found that exhausting underwater does not affect back pressure significantly.

[Silver Fox]

Stern Dip

Hello Rigspelt,

Yes on our Mark 2, we also experience that stern dip when under power. Like you said; at speed the exhaust tip is barely able to keep above water.

Haven't observed it at different speeds, etc to know when it starts, how low she can go, nor how to try and raise the stern...

Best,
Ralph.

[Mark S]

Rigs and Ralph,

Any boat is going to raise its bow and drop its stern when its speed through the water exceeds hull speed. I don't know anything about the design of your boats, but I would expect a speed of 6.0 to be near hull speed and 6.5 to exceed hull speed of a boat of LOA 27 feet.

Mark

[sastanley]

My C-30 squats some when I go 'fast', but the exhaust exit seems to stay clear...just barely.

rigs, Are you asking for our current specs?
If so:

Catalina 30
LWL - 25' 0"
theoretical hull speed = 6.7 knots
prop = stock 2 blade, 12" x 7"pitch??
cruise = 1,700 RPM - 5.1 knots - clean bottom & prop, flat water
WOT = 2,150 RPM - 6.1 knots - clean bottom & prop, flat water

I have calibrated my knotmeter to my GPS as close as I can. I have not verified it in varying conditions to see if it requires further adjustment.

I may need to adjust my cruising RPM/speed also, as it varies depending on where I am going