Low HP inboard engined C27s - prop size/pitch plea

[Paul Meyer]

I'm hoping to get a sense of how our two blade, prop size/pitch (12/8), and tranny ratio (2:1) compare with the few other C27s with low horsepower inboard engines. I'm am considering increasing the prop pitch of our Petter 6hp powered 1973 C27. The prop is 12 inches in diameter. The local prop specialist has examined it and confirmed that it is balanced and not bent and the pitch is 8, as marked (12 x 8). The engine moves the boat at about 5 to 5.2 knots at full throttle on smooth water. As waves and wind rise against us, speed eventually goes to zero, even at full throttle. Not surprising - its a small engine. But when this is happening, every now and then the engine abruptly over-revs, and we have to momentarily back off the throttle to let the engine recover rpm control. This only happens when the engine is being worked its hardest - it works fine otherwise. My questions: Has anyone else experienced this pattern of occasionally over-reving when motoring into wind and waves? Petter 6hp owners (and others with low horsepower inboards): What is the diameter and pitch of your prop, and transmission gear ratio? I'm considering having the pitch of the prop increased from 8 to 10. Anybody have an opinion as to whether that might help? I'm sure people will ask about current rpm at full throttle. I was going to install a Tiny-Tac late this past fall, but the boat had to be hauled before I could get it installed. So we can only guess, but I am considering taking the opportunity to add prop pitch while the boat is out of the water - just to see if it helps. Any knowledge, experience, and predictions would be appreciated. Paul Meyer, New Brunswick, Canada

 

[Alchemist]

Unfortunately, that's not uncommon

with outboard powered boats. When a wave lifts the stern, you can get air sucked down around the prop, the prop cavitates, and the engine over revs until you slow the engine and allow the cavitation to break. With a transom mounted engine, this phenomenom is more common than if the engine is mounted in an well(as in the C-27), but all outboard powered boats will experience this to some degree or another.By the way, you are at (or near) your hull speed with your 6 Hp engine, all putting a bigger engine on will do for you is burn more fuel. The only way to prevent this is to use a longer shaft on the engine to get the prop further under water.The symptoms are exactly as you describe: the engine revs up, and the boat stops moving forward, this happens especially when you want it least, big waves and high winds.SteveAlchemist C-320

 

[Jeff]

Prop size is ok

It's an inboard, not an outboard. However it still may be cavitation that overspins the prop during wave action. Or thrust from the sails (if deployed) or even a following sea may fluctuate the RPM's.Also, it's an old engine, it may be jumping out of gear, have fuel delivery issues, etc.I higher pitch prop may actually slow you down, especially when beating waves, when you need torque the most. The 6hp will not have enough power or time to recover an overpitched prop mid-swell.Your current pitch lets the engine rev up to where it should be, instead of bogging down. Remedy? Have the engine inspected and serviced by a diesel mechanic. When it is in order then use it to the best of it's ability. Augment it's power by motorsailing and keeping the bottom painted and clean.

 

[Paul Meyer]

Prop update

We have had transmission slippage in the past for a couple of other reasons - several times due to kicking the shift lever by mistake when moving around the cockpit rattled by challenging conditions, and also due to control cable misadjustment which I have since fixed. Now that you mention it, I'm also wondering about transmission fluid. The plate on the tranny says to use straight 10W diesel rated motor oil. I've been using 15W-40 hoping that it would slow down a leak that I believe I have now fixed. Having fixed it I'll likely go back to 10W oil. I wonder if using heavier oil might cause tranny slippage when the system is working hard? We have decided to leave the prop alone for now. Our local prop specialist says it is balanced and straight and confirmed the pitch at 8. According to the calculations at http://www.bomon.com/jprop/techinfo.htm (11 x 7) and http://www.alberg30.org/maintenance/MechanicalPropulsion/Propeller/propcalc.xls,(10 x 8) we are perhaps a bit overproped now at 12 x 8. I'll install the tach in the spring and determine it by speed/rpm curve. Thanks to everyone for their comments and suggestions. Paul Meyer