I recently received the following PM from Phoenician, which I am posting
here with his permission:
Dockhead, I realized from your recent post that we both have the exact same Yanmar engine
, with approximately the same number of hours - mine has 1700. In addition, we both have similar sized boats.
I haven't experienced your problem at high revs yet, (I actually never rev it this high), so i'll keep an eye for that and let you know if it happens with me.
However, I thought it might be interesting to benchmark our respective engines and compare experiences: (hence the PM)
I have a three blade MaxProp, pitched at 20 degrees, and when motoring on flat sea at 2300-2400 rpm
, my SOG is around 7 to 7.5 knots, which someone observed seemed on the low side. I wonder what is your own experience?
On another topic -probably "post worthy":-), a very good mechanic
, while servicing my engine
last month, recommended that I switch to 20W50 oil- His reasoning was that at 1700 hours on the engine
, with oil
droplets starting to appear on the exhaust
side of the airfilter, it would be reasonable to switch to a more viscous oil
, particularly considering the warm summer waters in the Easter Med. Any thoughts?
Kind regards, Jean"
It will be interesting to hear what other think about this. My thoughts:
1. It's hard to compare speeds and RPM
. On my boat, the speed achieved for a given RPM
varies very widely depending on cleanliness of the bottom and sea state. Moreover, my boat has a variable pitch
prop -- a Brunton Autoprop. So a constant RPM will give an even wider range of speed depending on the resistance -- the prop will pitch
up and move the boat faster and faster if there's no resistance or if there is, for example, a following wind
2300 RPM, these days, is my fastish cruising speed. If I'm not in a hurry, and especially in very calm weather
and/or with a favorable tide, I mostly use about 2000 RPM, taking advantage of my self-pitching prop. In a dead calm and glassy smooth water
and clean bottom, I guess I get about 8.5 knots out of 2300 RPM, but falls rapidly with a headwind or any bottom fouling.
I don't think there's anything wrong with 7 or 7.5 knots with a fixed pitch prop like the Max Prop. I don't know your waterline length, but if its like mine (47 feet) I am guessing that you will easily reach 9 knots without stressing the engine
-- is that correct? Even 9.5 to 10 if you open it up? If all this is true, then that's just the way your prop is pitched, and probably it's pitched right since you will need that somewhat lower gearing for tougher conditions.
2. I would never mess around with oil
viscosity of a marine
engine. They run much cooler than terrestrial engines. Our engines have direct sea water
to lube oil
heat exchangers, so the oil is never going to get very warm (in fact I wonder if it is simply too cool) -- nothing like the 120 degrees C plus which lube oil can reach in a car engine. I can't imagine that the recommended oil is going to be too thin under any possible circumstances.
Now that's my amateurish guess - maybe some real prop with actual knowledge could say something on the subject?