Yanmar sweet-spot
Aug 29, 2023 11:38:53 GMT
Post by rene460 on Aug 29, 2023 11:38:53 GMT
Trevor raises the interesting question on how we apply all the discussion to our little engines. I have attached the curves of the 3YM20 installed in my SO30i, a naturally aspirated engine rated at 15.3 kW at 3600 rpm. The curve is broadly similar to what NZL posted above for his engine, with the difference in shape mostly due to the turbocharger compared to the naturally aspirated characteristic. With the propellor curve also on both diagrams, we can see how the engine and propellor work together.
For the engine to run at a constant speed, the power has to be exactly absorbed by the propellor, so in practice, the engine operates on the blue line, which is where it matches the propellor curve. This is achieved by reducing the throttle.
If we talk about 80% power (12.24 kW), the propellor can only absorb this at about 3350 rpm, about 93% of rated speed.
At 80% rated speed or about 2880rpm, the propellor only absorbs 8 kW which is 52% of rated power.
In practice I found 3350 rpm was not a sweet spot, we found it noisy in the cabin and vibration levels were increasing though not severe. Hull wave making was starting to increase. When I throttled back to 80% speed or about 2880-2900 rpm, it seemed much more like a sweet spot for running any distance. Not 80% power, but well above idle speed. And at around 50% power, fuel consumption did not seem excessive. A bit on the low side compared with the 80% recommendation, but felt more comfortable.
In the channel out to to open water, the 5 knot limit was achieved at only 2200-2400 rpm, clearly very low on the power curve. Once in open water I would give a short time at wide open throttle, about 3800 rpm, before cutting back to 2800 if we were motoring or cut right back and turn into the wind to raise sails, before cutting the engine.
At the other extreme is running the engine to top up batteries. Now the Yanmar data sheet says the alternator is 125 A at 12 V, about 1500 watts, say 2 kW after allowing for the alternator efficiency. This is only 13% power, and I always felt this was pushing too far, and rarely used the engine for battery charging alone. However, I normally had access to shore power so rarely had a problem. With a different routine I would have gone for more solar panels, and on a larger boat possibly a generator. A generator engine is better matched to the load for battery charging so would be the preferred solution if solar panels were impractical.
However having followed several threads on this theme over the years, I have the impression that many settle at a somewhat lower speed than me. It takes a lot of engine hours to start seeing the effects, and I can remember only one adverse report from running slower, but of course many may not write of their experience.
rene460
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