Interesting chart thanks , we have a 2QM15 that red lines at 2,700 and is actually more happy at 2,650 . That is amazing that the 2GM20F red lines at 3,600 . Also our boat at full loaded up for a multi week cruise is 14,000 plus #s with a 23.5 water line so we don't go much over 5.5kt. at best (yes that's towing the dink) . I bet your Person 28 is a rocket a 3,600 !

 

The 2QM15 has a max output of 15 hp at 3000 rpm. the 2GM20 has a max output one hour rating of 18 hp at 3600rpm or continuous of 16 hp at 3400 rpm not much difference between the engines.

 

Well now you have me thinking , are you sure 3K rpm max for the 2QM15 ? Even if I put the throttle lever all the way forward most I get is 2,700 , I really do not think I have the wrong prop , but I could be wrong .

 

If you have the service manual for your engine, it likely includes a couple of important graphs...RPM vs HP or KW and RPM vs fuel burn (probably in grams or liters, but you can do the conversion to gallons if you think in the archaic British system). Using these two graphs and knowledge of your boat's performance (boat speed under benign conditions w/o current effects) vs RPM, you can determine the most efficient operating point for your engine. The effects of alternator loading needs to be included (do your test with minimal electrical loads, including a charged battery, especially if you have a high capacity alternator).

That point will likely be around 50 percent of rated power if your engine is properly sized for your boat, yielding about 70 percent of hull speed. When you know that point, you can use it as an input into your decisions on engine/fuel use.

You can do a similar exercise for using the engine for bulk charging the batteries periodically on a long voyage to estimate the fuel needed for battery charging if wind/solar are neither sufficient or available.

 

The 2QM15 is rated at 3000RPM. Mine (I have two) are mated to the Kanzaki KBW-10 (modified Hurth) transmission with reduction ratio of 2.14 in forward gear.

The engine is called a 15 because it is rated at a max output of a bit over 14HP, but has a continuous rating of 13.5HP.

I use a 13x10 prop on my 8500# (empty weight...probably more like 9500# loaded) and get rated RPM at about 6kts plus or minus with a 25 ft waterline.

 

Well I'm been keeping track of my fuel/hour use for 8 years and it doesn't come close to matching that chart (I'm using less).

 

Well.

:nerd

Chart please

Or estimates etc, whatever.

 

Any general chart of fuel usage vs RPM leaves a very important piece out - the prop. The consumption at a particular RPM for the same engine in the same boat will vary widely depending on the prop pitch, diameter, number of blades, etc.

 

I occasionally get to borrow, and have found generally that estimating a gallon of diesel per hour for most medium-displacement cruising sailboats in the 28-35-foot range (say 15-30 hp range), is a fairly safe WAG (wild-ass guess), and is usually on the conservative side. It's way safer to guess "high" and be pleasantly surprised rather than gasping fumes when you need power in a tight spot.

Variables are numerous, propeller is one, so also is motorsailing where you run at reduced revs. And having an accurate sounding stick or fuel gauge helps.