22 Cruiser RPM to speed table?

F

ferret30

New member
Hey brats,

I've had a feeling since we got our boat that the tach might be flaky. Each time I turn the key and the tach reads out the motor hours, it reads a slightly different amount (i.e. differing by a few hundred rpm).

Next time we're out and get into non-speed limited water (i.e. Lake Washington or the Sound), I'm going to do what some others have done and write down SOG vs. RPM at 500 RPM increments. But I was wondering if others that have done this (with a Suzuki FD90 or similar) could post their results.

The reason I ask is that I seem to have to go to very high RPMs to break 20 mph, with me and my wife (340# total) and not a lot of load other than 2 group 24s, 50' of 1/4" chain, and 3/4 full gas tanks.

I recall Sam (Retriever) blogging that he was at 4200 rpm to maintain 15 mpg, so that's the only data point I have currently.

BTW, I believe my prop is a 3 blade aluminum 14x17.
 
Hello ferret30,
2007 DF90, same prop, probably about the same weight.
1000 rpm =3.0 knots
1200=3.7
1400=4.2
1600=4.8
1800=5.3
2000=5.8
2200=6.4
2500=6.7
3000-8.2
3500=11.0
4000=14.0
4500=18.0
5000=21
If I'm by myself, with no wind or current, the boat will make 25 knots with WOT.
Jim
 
My numbers were almost exactly the same as Boat Test.com
This was with the boat light, and a Honda 90 with a 15" pitch prop.


RPM MPH Knots G/h MpG Nm/G
670 2.3 2.0 0.5 4.60 4.00
1000 3.9 3.3 0.5 7.70 6.70
1500 5.2 4.5 0.8 6.93 6.03
2000 6.6 5.7 1.1 6.00 5.22
2500 7.5 6.5 1.9 4.05 3.53
3000 9.4 8.1 2.2 4.35 3.78
3500 14.2 12.3 3.2 4.51 3.92
4000 19.1 16.6 5.0 3.81 3.31
4500 24.7 21.5 7.0 3.55 3.09
5000 27.4 23.8 8.1 3.38 2.94
5500 30.9 26.9 9.1 3.41 2.97
6100 32.9 28.6 9.6 3.45 3.00

Our numbers on Lake Powell were a little lower in the top speeds, and down at the Delta and San Juans, we were also a bit lower, because the boat was loaded for cruising.

The Suzuki's can be propped more aggressively because of lower gear ratio--and suspect that you will find that top speed and economy will improve with a few inches more of prop pitch.
 
It is always interesting to me to watch the tables that are published with the CD22. It seems to matter little what engine brand, single or twins, props, boat set up or load. The numbers still come in very close. My own numbers with my twin Suzies match pretty closely the numbers others are getting with a single ninety or twin 40's or fifties and so on. Yes, there are differences, but in general WOT's on plane are close, and 10 mph displacement are close, within 10 percent or so.

I guess that goes to prove that physics do rule the universe in that it takes x amount of thrust, which burns x amount of btu's to get x amount of speed with x amount of inertia to overcome, and the differences in the systems that do it are less relevant than the ruling elements of physics. (With a common hull design of course).
 
Thanks for the numbers guys. I am looking forward to running my own test as soon as I can. The small lock in Ballard is currently not operating (it was hit by a boat) so I'll probably head over to Lake Washington for some testing.

Bob, you say I could use a few more inches of pitch, so maybe a 14x19 or 14x21? Maybe Kreuger Props would let me demo one or the other. Hmmm.

Also, is the tach RPM the speed at the crankshaft or at the prop shaft?
 
Thanks Charlie. It makes sense for a car, but I was really wondering if it was the case for boats, especially since it's a fixed ratio (single forward speed), and RPM at the prop could potentially be useful in some cases.

Do outboards have an RPM limiter built in, or is it up to the user to watch the tach?
 
As charlie says, these are crank shaft (flywheel) speeds) However many of they are measured by the alternator, rather than by magnets in the flywheels as some of the earlier outboards.


The Suzuki final gear ration is 2.59/1 vs the Honda's 2.33/1. (the 150's are even more different: Suzuki 2.50/1 vs 2.14/1. Some of these engines have both a gear reduction off the crankshaft as well as in the lower unit.
 
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