Comparing MPH vs. RPM

[Thorne]

After two coats of bottom paint I can cruise at 24 mph at 4500 rpm with 4 boys and some fishing gear on board in a light chop. 1983 22' Angler with a new 90 4-stroke Yamaha. How does that compare?

[thataway]

I would say that is pretty good. The 83 boat is a flatter bottom--maybe a little lighter? About what I run with the Honda 90 on a 2006 boat. Props, trim etc all make a difference.

[MikeR]

Those sound like good numbers, assuming you also get recommended max rpm at WOT.
Rarely do I like to cruise that fast, but I did find one photo in my album taken at 4500 rpm, and the GPS was reading 22.3 kts, which is 25.6 mph. Give or take a couple mph depending on what tides were doing at the time. And that's just with me on board, not 4 boys in addition.

-Mike

[Kushtaka]

How is it propped?

Not knowing the redline RPM or the fuel consumption curve on the motor it's hard to say what is going on and if it's good, but go to:

http://www.boat-fuel-economy.com/yamaha-outboard-fuel-consumption-us-gallons

you will see that you should be using about 4.70gph at 4500rpm, and that would put you at 5+mpg cruising faster than most with a loaded boat.

It sounds a little too good to be true.

If you are burning more fuel than 4.7GPH you may want to consider a new prop.

If you aren't sure how to determine if your prop is the right pitch, the easy way to do it is to set up the boat the way you'll load it 90% of the time and on a nice calm day with as little current as possible, run it up to full throttle and note the RPM. Do not exceed the rated max RPM, but you should get pretty close to it. I think your motor should max at near 6000 rpm, so you will want to be at or near max RPM, but not over.

Depending on how that goes, you will know if you just have the sweetest fast-cruise CDory around, or if you need a different prop.

My guess is that perhaps your boat takes a little longer to get on step, and is working harder than it should to push your boat as fast as it is going, meaning you are generating more "push" than is ideal with the RPM of the engine.

Another way of looking at it is like a bike with a fixed gear. if it's too high you can go fast, but it's too hard to push the pedals to get it going. If it's too low you will be able to get it going really easily, but your legs will be pumping as fast as they can and you will be going painfully slow. I'd guess you are in the former, geared too high, category.

BUT, it may be perfect, and if it is, WOW!!!! Makes me want a new Yamaha!

[Kushtaka]

Another thought is that you might want to confirm that your tach is reading properly. They can be off by a enough to make a difference. It's an easy enough thing to do with a timing light, easier with a mechanic and some $.

But since I looked at your post again, for comparison, I have a 1993 CD22 cruiser, and a 2003 Honda BF90 turning a 13.25x15 prop and I get around 20 mph at 4500 RPM with just me.

[jennykatz]

We have a newer merc CT 115 engine on a 23 cc venture

We have a 14 1/2 in by 17 pitch S/S prop

At 4000rpm = 20mph
At 4500rpm= 25mph
AT5300rpm=31-33mph WOT depending on sea condition

I like it being quiet at 4000rpm you can carry a conversation These new Merc 's are very quiet compared to my old Suzuki or yamaha

[hardee]

[Thorne]

Thanks everyone for your well-considered replies.

I'll re-read several times to commit it to memory.

Speed was measured with GPS, wasn't considering tide at the time although it was a couple of hours after high tide in inner Port Chester, Metlakatla, AK.

I'll check RPM at WOT the next time I can get out.

I get what you say about straining to get up on plane, and relating the prop pitch to bike gears.

I don't have a fuel flow meter, so gauging gph is tough.

[Kushtaka]

You can still accomplish a pretty good analysis with what you have.

Just pick a nice calm day and do your WOT tests. It doesn't really matter how fast you are going at WOT, just that you are at WOT and not over or under revving. This will change with altitude and load, so it's important to set the boat up like you would normally. If you are interested in your speed and it isn't practical to test during slack tides just run with and then against the current and average your GPS speed. It will be close enough. But you could really do some damage to your new motor if you are over or under revving your engine.

To estimate your fuel consumption, first make sure you have the right prop with the above test. without a flow meter simply fill your tanks up and go run around normally for awhile, and note your engine hours (realize that sometimes the engine clock ticks on when the engine is not running but the key is switched on and being careful to account for time properly) when you refill your tanks at the end of the day(s). That will at least tell you what your average fuel consumption is and give you an idea of what to plan for your boat's range.

[hardee]

I found tracking MPG extremely frustrating because my numbers were all over the board. The variations, as far as I could track were due to lots of variation in throttle settings, tidal currents, sea state. I was measuring both time and distance by a Garmin GPS. The gallons per hour fluctuated also, but it was less of a variation than miles per gallon.

Kushtaka is right, you can run both ways on the bay and (with and against) the currents and take an average. BUT as he says too, the important part is not to be over revving or lugging your OB. Find out what your WOT should be at and prop so you get the right WOT RPM.

Harvey
SleepyC

[Thorne]

I finally got out today.

WOT was 5900 rpm, and I was going 33.9 mph. Getting the right motor trim was fussy at that speed. A little to little and it felt like the boat wanted to bow steer.

At 4800 rpm I was going about 26.2 mph into a slight chop, which was a heck of a lot more comfortable. Going 16.7 mph at 3800 rpm was down right laid back!

Seems to me there isn't a lot to be gained by fussing about with different props.

What are your thoughts?

[Kushtaka]

You would need to compare those results to the spec for your motor. Without knowing that, it's tough to tell you whether you are propped right.

I would say you are definitely not propped too low. Is your boat struggling to get on step? Does the engine fail to gain RPM quickly as you advance the throttle to get up on step or does RPM build slowly as your hull comes out of the water?

These would be good numbers on my engine. Basically if 5900 RPM is within spec for your boat at WOT, you're good!

[thataway]

Sounds close to perfect with the 4 stroke 90 Yahama! Good on the comfortable cruise at 3800 RPM.

[Kushtaka]

[Thorne]