Prop Efficiency vs. Top Speed

[doggunit]

My 17' boat is powered by an '04 Merc 40, mounted at 21.5" and at neutral trim, turning a Stiletto 10 1/2 X 12 hits 30.5 mph (GPS) at 5200 rpm WOT. Calculating the slip: 5200 (rpm) X .54 (gear ratio) X 12" (pitch) divided by 1056 = 31.9 mph and 30.5mph/31.9mph = 5% slip which seems unrealistic. Am I missing something here?

Also, I experienced an increase in efficiency as rpms increased. Should I consider adjusting the prop to get to the rated 5500 rpm and run in the most efficient range?

 

[Crownie2]

Re: Prop Efficiency vs. Top Speed

Actually, I calculated 5.5% slip, by not rounding as much.

Don't touch a thing!! Your boat is running great!!

No, really - could be that one of your numbers (tach or prop) is a little off.

Yes, efficiency gets better as the speed goes up - outboards perform best in certain design ranges, and the Max. suggested WOT is one of the keys to this - (although max. fuel econony is usually well below WOT, as you know).

IMO you are not going to gain much, if anything by "adjusting" the prop - the only exception being if you wanted to try mounting the engine a little higher - which is something you can undo if it doesn't work better. The next step would be to try a little setback - but I don't think you would want to fool with that.

Bob

 

[Texasmark]

Re: Prop Efficiency vs. Top Speed

The main reason for improved efficiency is reduced inefficiency. 8) As you speed up, more and more of the hull lifts out of the water. Hull drag is your #1 performance inhibitor. The prop slips less as the hull comes out of the water.

On the last prop testing I did, on a light, padded al boat with a big (for the size) engine, my slip calculated to be 35% at 18 mph where I got on plane and dropped to 9% at WOT (50+ mph). Calculations were via BAM online calculator.

HTH

Mark

 

[walleyehed]

Re: Prop Efficiency vs. Top Speed

Not sure where your numbers are coming from, but we need to know if this is a std 4-stroke, bigfoot or 2-stroke.
If it's a 2-stroke, the tach and/or speedo is way off.
Even if it is a 4-stroke, the numbers posted represent a negative slip....impossible.

 

[Crownie2]

Re: Prop Efficiency vs. Top Speed

Excellent point, Mark, and one I didn't even mention...

Kenny - if you assume a lightweight boat and a 2- stroke, the numbers work well in the Merc Calculator. I guess I assumed 2-stroke because of his stated drive ratio...

Bob

 

[walleyehed]

Re: Prop Efficiency vs. Top Speed

As I said.......don't know where those numbers came from...it's a 2:1 gear ratio, not .54...

 

[Crownie2]

Re: Prop Efficiency vs. Top Speed

As I said.......don't know where those numbers came from...it's a 2:1 gear ratio, not .54...

Oh - the Prop Selector is suggesting it is 1.83 and I went with that.

In his formula 1/1.83 = .54...

Bob

 

[walleyehed]

Re: Prop Efficiency vs. Top Speed

You may be right...I don't know why my official book from Merc is giving this info.....I'm going to make a phone call and find out why...
The only problem with 5-5-1/2% slip is that it is near impossible except for the most highly tweaked High-performance hulls that have been worked for every ounce......8% I would believe....5-6%, I don't.

 

[walleyehed]

Re: Prop Efficiency vs. Top Speed

Crownie2, My Merc contact is no longer "with" mercury marine.....
I'm a bit surprized at this...He'd been with them for 17yrs and all of a sudden...gone?
I have some new books coming, but the worst part is...this is the man I turned all of my prop data over to, via email, waiting for the OK to post results.
They set me up with the new director and tomorrow we will be figuring the details out.....what a deal.

 

[Crownie2]

Re: Prop Efficiency vs. Top Speed

Sorry to hear that - hope it works out OK for you...

Bob

 

[walleyehed]

Re: Prop Efficiency vs. Top Speed

The Data may get posted without permission...
I'm not sure why I need it anyway...I work for myself..

 

[MelLandry]

Re: Prop Efficiency vs. Top Speed

This is my first time playing with this forumula. (By the way, I'm a casual observer who just happened to be passing through). The formula I've found on the internet is not the same formula dogg uses in the first post. And when I use the internet formulas, they come out WAY different. Could someone share the correct formula that they use and tell me what they came up with.

In the meantime, I made my own formula. Anyone care to see if it makes sense to them?

5200RPMe x (60RPHe/1RPMe) x (1Rp/1.83Re) x (12"/1Rp) x (1mile/63360") = 32.29 MPH = 9.44% slip

In narrative: RMP engine x (60 Revolutions Per Hour / 1 Revolution Per Minute converts from RPM to RPHour) x (X Revolution prop / X Revolutions engine converts for gear ratio) x (pitch / 1 Revolution prop converts revolution of prop to theoretical distance coverd in water) x (1 mile / 63360" convets inches per hour to miles per hour)

Sequence of conversions through the formula:

Revolutions Per Minute (engine) - Revoutions Per Hour (engine) - Revolutions Per Hour (prop) - Inches Per Hour (prop) - Miles Per Hour (prop)

I don't mean to insult anyone's intelligence by describing it like that, I just want to make sure you see my logic. It looks so long because I'm only converting one factor at a time, thus making it easier to illustrate. Does this look right to everyone else?

 

[walleyehed]

Re: Prop Efficiency vs. Top Speed

Using the 1.83 ratio, I come up with 11.4%....very believable, and very likely.

 

[MelLandry]

Re: Prop Efficiency vs. Top Speed

Also... where does the "1056" in the formula come from?

 

[Crownie2]

Re: Prop Efficiency vs. Top Speed

Also... where does the "1056" in the formula come from?

The 1056 is a constant to convert inches-per-minute of revolution to boat speed - for miles-per-hour. JUst makes it easier.

Bob

 

[MelLandry]

Re: Prop Efficiency vs. Top Speed

The 1056 is a constant to convert inches-per-minute of revolution to boat speed - for miles-per-hour. JUst makes it easier.

Bob

Got ya... 1 MPH = 1056 inches/minute

Still want someone to look over my formula and see why I'm coming up with something different than everyone else.

 

[JESSIELSMALL]

Re: Prop Efficiency vs. Top Speed

CAN SOMEONE TELL ME IF YOU GO UP OR DOWN IN PITCH TO RAISE YOUR RPMS

 

[JESSIELSMALL]

Re: Prop Efficiency vs. Top Speed

I KNOW IM PROBABLLY ASKING AT THE WRONG SECTION, BUT IM NEW AT THIS AND NEED HELP. IM ONLY GETTING AROUND 3200 RMPS ON A BAYLINER CIERRA. CAN SOMEONE TELL ME OF A FEW SOLUTION OR POINT ME TO THE RIGHT SECTION.

 

[MelLandry]

Re: Prop Efficiency vs. Top Speed

CAN SOMEONE TELL ME IF YOU GO UP OR DOWN IN PITCH TO RAISE YOUR RPMS

Reducing the pitch will raise RMP. RMP will increase ROUGHLY 200 RMP for each inch of pitch.

Posting some more information about the size of your boat, the hull design, the model of the engine and drive, the prop you're currently using, and suggested wide open throttle of your engine will help guys here (who know a lot more than me) help you choose the correct prop.