Outboards M26

[Bobby T.-26X #4767]

Moe comes thru again!

[Robert]

Etec Saltwater 90
326 lbs. White
2.25 gear ratio and 5000 engine rpm 14" prop
25 amp charging
..
comes with 25" length, might be hard to mount up to Mac. I guess that's why the saltwater 90 is not on the chart.
..
Awsome chart Moe!

[Moe]

Thanks, guys! I have a New Version of the Chart.

It adds an "Estimated Prop Pitch" row.

Would you guys who've found a good prop pitch for your motor give it a look and let me know how I did on the estimate. The objective is to reach max prop shaft speed (and hence the top of the max engine rpm range) when very lightly loaded.

Thanks,
--
Moe

[Frank C]

Well, definitely a minor nit, and also definitely load-dependent, but ....

FWIW, I think 13" is too much pitch for an average loaded Suzi 70. My dealer repitched mine down from 15" to 13" and it's still too much. A friend tried it on his Suzi 70 and preferred his 11 pitch. Seems to me that 14 x 12 might be golden ...


And, just for clarity, I PM'd Moe asking what software he's using to create an HTML chart ...
and Moe responded by saying he never learned anything other than Notepad !!!

So you just type out HTML charts in Notepad, eh?
Do you also swim across the Mississippi for coffee breaks?

[joncon17]

Big thanks for the chart Mo: Ok I be thinking I sailing right, so dump the weight go for a Hon BF 50... DA 9 grand!!! can this be right? did 3 searches & got 2 email quotes from dealers average 9K plus shipping; holly molly

[Tom Spohn]

Frank C. Said:

FWIW, I think 13" is too much pitch for an average loaded Suzi 70. My dealer repitched mine down from 15" to 13" and it's still too much. A friend tried it on his Suzi 70 and preferred his 11 pitch. Seems to me that 14 x 12 might be golden ...

I am beginning to think you are right. My suzuki 70 is turning a 3 X 13.75 X 13 and I only get 5150 rpm at WOT. I am thinking of a 12 or 11 myself. Anybody else tried a lower pitch?

[Moe]

Tom, is this very lightly loaded? What's the mph when you achieve 5,150 rpm?

The rule of thumb is 200 rpm change for each 1" change in pitch. However, dropping from 15" pitch to 13", same manufacturer and model prop (Mercury Black Max), resulted in going from 5,200 rpm to 6,000 rpm at WOT on our Whaler... double the rule of thumb.

Thanks,
--
Moe

[on edit] If I leave max speed at 24 mph and change the slip to 12% for the 70HP Suzuki, the pitch comes out to almost exactly 12". However, given Tom's report, 11" (5-6% slip) may be the target. This is getting to what I'm used to seeing for slip with a powerboat at WOT on plane.

[Moe]

The formula I used for coming up with the pitch is:

((((MaxMPH/(1-SlipPercent))/60)/MaxPropRPM)*5280*12)=EstimatedPropPitch

where SlipPercent is a decimal fraction less than 1

The /60 converts the hour in mph to minutes in RPM

The *5280 converts the miles in mph to feet (use 6076 if using nautical miles per hour)

The *12 converts the pitch in feet to pitch in inches

The MaxMPH and SlipPercent were obviously WAGs, however, with real-world data, such as Tom's MPH at WOT 5,150 RPM and given prop pitch, slip can be calculated. Slip varies with loading (and motor trim) as well as speed, where it drops as speed increases.
--
Moe

[on edit] Added the 5280

[Bobby T.-26X #4767]

Moe,
Regarding pitch and my experience with the Tohatsu 90 TLDI...
Unloaded (captain only) - 14" x 13 pitch gets 5500 RPMs and 28 MPH.
Loaded (Captain, crew, gear) - 14" x 11 pitch gets you to 5500 RPMs with top speed near 25 MPH.
Note: the Quicksilver is recommended by Tohatsu technicians due to it's acclaimed Torque-Flow Hub. Additionally, the Q-Silver 11 pitch has huge "ears" which maximizes water surface area.
Therefore, my recommendation for the majority of uses on a MAC (and the recommendation from the Tohatsu technician) is the 11 pitch.

[Bobby T.-26X #4767]

One more comment about pitch...
If you go with an 11 or a 9 pitch to maximize RPMs (which is what manufacturer's recommend), make sure of two things:

1) that you get as large a diameter as possible
2) the surface area on the blades is noticably large

That is...if you compare three different brands of props with the same pitch you will usually see a difference in diameter and blade size.

If the diameter and blade size is not large enough for the weight of the MAC, you will cavitate excessively and not optimize your speed.

To prolong the life of your engine you need to be close to the top end of the recommended RPM range without exceeding it.

[Moe]

Unloaded (captain only) - 14" x 13 pitch gets 5500 RPMs and 28 MPH.

Thanks, Bobby! That calculates out to 5% slip. Using that number to get 5850 rpm at 28 mph, works out almost exactly to a 12" pitch, which unfortunately isn't available in the Mercury (and Quicksilver) lines. That 13" is probably the same prop (different hub) as what we have on the Whaler. Big elephant ear blades.

Given that you have to make a choice, you have to look at how you'll use the boat most of the time and go with the prop that doesn't cause the motor's max rpm at WOT to drop out of the manufacturer's recommended range, and the 11" appears to be that for you. Of course, if you're going to carry a spare prop, having the 13" also gives you a higher speed solution for running around lightly loaded. We have two such props for our Whaler, and it isn't a big deal to swap them.

I'll recalculate the 90HP TLDI and E-TEC using the lower slip number.

Thanks again,
--
Moe

[Terry]

Geez Moe sure is nice to see you have so much spare time, that chart solves a lot of decision making for folks, a very generous contribution to say the least.

The Honda BF50 was a $7200.00 CDN $ option for me in 2003 on my 26M, given the exchange rate it may be cheaper for folks near the 49th parallel to look north for one.

The dealer here puts 3 X 12 X 10.25 props on them (Honda's) with the occasional 3 X 10.75 X 12 I re-pitched my 3 X 10.75 X 12 to a 13 pitch and it gets me in the 55-6000 range but with more slippage. I bought a Solas 4 X 11.4 X 10 (which later turned out was 10.5 pitch) but it just made it to 5500 rpm so I had it re-pitched down to 9 for a drop of 1.5 inches or 300 rpm (which should have put me at 57-5800rpm) but in reality it boosted my rpm to 6200 so I guess it's not a linear translation. In retrospect I wish I had got 4 X 11.8 X 9 which I suspect would have been a perfect fit. for the Honda. The 4 blades provide more surfacce area for these barges.

Oh yea I get 20 miles per hour on that 4 X 11.4 X 9 re-pitched Solas and maybe 18 mph on the 3 X 10.75 X 13 one. (too much slippage)

Anyone have a Stainless Steel prop - if so can it take a higher pitch being steel as opposed to aluminium.

[Moe]

Robert tested the aluminum Solas 4 x 11.8" x 9" and it performed pretty well for him. He did mention that it had a ringing sound at a certain rpm.

Given the same blade profile, which is rarely the case, stainless won't make enough difference to change the pitch. In most cases, the stainless props are cupped though, and perform like a pitch 1" greater... which mean they load the engine more and you'll need 1" less pitch.
--
Moe

[nemo]

Data is great - thanks for posting it Moe and others. One caution, it can be misleading if one interprets it wrong..

For example, compare Pounds per HP for a Honda BF40 vs. a Suzuki DF70. They are both 5.1 lbs/HP. One could say they have the same power to weight ratio.

Not so when you bolt it onto a boat. Add the 3700 LBs for a ballasted 26M into the equation and now you have 97.6 LBs/HP for the Honda BF40 and 58 LBs/HP for the Suzuki DF70.

Moe, you might add a line in the spreadsheet incorporating boat weight too - it makes the comparisons a bit more revealing about what you get for the extra weight of the larger motors.