Cruise fuel mileage

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nowgrn4

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1652 Weld-Craft side console W/transom pods for a total length of 17'-11" and 6' width. 2020 Zuke DF60A W/Stainless 3 blade Power Tech 15" prop. Zuke operating range is 5,300-6,300 RPM. Currently getting 10.5-11.5 MPG @ 20-24MPH cruise @ 3,900-4,300 RPM. Top speed is 34 MPH @ 5,400 RPM, 900 RPM lower than Max RPM. This is at 3/4 throttle, the last 1/4 of throttle (around 2.5" of throttle range) does not increase RPM. This tells me I need to reduce pitch to at least a 14" to optimize RPM range. I'm not looking for more top speed, 30MPH is plenty fast enough. I'm looking for better MPG @ cruise. I know with a lower pitch my RPM will increase a little @ cruise speed but in theory I think matching the prop better to the engine will increase efficiency and help overall MPG. Anyone agree of not? Am I chasing my tail and just leave it as is? Thoughts?
 
Since your in the powerband the only reason i could see for changing out the prop is performance if there are any undesirable tendencies with your current setup.

You would most likely gain a better holeshot and plane a lil easier with the top speed staying about the same.
 
Great hole shot and cruise now. Top speed is plenty good now, anything over 30 is squirrely as hell.
 
You have the WRONG prop, too steep a pitch, regardless for if you ever run it up there, pitch it to give 100-200 off the max RPMs.

Otherwise you are lugging the motor carrying any load ... not good for engine life. Besides running an OB without any oil, using the wrong prop is arguably the next fastest way to destroy a motor.
 
Tech I spoke to this morning at Power Tech said it was an impossibility for my engine with 2:70 gear ratio and the 15" prop to run 34MPH at 5,400. Tech said even with a impossibly low 1% slip to go 34MPH the engine is turning a minimum of 5,750RPM. Conclusion, som ting wong with one year old factory Zuke Tach.

May snag a tiny tach to confirm or just run it as is.
 
Garmin 64st on a ram mount. Zeroed at departure from the ramp with a topped off fuel tank. We have done a lot of exploring this year with the new engine and totally rerigged/refreshed hull. Our longest day was 83 miles on the Sawannee, we used 8 gallons that day with 2 hours running back with the current at 25+ MPH. Lots of 30,40 and 50+ mile days on the St Johns river which is just minutes from us. My MPG numbers are very accurate.
 
DaleH said:
You have the WRONG prop, too steep a pitch, regardless for if you ever run it up there, pitch it to give 100-200 off the max RPMs.

Otherwise you are lugging the motor carrying any load ... not good for engine life. Besides running an OB without any oil, using the wrong prop is arguably the next fastest way to destroy a motor.


agreed. On 4 stroke especially. Will kill the engine eventually from lugging it. It'll also make oil in the meantime. On any 4 stroke outboard it's imperative that if your operating range is 6300 tops, prop it so that it runs 6300 with a normal load. If it's bouncing on the limiter with a light load, back off the throttle a little. With that in mind, at cruise, you'll use LESS throttle, less GPH, and of course with a lower GPH, better fuel economy. A lot of times MUCH better. And quieter operation. And longer engine life. And less maintenance (fewer oil changes especially since the oil level won't rise nearly as quickly as if it's lugged all the time). Propping a 4 stroke, there is very very little room for error. On a 2 stroke you can "get away" with more error but it's still critical. Lug a 2 stroke and the rings build up carbon deposits which hinder their ability to conduct combustion heat away from the pistons, resulting in overheating, and possibly holed pistons among other things. Best case, the rings never really break in properly and it'll be weak for it's entire operating life. Seen that a time or two.

you should have a 2.27 foot. With that, and 15" at 10% slip and 34mph, should be close to 6000 RPM. I'd still go down to a 14" given the same prop. Or 13" in 4 blade. Which powertech do you have? Hopefully not SRA3?
 
11.5" Dia, 15" pitch Power Tech 3 blade SRD3R15PYS50. Load out, Hull, engine, fuel, peeps, gear both Group 31 and 24 AGM batteries and everything else is around 1,550 lbs.

Question; Is 10% slip average/normal?
 
nowgrn4 said:
Question; Is 10% slip average/normal?
Normal ...

For kicks, run your exact rig (or a suitable alternate) through the prop calculator on the Motors Forum, as a sticky. Be honest on the weights, I even take it so far as to weigh everything carried aboard, but I admittedly get 'techie' like that.
 
Mercury calculator says 34MPH @ 5,750 RPM is 6% slip, doubtful. 34MPH @ 6,300 RPM is 14%, that's believable. I have never bounced off the rev limiter so I might be OK.
There is a mathematical formula to get true RPM with a meter set on the Hertz setting. I may have to make some jumpers and try this to see what is really going on.
 
nowgrn4 said:
Garmin 64st on a ram mount. Zeroed at departure from the ramp with a topped off fuel tank. We have done a lot of exploring this year with the new engine and totally rerigged/refreshed hull. Our longest day was 83 miles on the Sawannee, we used 8 gallons that day with 2 hours running back with the current at 25+ MPH. Lots of 30,40 and 50+ mile days on the St Johns river which is just minutes from us. My MPG numbers are very accurate.

Not bad on your long trip.
I did exactly 100 miles on the Suwanee, from Yellow jacket Campground all the way to Branford bridge and back, in a 16' Alumacraft with two people on board (Dan weighed almost 290), a cooler, 20 gallons of gas and safety gear. I burned 11 gallons total at a steady fast cruise with my 1958 Johnson 35hp. Ended up giving away 5 gallons to another boat along for the trip as he did not bring enough.
 
DW and I did the 19 Mi loop around our lake yesterday. 21MPH cruise is now an indicated 3.6K RPM on the tach, It used to be an indicated 3.9K RPM. Tach is obviously defective, and just out of warranty. The can of worms is it has the hour meter for the engine that displays total hours in RPM's and displays alerts for service intervals when the key is activated. Can a Suzuki dealer program a new tach to my existing hours so my hours and service alerts/prompts are correct?
 
nowgrn4 said:
DW and I did the 19 Mi loop around our lake yesterday. 21MPH cruise is now an indicated 3.6K RPM on the tach, It used to be an indicated 3.9K RPM. Tach is obviously defective, and just out of warranty. The can of worms is it has the hour meter for the engine that displays total hours in RPM's and displays alerts for service intervals when the key is activated. Can a Suzuki dealer program a new tach to my existing hours so my hours and service alerts/prompts are correct?

Not sure what year engine you have, but I think 2013 and newer engines, the ECM stores and sends this information to the tach. As long as you do not change the ECM, all the info is still there.

Some tachs have an independent hour meter and that normally cannot be changed.
 
Pappy said:
NO. Hours cannot be transferred from one tach to another. However if the engine can be hooked up to a diagnostic program then your true hours can always be pulled up. Stored codes can as well.

The Hours that are displayed in RPMs, when key is switched to on, is not stored in the tach, it is in the ECM.

Not positive, but pretty sure it can be viewed on any tach.
 
Not sure if this works on analog tachs and you have to have more than 50 hours to get a display.

When you turn the key to "on" (not start) after it has been off, the tach should jump to a reading between 0 and 5000. Divide by 10 to get the number of hours. So if the tach goes to 4000, that means it has 400 hours.

IMPORTANT: if the engine has more than 500 hours, the "check engine" light will flash once for each additional 500 hours. So for instance, if the tach goes to 3000 AND the check engine light flashes once, that means the engine has 500+300 hours = 800 hours. etc.
 
nowgrn4 said:
DW and I did the 19 Mi loop around our lake yesterday. 21MPH cruise is now an indicated 3.6K RPM on the tach, It used to be an indicated 3.9K RPM. Tach is obviously defective, and just out of warranty. The can of worms is it has the hour meter for the engine that displays total hours in RPM's and displays alerts for service intervals when the key is activated. Can a Suzuki dealer program a new tach to my existing hours so my hours and service alerts/prompts are correct?

Not sure if this is the tach you have, but it has some trouble shooting info. See the attached.
 

Attachments

  • 54687IN.pdf
    1.1 MB · Views: 5
Thanks Pappy and Insane for the info. Glad to find out the hours and service promps are in the ECU. That makes things much simpler.

Yes that is my tach model. It's 4 days out of warranty. Gonna talk to the dealer tomorrow and see if they will cover it.

Thanks again.

Good news. Diamond Marine In New Smyrna said they will swap it for a new unit because it's still so close to the expired warranty.
 

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