Prop pitch question

Gene&Mary · Posted: Fri Jul 22, 2016 1:54 pm Post subject: Prop pitch question

I have a 2014 Suzuki 90 which came with a 3x13.75x17 prop. The max revs I could get was ~53-5400 rpm. I changed to a 3x14x15 and could then get ~6000 rpm which is what I understand is what you should get from a Suzuki 90.

Judging from the feel I need to increase the revs of the engine to get the same speed with the new prop.......seems intuitively logical. I don't have a way of actually measuring the speed so I'm doing this just by feel.

I do get up on plane quicker but I didn't think that was necessarily a problem with the old prop.

My question is- Is the increased revs better for the engine so it doesn't lug or for some other reason? Is the mpg decreased because of the higher revs needed for a given speed? Which prop does everyone think is the best for primary use? I will of course keep the other one as a backup.

Thanks

Aurelia · Posted: Fri Jul 22, 2016 2:11 pm Post subject:

It sounds like your newer prop with lower pitch is better suited to how you ran the boat. If your testing up to 6000rpm included a normal load of fuel, passengers, and cargo for you and was at an elevation where you will normally run the boat, you have a good working prop and motor combo.

Lugging the motor is what happens when you can't reach the designed top RPM and that is not good for the boat in the long term. We also switched to a slightly larger but lower pitch prop on our boat for quicker planning and better overall handling (less lugging) of our heavy cruise loads.

You will run a higher RPM for a given speed than you did with the original prop and that might equate to a bit more fuel use at that speed. But....you can just slow down a bit and run at a speed that works well for you and stop comparing one prop to another with speed in mind. We lost some speed on our boat when we changed props but gained a better feeling boat with a little less stress on the engine.

Greg

thataway · Posted: Fri Jul 22, 2016 6:31 pm Post subject:

Generally the fuel consumption is related to the HP output of the engine. If the engine is fully loaded, then it is related to RPM. But when you are loading the engine less, and using the same HP, the fuel consumption should be almost the same (not measurable) and it will give less wear on the engine. There are a lot of factors--and Greg mentioned some--altitude, load, bottom condition, engine trim, engine height, etc.

Better to stick with the prop you have now. (The lower pitch),. There are some who run at displacement speeds all of the time, and they will over pitch the prop slightly.

capt. meares · Posted: Fri Jul 22, 2016 8:02 pm Post subject:

I have a 22 cruiser, with a 2015 DF90 Suzuki. The prop the motor came with could not get the boat above 5200 RPM's, with frequent cavitation. After much research I was able to narrow down which prop was best, and now get to 6500 RPM WOT, and no cavitation. The owners manual for this motor says the WOT needs to be between 6200-6800 RPM's

Discovery · Posted: Fri Jul 22, 2016 8:30 pm Post subject:

I guess I don't understand the reason to run at WOT on a 4 stroke engine. Your car is capable of running at 6500 RPM, but the manufacturer gears the trans to run the engine at 1800 to 2200. My Honda 135 is a car engine standing in its end. Explain?

thataway · Posted: Sat Jul 23, 2016 12:35 am Post subject:

Discovery · Posted: Sat Jul 23, 2016 10:53 am Post subject:

With the 135's on the TomCat. We usually run on a clean plane at 20 to 22 MPH at about 3800 to 4000 RPM, Honda 17" pitch stainless props. Never have I had the feeling I was lugging the engines. Now over 1000 hours on the engines. The FloScan tach and fuel gauge shows this to be our sweet spot for fuel consumption.

Gene&Mary · Posted: Sat Jul 23, 2016 10:54 am Post subject:

capt. meares- what prop did you finally settle on?

tsturm · Posted: Sat Jul 23, 2016 11:57 am Post subject:

capt. meares · Posted: Sat Jul 23, 2016 12:04 pm Post subject:

3x13.875x15

Discovery · Posted: Mon Jul 25, 2016 12:53 am Post subject:

You can downshift the car when going up hill.(or as the load demands)
Wink

[/quote]

If I'm running clean and fully on plane at 3800 RPM and 22 MPH, by going at 6300 RPM and 40 MPH, I'm not running any cleaner or further out of the water. I'm just going faster and burning more gas.

Lighthouse Express · Posted: Mon Jul 25, 2016 11:33 am Post subject:

Gene&Mary

You may want to take a look at this video to get a general idea of the proper sizing of a prop: http://propellerhub.com/sizing-and-selection.html

Enjoy,
Heinz