Max RPM 1996 88hp Johnson Special

[surrender]

motor is on a 17' McKee Offshore Fisherman. Getting ready to install new tach and going to do some runs to see if I'am reaching the rpms needed. Boat is new to me, did not have tach when I got it, no history. Did all recommended things prior like carb cleaning, fuel lines, ect. Boat runs good, just dont know rpm's. What should it run WOT and what should I cruise at. All assuming I have right prop on it. Kinda the point of this test.

 

[Dhadley]

Re: Max RPM 1996 88hp Johnson Special

Your target rpm is 5800 with an average load. However, if you're using ethanol fuel you may be looking at 5600 or so as a target rpm. Make sure your tach is set on position 6, not 4. Just an FYI - a common misconception.

 

[surrender]

Re: Max RPM 1996 88hp Johnson Special

Thanks sir. Tack should arrive today. I'll let you know how it goes

 

[psteurer]

Re: Max RPM 1996 88hp Johnson Special

To answer your question about cruise speed, you usually get your best mpg at 20% of WOT. So somewhere between 4500 and 4750 rpms assuming your boat will maintain plane at that rpm.

 

[surrender]

Re: Max RPM 1996 88hp Johnson Special

You mean 20% less than WOT? 5400X.8=4320 assuming 5400 rpm is WOT.

 

[trendsetter240]

Re: Max RPM 1996 88hp Johnson Special

You mean 20% less than WOT? 5400X.8=4320 assuming 5400 rpm is WOT.

Hi surrender, just fyi I've never seen/heard of that 20% less than WOT rule. It depends upon what motor you have, the hull of your boat, the weight you are carrying in the boat, your prop and the relationship between all three, among other factors.

You will need to experiment and find out for your specific boat and usage what will be the best cruising speed for fuel economy.

Here is an example of a test run with an Etec and a 17" boat. This test shows the best fuel economy at 3000RPM. Just an example, and most tests I've seen have not shown best MPG in the 20% less than WOT range.

http://www.evinrude.com/NR/rdonlyres/7CF4070B-3D48-4B8E-9818-576CDCFEDAF0/0/PE657.pdf

 

[SparkieBoat]

Re: Max RPM 1996 88hp Johnson Special

a lot of people I know run their motors at around 7/8 throttle, mostly on the idea it will get slightly better fuel mileage than WOT and also that keeping the RPMs down some motor will last longer...may be true, maybe not, I run WOT a lot.

 

[SparkieBoat]

Re: Max RPM 1996 88hp Johnson Special

I have a 17.5 McKee, I would be curious what year you have Surrender, also weight of your boat, and top speed achieved at around 5800 RPMs

 

[surrender]

Re: Max RPM 1996 88hp Johnson Special

Boat is the tri hull offshore fisherman, weight-1490 pound(hull only) I'll let you know how it goes

 

[emdsapmgr]

Re: Max RPM 1996 88hp Johnson Special

Sparkie is right on regarding cruising fuel economy. If you pull the cowling and observe the carb/throttle linkage, you can decide your best economy. From the full throttle handle position, slowly pull the throttle handle back. The carb butterflys will close slightly. Pulled back further, the spark advance lever will start to retard-and that's too far. You want to pull the carb butterflys back slightly, but leave the spark advance at full. Usually, that's about 7/8 throttle for best cruising economy. Once you know what you are looking for on dry land, try it while and plane and check your speed and rpm's for future reference.

 

[surrender]

Re: Max RPM 1996 88hp Johnson Special

The etec test does show 3000 rpm to be best for that boat but I would cruise a little faster and sacrifice a little fuel economy. 24-28 mph would be more like it for me on that boat. The problem comes when you run WOT on a boat like mine. You dont get much more speed but do burn a lot more fuel. I think the best would be around 3/4 throttle. I'll do some tests in the next few weeks to see how it goes. no fuel flow meters so I will test while running off a small tank and measure gas used over a certain time at a certain RPM. Do it multiple times over different RPM's and see how it comes out. I'll let you know. I think a 15 minute run straight down the waterway at slack tide would give a good result if you measure the fuel carefully.

 

[ezeke]

Re: Max RPM 1996 88hp Johnson Special

Someone has actually studied the question and published the results here: http://members.iinet.net.au/~pauldawson/Iame25_economy.pdf

 

[surrender]

Re: Max RPM 1996 88hp Johnson Special

I have a 17.5 McKee, I would be curious what year you have Surrender, also weight of your boat, and top speed achieved at around 5800 RPMs

Ran today. Something not right, i posted in my other thread the details(No tach, new rectifier, ect thread) Anyway, it ran to 5300 rpm with two people and did 32.9 mph on the gps. Could not do that twice though. After it fully warmed up it was not right andf could only get 5000 out of her. Still working on it. Prop is a 13 3/8X 17

 

[ezeke]

Re: Max RPM 1996 88hp Johnson Special

With a 17 pitch prop, your theoretical speed at 5300 RPM is 43 MPH. So you have something else going on here because that's a lot of slip at 32.9 MPH.

What HP is the boat rated for?

Use: http://go-fast.com/Prop_Slip_Calculator.htm

Enter 2.0 for gear ratio

 

[surrender]

Re: Max RPM 1996 88hp Johnson Special

Boat is rated to 160 hp. Dont know any that had that much hp put on them. It is a heavy boat, 1480 pounds dry. Flat bottom. Round chine at the stern, not built for speed. I have heard the 88 could be underpowered but others have had it that way and it does fine. Something is wrong but I think its motor wise not a boat issue, just a heavy boat.

 

[ezeke]

Re: Max RPM 1996 88hp Johnson Special

Try a 15 pitch prop. That should get you closer to your range.

At 5800 RPM with a 15 pitch you would have a theoretical speed of 41 with a better hole shot.

 

[surrender]

Re: Max RPM 1996 88hp Johnson Special

Thats probably what I'll do but I have to work out the other issue Iam having and test again. Details are in my other thread,:"No tack, new rectifier, ect.."