Cruising RPMs

[Jon Boat Jim]

What would be a good rpm reading for fuel consumption/speed for my 18HP 2 Stroke Nissan motor (NS18E2)? It has a 10in pitch stainless steel prop and turns about 5400 rpms at WOT. The speed is about 23 mph on my 15ft Jon Boat.

 

[steelespike]

Re: Cruising RPMs

A 18hp will use about 1.8 gallons per hour at wot. The ideal cruising speed could be anywhere from just on plane to about 1000
rpm higher.It's a trial and error technique. A 18 hp should make about 27 mph on a light reasonably fast boat.
Jon boats can be stubborn with all the rivets,seams,and some have 5 keels.
You may want to fine tune your setup as it stands your slip is a little high at 16% is the boat carrying a good load?
Is your speed by gps?

 

[Sea Rider]

Re: Cruising RPMs

A 18 consumes 2.2 Gal/Hr at wot, best fuel comsumption/speed is around middle throttle. If you place a 9 pitch prop you should experiment 150/200 rpm increase which will be best as min rpm at wot for that horse is 5300. Leg/transom height, proper trim is very important to achieve best hole shot, top speed and superb close turns.

On a very light boat, inflatable type using standard delivered prop probably engine will be bit over reving, must check with induction tach if wanting to go wot inside safe engine parameters. My 18 placed on a light 430 rib with me alone on flat calm, no wind waters is achieving 5800 rpm from max stated 5800 and running 42 Km/H GPS measured on a 1.0 mile test run at a rowing track. 27 MPH is too much, probably 23 top and with driver lying flat on deck

Happy Boating

 

[Jon Boat Jim]

Re: Cruising RPMs

Steelespike,
My jon boat would probably be on the heavy side.....a 6 gallon gas tank, two batteries (one is a heavy deep cycle), a front casting deck was added (3/4 inch plywood) and it has a 1/2 inch plywood floor with 3 keels and many rivets.
Maybe that 23mph at WOT is about all that I can expect to get. The speed was determined by a Garmin handheld GPS.
Your plane plus 1000 rpms sounds like a good area to be for cruising.

 

[Jon Boat Jim]

Re: Cruising RPMs

Sea Rider,

I agree, a 9 inch pitch would probably be ideal for max rpms. However, I would probably give up some top speed.

 

[Sea Rider]

Re: Cruising RPMs

Would preffer loosing slight top speed but achieve better hole shot & higher rpm for than engine. Nobody in their right mind runs wot all the time. Not opt to run at 5400 rpm when min is 5300, optimize it at least to 5500 for better engine overall performance.

Happy Boating

 

[jestor68]

Re: Cruising RPMs

The WOT rpm is 5200-5800 for the NS-18E2.

Right in the middle of the rpm range(5500) is where you would usually achieve the best fuel economy and speed. At 5400, you're right there.

Reducing pitch would only burn more gas and reduce top speed.

You can go to the Nissan web site and down load the owners manual if you don't have one.

Don't worry; be happy!

 

[Jon Boat Jim]

Re: Cruising RPMs

Sea Rider and jestor68,
Thank you for your comments. I'm very pleased with the overall performance of the 10 inch pitch SS prop that I have on. Even with two people on board (people weight ~ 325 lbs total) with fishing gear, a small cooler, plus all the rest of the stuff that I listed, it pops up on plane very nicely and maxes out at 22-22.5 mph at 5,400 rpms.

I get a little splash from the engine being mounted a bit low. I raised it up a bout 3/4 inch and eliminated most of the splash. It needs to go up another 1/2-3/4 inch, but I running out of transom!

Thanks for the tip on the owner's manual. I downloaded it a day or so after I got the engine.

 

[steelespike]

Re: Cruising RPMs

A 18 consumes 2.2 Gal/Hr at wot, best fuel comsumption/speed is around middle throttle. If you place a 9 pitch prop you should experiment 150/200 rpm increase which will be best as min rpm at wot for that horse is 5300. Leg/transom height, proper trim is very important to achieve best hole shot, top speed and superb close turns.

On a very light boat, inflatable type using standard delivered prop probably engine will be bit over reving, must check with induction tach if wanting to go wot inside safe engine parameters. My 18 placed on a light 430 rib with me alone on flat calm, no wind waters is achieving 5800 rpm from max stated 5800 and running 42 Km/H GPS measured on a 1.0 mile test run at a rowing track. 27 MPH is too much, probably 23 top and with driver lying flat on deck

Happy Boating

The OMC 18s were rated for 27 mph back in the last 1950s Granted they were being optimistic and would require a light reasonably fast boat. In the 60s had a big 15 ft runabout kit boat with a 61 18, big enough that our Great Dane could ride on the front deck with room to spare, it routinely reached 24 On the speedo of those days And on occasion 26 With just me
and the near empty gas tank on the back seat running down wind,down current. This was with no special setup or prop as I was uninformed at that point. JBs boat is decidedly slower as expected from his description.

 

[Sea Rider]

Re: Cruising RPMs

Perfectly clear about OMC, but we are talking about a Nissan 18E2 which is a rebadged same Tohatsu 18E2 engine...

Happy Boating