what RPM do you cruise at

[trollby]

I am buying a 1703 Trophy with a Mercury 90 hp Optimax. Top rpm is 5800 but wht is the maxium safe cruising rpm? I would like to cruise at 30 miles per hour in calm waters. This would mean running about 4500 rpms. Is this safe or to high for long periods of time? I know I would burn less fuel at say 3500 but looking for the extra 5 miles per hour the extra rpms will povide. What rpms do you cruise at?

 

[RLT III]

Re: what RPM do you cruise at

Well what i have learned is just on plane is a good idea for saving fuel.where you don't have any drag so find that rpm and run it record it and try a little higher rpm and see where your at every boat it different so it's trail and error with recording your way through it you can find the best rpm for you and your buget

 

[achris]

Re: what RPM do you cruise at

As a boatshop owner many years ago this was a question many of my customers would ask.... So I sat down and did some figures, and was quite surprised by what they told me. As a local magazine had just done a big article on all the different outboards on the same boat I had all the raw material I needed. This was for 20hp engines, all the makers. I mapped the fuel usage vs distance traveled. The surprise was that the best economy was at full throttle... Go figure. Yes, you burn more gallons per hour, but you also travel more miles per hour.

I have a stern drive (with a 4 stroke engine) so the figures are a little different. I mapped them a while back and found the best economy at 3300rpm, 27knots (WOT at 5000, 43 knots). At that speed the engine sounds different too. It sounds 'sweeter'... Sorry, can't be more objective...

So to answer the other part of the OP's question.... Outboard engines (the better brands ) have been race proven. That means lots of miles at WOT, they are designed to run all day every day at any speed you want. Provided your engine is not 'over-propped' then you shouldn't have any worries about damage. Your engine's making 5800rpm (as per the spec), so running at 4500 shouldn't cause any problems.

Chris.........

 

[Chris1956]

Re: what RPM do you cruise at

Outboards are capable at cruising at WOT, however, you pay a large fuel penalty. The most economical cruise is just solidly on plane(usually 3k - 3.5K RPM, depending on the boat and powerplant).

The fastest speed with the best fuel economy is when you advance the throttle until the timing is advanced fully, and not any further (likely 3/4-7/8 throttle). That last bit of throttle travel is all carb throttle plate movement and the economy really takes a dive.

 

[Mark_VTfisherman]

Re: what RPM do you cruise at

....gallons per hour, but you also travel more miles per hour.....

Gallons per mile, gallons per hour, miles per gallon.

I had a chevette back in early 80s that got better mileage at 75 than 65; volumetric efficiency and where the motor made power at the car's gear ratio is what did it.

Boats and motors will each have their own "best" cruising speed for economy, but unless time is a factor in what you are doing, MPG is still the way to best measure fuel economy. WOT usually gains some speed, but the percentage increase in fuel usually exceeds the additional distance traveled in a boat.

Each boat is different- my 14' Niagara with 40hp Suzuki does OK at 75% throttle, but it produces nearly the same MPG at trolling speeds. WOT fuel use goes way up depending on the seas.

Running a few miles on Champlain to a fishing spot and then back in rough water can burn 3 gallons. Other days, you fill up after being out and I am surprised at how little I burned when we felt like we ran a lot.

 

[Silvertip]

Re: what RPM do you cruise at

Actually, everyone is wrong on this point. The best economy is almost always at displacement speed. Yes -- that is very slow but almost without exception, that's where the best fuel economy is achieved. The proper term is what "on plane" RPM produces the best fuel economy. That is also not at wide open throttle in most cases but rather something less than that. You can get into an efficiency vs economy discussion very easily here. An engine is indeed most efficient (maximum hp from the available fuel) at wide open throttle. That however does not produce the best fuel "economy". If you like to run near or at wide open throttle, a little trick is to run wide open then pull back slightly on the throttle which leaves slightly less throttle opening but maximum timing. For best economy you need a fuel flow monitor (gallons per hour). Since most boats don't have odometers, this is a reading that is instantaneous and can be translated to MPG if you wish (which is an average) and as such cannot be determined until the trip is over and only if you know how far you traveled. Most engines will deliver their best economy in the 2/3 throttle setting which will typically be in the 4000 RPM range give or take a little. Do a search on this forum for a fuel economy discussion I posted some time ago. It dealt with several different 75 HP engines (two and four stroke) on the same boat.

 

[robert graham]

Re: what RPM do you cruise at

Understand you have a Mercury, but for general purposes this may provide some useful information: Yamaha Outboard Performance Bulletins, gives GPH, MPG, MPH at different RPM's for a variety of outboards and boats combinations, just plug in the rig that most nearly matches yours and check it out. For instance, I run a 90HP Yamaha on a 18' Express Jon boat and best cruise efficiency is 3500RPM. Good Luck!

 

[Mark_VTfisherman]

Re: what RPM do you cruise at

...The best economy is almost always at displacement speed. Yes -- that is very slow but almost without exception, that's where the best fuel economy is achieved....

Haha! You mean displacement speed, as running just under planing speed? I can run about 10-12mph on GPS (running fairly even without an extreme bow wave) and it seems as if it doesn't use much as much fuel as planing at 16mph...

If I am understanding correctly, that explains very well what I was trying to relate. So many times people have told me slow/moderate running wastes gas, but my experience trolling and running flat out indicates otherwise.

I feel vindicated, but I bet still no one (locally) will believe me. But then again- they should! I put a lot more hours on my boat motor in a season than virtually anyone I know.

 

[rodbolt]

Re: what RPM do you cruise at

dunno
I rigged my F150 with the command link square tach and the square fuel/speedo.
I can tell you instantly what my MPG or GPH is at any speed.
at 40 MPH I am burning 14.2 GPH.
throttle back to about 29MPH and I am burning 6 GPH.
its 11 miles from my dock to my work place dock.
time difference is only a few minutes but the round trip fuel burn is incredible.

 

[trollby]

Re: what RPM do you cruise at

Thank you to all. Real good info. My real concern is potential damage or abuse to the outboard. I'm not that concerned with the difference in fuel consumption. My run from dock to fishing grounds will be about 10-12 miles. In the evenings, espically in July and August the sound gets are smooth as glass and I know I will be pushing the throttle forward. If I go from say 4-5 GPH fuel consumption to 7-8 for 20 minutes it will cost and extra gallon on my return. So for $3.00 I can have some fun. I just don't want to blow the outboard doing it.

 

[achris]

Re: what RPM do you cruise at

I have a dive site that's 16 miles out (real miles, not those silly little land ones ), and I've run out there quite a few time at WOT... No problems. My brother and I used to race each other out there (he has 1996 90 hp Mariner).

Chris.........

 

[sschefer]

Re: what RPM do you cruise at

Thank you to all. Real good info. My real concern is potential damage or abuse to the outboard. I'm not that concerned with the difference in fuel consumption. My run from dock to fishing grounds will be about 10-12 miles. In the evenings, espically in July and August the sound gets are smooth as glass and I know I will be pushing the throttle forward. If I go from say 4-5 GPH fuel consumption to 7-8 for 20 minutes it will cost and extra gallon on my return. So for $3.00 I can have some fun. I just don't want to blow the outboard doing it.

I just read this post, not sure how they got you all sidetracked but that happens. The answer to your question is that running your engine up to 5800 rpms all day long is not a problem. Two stroke engines don't like to be heavily loaded so if you prop it so that you can turn 5800 rpm and no more, then you can run there just about forever.

Just an FYI, turning a 2 stroke at 7000 rpm doesn't mean the engine is going to blow up it just means it will wear out faster. The engine is factory percentage balanced for 6000 rpm.

 

[Texasmark]

Re: what RPM do you cruise at

For me, whatever mood I am in. I may run around 20-25 around 3000 or kick it up to 3500 or 4000. If I go over that usually I let the hammer down and take it to the wall....5600.

Most of the time the water condition dictates the mood. However, I have been boating lately on a convenient 1000 acre city lake with all the amenities which most lakes in Texas don't have and it doesn't get very rough. Takes 2-3 minutes at WOT to run end to end then a high speed uturn and back again. I usually go during the week around 10am to noon and there are few boaters to annoy which I steer way clear of anyway.

Mark

 

[saumon]

Re: what RPM do you cruise at

I've always done the "gun out of the hole then throttle back" thing, and i adjust it by the sound. On my particular setup (17 ft aluminum side console fishing boat + Merc. 60hp 2-strokes) the engine is happy and made a sweet sound at around 4000 rpm, wich translate to 26 mph. If I push it to 5500rpm, i only get 33 mph (and a winding high pitched sound), wich is not a great gain for 1500 more rpm.

It seems to be confirmed by theses tests on 50, 90 and 150 2-strokes OB on various setup where the peak efficiency (MPG) is around 4000 rpm, ranging from 3500 to 4500.

http://www.mercurymarine.com/look_de...p5090150efi=20

Naturally, for all this to work your rig has to be set right i.e. being able to reach the engine upper range of the recommended rpm at WOT.

 

[Mark_VTfisherman]

Re: what RPM do you cruise at

trollby- I got your PM a few weeks ago (or more!) and responded. Almost time for the ice cubes to go away and can't wait to surface troll for landlocks on Champlain. Are you still out there somewhere? Would like to invite you aboard my little Niagara for a day and maybe invite myself aboard your hole-in-the-water for a day if that's welcome. I hope you see this and get back to me.

 

[saumon]

Re: what RPM do you cruise at

trollby- I got your PM a few weeks ago (or more!) and responded. Almost time for the ice cubes to go away and can't wait to surface troll for landlocks on Champlain. Are you still out there somewhere? Would like to invite you aboard my little Niagara for a day and maybe invite myself aboard your hole-in-the-water for a day if that's welcome. I hope you see this and get back to me.

Hey, i love this place (an hour from home). I usually go there a couple of times a year before our opener (last friday of april). I usually launch at South Hero and troll aroud the sandbar and Keeler Bay. OK, i had to go tying some streamers

 

[Texasmark]

Re: what RPM do you cruise at

Actually, everyone is wrong on this point. The best economy is almost always at displacement speed. Yes -- that is very slow but almost without exception, that's where the best fuel economy is achieved. The proper term is what "on plane" RPM produces the best fuel economy. That is also not at wide open throttle in most cases but rather something less than that. You can get into an efficiency vs economy discussion very easily here. An engine is indeed most efficient (maximum hp from the available fuel) at wide open throttle. That however does not produce the best fuel "economy". If you like to run near or at wide open throttle, a little trick is to run wide open then pull back slightly on the throttle which leaves slightly less throttle opening but maximum timing. For best economy you need a fuel flow monitor (gallons per hour). Since most boats don't have odometers, this is a reading that is instantaneous and can be translated to MPG if you wish (which is an average) and as such cannot be determined until the trip is over and only if you know how far you traveled. Most engines will deliver their best economy in the 2/3 throttle setting which will typically be in the 4000 RPM range give or take a little. Do a search on this forum for a fuel economy discussion I posted some time ago. It dealt with several different 75 HP engines (two and four stroke) on the same boat.

Silver, I can't believe this is coming from your lips, the lips that I have supported for at least 5 years.

Personal experiences and all....all the data that I have assessed certified the fact, that just over planing speed is where max economy is realized. Realizing what you said about the engine efficiency and all, as you well know, you have to take the whole package into perspective. I'm going to leave this right here for now.

Mark

 

[Mi duckdown]

Re: what RPM do you cruise at

Hammer it and get on Plane. Then Back it off to stay on Plane. pretty simple.???