Cruising Speed vs WOT RPMs

[jmbfin]

Which is more important in selecting a prop, cruising speed or higher WOT RPMs? Should I sacrifice cruising speed for higher WOT RPMs?

My Yamaha F115 outboard has a recommended max range of 5000 to 6000. Here are the results of my recent test. These are all 4-blade props.

Solas Aluminum 17" pitch:
cruising 4000 rpm @ 25 mph
WOT 5600 rpm @ 39 mph

Powertech SS 17" pitch:
cruising 4000 rpm @ 29 mph
WOT 5300 rpm @ 40 mph

Powertech SS 15" pitch:
cruising 4000 rpm @ 25 mph
WOT 5800 rpm @ 40 mph

A 4-blade prop was recommended because of the weight of my Sailfish 1900 Bay Boat with a full tank of fuel, 2 adults, and F115 Yamaha is about 3000 lbs. The only Yamaha Performance Bulletins I was able to find for the F115 on a similar boat and weight used 3 blade props.

Would it benefit me to use a 3-blade prop to try to increase WOT rpms to 6000? How would performance likely be affected coming from a 4-blade?

 

[imported_Curmudgeon]

Re: Cruising Speed vs WOT RPMs

Powertech SS 15" pitch:
cruising 4000 rpm @ 25 mph
WOT 5800 rpm @ 40 mph


I don't know what 4 blades do over 3 blades, but your RPM is about where you want it unless the readings were taken at extremely lite weight. The name of the game is to prop for WOT and take whatever cruise you get (which is subjective, anyway).

 

[Crownie2]

Re: Cruising Speed vs WOT RPMs

Looks like the 15" Powertech is about perfect.

I guess the only way to know for sure about the 3 blade is to try one - say about 16"??

 

[72SideWinderSS]

Re: Cruising Speed vs WOT RPMs

What's your setup like?
Where is your A/V plate at in relation to the hull bottom (above or below)?

jimmy

 

[Ron G]

Re: Cruising Speed vs WOT RPMs

What's your setup like?
Where is your A/V plate at in relation to the hull bottom (above or below)?

jimmy

Very good question!and hows your W/P and trim?can you trim all the way up?by what you have given the 15 is right there.imo

 

[JUSTINTIME]

Re: Cruising Speed vs WOT RPMs

4 blade
u will get better hole shot and grip

3 blade more top end

 

[QC]

Re: Cruising Speed vs WOT RPMs

How did you select your cruise RPM? What do you do most? Cruise? Ski? Haul arse? If it was me, and the hole shot was acceptable, and your test load was typical, I'd want the SS 17". Should be better on fuel too.

 

[jmbfin]

Re: Cruising Speed vs WOT RPMs

1. The A/V plate is even or slightly above the bottom of the hull.

2. I can trim about 75% all the way up.

3. I selected my Cruise RPM from Yamaha Performance Bulletins for the F115. The motor seems to operate the most efficient at around 4000 RPM. I am not certain the motor is more economical with the 17" SS traveling 29mph or with the 15" SS traveling 25mph.

Because all the Yamaha Performance Bulletins were measured with a 17" 3-blade prop, I was wondering if my boat would perform better overall with a 3-Blade, either a 15" or 17" pitch.

Based upon my limited knowledge of props, I was thinking perhaps I could get a couple hundred more RPM and maybe 2 or 3 mph more top end with a 3-Blade. I would consider changing if that were possible as long as my cruising speed did not drop below the 25 mph.

 

[QC]

Re: Cruising Speed vs WOT RPMs

I am not certain the motor is more economical with the 17" SS traveling 29mph or with the 15" SS traveling 25mph.

From almost all test data that I have seen, it would be most economical at 25 MPH. I guess my thought was it could be even more economical with the 17" @ 25 MPH . . . With that said, we are probably debating about less than $5 a weekend :} 8) I guess then, I'd probably go with the best performance.

 

[Silvertip]

Re: Cruising Speed vs WOT RPMs

I get a kick out of the cruise vs wot discussions as they relate to fuel economy. While the engine definitely burns less fuel at cruise, it also takes you longer to get where you are going. My boat burns exactly 7.5 g/hr at wide open throttle as measured by a fuel flow monitor and at that burn rate I'm a tick under 40 MPH. So in an hour I go 40 miles and burn 7.5 gallons of fuel. I can throttle back to about 4000 - 4200 RPM and burn 5 - 5.5 g/hr. But instead of going 40 MPH I am now going around 30 MPH. So after an hour I still need to travel another 10 miles to my destination. That 10 miles requires another 1.38 gallons of fuel. So the total fuel required to reach my destination at cruise is 5.5 + 1.38 = 6.88 gallons. That 0.62 gallon savings is not a major consideration for me. If you are running 250 HP engine running very long distances that may be of more importance. If you aren't in a hurry, run at cruise and enjoy the ride. If time is of importance, run wide open and the difference in fuel consumption becomes a moot point.

 

[Crownie2]

Re: Cruising Speed vs WOT RPMs

Silvertip - as you noted, just because your small engine saves you only .62 gallons, cruise vs. wot doesn't mean that holds true for all boat/engine combinations.:^

Also, cruising for most people is very enjoyable due to the noise and stress reduction, and the reduced wear and tear is a big factor for many boat owners, as well.

Just my opinion...

Bob

 

[Silvertip]

Re: Cruising Speed vs WOT RPMs

Didn't I say that? If you aren't in a hurry, there is no need to run wide open -- after all that's why boats have a throttle. I'm a fisherman and run very long distances. I want to get from hole to hole in a hurry. My only point in this is that if you feel you will save a bunch of fuel getting from point "A" to point "B" by cruising rather than running wide open you will be disappointed at the actual difference. I just communicated with a gent that has two 120 HP engines on his verrrrry heavy boat. He burns 18 g/hr each at WOT so the 10% calculation doesn't apply in his case but it does point out a problem. This is an example where bigger engines loafing would be more economical than small engines working their guts out. Twin 200's would burn about 20 g/hr at WOT but at reduced throttle they would be in 15 g/hr range at the same speed the 120's are running at WOT.

 

[Ron G]

Re: Cruising Speed vs WOT RPMs

i havent said anything because 99% of the time its balls to the wall for me.dand i still cant go fast enoughdeven when im out just looking its running over 50 or 60mph the only time i go slow is at idal looking for stuff on the finder.dd
But the key is to prop and setup for wot rpms and the cruise factor falls were it be,in my opion.

 

[Silvertip]

Re: Cruising Speed vs WOT RPMs

I guess at least the two of us agree Ron! "Crusing is nifty but wide open is nearly as thrifty." d

 

[kenmyfam]

Re: Cruising Speed vs WOT RPMs

For me cruising rpm is a very pleasant ride with time for conversation and enjoying the ride. WOT is just as much fun but the "family time conversation" is always a lot more limited due to the noise factor. Bottom line is a few bucks difference doesn't bother us as long as we are enjoying our time out there.

 

[tommays]

Re: Cruising Speed vs WOT RPMs

RPM MPH Knots Total GPH MPG NMPG Range

3500 23.7 20.6 3.8 6.24 5.42 135 117
4000 28.1 24.4 5.2 5.39 4.69 117 101
4500 32.3 28.1 6.6 4.93 4.29 107 93
5000 36.2 31.4 8.5 4.25 3.70 92 80
5500 39.8 34.6 10.3 3.88 3.38 84 73
5850 42.6 37.0 11.0 3.84 3.34 83 72



Every test i read shows a BIG difference in your wallet

Tommays

 

[Silvertip]

Re: Cruising Speed vs WOT RPMs

Most of those tests show reduced fuel burn at lower rpm. That's different than fuel consumption over a trip which is the point of this post. Something is dreadfully wrong in your chart. You show 6.24 g/hr @ 3500 RPM yet at 5850 you show 3.84 g/hr. Isn't that interesting that the engine burns less fuel wide open than it does at 3500 RPM. Ain't possible. What is the "Total" column. Total of what? I will assume the g/hr column is inverse from what you actually show. Since the speed at 3500 rpm and at 5850 rpm are almost double or half of each other this makes it easy to show another example. Problem: You take a 46 mile trip. At 3500 rpm your speed is 23.7 MPH so it takes you two hours. Your burn rate is 3.84 g/hr but it takes two hours to reach your destination so you burned 7.68 gallons. If your ran wide open at 42.6 mph you'd make the trip in just a tick over an hour. At that speed you would burn 6.24 gallons. So now where is the savings? This is even worse if your table is actually correct. Do the math.

 

[QC]

Re: Cruising Speed vs WOT RPMs

Silvertip,

Reread the data, the second #s are MPG and Nautical MPG they do indeed go down as speed goes up which actually disproves your original point . . . MPG is roughly the combination of GPH and distance traveled over that same hour. It is absolutely the ONLY way to calculate your best range and your lowest fuel operating cost point. Oh, and if that chart went all of the way down to idle speed you'd find that usually, just barely off of idle is a planing hull's best MPG. Which again translates to the lowest operating cost over a given distance.

 

[Silvertip]

Re: Cruising Speed vs WOT RPMs

OK -- you still didn't answer my question. What is the "total" column. I see eight columns of data. Column 5 is titled g/hr.
Are you actually claiming that the engine in this example burns more fuel at 3500 rpm (6.2 g/hr) than it does at 5850 (3.84 g/hr)? If that's true, then either you need a new flow meter or i want to know what engine you have so I can get one. And I also take issue with the statement at MPG or NMPG are the absolute only way to measure your fuel economy and cost. The airline industry measures their fuel use in pounds/hour or gallons/hour distance per gallon of fuel is not a constant. Time to destination, burn rate and fuel remaining is far more meaningful. MPG in a boat is way to variable depending on sea and weather conditions. I can turn total fuel burned over time into any arguement you wish. Say you have a 40 gallon tank. At 23 MPH you are using 3.8 g/hr. 40/3.8=10.5 hrs of run time. 10.5 x23 = 242 mile range.
At 43 mph you are using 6.8 g/hr. 40/6.8=5.8 hours of run time. 5.8 x 43 = 252 mile range. Show me the saving? Something is still wrong with your chart.

 

[Silvertip]

Re: Cruising Speed vs WOT RPMs

Here are two snapshots of fuel economy from Yamaha's performance tests for a 90 HP engine and 70 HP engine.

90 HP:
1000 RPM -- MPH = 3.8, GPH = 0.9 MPG = 4.22
4000 RPM -- MPH = 26.9, GPH = 5.0 MPG = 5.38 (best)
5550 RPM -- MPH = 38.2, GPH = 8.6 MPG = 4.44
As this shows, 50% increase in speed results in only .9 MPG increase in fuel consumption.

70 HP
1000 RPM -- MPH = 4.1, GPH = 0.6 MPG = 6.83
4000 RPM -- MPH = 25.6, GPH = 3.8 MPG = 6.74 (best)
5600 RPM -- MPH = 38.6, GPH = 7.1 MPG = 5.44

As this shows, 50% increase in speed results in a 1.3 MPG increase in fuel consumption so the small engine loses as it has to work harder. And again, the burn rate for both engines is roughly 10% of hp at WOT. Run all the calculations you want, but if the difference in dollars and cents gives one major heartburn one should probably not own a boat. I have not seen charts for very large vessels nor am I interested in them so I won't get into that discussion.

As I said: "Cruising is nifty but wide open is nearly as thrifty"