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Author Topic:   Revenge 25 Walk Through Fuel "Economy" (chuckle)
Tom W Clark posted 10-23-2005 12:57 AM ET (US)   Profile for Tom W Clark   Send Email to Tom W Clark  
I have some data from my 1988 Revenge 25 Walk Through relating to the fuel consumption. I thought I would toss this information out here for others to use as a point of reference.

I am working on organizing my information to start another thread about the various propellers I have been trying out, so I won't get into a discussion of propellers here.

My Revenge is powered by a pair of non-counter rotating 1989 Mercury 150s. Fairly standard issue original power. A photo of the boat can be seen here:

http://home.comcast.net/~tomwclark/Revenge25WT_at_LonesomeCove.jpg

Fuel consumption is monitored by a FloScan TwinScan fuel flow meter which I have calibrated over the last few months to amazing accuracy.

Speed is measured by a Furuno GPS with DGPS correction. Speed figures presented here are the averages of multiple observaions taken going opposite directions.

Yesterday I was out testing by myself with little gear on board. Today I made a round trip to Port Townsend, WA from Seattle with four friends on board, basic gear plus a small ice chest (full) and numerous bags & purses (my four passengers were all women). I started today with 58 gallons of fuel on board. The full Mills canvas was set both days.

Our transient moorage destination was Point Hudson Marina in Port Townsend (a VERY nice little marina):

http://www.portofpt.com/point_hudson.htm

The weather both days was nice, mild with wind speeds near calm yesterday and from 0 to 12 knots and wind waves of 1-1/2 feet or less today.

Here are yesterday's numbers:

RPM -- MPH - GPH - MPG (Note: GPH is both motors combined)

1000 - 6.1 -- 5.0 - 1.22
1500 - 8.4 -- 6.0 - 1.40
2000 - 11.3 - 8.4 - 1.35
2500 - unable to get both motors to run at this speed
3000 - 23.9 - 13.0 - 1.84
3500 - 28.8 - 14.6 - 1.97
4000 - 33.0 - 15.6 - 2.12
4500 - 36.8 - 17.0 - 2.16
5000 - 41.0 - 22.4 - 2.01
5700 - 46.8 - 32.0 - 1.46

One of the surprising things I have come to learn is that the most fuel efficient speed for my boat is much faster than I would have thought. It turns out that it gets better gas mileage if it is run at 35 mph instead of 25 mph.

This can be a problem if the water is anything less than very calm. 35 mph is not all that comfortable is the chop is even one foot. For comfort, I often run the boat at an engine speed of 3150 RPMs which is about as slow as the motors will run and still easily keep the boat on plane.

At this engine speed the boat will make 24-25 mph and the engine noise is relatively low with a good "easy" sound to it. It just sounds and feels right. Unfortunately at this speed the fuel mileage suffers and seems to amount to about 1.8 MPG.

If I advance the throttles just a wee bit the motors really come alive and the boat speeds picks up dramatically, yet the needles on the fuel flow meter barely advance.

I guess the lesson here these old Mercs like to run fast, which has always been the Mercury outboard reputation.

To compare, today's cruise figures (incomplete) are:

RPM -- MPH - GPH - MPG

3610 - 29.9 - 15.0 - 1.99
4000 - 32.9 - 15.4 - 2.14
4500 - 37.2 - 17.4 - 2.14

Even with four addition persons, more fuel and more gear the performance is not changed much at all. The only thing I can think of that the wind was greater today and (luckily) was always going with us (we benefitted from an afternoon wind direction change.) More light chop and ripples means more air under the hull and less resistance.

I am curious what other owners of the Revenge 25 are getting for fuel consumption figures.

Bulldog posted 10-23-2005 07:38 AM ET (US)     Profile for Bulldog  Send Email to Bulldog     
Tom , my little 20' Revenge with twin 1987 70HP Yamahas gets around 2.5 MPG, with the sweet spot of noise, speed (25MPH), and "feel" being 3900RPM..........Jack
jimh posted 10-23-2005 11:53 AM ET (US)     Profile for jimh  Send Email to jimh     
Tom--I feel your pain, albeit more severly, perhaps, because 2.1-MPG is all I can coax from my single 225-HP two-stroke.

Eagleman (Russ) should be a good source of comparison. He as a REVENGE 25 WT WD with twin Mercury motors.

Tom W Clark posted 10-23-2005 12:17 PM ET (US)     Profile for Tom W Clark  Send Email to Tom W Clark     
I forgot to include the most surprising bit of data, the single engine performance figures.

While testing new propellers Friday I decided to compare one motor to the other and ran each with the other feathered.

The maximum RPM for my Mercury 150s is 5500 RPM which I am able to reach at WOT with my 19" pitch props.

Running on just one motor, I can reach 5200 RPM and a top speed of 33 MPH, a fuel burn rate of 10 GPH for an amazing mileage of 3.3 MPG.

Does this surprise anybody? It sure surprised me.

Peter posted 10-23-2005 12:47 PM ET (US)     Profile for Peter  Send Email to Peter     
Yes, 10 GPH at WOT at 5200 RPM does surprise me when the pair consume 22.4 GPH at 5000 RPM.
Tom W Clark posted 10-23-2005 08:28 PM ET (US)     Profile for Tom W Clark  Send Email to Tom W Clark     
Peter,

Oops. I've made a mistake in reading my notes. 10 GPH at 4500 RPM with a speed of 25 MPH. This was true of both motors. Still, it comes to 2.5 MPG which is significantly better than ANY speed with both motors.

There is a school of thought around here (to which I have subscribed) that says having extra horsepower which works less hard will yield better fuel economy and longer engine life. While it may be true of engine life, it does not seem to hold for the fuel economy in my case.

I am still very surprised.

jimh posted 10-24-2005 11:00 AM ET (US)     Profile for jimh  Send Email to jimh     
Tom--that is very good information. In the past most of the information about fuel consumption of twin engines was anecdotal and depended on rough estimations of fuel economy.
Peter posted 10-24-2005 11:29 AM ET (US)     Profile for Peter  Send Email to Peter     
With respect to the maximum efficiency at higher than expected speed, I wonder whether that is in part setup related -- that is with respect to motor alignment, height and propeller type. The fixed toe-in/out alignment of the motors can't be ideal for all boat speeds. Except for some optimal speed where the motors are aligned best with the flow of the water under the boat, the twin motors are at a disadvantage relative to a single motor which is, in theory, optimally aligned at all speeds.

What I would like to see is the fuel consumption report of a single Mercury 225 or 250 HP (carbureted) at 25 MPH probably turning 3500 to 3700 RPM vs. the single 150 turning 4500 RPM to achieve the same 25 MPH. My guess is that a single 225/250 would be burning less than 10 GPH to go 25 MPH because its would be running in the 3000 to 4000 RPM band which is usually the most efficient zone for a 2-stroke V6.

andygere posted 10-24-2005 11:35 AM ET (US)     Profile for andygere  Send Email to andygere     
Tom,
Interesting data and a great photo of the boat. GPH, MPG,RPM, blah, blah, blah. What we really want to know is how you managed to arrange to have 4 women as your crew!

All kidding aside, it's interesting to note that your fairly heavy Revenge will not just plane out on a single 150, but run 25 MPH while loafing along at 4500 RPMs. Even more interesting is the great fuel economy. Are your 150's built on the 2.4 liter block, or do they have less displacement? The single engine data certainly shoots a few holes in the "power it to the max" school of thought, and seems to suggest that even Whaler's best offshore hulls are easily driven and don't require the obscene power that seems popular in new boats of the same size. It is certainly food for thought on my own repower decision.

Bthom posted 10-24-2005 11:47 AM ET (US)     Profile for Bthom  Send Email to Bthom     
Tom,
That is most excellent information and very interesting data about the single engine performance. I'm sure it will add fuel to the single/twin debate that is forever ongoing.
I can't add much useful data as I don't have accurate fuel metering equipment aboard, but I can say that my boat, a 1989 25' Revenge Walk Thru with Whaler Drive, powered by 150 HP counter rotating Black Max outboards,gets better fuel economy and handles best at 4200 to 4400 rpm, depending on load and weather conditions.
My GPS indicated speed is 32-33 mph at that rpm,climbing to 35 mph when the boat is light, as in 1/2 tank of fuel.
The slowest I can go and stay on plane is 3500 rpm where my speed is 22 mph, but that is always in confused seas.
My top rpm is 5500 and my speed(gps) is 44 mph.
These numbers are with all standard salmon and bottom fishing gear on board, as well as a generator and an 8 cu. ft. freezer,and at least 2 adults on board.
I am also packing an aluminum hardtop with a lot of glass in it, to add to the weight penalty.
This will give you some performance figures to compare yours with, if not fuel data, but it would seem our boats are pretty comparable.
Hope it helps,
Brian
Peter posted 10-24-2005 12:34 PM ET (US)     Profile for Peter  Send Email to Peter     
Andy, careful... The "power to the max (or near max)" rule still applies. This data actually is good to illustrate the point.

That poor old 150 at 4500 RPM isn't going to last nearly as long as a larger 225 that would only have to turn say 3700 RPM to go the same 25 MPH speed. For each hour at a 25 MPH cruising speed, the 150 will turn the flywheel 48,000 times more than a bigger motor. Each piston ring under load in the 150 will have have traversed 48,000 times more over its same path in that one hour. More loaded piston ring travel means more wear per hour. Even the gearcase is undergoing greater wear because it has to turn faster to keep up. Greater wear per hour means repower time comes sooner.

Also something to think about with respect to 2-strokes is that typically, with variable ratio oiling systems, the oil consumption goes up proportionally as the engine speed increases. I would expect a 225 loafing along at 3700 RPM to use less oil than a 150 turning 4500 to go the same speed.



JMARTIN posted 10-24-2005 01:02 PM ET (US)     Profile for JMARTIN  Send Email to JMARTIN     
Interesting information especially on the figures from running one engine. The differences between 150 hp and 200 hp is more than I realized. I can coax 2.3 MPG out of my 1992 OMC 200hp Looper on a 1983 V-22 Revenge notched transom. As predicted it is at 3500 rpm about 26 mph burning 11.3 gph. If Tom and I were running together and we both wanted to maxamize our fuel economy, he would be going 10 mph faster than I would. I do not know what my fuel economy would be if I tried to keep up. Next time out I will try to remember to make a chart. Tom, is that a public dock at Lonesome Cove? I did not know where Lonesome Cove was, but I reconized Spieden and found it. John
elaelap posted 10-24-2005 01:59 PM ET (US)     Profile for elaelap  Send Email to elaelap     
Taking Peter's comments into consideration about increased engine wear at higher turns, it still seems worth some analysis to determine whether or not to cruise while only using one motor.

I frequently 'deckhand' on a 26 ft Thunderbird 6-Pak with twin 4-cylinder Volvo 4/s I/Os, and the boat's skipper alternates use between the engines as often as possible while trolling and while moving from place to place where the distances are not great. He needs both motors to get up onto plane and stay there, however, so cruising any distance at all would be impracticable for him on only one motor.

Tom's situation is very different. Seems to me that one reasonable way to do it on a fairly long trip in calm weather would be to get quickly up onto plane with both motors, then shut one down and cruise with the other. Wouldn't alternate motor use and prolong engine life--notwithstanding their use at higher revs--by putting far fewer hours on each motor than if both were always in use together.

Tom--You didn't mention whether you tested tilting the non-used motor up when you're running with a single motor...I assume not since you discuss 'feathering' the other motor's prop. Might not tilting up one motor improve single engine performance even more?

Tony

Tom W Clark posted 10-24-2005 02:56 PM ET (US)     Profile for Tom W Clark  Send Email to Tom W Clark     
Peter,

The toe-in angle of the motors is not a disadvantage for the twins. Remember, when I was operating one motor, that motor is 15 inches off-center! I guarantee you it FAR less aligned with the water flow than the twins when they are running.

Andy,

I wouldn't exactly say the single motors were "loafing" at 4500 RPM but I don't necessarily agree with Peter that a 225 at 3700 is loafing either. Based on the 10 GPH fuel flow it seems the 150s were operating at a power level (throttle position) about equal to about 5000 RPM is they were running together. That is approaching full throttle.

But still, I think it is very impressive single engine performance.

Peter,

I can only partially agree with you about more power being better from an economic point of view. While you are correct that a smaller motor turning faster will see great travel of the pistons, rings, etc., very few outboard motors EVER truly wear out. Usually something catastrophic happens or they just get old and we want new ones with more bells and whistles.

I have owned something like 16 different outboards in my life. I have yet to wear one out. A larger motor will also cost more to buy and maintain so even if if has a longer life span on a given boat, the cost of ownership will not necessarily be less and it could well be more.

John,

Good eye. Most people wouldn't recognize Speiden Island or perceive that as the San Juan Islands. That photo was taken at Lonesome Cove Resort http://www.lonesomecove.com/

My brother took the boat up there in July. His family and some friends take a cabin or two there every summer. It is one of those places where you make your reservation a year or two in advance.

Tony,

When I say "feathered" I mean I shut the motor down and tilted it clear of the water. I do not think the boat would plane if it were dragging a "dead" gearcase around.

It is very difficult to get the boat up on plane with one motor. You must accelerate very gently or the prop will ventilate violently. In fact it is harder to do this with the starboard motor than the port motor. This puzzles me.

At any rate once the boat is on plane it accelerates well. Interestingly, the props were continuously ventilating which is how I was able to get the RPMs as high as they were.

It is something of a balancing act to keep the motor trimmed and the RPMs under control. They wanted to race ahead and then the prop would catch and the RPMs would drop. I would not want to HAVE to cover any serious ground like this. It is a lot of work.

At the 5200 RPM single engine WOT speed, I calculated the propeller slip was something like 30 percent! But this is good in that it acts like a variable pitch prop or automatic transmission.

Peter posted 10-24-2005 02:58 PM ET (US)     Profile for Peter  Send Email to Peter     
Q: "Wouldn't alternate motor use and prolong engine life--notwithstanding their use at higher revs--by putting far fewer hours on each motor than if both were always in use together."

A: No. The reasons are simple.

First, the two engines are optimally propped as twins, not singles. That means they run with a higher pitch than they would if there was only one of them on the transom. Thus, to run on plane with only one of the two engines causes that running engine to be severely overpropped. Not good for the engine over the long haul as puts even more stress at a higher RPM on the motor than would be the case just running at a higher RPM with the correct prop pitch. Thus to run routinely on a single would really require the ability to drop the pitch of the propeller on the fly.

Second, twin engine boats runing on a single engine that is offset from the center line do not handle very well on plane as compared to when they are running with both engines. They usually require quite a bit of correction from the trim tab on the opposite side of the boat because the stern of the boat is not being "lifted" by the operation of the other motor.

For at least these two reasons, you don't see people doing what is proposed by the question. I've often considered taking very low pitch propellers, like 15 or 13P, along on long trips just in case I have to make an extended run home or to some repair port on a single engine.

Peter posted 10-24-2005 03:17 PM ET (US)     Profile for Peter  Send Email to Peter     
Tom -- I've seen many repowers of underpowered boats caused by the motor basically wearing out or failing prematurely. I've seen many repowers of max powered boats caused by the motors simply corroding apart. That's the trend lines I see around here. I too have never worn a motor out but that is because they've always been right at the max for the boat.

Your motors are not counter-rotating if I recall correctly. If so, that is why one is easier to launch than the other. Use of trim tabs on the inoperative motor side helps with boat balance and launching on a single.

With respect to efficiency, toe-in angle does matter because the angle of the water flowing off the bottom of the boat varies with speed. I guarantee you that the drag is higher at most speeds on twins than it is on a single because the two engines are not perfectly aligned with the flow coming off the bottom of the boat accept at a certain speed, whatever it is for the angle between the two motors.

Take some time and play around with the toe-in angle and you should see that a 1/4 inch difference in toe-in angle (about as much as you can vary them without cutting the tie bar) makes a difference in fuel economy, at least it does on a 27 Whaler WD. I have my toe-in optimized for the cruising speed I usually do based on my Navman 3100 readings.

elaelap posted 10-24-2005 03:20 PM ET (US)     Profile for elaelap  Send Email to elaelap     
Gotcha, guys...thanks. Interesting fuel useage figures, Tom. With gas prices the way they are, I'd be spending more than $150 per week on gas...ouch. Another reason to stick with my 'little' 'underpowered' OR 18 skiff ;-).

Tony

Tom W Clark posted 10-24-2005 03:45 PM ET (US)     Profile for Tom W Clark  Send Email to Tom W Clark     
Peter,

Regarding Toe-in angle, I disagree with you that a twin engine boat operating on a single motor will have its gearcase operating more in-line with the water flow and thus more efficiently.

Because the thrust is grossly offset, the single operating motor must be turned away from the flow of water at a MUCH greater amount than the less-then-perfect toe angle of twins in order to counter the (now) asymmetrical load.

Remember, the toe-in we use on our twins is there to align the gearcases with the water flow, not too misalign them. That's the whole point. Water does not flow out from under a V-hulled boat in straight line. So even if at one speed the gearcase is slightly less than perfectly aligned, the deviation is tiny.

Whaler recommends 1/2"- 3/4" toe-in for twins. When I bought my boat I noticed a tendency for the boat to "hunt" a bit when traveling parallel to waves int he trough. I suspected the toe angle was not right.

I measured the toe-in as 1/8" (propeller o.c. distance 1/8" greater than the distance between the leading edge of the gearcases).

When I increased the toe-in to 5/8" the handling was perceptibly better, but I don't think for a second the fuel mileage would be measurably increased by moving the props laterally 3/16". It's not that radical of a change.

Regarding one motor getting my boat easier than the other: Yes, I agree that it is my non-counter rotating motors that is the cause of this. Each prop is turning in a different direction relative to the V of the hull. What puzzles me is that it is the starboard motor that has the trouble.

On a counter rotation pair it it (usually) the port motor that receives the counter rotation motor. But my experience suggests it should be the other way around if having the blades travel towards the centerline in the upper half of their arc results in amore efficient "grip" on the water.

Peter posted 10-24-2005 04:24 PM ET (US)     Profile for Peter  Send Email to Peter     
Sorry Tom, I think our arguments are a little askew here and we may be actually in agreement. My point was with respect to a single, centered engine which should not have any alignment factors with respect to efficiency. In other words, the motor is always aligned well with the flow of water at all speeds whereas that is not the case with a twin because the angle of the water coming off the bottom varies with speed. At some speed the twins are best aligned with the water flow and at other speeds, not so.

Both of my motors are somewhat difficult to launch alone and take a careful play of the throttle and tabs to get the boat on plane. The propellers rotating towards the near side of the boat lifts the near side out. Your starboard motor is lifting the starboard side. Your port motor is still trying to "lift" the starboard side too but there is more boat to lift between it and the starboard side.

Tom W Clark posted 10-24-2005 04:40 PM ET (US)     Profile for Tom W Clark  Send Email to Tom W Clark     
Peter,

I hear what you are saying but I still maintain that the very slight out-of-alignment of a gearcase relative to the flow of water is negligible in terms of speed and efficiency. Handling, yes, certainly but not speed and fuel economy. I just don't think it a significant amount of drag.

A single engine boat's gearcase is *not* always in alignment with the flow of water. Water conditions are always changing and, for example, any cross wind at all will result in the gearcase being turned to one side or the other with a resulting misalignment to the water flow relative to the gearcase.

Even operating in a windless conditions, the gearcase of a single engine boat is its rudder and is thus constantly being turned away for the water flow.

We're getting a little off track here. My original surprise was at the improved fuel mileage of my boat operating with only one motor in spite of the fact the motor is off center and operating with a less than perfectly pitched prop for single engine operation, not mention the added weight of a motor that is not being used.

The conclusion I reach is that we pay a significant toll from the drag of an extra gearcase with a twin engine powered boat. This is something to be considered by anybody contemplating the twin vs. single decision.

Plotman posted 10-25-2005 01:25 PM ET (US)     Profile for Plotman  Send Email to Plotman     
Tom -

Interesting data. I'll have to check single engine economy on my boat compared to what I get with both running.

Your data do seem to fly in the face of the performance data that whaler posts on their website for the boats where the offer both single and twin engine configurations. When making comparisons between engines of the same technology (Optimax vs Optimax and Verado vs Verado) in all cases I looked at, the twin-engine set up gave better mileage than the singe engine.

On the 240 Outrage:
Twin 150 Optimax - best mileage 2.86 mpg
Single 250 Optimax - best mileage 2.36 mpg

Twin 150 Verado - best = 2.71 mpg
Twin 175 Verado - best = 3.13 mpg
Single 250 Verado - best = 2.60
Single 275 Verado - best = 2.38


On the 255 Conquest:
Twin 150 Optimax - best mileage = 2.43 mpg
Single 250 Optimax ( no MPG data)

Twin 150 Verado - best = 2.29 MPG
Twin 175 Verado - best = 2.47 MPG
Single 250 Verado - best = 2.36
Single 275 Verado - best = 2.45

David

Tom W Clark posted 10-25-2005 06:25 PM ET (US)     Profile for Tom W Clark  Send Email to Tom W Clark     
David,

That's not quite right. The data you've provided seems to be mixed. The single 275 Verado power on the Conquest seems to equal to or better than the twin power. Either a 250 Verado or a 275 Verado do better than twin 150 Verados, but the difference is negligible.

I am under no illusion that my 17 year old carburetor motors are Verado power. I am however, very gratified to see that my boat's fuel economy is not really all that much worse than the comparably sized 240 Outrage with twin 150 Verado power.

Contender25 posted 10-27-2005 03:57 PM ET (US)     Profile for Contender25  Send Email to Contender25     
Thanks for the detailed information

I am amazed at those numbers

Tom W Clark posted 10-27-2005 06:51 PM ET (US)     Profile for Tom W Clark  Send Email to Tom W Clark     
Jason,

Amazed at exactly which numbers? Why?

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