Gas consumption full power

[rsn48]

So I know I am asking for speculation, but please speculate. Boat speed is determined by power, hull construction, wind resistance, affects of wind and direction, current, wave action; this I know.

So my 1969 Mariner cabin cruiser (no one has heard of this boat, built in Vancouver, nothing on the net either), newly acquired, with Merc inboard stern drive with 270 HP's can push the boat up to roughly 11 knots but I've been told that the boat's most fuel efficient speed is around 7.5 knots. The chap I bought it from says the fuel burn is roughly 13 litres/hour pushing the 5 ton boat at 7.5 knots. He said get out your credit card if you want it to go faster, you'll be burning more fuel, all this I get.

But at 11 knots on a calm day, little current, no wind, yada yada, what should I expect for fuel consumption?

[ranger58sb]

No clue, but things you can learn in the meantime...

Which engines, exactly, and what's rated RPMs at wide open throttle (WOT.) See also if you can find nominal rated prop curves, nominal performance curves, nominal fuel consumption curves on line somewhere...

Once you know that, see if the boat will actually reach WOT for a few minutes (decent way to check whether you're propped correctly).

Then try running at approx 80% of WOT, see what RPMs/speeds you get. (Maybe a 2-way test, if you've got current or tide to deal with, then average.)

From that, you might at least be able to compare RPMs/speeds to nominal fuel consumption.

Eventually, take a trip with full tanks, fill up afterwards, do the math.

In the meantime, your best fuel consumption will likely be at 1.0 * SQRT(WLL) knots where WLL is your waterline length. Usually any increase in RPMs will mean an increase in fuel consumption relative to speed, so a decrease in NMPG.

-Chris

[a64pilot]

Larger four stroke outboards pretty much will use 1gl an hour per 10hp of the engine at full tilt.
So 15 an hour for a 150 and 30 an hour for a 300.
I’m betting your slightly more efficient, but not by a whole lot, my guess is about 25 gls an hour.

Actually the faster you go, the more fuel you burn period, just the curve of fuel over speed gets real steep above hull speed until you climb onto plane, it flattens out some then, but never comes down.
So your best fuel mileage is at idle, but who can live with that? Most can accept hull speed even though it’s more fuel, just above hull speed, your fuel consumption goes up astronomically.

[denverd0n]

Originally Posted by a64pilot

Actually the faster you go, the more fuel you burn period, just the curve of fuel over speed gets real steep above hull speed until you climb onto plane...

That, of course, is IF you ever actually fully climb onto plane. Knowing nothing about the boat except the fact that max speed, with 540 hp of engines pushing it along, is still only about 11 knots, my strong suspicion is that this is a boat that can only "semi-plane." It most likely will never see the efficiency benefits of getting fully into planing mode.


Of course, the other thing is that this all assumes a propeller that is proportioned properly. I don't think the OP's is. If it were, he should be able to get a whole lot more than 11 kts out of 570 hp at wide-open throttle. So the empirical approach, actually measuring things, suggested by ranger42c, is probably the only way to get into the neighborhood.

[Lepke]

Sounds like it's a single engine, otherwise it should plane. Every hull I've seen has either a bottom that would plane or has planing flats built into the bottom. My experience is bigger boats. After WWII a lot of people bought surplus wood aviation rescue boats (about 65') and installed surplus Detroit Diesels (about 165 hp)from landing craft. Flat out they could do 21 knots burning about 12+ gallons an hour, but still planing at 19 knots burned about 8-9 gallons an hour. Planing is the solution to go faster (in a short boat) somewhat economically.

I have a 16' Bayliner with a single 4 cylinder Volvo gas engine that burns 3 gallons an hour flat out, planing at somewhere above 25 knots. But it's much lighter than a cabin cruiser.

[Cheechako]

It sounds like boat the doesn't plane, which is a bit strange for a boat with 270 HP. . If that's the case I see no point in using all that HP. You are operating as a displacement hull. A 29 ft sailboat will have maybe 30 HP and only burn 1/2 gallon of diesel an hour at hull speed.
At 11 knots or so you are pushing a huge bow wave to get up on plane but not getting there. Very inefficient.
You hull is maybe 7.2 knots hull speed. If you operate about there, you will be most efficient and only really need maybe 25 HP to do so.

[rsn48]

1. The boat is single stern drive so max 270 Merc horsepower.
2. I do know the prop was swapped out, he said more power at lower revs, great for fuel economy, not so great for speed.
3. I don't think this boat has ever planed.

This link will only last for about another week from today's date (yes I know the galley is ugly, already have bought a 2 burner force 10 propane stove for it). In the link, you can go through the slide show and you will see the leg, engine, etc.

https://www.facebook.com/marketplace...22498835044761

[denverd0n]

Okay. My mistake. I read it as twin engines, 270 hp each, rather than as a single.


I still think empirical testing is the only way to get a meaningful answer, but a ballpark guess (and I'm talkin' a ballpark the size of Coors Field!) would be in the 11-12 gph range (40-45 lph).


Yeah, the gas consumption really does get THAT MUCH worse when you push the speed. These kinds of boats really aren't made to go that fast.

[a64pilot]

He has overpropped the boat in an attempt to get better fuel mileage, this works to an extent, but if you run it at high power for very long, it will likely damage the engine.

Think about a modern automobile being locked into it's highest gear, cause that is what over propping is.

If you want to go fast you need the engine to reach Max continuous RPM at full throttle, or at least within the range that the engine manufacturer recommends.

Many, many people overprop, and it works as long as you realize it's limitations and stay within them.

[Bill Seal]

Originally Posted by Lepke

Sounds like it's a single engine, otherwise it should plane. Every hull I've seen has either a bottom that would plane or has planing flats built into the bottom. My experience is bigger boats. After WWII a lot of people bought surplus wood aviation rescue boats (about 65') and installed surplus Detroit Diesels (about 165 hp)from landing craft. Flat out they could do 21 knots burning about 12+ gallons an hour, but still planing at 19 knots burned about 8-9 gallons an hour. Planing is the solution to go faster (in a short boat) somewhat economically.

I have a 16' Bayliner with a single 4 cylinder Volvo gas engine that burns 3 gallons an hour flat out, planing at somewhere above 25 knots. But it's much lighter than a cabin cruiser.

But then again, that 83 footer of yours in USCG trim sucked up over 100 GPH of avgas while blasting along at a blazing 17.5 kts.

[rsn48]

I do know the current owner never took it above 8 kn except to demo for me. He wanted the lower gas consumption and not the speed. He said based on his calculations you can almost get 40 hours with the dual 50 gallon tanks (100 gallons total). If a storm blows in I'll say "to hell with the fuel savings" and go for "any port in a storm."

The great thing about where I'm moored is that I'm close to Desolation Sound and Jarvis Inlet, lots of areas to play. The last time I was in Desolation Sound was in 1974 as a young Canadian Navy Officer, by that point my "new" boat was 5 years old...lol!

One last question, again with the link above that will disappear shortly, if you scroll through you'll see a pic of the motor which looks incredibly clean and if you scroll further you'll see the motor has 366 hours on it. For a boat built in and launched in 1969 that number is almost impossible. Another speculation, does the motor look newer, it does to me.

[parkstone bay]

To be pedantic, the Hours Run meter has 366 on it. You don't know what the engine has done (unless you have some other evidence you haven't shared with us). I suggest either the Hours Run meter is not original, or the engine isn't original, or, wow, neither of them is original! Weekending a little MB, we clock 50 hours max per year. Other people probably do more. But you'd have to be very mean to do as little as your instruments show.

[David M]

To double the speed of a non-planing vessel one must cube the fuel consumption per mile...yes, cube.

So to go from 7.5 knots to 11 knots will be a huge difference in fuel consumption.

Overpropping an engine is a really bad idea. Overpropping meaning the boats engine can't reach it's governed speed with a healthy engine, prop in good condition and a clean hull.

Eventually you will cause engine damage which is going to cause you a lot more than you ever thought you were saving in fuel costs.

It's not the same thing as having a 5th gear in a car. The physics are different.

[rsn48]

The boat is going in for a retro fit (renovation) of the interior and will be on the hard from November to early March 2019. While in, the correct prop for this motor will be installed, the engine gone over and usual maintenance performed, and the hull bottom cleaned and re-coated. It will be under cover for all of this so it will be out of the usual rains that occur along coastal BC.

[Muzza61]

From a recent delivery of a 60ft powerboat - top speed 34 knots, advertised cruise speed 24 knots, here's an example of its fuel burn. Boat had twin Volvo D13-900.

We were keen to stretch the fuel, aiming for low consumption, so knew we wouldn't get to experience any of these advertised figures - our longest leg was 800nm - hence we didn't bother testing any faster!



I can only imagine what it was at full power!