Which is more efficient?

[av8erdunn]

When it comes to boating I enjoy just putting along at 6-8 knots as I have done in our sailboat for years. We have recently purchased a 2004 Chaparrall 290 Signature with twin Volvo Penta 4.3 for a change in pace. I was wondering if its more efficent to run these engines at 6-8 knots rather than being up on plane around 20? Would running one engine at 6-8 knots be more efficent than either of the above mentioned methods? It doesnt matter either way, I just dont want to be consuming 2-3x the fuel poking along if it makes that much of a difference in economy.

 

[Mischief Managed]

Find the length of the boat at the water line in feet. Find the square root of that number. Multiply that by 1.34 and that will give you the hull speed for your boat in knots. Run at that speed or lower, or cleanly up on plane and you will get the best efficiency. My boat (a planing hull like yours, but only 25 feet) is most efficient between 28 and 33 MPH with a typical load. It's almost as efficient at 4 MPH. Any other speeds and it's worse. 8 knots is HORRIBLE for fuel efficiency on my boat.

 

[southkogs]

My boat at idle / no wake (165HP inline 6) will drift along at about 5 MPH (by the GPS). It's not running as efficiently as up on plane around 22MPH, but it's not double the fuel usage. Making turns for 10MPH, I'm starting to push a buncha' water as the hull wants to start climbing up on plane ... and I am chewing through a lot of fuel in that scenario.

On almost any planing hull, typically they run most efficiently up on plane at the lower end of the RPMs required to hold 'em on plane. For example, I get up on plane, then drop the throttle back to a point where I'm starting to "slush" a little ... then re-apply a little more power back on and hold it (not sure the RPMs exactly - my tach isn't workin' right). That seems to be the most efficient run for me. But we also do a lot of "putting" along at no wake ('cuz that's what the Admiral prefers) and as long as keep it from trying to push a lot of water ... it's not too much more fuel (not double anyway).

 

[alldodge]

When it comes to boating I enjoy just putting along at 6-8 knots as I have done in our sailboat for years. We have recently purchased a 2004 Chaparrall 290 Signature with twin Volvo Penta 4.3 for a change in pace. I was wondering if its more efficent to run these engines at 6-8 knots rather than being up on plane around 20? Would running one engine at 6-8 knots be more efficent than either of the above mentioned methods? It doesnt matter either way, I just dont want to be consuming 2-3x the fuel poking along if it makes that much of a difference in economy.

Couldn't find a 290 which was tested at Boat Test but did find a 310. Looks like your best mileage is either 600rpm (3.6 mph) or 1000 rpm (5.6 mph), with a range of 212 miles. Best planning speed is around 3500 rpm (28.5 mph) with a range of 165 miles. Keep in mind running an engine at slow speeds all the time can also cause issues as carbon build up and the like.

 

[Starcraft5834]

Trolling speed for my 3.0 is about 2-3 mph.. uses very little fuel... purposing to go 6-8 knots would not be efficient... your better off at base speed (just tossed in gear) or up on plane... two most efficient modes..

 

[av8erdunn]

Thats exactly what I was looking for.
Thanks for the help.

Just noticed my title " Which is more efiiecent?"
LOL... guess I shoud use spell check more often.

 

[rallyart]

Also, running on one engine will be less fuel efficient than running on both, even at idle. You can actually here when the engine starts to work harder as you go from idle to fast idle to on power a bit. Around 6 or 7 mph in your boat you'll here the throttle noise increase without a speed increase and you'll see more turbulence at the transom.

 

[UncleWillie]

Your boat will have a Hull Speed of around 6 knots. Any faster will have the hull plowing.

Your closest comparable Chaparral, currently manufactured, is the twin engine 30 footer.
BoatTest "Test Results" shows the most efficient speed is at dead idle at 4.7 mph.
The next best speed is at 3000 rpm and 29.6 mph.

The worst speeds to run at are Wide open followed by just below plane at ~12 knots.

Running on one engine at idle will drop the fuel used by half but only lower the speed by 30% (~5mph becomes ~3-4mph)

A small sailboat running at 8 knots is likely plowing too fast for the hull and not making efficient use of its sail power.
But since the wind is free, no one cares.

 

[JoLin]

Everything these guys have said is correct, including the fact that you don't want to run at idle speed continuously. It isn't good for the motors. You can continue to putt along at 4-5 mph and use maybe a gallon of fuel per engne (tops) per hour. Chances are, though, once you've pushed it on plane to actually get somewhere you won't be putting along very often. Having the power to 'move along' at a 25 mph cruise is addictive.

My .02

 

[dingbat]

Fuel is the cheap part of boat ownership....

If you're worried about fuel efficiency, best take up another hobby....lol

 

[av8erdunn]

Fuel is the cheap part of boat ownership....

If you're worried about fuel efficiency, best take up another hobby....lol

I realize fuel is the cheap part and the cost isn't a problem. I was just curious as I didn't want to burn 2-3x more fuel putting vs being on plane.

You think boat ownership is expensive, buy an airplane and report back. Boats are cheap in comparison.

 

[southkogs]

You think boat ownership is expensive, buy an airplane and report back. Boats are cheap in comparison.

... that's why I've got a boat. Even the glider tows are getting expensive! :faint2:

 

[UncleWillie]

I realize fuel is the cheap part and the cost isn't a problem. I was just curious as I didn't want to burn 2-3x more fuel putting vs being on plane.
You think boat ownership is expensive, buy an airplane and report back. Boats are cheap in comparison.

+1
I decided to get a boat once I figured that an aircraft that carried 4 would cost $100 per hour, and a boat that carried 8 would cost $50 a day!

Assuming that idle speed is the most effecient....
Cruise speed at ~3200 rpm will burn ~7 times more fuel per hour while traveling about 6 times faster. Your MPG will be about 90% of idle speed.
You need to decide if spending more time on the water is more important than traveling more distance.
The fuel used per mile varies well less than 2:1 no matter what. The fuel used per hour can vary nearly 20:1 from idle to Wide Open.

 

[av8erdunn]

+1
I decided to get a boat once I figured that an aircraft that carried 4 would cost $100 per hour, and a boat that carried 8 would cost $50 a day!

Assuming that idle speed is the most effecient....
Cruise speed at ~3200 rpm will burn ~7 times more fuel per hour while traveling about 6 times faster. Your MPG will be about 90% of idle speed.
You need to decide if spending more time on the water is more important than traveling more distance.
The fuel used per mile varies well less than 2:1 no matter what. The fuel used per hour can vary nearly 20:1 from idle to Wide Open.

That's a good way to put it. I just enjoy being on the water and very seldom venture more than 20-30 miles from my marina. Most times we go about 6 miles away to a local hangout to spend the day. Then we go back and spend the nights at the marina.

 

[smokeonthewater]

fuel has always been the single highest expense for me boating.... 25 years ago in a 12' tinny with a '57 7.5 rude, at $.87/gallon and now with 2 900 cc jetskis, a 701 cc jet boat, and a 30' twin engine cruiser... I spend WAY WAY WAY more on fuel than anything else.

That said, in the cruiser I do much better on fuel with one engine running 1000 rpm or less than both turning 3000 planing at 30 mph plus the river really doesn't look any different 50 miles away.

 

[smokeonthewater]

let me add this.... in current and wind the figures go out the window.... idling against a current or strong wind makes for poor economy.... for best mpg the slowest engine speed down river and on plane up river...

I was once in the Ohio with a friend on his 38'er over flood stage... 14 mph current and we had to go 8 miles up river ... he was cruising at 3200 rpm and 17 mph ..... after an hour (and 3 miles of headway) he was getting well below my comfort level on fuel and I wasn't certain we'd make it.... running out very possible meant going over the falls so although I had previously made several (ignored) suggestions I had enough and I walked to the helm and pushed the engines to full throttle... he (inexperienced) objected but I firmly explained the situation.... we made the rest of the trip in 20 minutes instead of the nearly 2 hours it would have taken.... We used twice as much fuel/hr but for 1/5 the time... net result... 400% increase in MPG's and range.


Drastic example but it clearly demonstrates the point and hey it's a good story lol


FWIW I wouldn't have even gone out in that mess (deadly) but he and his dad were going with or without me and I didn't think they had the experience to make it back.

 

[Brian 26]

This may be obvious to some but just to put it out there for this winter discussion: current and wind give back everything they take (assuming they do not change on your return trip).

Example, if your boat get's 4 mpg in flat water and going upstream you are seeing 2 mpg. You will get 6 mpg on your return downstream.

 

[smokeonthewater]

That's not exactly true... you DO get some back but not all....
Example 1:

you are idling at 6 mph burning 1 gph getting 6 mpg on dead water.... you head upstream into a 4 mph current for 10 miles... now you are moving 2 mph burning 1 gph getting 2 mpg... on the return trip you will be moving 10 mph burning 1 gph getting 10 mpg so it SOUNDS like you are right BUT there is a catch!
it took 5 hours to go upstream at 2 mph and you burned 5 gallons but it only took 1 hour to come back and you burned 1 gallon for 6 total For the same round trip without current you would have burned 3.33 gallons and taken 3 hrs 20 min (3.33 hours)

Example 2:

You are still traveling at 6mph 1 gph and 6 mpg and now you head into a 6 mph current..... obviously you are getting 0 mpg and you can run all day and get nowhere.... After 10 hours and 10 gallons expended the return trip is instantaneous and thus you burn 0 gas, and travel 0 miles, in 0 time....... 0 mpg for the trip

the faster your travel speed and the slower the current the less it matters hence my advice to go fast upstream and slow downstream but it never gives back all that it takes.

 

[alldodge]

Looks like this discussion has gone from the OP asking about efficiency of his type/style boat, and is now getting into utopia and boating in a vacuum.

 

[H20Rat]

On almost any planing hull, typically they run most efficiently up on plane at the lower end of the RPMs required to hold 'em on plane. For example, I get up on plane, then drop the throttle back to a point where I'm starting to "slush" a little ... then re-apply a little more power back on and hold it (not sure the RPMs exactly - my tach isn't workin' right). That seems to be the most efficient run for me. .

I bet if you find a boattest chart for your boat or a similar it might surprise you! Take the chart that was posted for example. Boat is most likely comfortably on plane at 20 mph, but is at 1.2 mpg. At 28.5 mph, it is up to 1.6 mpg's. And at 34.4 mph, it has only dropped to 1.5 mpg, which is still substantially better than the 20 mph mpg.

Boat speed and efficiency are massively complex equations, but part of that is that the faster you go, the less hull you are having to drag through the water. Also, engines to be much more efficient when they are near their torque peak. Depends on the particular engine, but the 4.3 torque peak is > 3000 rpm.