Fuel consumption question

[jddens]

While out for a cruise this weekend I did alittle experimenting with power settings.......crusing at 8 knots and 2000rpm i shut one engine down, my speed only dropped 2 kts while fuel consumption was reduced by 50%.........increasing rpms on the remaining engine to 2400 brought my speed back up to 8 kts. I'm assuming my fuel consumption would still be reduced by 25-40 percent vs. running with both engines at the same speed......my thought would be to use this pratice to reduce fuel comsumption this summer and alternate engines to keep hrs equal, The boat handles fine without pulling excessivly either way........anybody see any reason I shouldn't do this???????? Thanks......John

[etpence]

Hello John

it should not be a issue
I use only one all the time while trolling offshore
and it sure helps on fuel fo sure
and ware and tare

todd
30'Sea Raider
Mio Mio

[jav]

John-

just a couple of thoughts:

1) make sure the transmissions are of a design that can free wheel without gear damage. (some gears can't do this and require that you lock the shaft)

2) Your thought that you save 50% by shutting off one engine may not be accurate. Did you try this experiment with a flow scan or are you guessing at 50%? Turning one engine off should place quite a bit more load on the other engine and raise it's consumption even without adding throttle. Further, the gear from the "off engine" adds additional drag that otherwise would not be there to such a degree further adding load. At 8 knots, it may be sliught but I wouldn't expect the gains your posting.

[jimbo36]

Running with one engine at very low RPM while trolling is one thing. Cruising with one engine is quite another. Most planning hull boats over 28' have twin engines. Why? Because they need the power ( horsepower & torque) to effectivly push the wieght of the vessel through the water. Marine engines, unlike cars and trucks, are under load 100% of the time. If you are using a lot less fuel, which I doubt, by running one engine you will pay in the long term by lessening the life of your engines. A friend of mine once owed a 28' Bayliner with a single 454 V8. (cheaper to build, cheaper to sell) He ran into owners of the same boat equipped with optional twin V6's who got better fuel consumption then he did.

[rossjo]

Hull Speed is the natural speed for a given displacement hull length when the shape of the trough created by the bow wake is equal to the water line length of the vessel. It is therefore more efficient, since you're not trying to push the boat up over the bow wake, out of the trough.

Hull speed is expressed as 1.34 X the square root of LWL, or length of waterline.

For the 30' Sea Raider above, the LWL is about 28', and the Hull Speed is 7 knots.

My F32 is 32'OverallLenght(OAL) = about 30'LWL = 7.34 knots

25'OAL ~= 23'LWL = 6.4 knots
27'OAL ~= 25'LWL = 6.7 knots
30'OAL ~= 28'LWL = 7.1 knots
32'OAL ~= 30'LWL = 7.3 knots
36'OAL ~= 34'LWL = 7.8 knots
40'OAL ~= 38'LWL = 8.3 knots

So - you're best efficiency is at hull speed - which is 6.5-8 knots for most of us here! If in doubt on your exact speed, run a little slower and let your stern wave help push you forward.

You can run at hull speed on one engine (assuming your trannies can free wheel), and save about 20-30% (50% is not possible).

[jddens]

Thanks everyone for the responses and the detail. Not sure about the free wheel capability of my Hurth transmissions. Anybody know if they can safely freewheel?.....Thanks again.............John

[etpence]

Hello again John

I have hurth trannys and when I spoke to the rep about this I was informed that there is no issue with free wheeling it
I have been doing this while trolling for a couple yrs and have had no issues as of yet

Todd
30'sea raider
Mio Mio

[jddens]

Thanks Todd, sounds like we have about the same setup except you've got a few cubes on me.....thanks again.........John