Rotating twin engine 4.3 gas engines on voyage , will it converse fuel ?

[alan ber]

I have a 2004 Regal Commodore . Model 2860 twin 4.3 gas engines . Boat is around 30 ft
I have a trailer for Regal , not a fan towing long distance .At Boat ramp putting in myself is OK , taking out STINKS
The voyage is via water not on highway
. Retired spend a lot of time aboard , Marina kept , Our water condo here in Fla .
At this stage in our life, boat is not for ME anymore.
Weekender has limits , aft cabin has become storage, engine room with twin engines & generator is very tight
Just installed new long blocks and putting up for sale . Until sold this is all I have and will enjoy
I take boat out too ocean few times a week , stay local
I am getting the bug too take a voyage . I have done weekend voyages at 3000 RPM plane I burned gallon a mile ridding ocean on a 50 mile journey
Thinking about a voyage by water, too Keys, apx, 800 miles round trip .
Regular gas is around 4 bucks a gallon on the water, ( presently less )
Averaged out That's $ 3600 in fuel ., round trip ??? I am too cheap .
I have sailed and wanted a sail boat for the voyage but wife & I could not agree on anything , but that's a topic for the personnel section ,
I generally keep around 12 knots . I don't have too go fast just being on the water is what I enjoy
My buddy has a 40 ft sail boat. , living in Ft Lauderdale . pick me up will join
He said he did the math , go sail boat speed, sleep out one night , try too go with southern tide, stay intra coastal , rotate engines, hour on each too keep hour meters same , Power steering on starboard engine . steering very difficult with no PS but going straight could work
keep at sail boat speed , running one 6 cylinder engine slowly will save fuel , not pushing boat too do any engine damage

After a long winded post , MY QUESTION , running on one 6 cylinder engine at a time, around 10 knots, Will it save fuel , has anybody done it ? what was your hourly burn at that speed ?
Boat has twin IO Volvo penta duo prop . Can raise lower unit , even with bottom of transom, I don't see drag of non used lower unit or spinning prop , can leave in non running gear

thanks as usual . .

 

[JoLin]

10 knots is about 11.5 mph. Terrible speed on a planing hull. My engines turn more than 2500 rpm to maintain that speed, while another 6-700 rpm will add 10 mph to my speed.

You'll do better maintaining a low planing speed than you will plowing along off plane, regardless of whether you use one or both engines. I feel your pain (I also average about 1 mpg), but you aren't going to save anything by going slow on one engine. I've traveled in convoy many times with boats that cruise slower than mine. I just had to grin and bear it and to hell with the fuel consumption.

My .02

 

[Scott Danforth]

No, you wont save any money and at 10 knots you will burn more.

it takes X horsepower to push a boat. the faster you go, the more you burn.

doesnt matter if you run one motor or two motors, the thrust required is the same, so the burn rate is the same

Here is the funny thing about fuel economy in a boat....your fuel burn rate is directly related to how much power you command. 0.4 #/hp/hr is the BSFC for internal combustion spark-ignited marine motors.

so....you burn 0.4 pounds of fuel per hp per hour. doesnt mater if the boat is 4 cylinders, 6 cylinders, 8 cylinders or 12 cylinders

lets assume that it takes 180hp worth of prop thrust to push your boat. either you have two motors putting out 90hp just loafing about around 1500 rpm or you have one motor putting out 180hp at about 3500 rpm. same fuel burn rate.

at 10 knots, your too fast to simply idle along and are trying to climb out of the hole and are using more power than a few knots past your planing speed so your load is more, therefore your fuel burn is more. As JoLin pointed out, increase your speed a bit and you will burn less and get there twice as fast.

if you want fuel economy in a boat, get one with sails or oars.

 

[H20Rat]

Here is a graphic blatently ripped off from boattest showing a very similar boat to yours. (2x4.3l's, 31 ft)
http://www.boattest.com/review/cruis...s/1229_300-cxi

If you look at the MPG column, that is typical for any planing hull. Dead idle is usually the best, and until you get onto a clean plane, the MPG's go down. In very light, very high speed boats, MPG's near the top of the range get actually approach idle mpg or sometimes even better.

​

 

[ahicks]

A couple of thoughts. First, regarding the speed you're considering, with absolutely no experience in a boat like this, if fuel mileage were the issue, I would look at keeping the speed at "hull" speed - well shy of planing speeds. Maybe something in the area of 5-7 mph? Trawlers run at hull speed, as they are designed to maximize range/fuel mileage and use very little power to do that.

Second, You're retired, and you have a boat you trust (hopefully). There's the thought that you might want to get while the gettin's good? Our health does not get better as we age. If a trip like you're considering is on your bucket list, just do it. Cheaping out on something like this is something that will likely come back and haunt you later on, when you are no longer able.

 

[QBhoy]

Agree. Likely about the worst possible speed you could do with a planing hull. Trying to push the big flat underside through the water. Best just above tick over or just a little more than planing speed. Maybe about 3000 rpm or just over.

 

[mr 88]

I can save you 400.00 just by correcting your math. 800 miles at 1mpg at 4.00 per gallon = 3200.00 not 3600.00 Your hull if properly propped and trimmed out should be getting closer to 1.3 - 1.7 mpg. If you can lose some dead weight and hit that figure- 1.3 - you would be spending around 2450.00 in fuel. Look I just saved you 1150.00 so you can spend that money on food and slip fee's !!!!

 

[roscoe]

The way I see it,
you idle along at under 5 mph or 4 knots for 80 hours each way, 160 hours, 400 gallons of fuel, $1600 @ $4 per gallon.

Or you crank it up to 25 mph or 22 knots for 16 hours each way, 32 hours total, 600 gallons of fuel, $2400 @ $4 per gallon.

Maybe the sweet spot for your rig is 18 or 20 knots, which only you can determine.
Perhaps a fuel flow meter would help, but that would set you back several hundred.

Do you want to spend 1.5 days getting there - 3 days round trip, or 8 days getting there? And 8 days getting back.

Is an extra 13 days sitting behind the wheel worth the $800 in fuel savings?

Only you can answer these questions.