Cruising Speeds

[CapPap]

Just curious. How many of you choose to cruise a long distance at low speed (<10 knots) vs getting up on plane to get to your destination quicker?

Does the slow speed trawler approach save you anything on fuel costs?

[geoffm70]

I'd like to know also; surely, it must depend on vsl size.

[CapPap]

Early Boating Magazine Reports suggest that if I cruised at 1500 rpm (7.3 mph), I would get 38% better mpg than I would at 3500 rpm (26 mph). Just wondering if others actually run that way for 30-60 mile runs.

[mmwjr]

Not sure what boat nor engines you have but assuming twin small blocks would burn around 12 gallons per hour each at 3500 rpm times two to travel 52 miles for a total of 48 gallons. The same engines at 1500 rpm would burn around 3 gallons per hour each and it would take approximately 7 hours to travel the same distance for a total of 42 gallons.

[SteveP]

We have a 4600 with Cummins 6CTA motors. Usually cruise at 2100-2200 RPM but often slow down to 1000 RPM if we have a current or tide assisting. Easier on the motors and if I hit any debris chances are it will not do as much damage. Only over temp I’ve had in 14 years owning the boat was while slowly cruising and easily identified/mitigated.

[Jorgeinseal]

Have a 4100 SCA with twin 5.9L BTA's, when heading out to Catalina I keep the rpm's at 1800, according to Cummins burn rate that translates to almost 9.8 gph, at 2000 rpm the burn rate jumps significantly to 13 gph.

[CapPap]

Originally Posted by mmwjr

Not sure what boat nor engines you have but assuming twin small blocks would burn around 12 gallons per hour each at 3500 rpm times two to travel 52 miles for a total of 48 gallons. The same engines at 1500 rpm would burn around 3 gallons per hour each and it would take approximately 7 hours to travel the same distance for a total of 42 gallons.

I have the 3500 SCR with twin 7.4L 454 ci gas engines. Test data says 24 mph @ 3500 rpm & 33.8 gph; 7.3 mph @ 1500 rpm & 6.6 gph. I figure that should save 36% (24/7.3*6.6/33.8=.64). Curious if those with express cruisers often travel this way.

[mmwjr]

Originally Posted by CapPap

I have the 3500 SCR with twin 7.4L 454 ci gas engines. Test data says 24 mph @ 3500 rpm & 33.8 gph; 7.3 mph @ 1500 rpm & 6.6 gph. I figure that should save 36% (24/7.3*6.6/33.8=.64). Curious if those with express cruisers often travel this way.

As I said I assumed small blocks since you had not said what you have. A 7.4 will burn around 15 gallons each so your 33.8 is in line.

[Germanivette2010]

hi happy 2024 question i have a 1998 3700 scr with twin 7.4 mpi engine does anyone knows how many gallon both engine consume in one mile

[mmwjr]

Originally Posted by Germanivette2010

hi happy 2024 question i have a 1998 3700 scr with twin 7.4 mpi engine does anyone knows how many gallon both engine consume in one mile

Not sure anyone will know but we would need to know more such at what speed you are traveling and or rpm.

[Germanivette2010]

Originally Posted by mmwjr

Not sure anyone will know but we would need to know more such at what speed you are traveling and or rpm.

thanks in advance i travel 26 miles and i was doing 20 knts at 3800 rpm

[mmwjr]

Originally Posted by Germanivette2010

thanks in advance i travel 26 miles and i was doing 20 knts at 3800 rpm

With a similar boat to CapPap whose rpm of 3500 burns 33.8 gph you need to convert your 20 knots tp mph and multiple the inverse by 33.8 gph to get gallons per mile.

[Germanivette2010]

Originally Posted by mmwjr

With a similar boat to CapPap whose rpm of 3500 burns 33.8 gph you need to convert your 20 knots tp mph and multiple the inverse by 33.8 gph to get gallons per mile.

it took me 2 hrs
the equation will be 2hr x20 knots= 40 mph
then 40 mph ÷ 33.8 gph = 1.18 gallons per miles is that right?

[mmwjr]

Originally Posted by Germanivette2010

it took me 2 hrs
the equation will be 2hr x20 knots= 40 mph
then 40 mph ÷ 33.8 gph = 1.18 gallons per miles is that right?

Nope, 1 knot = 1.15 mph so 20 knots = 23 mph. 33.8/23 = 1.47 gallons per mile

[Germanivette2010]

Originally Posted by mmwjr

Nope, 1 knot = 1.15 mph so 20 knots = 23 mph. 33.8/23 = 1.47 gallons per mile

would that be for both engine or each engine consume 1.47 gallon per mile.

like today i did the test again 26 miles ride i was doing 21 knots at 3800 rpm and when i got back to the marina i refuel again and took 20 gallons each tank so in this case what it will be sorry to ask you so many times

[mmwjr]

That calculation was for both engines.

[mmwjr]

So you used 40 gallons and traveled 26 miles? That would be 1.54 gallons per mile.

[Germanivette2010]

Originally Posted by mmwjr

So you used 40 gallons and traveled 26 miles? That would be 1.54 gallons per mile.

thanks ill apreciate your help really i was thinking in selling the boat and find a diesel one but i think i stay with this one

[PJHoffnet]

Merritt Island => Key West and back ~650nm,
Merritt Island => St Augustine and back ~420nm

All at 9kts or below (best fuel economy for me) wte of a ~10 minute/day after engines got warm (and out of a no wake/slow speed zone) run up on plane ~17 knts to keep things juicy.

4100 SCA twin Cummins 330HP 6BTA 5.9 M-3; about 1,500 hrs on both, normal maintenance

[CapPap]

Originally Posted by PJHoffnet

Merritt Island => Key West and back ~650nm,
Merritt Island => St Augustine and back ~420nm

All at 9kts or below (best fuel economy for me) wte of a ~10 minute/day after engines got warm (and out of a no wake/slow speed zone) run up on plane ~17 knts to keep things juicy.

4100 SCA twin Cummins 330HP 6BTA 5.9 M-3; about 1,500 hrs on both, normal maintenance

Thanks PJ. That’s what I am curious about. However, I wonder if the larger yachts such as yours with diesels are more likely to travel at the slower speeds. My 35 footer has gas engines. I may try a journey at slower speed this summer to test it out. Time is less of an issue for me these days.