proper cruise rpm for extended trips

[hudman]

i have a new engine in my 97 rinker fiesta vee 266,
engine is a 383 mpi from mercruiser, basically a 6.2 mpi that has been bored .30 over. turning a bravo 2 lower with a 4 blade ss prop. motor now has 65 hours on it, and it really goes, will run 42 at 4960 rpm , cruises great, good out of the hole power. I am planning a 75 mile trip down the river, and then back up , 3 days later. what would be a safe crusing speed for the engine?
3000 rpm? would 3500 be too much?
I am on the mississippi river, 3000 would get me 28 mph going down , but only about 21 coming back up, time is not a factor, we have several boats making the trip together( KING BISCUIT BLUES FESTIVAL ON THE RIVER ,,, HELENA ARK, OCT 5 -7)
thanks in advance
hud

 

[Don S]

Re: proper cruise rpm for extended trips

What you need is a FloScan or a lot of time testing to find the best MPG for your boat. And it may change depending on how you load it.
With a FloScan hooked up to a GPS so you can get MPG, you can find your best Speed/Rpm/MPG for that particular day.

 

[Boomyal]

Re: proper cruise rpm for extended trips

IMHO, without the Floscan that Don mentioned, I would say your sweet spot would be between 27-3300 rpm.(pretty safe huh?) I you have any motor genes in your system you'd be able to feel/hear the sweet spot.

I would like to verify that on my boat with a FloScan, but I don't want to cough up the $$$$. I bet I would be within 5%, just doing it by ear and feel.

 

[Reel Poor]

Re: proper cruise rpm for extended trips

Like Don said the best cruise rpm range is for economy. As far as a safe range for your engine (comfort zone) 2800-3400 rpm.

Your WOT is within safe operating range. While it may not be economical to run at WOT for an extended period of time, it is safe and should not cause any damage. Just remember when running at, or near WOT consistantly, requires a lot of attention to a rigorous service schedule, with special attention given to the drive and drive componants.

 

[Don S]

Re: proper cruise rpm for extended trips

I would like to verify that on my boat with a FloScan, but I don't want to cough up the $$$$. I bet I would be within 5%, just doing it by ear and feel.
.

Don't bet on the 5%, I would bet you would loose. Most of the time.
On smooth water, at 3500 to 3700 you may get your best MPG.(say 2 mpg as an example) then the chop builds up and the 2mpg drops to 1.5mpg. So you back of the throttle to 3400 and the mpg goes to 1.7. Then you are going into the wind and turn and go with the wind.
See what I mean? there is no perfect rpm for all conditions.

The BEST thing to do is find the most comfortable speed, enjoy the trip, and pay the fuel costs.

 

[DJ]

Re: proper cruise rpm for extended trips

Great advice from Don S.

I've had much success finding the "sweet spot" using a simple vacuum guage. The highest vacuum reading (off idle) is the engines most efficient RPM/load.

 

[hudman]

Re: proper cruise rpm for extended trips

I dont know what a flowscan is really, but i do have the mersruiser smartcraft sc1000 speedo and tach, and the tach has the fuel flow per hour ,,,,(GPH) feature, i just have to do the math, if it is at 15 and im running 30 mph, its getting 2 mpg.
my main concern is that 35oo rpm for 3 hours would be too hard on the motor, this thing is so smooth its easy to get in the 3800-4200 without trying.with several other boats going down,,, we may just cruise/float at about 2000 and enjoy the ride

 

[Don S]

Re: proper cruise rpm for extended trips

You should be able to run at 3500 rpm all day if you want. No problems at all.

 

[f_inscreenname]

Re: proper cruise rpm for extended trips

Hell my sweet spot is around 3800rpm's but everyone's is different and like Don said it all depends on the conditions. My boat hates flat water and in return it loves gas and the throttle but throw a good chop in and it turns into a economy hotrod.

 

[tashasdaddy]

Re: proper cruise rpm for extended trips

hudman, if your going with a group of boats. customarily it is the slowest boat that sets the speed, or the smallest in rough conditions. everyone needs a vhf, to communicate, and you need a group commander to set the pace. it's all team work.

 

[hudman]

Re: proper cruise rpm for extended trips

we have a head bartender, will that work as a group commander? just kidding. I have taken some readings going upstream , and down, after work yesterday, at different rpms, to get gph, and mph readings, going to plug them into the computer and see what i get. the problelm in this trip is there are about 24 boats , but are leaving different days, and times, festival starts thurs, but im not going till fri morning. some are going wed, some thurs, and some fri. one guy with a 40 ft powerplay is going sat morning, he will cruise at70, and be there in an hour. I think that there will be 3 or 4 boats leaving fri morning, we probably will cruise at about 15-20 and make the day of it. We all have VHF radios and cell phones.

 

[Boatist]

Re: proper cruise rpm for extended trips

The best cruse speed and best Fuel mileage is when your boat first comes up on a natural plane with the trim in center position and no trim tabs. After that adjust for best ride. This is called a clean plane. To reach this point advance throttle a little at a time and wait, when you get to the point the boat climes up on plane the bow will come down and you speed will pick up. Many times this is slower than you want to go but that is the best cruse speed. Trim the outdrive out until the bow starts to bounce then back down just until it stops. In rough conditions may have to trim down more. Most boat I have been on this is between 2500 and 3000 rpms.

Of course if you have a way of figuring fuel mileage on the fly use that.

Make sure you practice the rule of 1/3. Use 1/3 of the fuel out, 1/3 to return and 1/3 in reserve for ruogh conditions or wind.

Some boats with a steeped hull a little more speed the boat may ride higher in the water and also be a good cruse speed.