Volvo AQ131D cruising speed

[MONC440]

What rpm would you guys recommend for sustained engine RPM for an old 131? I live on a small lake only a mile long and I have only ran the engine at 4500 rpm for maybe 15 minutes at a time. We are going out to Lake Michigan this weekend and will be traveling miles at a time. The manual says to cruise at 500 rpm lower than max so 4600 but I’m thinking that 4000 would be better if I was going to be cruising for 30 minutes or more. Your thoughts?

Thanks

 

[captmello]

cruising at 4000+ rpm sounds like a waste of gas. unless you're in a hurry. I would cruise at a speed that keeps the boat on plane.

 

[Jaegerboy]

For what it's worth, I have always used my cruising speed (RPM) at 75% of WOT. That of course depending on water conditions and as Captmello states, getting on a plane.

 

[Lou C]

The 75% of redline rpm works for most engines. I run my 4.3 V6 at 3600 rpm on plane, that works out to only about 25 mph, but fast enough given the cost of fuel. Most times due to water conditions, wind, boat traffic etc you can't really go much faster safely at least.

 

[MONC440]

Ok on our small lake it will get up on plane quickly and I can back it down to about 2000 and keep it on plane. I’m going to take it out today I will see what the speed is at the different rpm levels.

Another question the service manual says to retorque head bolts on these engines at certain intervals. I have not retorqued head bolts on an engine since using the old metal gaskets. Do you guys retorque head bolts on these engines?

 

[MONC440]

One other thing I was wondering about is fuel usage. My boat has a 20 gallon tank and around our small lake at home fuel isn't a big deal, if I'm low I can run to the marina or take gas can down to the station. So I guess my question is if I'm cruising up the coast of lake Michigan, how long that 20 gallons is going to last. The boat is a carbureted AQ131 engine with the stock prop, 18 feet. Will only be the wife and myself and a cooler for lunch. I know this is probably a hard question to answer but if anyone has experience with cruising a similar boat in calm waters I would like to hear it. Thanks

 

[Scott Danforth]

too many variables.

what direction is the wind blowing?
how loaded will you be?
where is the load in the boat, Bow, Stern, amidship?
do you have the timing adjusted correctly?
do you have a perfect prop or a dinged up prop?
what angle of trim will you be running at?

all those things affect fuel burn?

Just use the 1/3 rule. with a full tank of fuel, turn around when you burn 1/3 of your fuel in the tank. that way, when you get back, you may or may not have 1/3 left in the tank.

 

[Lou C]

Even carb choice influences fuel consumption, combined with driving habits.
A 4 bbl like the Quadrajet with the small primaries and large secondaries will be as economical as a 2 bbl or even more so as long as you stay out of the secondaries. I think on mine they start to open above 4000 rpm so I stay below that most of the time. While I feel my boat is somewhat under powered with the 4.3 V6, driven properly it is very economical for a boat. For water sports it should really have a V8, but for cruising and fishing where I'm idling much of the time the V6 is fine. Can't go that fast here consistently anyway due to water conditions and boat traffic.

For the OP to get a rough idea, I'd look for Volvo information about fuel consumption with 4 cyl engines, even though later modern VP engines used GM engines, that 4 cyl was larger in displacement (3.0 liters, your genuine VP engine is smaller) those #s would probably be somewhat close.
If Volvo doesn't publish such a chart, I think Merc did, I'd look up a 3.0 4 cyl Mercuiser and see how many GPH at various RPM levels.

 

[MONC440]

Thanks guys. I like the 1/3 rule. I did find a chart that showed the 3.0L at 5.3GPH at 3000rpm. My boat is about 2000lbs with fuel, the wife and I and gear I think 3000lbs is good. My 131 is tuned correctly with a single carb, stock prop. I'm going to use 4GPH for planning and use the 1/3 rule. I will probably stash a 3 gallon tank just in case.

 

[dingbat]

The biggest influencer of fuel consumption is the weather.

What conditions will you be facing the day of your journey?

It's 80 miles each way to the canyons to fish. Typically cruise 27 kts, 4400 rpm (5950 rpm red line) in a 22' outboard powered boat

In flat, windless conditions I get 2 mpg.

In 5-10 kts winds, 2' seas that drops to 1.7 mpg.

In 10-15 kts winds, 3' seas I'm down around 1.0 gpm.

How much fuel should I carry on my next trip?

 

[MONC440]

So I took the boat out today and it is running about 20mph on my gps at 3800 and maxed out at 31 at about 5000. It will stay on plane down to 3000 but it is starting to come down and push a little. At 3400 it is on plane good. 3800 is over 1000rpm below max so I would think that is a good cruising speed? The chart above shows about 7gph at 3800 so I should be able to get about 3 hours out of 20 gallons in calm water with no wind. I have the trim rod on the middle position. I might try it in the front position to see if the boat will stay on plane better at lower RPM.