Volvo duo prop cruise RPM?

[FAST FRED]

Went over to the really dark side and now have a summer toy with a small block 301 ci Volvo Duo-prop .

The boat runs fine tho it takes 3000RPM to stay on the plane.(frightening for a diesel guy!)

Does anyone have experience with the cruise RPM for these engines?

Have been running 3300 rpm , but locals claim 4000 "all day" is reasonable.

Economy cruise??

Comments gents?

FF

 

[gonzo]

Are you sure it is a 301? Most likely a 302 Ford block or a 305 Chevy. They are rated to 4400RPMs.

 

[CDK]

3000 rpm to stay planing is not unusual, 2600 is more "economical" but requires a steeper prop and affects acceleration.
For economy cruise buy another toy.

 

[FAST FRED]

Volvo calls it a 5.1 , tho it is a 305 Chevvy.

With a duo prop replacing a set of props might never pay .

Rated at 4400rpm is fine but I doubt it is the cruise RPM .

This toy was expensive enough , finding out where the engine will operate over the long term with out harm is the goal.

Economy cruise ,(3150 RPM so far) so far is not that bad , 7 -8 GPH at 20K is better than 2.5 mpg .

Just wonder if faster , higher on the plane would work even better

The boat feels great with another 200 -400rpm , will have to keep a detailed log to find out whether it actually is cheaper.

Can a 305 handle 3600 RPM for hours ? Only a 50 gal tank, so its not many hours!!

FF

 

[CDK]

It can handle 3600 rpm without harm (except when brand new) for about 4 hours, then you must row to the nearest gas station.

 

[Frosty]

It might pay back if you got a fuel flo meter.

 

[tuantom]

My guess is there is probably very little difference in economy between 3100 - 3500 rpm's - so where you and the boat feel happier is probably the better choice.
Depending on the model you have, some call for a wot rpm of 5000.

 

[FAST FRED]

While it wont give me a fuel burn , next run I will make a Speed/RPM graph .

Should help a bit with knowing when advancing the throttle will not do much.

Add 100rpm , gain a K or 2 great ,

add 400 rpm and gain a 1/2K , perhaps not cruise area.

FF

 

[MechaNik]

Looking at a volvo 350 today startled me a little.
On the engine it seems to state that the expected life span of the engine to be 1000hrs or 10 years? Maybe that is what you will be expecting with the poor thing crying out down there. It also states not for not commercial use....no wonder.

 

[Frosty]

American iron blows all the time the stresses on the old push rod engines is too much --Ive blow every high RPM american iron I ever had.

The race guys keep many for spare

 

[FMS]

These engine blocks date from when cars were considered fair to get 100,000 miles.
Consider that 350 will push a 22 foot boat to cruise at 45 mph. 45 mph x 1000 hours = 45,000 miles on the water. That's under much more stress than the engine in a car. The positive is that these longblocks are very inexpensive and parts are easy because they have been the same for years and years.

 

[Frosty]

100,000 miles of automatic transmission work in down town cities.

A few seconds of low RPM toque occasionaly ,,--not 4,400 RPM kicking out all it can,-- for hours.

Thats why the big boys went Lamborgini or Detomaso or Ferrari.

Ive even destroyed a Mastercraft Ford Escort motor. Ive blown everything made in USA even my Oldsmobile went cranky.

 

[MechaNik]

Out of curiosity looking at boat test today it is surprising where such engines find their most efficient cruise speed. For boats well on the plane few showed that slower and lower is better, maybe you can find a close comparison to what you have, otherwise some de-tuning may be cheaper than props.

 

[KnottyBuoyz]

We ran the 5.7 VP with duoprop for 7 yrs. Went down two steps in props from B6 to B4's before we got decent performance. Top end and mileage suffered but at least it could haul it's own arse out of the hole.

Sweet spot with the B4's was around 3200 RPM @24-25 kts. WOT was 4400 but I never pushed past 4000. I had a few flow meter but never got to use it before we sold the boat. On plane cruising at 3200 was probably 5 mpg or there abouts.

Proper trim of the drive and trim tabs helps a lot.

 

[WestVanHan]

A few seconds of low RPM toque occasionaly ,,--not 4,400 RPM kicking out all it can,-- for hours.

Thats why the big boys went Lamborgini or Detomaso or Ferrari.

Ive even destroyed a Mastercraft Ford Escort motor. Ive blown everything made in USA even my Oldsmobile went cranky.

Frosty,what you mean re: Italian auto maker reference?

And what is a Mastercraft Escort motor? Mastercraft is Ford's parts department,and Mastercraft built ski boats with V8s in them


BTW FF with boats such as this,usually once you get up on plane and if it's reasonably aerodynamic the nmp usually doesn't drop a whole lot.
For example my 22 ton boat,from 15 to 32 knots it goes from .7 nmpg to .55 nmpg.