Cruising a twin screw Semi-displacement boat for minimum fuel consumption

[Codger2]

Marin wrote:Frankly, I think debating a single vs twin on the basis of fuel burn (assuming the same kind of boat and engines) is perhaps debating the wrong thing....*

But our preference for a twin over a single has nothing to do with fuel burn. The factors of our preference are redundancy, maneuverability, and the fact I like running multiple engines *

******** Ditto... I think that the concern over "fuel burn" which seems to dominate this site is really misplaced. There are many other components of buying a good boat that should take preference over "fuel burn."

 

[jleonard]

SeaHorse II wrote:Marin wrote:Frankly, I think debating a single vs twin on the basis of fuel burn (assuming the same kind of boat and engines) is perhaps debating the wrong thing....*

But our preference for a twin over a single has nothing to do with fuel burn. The factors of our preference are redundancy, maneuverability, and the fact I like running multiple engines *

******** Ditto... I think that the concern over "fuel burn" which seems to dominate this site is really misplaced. There are many other components of buying a good boat that should take preference over "fuel burn."

*I also agree. Fuel is the cheapest part of boating.

 

[rostan]

Correct if you're speaking of FL120's or 135's; but what about Cat 3208 at 375hp each on a 50-55' boat to be used as a cruiser?* Fuel then becomes a cost driver.

 

[Marin]

rostan wrote:
Correct if you're speaking of FL120's or 135's; but what about Cat 3208 at 375hp each on a 50-55' boat to be used as a cruiser?* Fuel then becomes a cost driver.

True enough.* But Tony doesn't seem to be considering a boat of this type.* He's looking at lower powered boats.

 

[rostan]

Except I raised the original question and am still looking for as much info as possible.* One piece of info I'd like is what is the fuel consumption (SFC) for a 375hp 3208 engine operation below 2000 RPM.* Catapillar's spec sheet only goes down to 2000 RPM.* Can't even expolate the data since the 210 HP version's graph goes below that but shows no SFC's.*

 

[RickB]

rostan wrote:Can't even expolate the data since the 210 HP version's graph goes below that but shows no SFC's.*

You could use the ratings for the 435 but what is the point? Exactly what are you trying to determine?

If you have the power curve for your prop you can use*an average*BSFC figure for your engine and calculate it. That should be close enough for short duration recreational use unless you are planning on a non-stop non-refueling round the world trip and need to calculate tankage.

The tiny amount of fuel difference will be buried in the real life performance that is altered by temperature, barometer, wind and waves, how good the helmsman,*and how the boat is trimmed. If you really want accurate data then the only way you can get that is to meter the fuel through laboratory grade instruments while collecting over-ground and through water speeds and distances to a very high accuaracy. Even then it would only tell you how you did that day and time.

Is it that important?

 

[markpierce]

DavidM wrote:
*Trawlers use about 15 hp per 1000 lbs to reach hull speed.

*That horsepower-to-weight ratio doesn't apply to my steel*boat.* With a 31.75-foot waterline, the theoretical hull speed is 7.5 mph.* The boat weighs 28,000 pounds, so you would predict 420 horsepower to be needed to reach that speed.* But my boat has only an 80 hp engine at 2500 rpm.* With the engine at 2400 rpm (as limited by the governor), the boat moves at about 7.7 knots.* That's*about 3 hp per 1000 pounds displacement.

 

[markpierce]

DavidM wrote:

BTW, your theoretical hull speed is 7.5 kts, not mph which is what I suspect you meant to say.

*Doh!*

 

[markpierce]

At 1400 rpm (maximum crankshaft torque), the Coot (with a John Deere normally-aspirated 4045 engine)*moves at 5.8 knots with 50 horsepower at the crankshaft.

*


-- Edited by markpierce on Thursday 25th of August 2011 12:07:30 AM

 

[RickB]

markpierce wrote:
At 1400 rpm (maximum crankshaft torque), the Coot (with a John Deere normally-aspirated 4045 engine)*moves at 5.8 knots with 50 horsepower at the crankshaft.

Mark, your engine lives in a boat, it drives a propeller. The numbers you posted are dynomometer developed numbers which reflect the potential output of the engine at various speeds into a brake. The only numbers in your post that mean anything are 1400 rpm and 5.8 knots.

Your engine in your boat with your prop - probably - produces around 25 horsepower at 1400 rpm. The torque produced at that power and rpm is slightly under 94 lb-ft.