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Old 01-30-2015, 08:46 AM   #81
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With the newer more powerful common rail diesels and an effectively designed SD hull, you can choose to operate over the ranges that Marin and others desire.

For example, an AT 34 is efficient at 7-8 kn and 2-3 gph, or planes without plowing at 17-18 kn and 17-18 gph. No over propping required.
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Old 01-30-2015, 08:56 AM   #82
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The new modern engines with great brain boxes may be able to operate at WOT and not be too fuel inefficient when operated 99% of the time at 6K,

BUT the question of how many brain boxes are aboard , and how much time is spent say,in central FL where the lightning strikes are measured in the thousands per day remains.

AT $10K a brain box , a lot of fuel would need to be saved daily!
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Old 01-30-2015, 09:11 AM   #83
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Quote:
Originally Posted by djmarchand View Post
I am late to the party on this thread. There have been dozens of posts but only two: FF's and Ski's addressed the benefits of overpropping. FF said that the fuel consumption would drop 50%. Ski said that fuel consumption would improve slightly and the main benefit is noise

Would 1,400 rpm be quieter than 1,800 rpm? Sure but it depends on engine installation and soundproofing whether it would be meaningful.

David
On many vessels including ours, as Dave mentions the engine mountings, cabin structural design and soundproofing render small changes in RPM non discernible.

As to dropping fuel consumption by 50%, I wiil provide the vessel, props and testing gear along with a certified naval architect to view and comment on all results. My bet is for costs, pay for engine damage plus 100% that a 50% fuel savings will not result at hull speed. I estimate this test on my vessel to cost about $50K.
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Old 01-30-2015, 09:36 AM   #84
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sun: You have zero risk of loosing that bet.


50% fuel reduction claim at any fixed speed is absurd. People just don't understand props and think of them as fixed gears in contact with hard surface.




Now I can and do reduce fuel consumption greatly when slowing from 20KTs to 8 Kts but There isn't much that will change when dragging 38,000# through the water.
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Old 01-30-2015, 12:17 PM   #85
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I wish people would study what Larry said above. The same istrue for most SD or SP hulls even with older mechanical engines..
Quote:
With the newer more powerful common rail diesels and an effectively designed SD hull, you can choose to operate over the ranges that Marin and others desire.

For example, an AT 34 is efficient at 7-8 kn and 2-3 gph, or planes without plowing at 17-18 kn and 17-18 gph. No over propping required.



My 44' ACMY with a semi planning hull at 38,000# delivers similar 2-3 NMPG at bow down speeds and will also do 20 Kts easily.
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Old 01-30-2015, 12:21 PM   #86
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NOPE , I never wrote fuel consumption would drop 50%.

I wrote basically , that IF the mfg desire to have every engine regardless of boat size shape or use install a propeller that would reach WOT , it was not his concern if it cost 50% more fuel at cruise.

I never stated that any prop would make every boat 50% better at cruise .

The folks that cruise at 1/2 NMPG to make big waves and go fast , are delighted to pull back to slow displacement speeds and enjoy savings .

If the go fast had a different engine , matched to a cruising prop , it could come closer to efficient than a big engine at almost idle.

If course if the Go Fast had a better displacement speed shape , that would help too, but it would never get up on the plane.

All is compromise ,,,,
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