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Old 08-28-2015, 05:14 PM   #21
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When I bought my trawler with twin 85HP 4.236 Perkins, the PO told me he ran at 8 kts and 4 GPH. That's what I got for the first year or so. Then I decided to play with the power settings a little bit and sought the sweet spot.

I found that at 7.4-7.5 kts, I was running at 200-250 rpm less than the 8 kt power setting and my resultant fuel flow dropped to 3.2-3.3 gph. This has been consistently replicated after many hundreds of hours at the new, reduced power setting. Calculations after my fill up last week averaged 3.2 gph after 50+ hrs of operation.

7.5 Kts:



10 Kts:

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Old 08-28-2015, 05:34 PM   #22
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Wow, great discussion so far, although my question was how to physically recognise when I am at or near it, not how to theoretically calculate it. It seems to be the sweet spot that we all want to be able to peg for fuel burn.

So....... as I said I should have taken a picture however if I can describe it - the second wave was coming from the stern quarter, with the third being generated just aft of the transom. IF, I was in smooth water and I played with speed, should that second wave be generated from the edge of the transom, just like psneeds picture?
I dunno whether you could tell much about your waves -- very precisely -- from onboard...

I've found it easier to start with the calculation and then modify RPMs from there, comparing speed deltas per 100s of RPMs. Lots o' more RPMs with little gain in speed on ours pretty much indicates we're at the threshhold.

Modified somewhat by wind and current.

We gain much fuel economy by running at about 1 kt below our "calculated" hull speed.

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Old 08-28-2015, 05:38 PM   #23
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My boat uses 2.1 GPH, 2000-2100 rpms at 7.2 knots which is slightly above calculated hull speed. At 1900 RPM's (7 knots) hull speed, it will use less than 2 GPH. You need to find the sweet spot
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Old 08-28-2015, 06:18 PM   #24
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I let my wallet tell me. I do the largest amount of my boating on the Delaware River and Bay. Both have a fairly consistent 3 kt current that changes direction twice a day with the tide. I use my tach. My most efficient rpm is 1900. depending on the tide and the direction I'm going, 1900 rpm will give me roughly 7 kts or 10 kts.
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Old 08-28-2015, 06:56 PM   #25
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This has been one of the better threads as far as good technical analysis of the issue of “hull speed” is concerned. No one claimed to be an official naval architect but several posters certainly have good engineering insight and knowledge. The graphs presented by TDunn really say it all. There is not any exact speed beyond which the boat will go no faster, but the resistance increases nearly exponentially near hull speed and any prudent skipper will operate somewhere just below that steep rise. It does, of course, depend on hull shape and length-to-width ratio among other things.

The water is undisturbed before the boat enters it and leaves large waves in its wake. Those waves require energy of be formed and the energy comes from your fuel tanks. Look at the wake behind a sailboat operating below hull speed (which is almost all the time) and you will see why sailboats can cruise nicely with very small engines.
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Old 08-28-2015, 07:53 PM   #26
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I let my wallet tell me. I do the largest amount of my boating on the Delaware River and Bay. Both have a fairly consistent 3 kt current that changes direction twice a day with the tide. I use my tach. My most efficient rpm is 1900. depending on the tide and the direction I'm going, 1900 rpm will give me roughly 7 kts or 10 kts.
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One of the first tasks after acquiring my boat was to equate engine speed (RPM) to speed over ground. Did this by going up and down river in Mare Island Strait and averaging to cancel out tidal current effects.

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Old 08-28-2015, 08:06 PM   #27
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You can tell what a good efficient speed for your boat is simply by using the throttle.

You will find a speed at which further small advances in throttle do not yield a proportionate increase in speed.

That is what I as a layman would call your hull speed.
Also, if the throttle push lifts your bow, back off and listen to the engine(s). Even the rattles around the galley tell you something - it's a calming trim operation.
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Old 08-28-2015, 08:23 PM   #28
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so why such a large disparity between hull speed and best mpg's?
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Old 08-28-2015, 08:56 PM   #29
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... I found that at 7.4-7.5 kts, I was running at 200-250 rpm less than the 8 kt power setting and my resultant fuel flow dropped to 3.2-3.3 gph. This has been consistently replicated after many hundreds of hours at the new, reduced power setting. Calculations after my fill up last week averaged 3.2 gph after 50+ hrs of operation.

7.5 Kts:

Al, what's your fuel consumption at six knots? Bet it's around two gallons an hour.
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Old 08-28-2015, 09:00 PM   #30
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so why such a large disparity between hull speed and best mpg's?
Similar to the question "why do automobiles get better mileage at 55 mph versus 70 mph."
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Old 08-28-2015, 09:22 PM   #31
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so why such a large disparity between hull speed and best mpg's?

A knot below hull speed settles the boat in the trough of the wave not significantly climbing the the bow wave. One trades the extra power required to significantly climb the bow wave for riding a bit on the following wave getting a little push. Go hull speed and you loose the push of the following wave and gain the monsterous job of going "uphill" on the bow wave. Approximately one knot below hull speed is the "sweet spot" but that expression is usually directed to minimal vibration. But for this thread it means at the end of the relatively flat curve of power v/s speed that ramps up getting really steep about 1/2 a knot below hull speed.
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Old 08-28-2015, 09:25 PM   #32
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This discussion needs a real-life naval architect to chime in and say 'I am a licensed/registered naval architect and my position on this is ... " in order to either answer the question or add fuel to the discussion.
For some background on my post #11 on this thread, see Beebe, Robert P., Voyaging Under Power (first edition), pp. 56-7.

If you don't know Beebe's credentials, you can look them up.
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Old 08-29-2015, 01:24 AM   #33
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Al, what's your fuel consumption at six knots? Bet it's around two gallons an hour.
Might be, but I don't know. I can't stand going that slow so it doesn't matter to me. I'll spend the $$ on fuel to go 25% faster or 7.5 Kts.
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Old 08-29-2015, 01:33 AM   #34
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Might be, but I don't know. I can't stand going that slow so it doesn't matter to me. I'll spend the $$ on fuel to go 25% faster or 7.5 Kts.
Al, it must be the sound of the engines. ... Traveling at 6.3 knots, I can leave less than an hour before you and in four hours get to the destination before you. "You take the high road and I'll take the low road, and I'll get there afore you." Slowed down to let you pass as I was unfamiliar with our Treasure Island destination:

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Old 08-29-2015, 02:07 AM   #35
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7.5, 6.3, you guys are just a bunch of speedsters.
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Old 08-29-2015, 02:13 AM   #36
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As an ex-mono-hulled sailor, a consistent six-something knots seems fast!

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Old 08-29-2015, 04:15 AM   #37
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Might be, but I don't know. I can't stand going that slow so it doesn't matter to me. I'll spend the $$ on fuel to go 25% faster or 7.5 Kts.
Is the Californian 34 a semi hull, or displ.?

Lovely looking boat, has a sort of custom down east look to it.
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Old 08-29-2015, 07:27 AM   #38
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"my question was how to physically recognise when I am at or near it, not how to theoretically calculate it. It seems to be the sweet spot that we all want to be able to peg for fuel burn."

USE YOUR FLOWSCAN and a bit of graph paper.

"so why such a large disparity between hull speed and best mpg's?"

Look again at the graph of wave making resistance , and notice at some point it rises like a rocket.

Beyond there is when your wallet empties at the accelerated rate.

Hull speed is only of interest to sailors , who don't have to pay for it,

IF their rig is robust enough.

For most cruisers the Sq RT of the LWL is a great place to start as cruise speed (in Knots)
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Old 08-29-2015, 07:36 AM   #39
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If you don't know Beebe's credentials, you can look them up.

I know he was not a naval architect, he was an aeronautical engineer and a very talented yacht designer.

He is also dead so the chances of his "chiming in" with a response to the questions and comments posted here are pretty slim.
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Old 08-29-2015, 08:40 AM   #40
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so why such a large disparity between hull speed and best mpg's?

because hull speed has nothing to do with best economy despite all the dock talk myths.
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