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08-28-2015, 04:14 PM
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#21
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TF Site Team
City: California Delta
Vessel Name: FlyWright
Vessel Model: 1977 Marshall Californian 34 LRC
Join Date: Apr 2008
Posts: 13,728
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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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08-28-2015, 04:34 PM
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#22
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Guru
City: Annapolis
Vessel Name: Ranger
Vessel Model: 58' Sedan Bridge
Join Date: Aug 2013
Posts: 7,068
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Quote:
Originally Posted by Brisyboy
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?
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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.
-Chris
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Chesapeake Bay, USA
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08-28-2015, 04:38 PM
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#23
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Guru
City: Cypress Landing Marina (NC)
Vessel Name: Heron (2)
Vessel Model: '88 Cape Dory 28 Flybridge #115
Join Date: Nov 2014
Posts: 1,305
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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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Steve
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08-28-2015, 05:18 PM
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#24
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Guru
City: Philadelphia
Vessel Name: Dreamers Holiday
Vessel Model: Mainship 390
Join Date: Jun 2010
Posts: 572
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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.
John
2003 MS390
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08-28-2015, 05:56 PM
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#25
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Veteran Member
City: Ventura CA
Vessel Name: Proud Mary
Vessel Model: Pacific Trawler
Join Date: Apr 2015
Posts: 92
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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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08-28-2015, 06:53 PM
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#26
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Master and Commander
City: Vallejo CA
Vessel Name: Carquinez Coot
Vessel Model: penultimate Seahorse Marine Coot hull #6
Join Date: Sep 2010
Posts: 12,559
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Quote:
Originally Posted by johnma
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.
John
2003 MS390
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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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Kar-KEEN-ez Koot
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08-28-2015, 07:06 PM
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#27
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Senior Member
City: Port Moody
Vessel Name: Twilight1
Vessel Model: Permaglass Sedan
Join Date: Apr 2012
Posts: 217
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Quote:
Originally Posted by ksanders
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.
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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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08-28-2015, 07:23 PM
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#28
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Guru
City: kemah
Join Date: May 2015
Posts: 1,135
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so why such a large disparity between hull speed and best mpg's?
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08-28-2015, 07:56 PM
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#29
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Master and Commander
City: Vallejo CA
Vessel Name: Carquinez Coot
Vessel Model: penultimate Seahorse Marine Coot hull #6
Join Date: Sep 2010
Posts: 12,559
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Quote:
Originally Posted by FlyWright
... 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:
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Al, what's your fuel consumption at six knots? Bet it's around two gallons an hour.
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Kar-KEEN-ez Koot
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08-28-2015, 08:00 PM
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#30
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Master and Commander
City: Vallejo CA
Vessel Name: Carquinez Coot
Vessel Model: penultimate Seahorse Marine Coot hull #6
Join Date: Sep 2010
Posts: 12,559
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Quote:
Originally Posted by what_barnacles
so why such a large disparity between hull speed and best mpg's?
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Similar to the question "why do automobiles get better mileage at 55 mph versus 70 mph."
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Kar-KEEN-ez Koot
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08-28-2015, 08:22 PM
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#31
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Guru
City: Concrete Washington State
Vessel Name: Willy
Vessel Model: Willard Nomad 30'
Join Date: Oct 2007
Posts: 18,738
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Quote:
Originally Posted by what_barnacles
so why such a large disparity between hull speed and best mpg's?
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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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Eric
North Western Washington State USA
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08-28-2015, 08:25 PM
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#32
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Senior Member
City: San Diego and Gabriola
Vessel Name: Skookum Maru
Vessel Model: Ed Monk design #1924
Join Date: Dec 2011
Posts: 228
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Quote:
Originally Posted by RickB
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.
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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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08-29-2015, 12:24 AM
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#33
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TF Site Team
City: California Delta
Vessel Name: FlyWright
Vessel Model: 1977 Marshall Californian 34 LRC
Join Date: Apr 2008
Posts: 13,728
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Quote:
Originally Posted by markpierce
Al, what's your fuel consumption at six knots? Bet it's around two gallons an hour.
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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 $$ on fuel to go 25% faster or 7.5 Kts.
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08-29-2015, 12:33 AM
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#34
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Master and Commander
City: Vallejo CA
Vessel Name: Carquinez Coot
Vessel Model: penultimate Seahorse Marine Coot hull #6
Join Date: Sep 2010
Posts: 12,559
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Quote:
Originally Posted by FlyWright
Might be, but I don't know. I can't stand going that slow so it doesn't matter to me. I'll spend the $$ on fuel to go 25% faster or 7.5 Kts.
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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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Kar-KEEN-ez Koot
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08-29-2015, 01:07 AM
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#35
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Guru
City: Sydney
Vessel Name: Sojourn
Vessel Model: Integrity 386
Join Date: Oct 2011
Posts: 13,293
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7.5, 6.3, you guys are just a bunch of speedsters.
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BruceK
2005 Integrity 386 "Sojourn"
Sydney Australia
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08-29-2015, 01:13 AM
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#36
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Master and Commander
City: Vallejo CA
Vessel Name: Carquinez Coot
Vessel Model: penultimate Seahorse Marine Coot hull #6
Join Date: Sep 2010
Posts: 12,559
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As an ex-mono-hulled sailor, a consistent six-something knots seems fast!
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Kar-KEEN-ez Koot
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08-29-2015, 03:15 AM
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#37
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Guru
City: I need a bigger boat!
Vessel Model: Cheetah 25' Powercat.
Join Date: Aug 2014
Posts: 925
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Quote:
Originally Posted by FlyWright
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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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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Peter.
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08-29-2015, 06:27 AM
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#38
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Guru
Join Date: Oct 2007
Posts: 22,553
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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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08-29-2015, 06:36 AM
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#39
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Scraping Paint
City: Fort Lauderdale
Vessel Model: CHB 48 Zodiac YL 4.2
Join Date: Oct 2007
Posts: 3,804
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Quote:
Originally Posted by QB
If you don't know Beebe's credentials, you can look them up.
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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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08-29-2015, 07:40 AM
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#40
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Guru
City: gulf coast
Join Date: Jun 2014
Posts: 3,440
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Quote:
Originally Posted by what_barnacles
so why such a large disparity between hull speed and best mpg's?
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because hull speed has nothing to do with best economy despite all the dock talk myths.
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