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Old 07-31-2015, 08:13 AM   #21
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All the replys show almost the same answer, it depends on how quickly you wish to empty your wallet...
I like that answer. For us our most comfortable cruising speed is 2200 rpm, which gives us around 10 or 11 knots give or take with wind and chop and current, bit it's also a very fuel efficient speed, around 2 mpg which is pretty good for twin 454's on a 26,000 lb boat I think. "Cruising speed" has never made much sense to me, it's like asking what the cruising speed is on your car.
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Old 07-31-2015, 08:17 AM   #22
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Originally Posted by kthoennes View Post
I like that answer. For us our most comfortable cruising speed is 2200 rpm, which gives us around 10 or 11 knots give or take with wind and chop and current, bit it's also a very fuel efficient speed, around 2 mpg which is pretty good for twin 454's on a 26,000 lb boat I think. "Cruising speed" has never made much sense to me, it's like asking what the cruising speed is on your car.
That has to be the most efficient gas boat I have ever heard of!

Has to be fuel injected....yes?
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Old 07-31-2015, 08:32 AM   #23
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Comfort or fuel consumption has nothing to do w what speed the hull was designed for. Cruising speed is established by the NA that designed her.
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Old 07-31-2015, 08:48 AM   #24
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that has to be the most efficient gas boat i have ever heard of!

+100
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Old 07-31-2015, 08:50 AM   #25
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Cruising speed is whatever you want....

Engine manufacturers recommend an operating range, and a NA can design an efficient speed for the hull...but that is a fleeting number based on load and conditions....

So.....cruising speed is whatever you want...

Now if you want to talk about efficiency for a particular boat...then a few specifics might come into play....
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Old 07-31-2015, 08:59 AM   #26
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What is 'cruising speed'

In the original question, it appears the answer is more related to the window stickers on vehicles stating 'estimated MPG highway/city'. What is theoretical under optimum circumstances is rarely (if ever) seen in use, Just as there are no real ways to unequivocally state cruise and max speed for any particular vessel. Cruise and Max differ wildly from stated numbers to what is seen in real life.
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Old 07-31-2015, 09:04 AM   #27
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[Q catUOTE=bayview;353543]11-13 on a 42' is not cruising speed it is a lie. That is too slow to be on plane and too fast for any reasonable fuel use.


Typically cruising speed on planning hulls is about 15% lower RPM than WOT and well on plane.
On my 44. cruising speed At cats. recommended 2200 rpm. Is 13[/QUOTE]


Astounding that a 44 is well on plane at 13 Kts.
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Old 07-31-2015, 09:56 AM   #28
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My FD boat has a plate near the helm, placed by the engine manufacturer (Volvo Penta). It says:-

"Cruising speed 200rpm below obtained top speed"

obtained top speed will be at max revs - 3600rpm officially,
I generally cruise at around 2000rpm, or 7.1 kn which seems the best speed for noise / vibration / speed / fuel consumption compromise.

.
My Volvo is the same...and I also cruise at 2K at 7.5 knots or so. Seems the most comfortable for the engine and SD hull form and averages 2 GPH at that number. WOT at 3600 is around 17 knots
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Old 07-31-2015, 10:15 AM   #29
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Folks, for virtually any planing boat the manufacturer will have a chart and graphs like this.

http://www.bostonwhaler.com/boat_gra...14101501AM.pdf

See what happens at 4000-4500 rpm?
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Old 07-31-2015, 12:33 PM   #30
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this is a very interesting thread.

I would say to take anything in an ad for a used boat with a big grain of salt. Our current boat was advertised with a max speed of 16 knots. On the test ride, the owner, salesperson and I were below and my husband was on the flybridge driving with the surveyor. The owner pointed to the VHF and said to the broker "see, I told you it would do 16."

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Old 07-31-2015, 07:39 PM   #31
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When I was gathering data to purchase our Monk, I ran across a old magazine article for a 42 Grand Banks with 375 hp Cat 3208TA's. It stated "She will cruise at 18 knots at 10 GPH". NOT!!!
Be very skeptical of any such claims, even by "experts".
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Old 07-31-2015, 07:59 PM   #32
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For a displacement boat, hull speed (most efficient) is computed at 1.34 x square root of the water line length. On my Gulfstar, a water line length of 33 feet gives me a theoretical hull speed of 7.69 knots. When I first got the boat, someone on this forum suggested to increase rpm 100 at a time and watch the bow. When it starts to rise, back off because you're wasting money trying to climb over your bow wave. I found a sweet spot at 1750 rpm on my two 85 horse Perkins, which will give me 7 to 7.5 depending on wind and waves. I plan on 3 gallons per hour usage, and usually do better. Canal cruising at 6 knots or less does even better, hovering above 1 gph. Slow and steady wins the day (and the wallet).
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Old 07-31-2015, 08:27 PM   #33
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Quote:
For a displacement boat, hull speed (most efficient) is computed at 1.34 x square root of the water line length.
Nope. That is the speed beyond which inefficiency goes up exponentially, which is what you describe later in your post.
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Old 07-31-2015, 09:27 PM   #34
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Cruising speed for most FD boats will be well below hull speed. Cruising speed for a FD boat is only a very narrow range usually 1/2 to one knot below hull speed.
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Old 07-31-2015, 09:39 PM   #35
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On this boat, a knot below hull-speed is at least twice as efficient as far as gallons per hour goes (less than two compared to nearly four gallons an hour at 6.3 versus 7.3 knots).
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Old 07-31-2015, 09:46 PM   #36
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On this boat, a knot below hull-speed is at least twice as efficient as far as gallons per hour goes (less than two compared to nearly four gallons an hour at 6.3 versus 7.3 knots).
Fuel burn and speeds almost exactly like mine and mine is supposedly semi-displacement...but it leans heavily towards the line between the two.
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Old 07-31-2015, 09:47 PM   #37
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With our mainship 400 (twin 240 hp yanmars) which I think of as sort of a hybrid trawler/cruiser I often get this same question. "What's your cruising speed?"

I always say that we have two: Slow cruise, which is most efficient, is at 1850 rpm, about 8.5 kts. Fast cruise, which we use when we want to get somewhere or when it is rough is at 2850 rpm, about 15 kts.

So for me there are two cruising speeds. Depends on the day.
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Old 07-31-2015, 11:11 PM   #38
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Anything over 7 knots is unsafe, sucks the wind out of one's lungs. When we do see 7 knots we consider breaking out the skis!

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Old 07-31-2015, 11:16 PM   #39
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I looked on the can of ablative bottom paint I used this spring. It said NOT to exceed 7 knots or the bottom paint would slough off too fast. Who am I to argue? Or was it 7 coats? Maybe 7 beers between coats. I forget.
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Old 07-31-2015, 11:16 PM   #40
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The NA of your boat knows what the cruising speed or speeds is.

But conversationally cruising speed is just the speed you run at most of the time.

Mine is 6.15 knots.
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