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Old 21-07-2017, 06:17   #1
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Diesel cruise

Am looking for a boat to purchase, McKinna, Navigator, Viking and Ocean Alexander, in the 44 - 48 foot range.
I find cruise speeds and WOT to vary between boats and even models.
Floscan seems to me a method of calculating gallons per hour.
But, what does "cruise" mean?
Thanks, Karl
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Old 21-07-2017, 06:40   #2
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Re: Diesel cruise

Interesting Question, Probably means different things to most boaters but for me it means running your engine at the most fuel efficient RPM to gain the best knot speed/mileage per day,

Every boat has a different sweat spot depending on hull design /engine type propeller etc and normally comes back to sea trials and good recording of these to come up with the answer,

In reality how ever it's always a compromise

Cheers Steve
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Old 21-07-2017, 06:49   #3
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Re: Diesel cruise

Got it
Then,running engines at idle or idle plus while traveling is not necessarily the best way to conserve fuel.
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Old 21-07-2017, 06:49   #4
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Re: Diesel cruise

Long range cruise speed of a displacement motor boat is

Square root of LWL.

in knots.

The Hull Speed is Sq root of LWL x 1.34
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Old 21-07-2017, 06:52   #5
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Re: Diesel cruise

Favorable cruise speeds for boats like that will likely be 10-12 knots.
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Old 21-07-2017, 07:27   #6
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Re: Diesel cruise

Knucklehead, the above calculations give you the theoretical max hull speed NOT your cruise speed and to note the previous reply saying 10-12 knots is absolutely incorrect as I see your vessel is a Beneteau 473 which means your as built water line is only 43.8 ft , the sq root of that is 6.618 x 1.34 = 8.86 knots max speed, REMEMBERING that is the maximum hull speed possible no matter how big your engine is in horsepower.

THEREFORE: i would suggest depending on engine horsepower used the cruise speed would be in the range of 6 to max 7 knots to gain the most efficient speed and per day mileage at a sensible fuel burn per hour.

Cheers Steve
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Old 21-07-2017, 07:41   #7
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Re: Diesel cruise

Quote:
Originally Posted by Knucklehead View Post
Got it
Then,running engines at idle or idle plus while traveling is not necessarily the best way to conserve fuel.
Yeah, maybe to conserve fuel that will save you the most but heck, you wouldn't ever get anywhere. Somewhere above idle but way short of plane you should find that sweet spot the others are speaking of.

You will probably want to find two sweet spots. One for the slow speed cruising that will give the best mileage while still offering some speed. Another for faster speed when you need to get somewhere, i.e. beating bad weather into port. That spot is probably somewhere just above planing speed. Get the vessel up on plane then back off to where you are maintaining plane without struggling.

Other power boaters can possibly explain that a little better.
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Old 21-07-2017, 07:44   #8
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Re: Diesel cruise

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Knucklehead, the above calculations give you the theoretical max hull speed NOT your cruise speed and to note the previous reply saying 10-12 knots is absolutely incorrect as I see your vessel is a Beneteau 473...
Better read the first post again. OP was not inquiring about his own vessel, the cruise speed for which he already knows.
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Old 21-07-2017, 09:10   #9
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Re: Diesel cruise

Cruise is sort of a marketing term, if you ask a salesman how much fuel it burns in cruise, you will most likely be told what it burns at hull speed, if you ask how fast will she cruise, you'll likely get an optimistic number at full throttle and empty tanks etc.
Need to define an RPM that you consider cruise speed to be, and of course that RPM will vary from one engine to another
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Old 21-07-2017, 09:41   #10
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Re: Diesel cruise

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Originally Posted by Captsteve53 View Post
...which means your as built water line is only 43.8 ft , the sq root of that is 6.618 x 1.34 = 8.86 knots max speed, REMEMBERING that is the maximum hull speed possible no matter how big your engine is in horsepower.
I really don't wish to offend, but this statement is absolutely and completely wrong. It is a very common misconception, but it IS a misconception!

Hull speed is simply the speed at which you cannot go any faster, unless you have enough power available to climb on top of your own bow wave. With enough horsepower, almost any boat can exceed its theoretical hull speed.

There are a LOT of trawler-type powerboats out there that can easily exceed their theoretical hull speed. Most of them are designed to operate best, and most efficiently, at right around hull speed. Nonetheless, a lot have many times more horsepower available than they need to achieve hull speed, and pushed to WOT will readily exceed it.

To answer the OP's question, as others have said, "cruise" pretty much means whatever you want it to mean. There is no, single, universally-accepted definition of the term.
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Old 21-07-2017, 09:47   #11
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Re: Diesel cruise

I think of cruising speed like max VMG sailing.

It's all a compromise and you'll figure it out after a few tanks and seeing what works and feels best. Often there is a sweet spot where things other than fuel economy come into play. I know on my sailboat, there is a certain RPM where everything feels right. It may not give me the best range/speed/economy but the engine sounds happy, the harmonics are good.
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Old 21-07-2017, 11:11   #12
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Re: Diesel cruise

Quote:
Originally Posted by Knucklehead View Post
Am looking for a boat to purchase, McKinna, Navigator, Viking and Ocean Alexander, in the 44 - 48 foot range.
I find cruise speeds and WOT to vary between boats and even models.
Floscan seems to me a method of calculating gallons per hour.
But, what does "cruise" mean?
Thanks, Karl

Yes, Floscan would give you more empirical data.

"Cruise speed" is a variable mostly depending on boat and engines. "Cruise RPMs" is likely more useful, and often starts with the engine manufacturer's recommendations for RPMs and load.

Often you'll hear recommendations to cruise "200 off the top" (200 RPMs below WOT), or maybe "90% of WOT" (or 80, or 70). Varies per maker and actual engines.

That's usually then modified by the hull. For example, some planing hulls are more efficient/comfortable at 22 kts than they are at 20 kts. That could mean 90% of WOT works better than 80%, or maybe 200 off the top works better than 400 off. Et cetera.

And then that approach is also modified by load, sea states, wind, tide/current, etc. What works for flat water might need modification in chop.

Another "cruise speed" is often based on hull length, and usually somewhere between .9 or 1.1 * SQRT(LWL) is the most economical. (See Beebe Cruising Under Power.)

IOW... it depends.

You might want to check out trawlerforum.com (sister site) since there are maybe more folks there with those brands of boats.

-Chris
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