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Old 03-12-2017, 01:37 PM   #1
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Cruising speed...

A boats speed is often broken down by cruising speed and maximum speed. Maximum speed is self explanatory but I need help in defining what is cruising speed, is it the most efficient. If a boat is described with a cruising speed of 12 knots what is the effect of going 8 knots, is it less efficient at that speed?
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Old 03-12-2017, 01:57 PM   #2
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Imagine it being a slippery word that somewhat anticipates the boat's planned (expected?) use, throws a dart at a speed that matches that expected use, and then matches things like fuel usage, comfort, noise levels, whatever... to that "darted" speed.

IOW, it depends to an extent on hull form, and expectation of how the user might use that hull form.

A planing hull might be expected to plane, so "cruising speed" for that hull is likely to be "on plane" at a reasonable mix of noise, comfort, fuel consumption, etc. (For us, that's about 19-21 kts.)

OTOH, a full displacement hull might be expected to putter along nicely, so cruise would well be an expected "hull speed." There's a formula for that, relative to a boats overall length at the waterline, hull speed is actually more like "the fastest you can go without trying to climb your own bow wave and quadrupling your fuel use" or something like that.

In both cases, "efficiency" is maybe in the eye of the beholder. For us, puttering along at displacement speeds is cheap (in the grand scheme of things) even though that's not the way the boat use was envisioned and even though others with the same style of boat might blast around at 20-25 kts. For the full displacement guys, usually fuel economy is best at speeds lower than that maximum theoretical hull speed (a slight modification to the same formula)... but then the FD boater may value arrival time (speed related) more than fuel savings.

IOW...

It depends.



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Old 03-12-2017, 02:16 PM   #3
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NJ, Chris did a good job of explaining it. You can have two folks with the same boat and they will "cruise" at different speeds.

The PO of my boat told me that he said normally run the boat at 10 knots. Max speed is 12.

He was a retired banker with lots of money to burn. I am not. I normally cruise at 7 knots.

Most boats that I am familiar with have a relatively linear relationship between fuel burn and speed at low to moderate speeds. Past that and the graph becomes exponential. I like to run at about that break in the curve.

Not exact at all but most folks seem to know what their fuel burn is at different rpms and decide based on time vs dollars.
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Old 03-12-2017, 02:21 PM   #4
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There is no real definition for "cruising speed". Maximum cruising speed might be the fastest you can go and not ruin your engine prematurely, but you will find plenty of debate (here and elsewhere) as to what that means also.

I think of my boat, a semi-displacment hull with a high output engine, as having two cruisng speeds: slow at 7 kts and fast at 14 kts. The engine uses four times as much fuel to go 14 kts as 7 kts. That ratio is fairly common and until you get to really slow speeds, all boats get poorer fuel mileage the faster they go, just llke cars.

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Old 03-12-2017, 02:33 PM   #5
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I think of my boat, a semi-displacment hull with a high output engine, as having two cruisng speeds: slow at 7 kts and fast at 14 kts. The engine uses four times as much fuel to go 14 kts as 7 kts. That ratio is fairly common and until you get to really slow speeds, all boats get poorer fuel mileage the faster they go, just llke cars.

David
My situation exactly (only 14 knots is my WOT speed)....with a 27' waterline length, anything over 7-8 knots is pushing the efficiency envelope. My boats happy spot is right at 2K rpms and about 7.4 knots. Over 2 years of cruising at this speed my burn is just under 2 GPH. You can "feel" it when you hit the optimum speed/efficiency combination....At least I can..(but I'm a sensitive guy..:-)
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Old 03-12-2017, 02:35 PM   #6
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I cruise my boat at 7 knots.

6 Knots = 5.0 MPG
7 knots = 3.5 MPG
8 Knots = 2.0 MPG

All work. It all comes down to time and money. If I were buying, I would want to know fuel consumption, engine RPM, and speed at cruise. Depending on the boat, interior noise level can also be a big factor at cruising speed. I can afford the time to cruise quietly and economically.

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Old 03-12-2017, 03:09 PM   #7
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So I get the sense there is s maximum speed, maximum cruising speed and a personal cruising speed.
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Old 03-12-2017, 03:11 PM   #8
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So I get the sense there is s maximum speed, maximum cruising speed and a personal cruising speed.
you've pretty much nailed it....
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Old 03-12-2017, 04:19 PM   #9
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So I get the sense there is s maximum speed, maximum cruising speed and a personal cruising speed.
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Old 03-12-2017, 04:45 PM   #10
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If I were buying, I would want to know fuel consumption, engine RPM, and speed at cruise. Depending on the boat, interior noise level can also be a big factor at cruising speed.
Great advice by Ted. When I bought my boat I knew what fuel burn to expect at 7 knots and I knew what the max speed would be approximately. I really wasn't sophisticated enough to get (and fully understand) the fuel/rpm/speed curve for the boat. Other NP owners (Rogue) were helpful as well as Trevor Brice from North Pacific, but I didn't know what questions to ask.

In your shoes I would ask the broker/owners what the rpm and fuel burn are at various speeds, say from 6 knots to Max speed. That would give you an idea of how you likely would be using the boat. Also finding out from other owners of that boat type what speed they normally run at. In general, I think most folks will settle on very similar speeds due to fuel and noise. I did NOT do that and I wish I had. It wouldn't have changed my buying decision as my boat is reasonably efficient for a 43' FD hull.
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Old 03-12-2017, 05:06 PM   #11
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IMO cruising speed is the speed used by the seller to optomistically describe how fast the boat will go without being at WOT. On planning boats it is usually the highest rated RPM of the engine maker for other than brief operation. It is always an exaggeration IMO of reasonable operation if engine life and fuel use are considered important..
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Old 03-12-2017, 05:08 PM   #12
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IMO cruising speed is the speed used by the seller to optomistically describe how fast the boat will go without being at WOT. On planning boats it is usually the highest rated RPM of the engine maker for other than brief operation. It is always an exaggeration IMO of reasonable operation if engine life and fuel use are considered important..


For example, Yanmar says that all of their engines can be run continuously at 200 rpm less than rated rpm/hp. To say that my 370 hp 6LY, a high output engine that makes 70 hp per liter, can be run at 55 hp per liter continuously is total BS. And it is even worse for their 80 hp per liter at 4,000 rpm BY engines.

You need to get down to about 40 hp per liter or 400 rpm less than wot for the engine to have any decent life.

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Old 03-12-2017, 05:17 PM   #13
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...
OTOH, a full displacement hull might be expected to putter along nicely, so cruise would well be an expected "hull speed." There's a formula for that, relative to a boats overall length at the waterline, hull speed is actually more like "the fastest you can go without trying to climb your own bow wave and quadrupling your fuel use" or something like that.

...

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On my full-displacement boat, full speed is hull speed (2200 RPM). Normal cruise speed is one knot below hull/max speed, where fuel consumption rate is 50 percent of max (1800 RPM).

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Old 03-12-2017, 07:24 PM   #14
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Not mentioned but there is actually a cruising speed.

It's not specific. That is it's not 8.25 knots, or in my case 6.15. It's a range of speeds that the boat is designed for. For a FD boat that range is very narrow because operating a FD hull at hull speed is overdriven .. as Mark says by quite a bit. Over hull speed is insane. Much under a knot less than hull speed is not running most all FD hulls the way they were intended by the designer.

SD hulls have a wider range of speeds that the boat is designed for. The designer may smile at a SD 36' boat running 10 knots. 14 knots? Maybe or maybe not. One 36' SD boat could be great at 14 knots and another may be just burning excess fuel and making huge waves. Lighter boats are usually faster. Heavier boats slower. The straightness of the hull lines aft are very important elements of the design that would be likely to put a certain boat high or low in the range of speeds the boat was designed for.

A Naval Architect could consult the specifications of a specific boat and a hull lines drawing and predict a good range of speeds to cruise at. One can look up specifications for many boats and read the speeds recomended or intended for a specific boat. Ideally one could contact the designer of your specific boat and discover the intended speeds the boat was designed for. Most boats will have specs relative to a specific engine or engines. Exact rpm with x amount of weight and fuel burn too.

Planing boats
Since this is a trawler forum I'll only mention that weight becomes much more important and speeds on plane consume much more fuel and basically are only practical driven quite to very fast or very close to hull speed. This is rather general as there are a wide range of planing hulls.
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Old 03-12-2017, 07:50 PM   #15
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A boats speed is often broken down by cruising speed and maximum speed.
If you have a trawler in BC, e.g. you may want a possible WOT speed of 10-12kn to get through passes. After getting through, then reset your AP and go back to surfing your Poort on your new data plan.

(Disclosure, I flog Poorts on the theory that steering ain't so bad with an iPad aflame before you. Do set your alarms too...)
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Old 03-13-2017, 06:46 AM   #16
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" hull speed is actually more like "the fastest you can go without trying to climb your own bow wave and quadrupling your fuel use" or something like that."

Actually hull speed IS the speed where the bow wave is impossible to climb on top of , and you are using 2-4 times a cruise speed fuel burn..

The transom may be down a foot or two from normal trim,making huge waves.

As we all have calculators the rule of thumb is the square root of the length of the boats water line times 1.34 or even 1.4 for much lighter finer boats.

Normal trawler crawl is simply the SQ RT of the LWL times somewhere between .9 and 1.15

So a 36 ft waterline boat would have a hull speed of about 9K and be burning mucho GPH.
or a cruise speed of between 5 and 7K with a very modest fuel bill.

Many trawler engines will not operate at all long term at full throttle .

Hull speed is mostly for sailors where with free wind a 25K breeze might get them to hull speed.

A God Send for many cruisers is the ICW is measured in land miles so 6K becomes "seven miles an hour" when at the bar.
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Old 03-13-2017, 07:43 AM   #17
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So I get the sense there is s maximum speed, maximum cruising speed and a personal cruising speed.

Yep.

Or maybe max (at WOT), max cruise (engine maker's spec for max continuous operation; ours is 200 RPMs below WOT), and then personal high (maybe planing, for some hulls), and personal low (loafing or puttering).

Edit: Maybe even a personal "in the middle somewhere" influenced by time of day, location, destination, weather, sea states, and so forth...

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Old 03-13-2017, 07:50 AM   #18
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From the above posts, you can see that there is set definition of "cruising speed". Basically, it's whatever you find comfortable and economical (if that matters to you).
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Old 03-13-2017, 12:02 PM   #19
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Many folks will dig up the engine manual and cruise close to the Torque Peek if that gives a reasonable speed.

More industrial style HD engines will frequently find the engines gen set operating RPM as a sweet spot for long cruising.
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Old 03-13-2017, 12:54 PM   #20
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My hull goes no faster than 10 knots WOT when loaded. Fuel burn way up there. I cruise my 30 Atlantic trawler with a TAMD 40 B (165hp) Volvo engine at 16 to 1800 rpm. Makes between 6 to 8 knots depending on conditions. Burn a little over a gal per hour. Don't really worry about speed. Just like the fuel burn and noice level at those RPM. If you are concerned about speed you have the wrong boat
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