Planning And Displacement Speeds

[Cruising Roo]

Hello Fello Thalassophiles, I have no double what so ever that this post will invoke so much comment and I say this because it is not far from the old "how long is a piece of string" scenario. A motor boat that has a max speed of 22 knots and a cruise speed of 18... if one is not in a hurry, what is the most cost effective steaming speed to go from point A to B with EVERYTHING else being neutral in that wind, tides, load weights etc need to be taken out of the picture. I was asked this buy my 33 year old son (as if I know !!!) so I thought I would throw it out to the brains trust. Thanks all...

[Greatest Lakes]

Cruising speed on a power boat is generally its most efficient speed. It will vary based on the type of hull, so there isn't a set, most efficient speed. Most often, people calculate their gallons per hour at different speeds. Aircraft find their cruising speeds this way too.

[Cruising Roo]

Originally Posted by Greatest Lakes

Cruising speed on a power boat is generally its most efficient speed. It will vary based on the type of hull, so there isn't a set, most efficient speed. Most often, people calculate their gallons per hour at different speeds. Aircraft find their cruising speeds this way too.

Well there you go. Already I have learnt something. Yes I was not sure what "cruising speed" actually means so thank you. Wow..the boat I am looking at has a cruising speed of 18 knots which to me is "bloody fast. Again...Thank you "Great Lakes"

[StuM]

Making assumptions here about the size of vessel and type of hull based on your 18 and 22 knots

The most "cost effective steaming speed" is well below hull speed / displacement speed ( actually idle speed or just above) - but you will need plenty of time to get anywhere. But a good trade off between speed and fuel comsuption would be a bit below hull speed. That could be around 6-7 knots

Are you talking about diesel or petrol engine(s)? That can make a difference.

"Cruising speed" is generally a trade off between comfort, speed, fuel consumption and engine wear.

As you approach hull speed, fuel consumption per mile starts to increase rapidly until you get on the plane. Once you get on the plane the rate of increase of consumption per mile drops off quite a bit but will still increase as your speed increases.

Cruising speed will be a bit above planing speed. If you're running diesel, it's likely to be closer to WOT than with a petrol engine since diesels are more tolerant and suffer less wear/strain when pushed up to about 10% below WOT.

That said, in open water, cruising speed is often dictated by water conditions

Basically you need to try running at different speeds, and compare comfort, consumption and speed to find the "sweet spot" for the individual boat.


(With a "cruising speed" of 18 knots, your boat will probably get up on the plane around 15 knots - between 7 and 15 knots, it will be trying to climb up the bow wave and will not feel comfortable at all.)

[FPNC]

4.5 kts.

[rslifkin]

If it's a really small, light boat with gas engines, then whatever speed is most efficient on plane is likely to be as good as it gets (as the light boat takes a small enough fuel penalty for being on plane that the efficiency loss from running a gas engine lightly loaded to go slow ends up worse).

For pretty much anything with diesels, or a bigger, heavier boat with gas engines, best fuel economy will come somewhere between idle and hull speed. Usually about a knot below hull speed is the best speed / fuel economy compromise.

Keep in mind the effect of current. If you have fuel flow meters it's easier to optimize for current. If you're fighting a current, your "best efficiency" speed will likely be faster than in still water. If you're doing 6 kts against a 2 kt current, you're only going 4 kts over ground. So speeding up to 8 kts is now a 50% increase in speed instead of the typical 33%, which may change the math for "extra fuel vs extra speed".

Out of curiosity, what's the boat in question?

[mvweebles]

For most displacement and semi-displacement motoryacht/trawler style boats, best fuel economy will be in the range of 1.0-1.2 S/L ratio. So a boat with 36 feet of waterline will be most fuel efficient at between 6.0-7.25 kts.

[Be Free]

It is going to depend on your boat and conditions. You will need to take measurements in order to find the answer. All of the things you want to take out of the picture are very much in the picture.



Miles/hour (over land) divided by gallons/hour will give you miles per gallon. Whatever gives you the largest number under the current conditions is the speed you are looking for. (KM/HR divided by Liters/HR = KM/Liter if you are not in the US).



If you want a WAG (Wild-Ass Guess) it's probably going to be somewhere just below hull speed or somewhere just over planing speed. For me it's the former, for you it could very well be the latter.

[Adelie]

Originally Posted by Cruising Roo

Hello Fello Thalassophiles, I have no double what so ever that this post will invoke so much comment and I say this because it is not far from the old "how long is a piece of string" scenario. A motor boat that has a max speed of 22 knots and a cruise speed of 18... if one is not in a hurry, what is the most cost effective steaming speed to go from point A to B with EVERYTHING else being neutral in that wind, tides, load weights etc need to be taken out of the picture. I was asked this buy my 33 year old son (as if I know !!!) so I thought I would throw it out to the brains trust. Thanks all...

There are at least 3 “economy” speeds.
1. The best mpg speed is probably in the range of 3.0-4.0kt. Slower has less power demand to propel the boat, but the power to keep the engine turning over is flat regardless of speed. This may be at idle or slightly above. Long term running of a diesel at this rpm has maintenance and longevity implications. For a powerboat with a 36’ waterline length you can expect to get 5-8nm/gal
2. Displacement cruising speed is usually about 1.0 X sqrt(LWL). This is a trade off between speed and fuel economy. For a boat with a 36’ waterline length is will be about 6kt. For that size power boat you can expect to be getting 2-4nm/gal.
3. Planing cruising speed. This is when you are just barely on a plane so 2.7-2.9Xsqrt(LWL), about 17kt or so. Getting 1nm/gal at best. Slower than that MPG goes down.

Cruising speed is a bit subjective. Nobody that owns a power boat is going to be content going 4kt, let alone 3kt. So then it becomes tradeoff between fuel costs and time underway which is an apples to oranges tradeoff and varies skipper to skipper.

[Tedd]

Some data might help. This is from data posted on another forum for the Ranger Tug 31.

[Adelie]

Was the data estimated by naval architect or actually measured?

[Tedd]

Originally Posted by Adelie

Was the data estimated by naval architect or actually measured?

I assume you're asking me?

The data was measured and posted by the boat's owner, so far as I recall. Definitely not for planning or navigation purposes. I post it here in answer to OP's original question, not to provide a reference or for any other purpose.

[MicHughV]

I can give you this.

I have a twin engined fishing boat...twin 150 outboards on a hull of 24'.

The boat is equipped with a fuel flow meter so I can tell at a glance what it is.

Just putzing around at a slow " displacement" speeds, ie, around 5-10 knots, the engines running at about 2,000 rpm. At this rpm, the engines are somewhat laboring, trying to "push" this boat thru' the water...fuel consumption is surprisingly high.....15-18 gals/hour. I have to slow down to a crawl, maybe 5 knots, where the engines are not laboring and fuel consumption comes down.

Getting the boat on plane requires putting the hammer down, 4,500 rpm plus, with fuel consumption jumping into the 30- 35 gal/ hr.

Once up and running, I can throttle back. 3,000 rpm will keep the boat on plane, but I can hear the engines are not happy at this speed. The boat is on plane, but is wallowing, fuel consumption is again at 15-18 gal /hr.

Increasing the rpm to 3,500 rpm gives only a small increase in speed, but at this rpm the engines are "happy" , and fuel consumption is at around 13-15 gal/ hr.

Increasing engine rpm to around 4,000 rpm is really the " sweet spot" for these engines, with little noticeable increase if fuel consumption.

The bottom line is that most engines run best at an rpm that gives the best torque for that engine.
Most diesel engines have a power graph showing max torque at what rpm. This generally gives the best fuel consumption.

[Adelie]

Depending on the particular boat, motor and engine. The propellor curve drawn on a fuel consumption map will look something like this with the RED curve being prop demand. This is a curve for displacement speeds not planing, and the engine is for a Camery so not actually a marine engine.

As you can see the prop curve approaches peak efficiency at max throttle. If a controllable pitch propellor (CPP) were used the motor could be run in the most efficient fuel range for a much larger portion of the vessels operating speed range.

In Europe where fuel is more expensive CPPs are much more common than in the US where fuel prices have historically been much lower.

[Tedd]

Originally Posted by Adelie

As you can see the prop curve approaches peak efficiency at max throttle.

The BSFC of the engine alone should be about 255 g/kW.hr at WOT, so that would mean the maximum prop efficiency from your plot is about 85 percent. Does that sound right?

[Whoops. I guess the maximum BFSC from the plot is more like 330 g/kW.hr, so that would mean maximum prop efficiency would be about 77 percent, not 85 percent.)