Cruising speed

[Adetheheat]

The Cruising speed of my boat is 20 knots. Is Cruising speed the most efficient performance of the boat? E.g. if I do 20 miles at 20 knots then less fuel is used if I do 20 miles at 5 knots?

 

[B27]

A planing hull will probably tow a huge wave at 5 knots, and the engines won't be in their most efficient mode.
Some bigger boats with 20knot cruising speed might be long enough to not be making waves at 5 knots, so might be very economical.
It's dangerous to generalise. Our RIB is quite economical on the plane at 25 knots, at 12 knots it's terrible making a big wave. At low speeds maybe 4 knots, it's not too bad, if you keep the weight forwards a bit to get the transom mostly out of the water. Flat out in waves, it's thirsty again.

 

[Alicatt]

Depends!
What boat is it? what engine is it?
Usually at displacement speed is the most economical in distance per litre/gallon but not always.

In theory at 6knots my boat does 2litres/h (Wide open Throttle) at 4knts it is 1.2litres/h (cruising speed) Therefore to do 6km @ 6knt =2litres or 6km @ 4knt = 1.8litres so 4knts would be more efficient to cover that distance, but it would take half as long again ie. 1hour 30 minutes against 1 hour @ 6knt

It is something I discovered over the weekend is that my make of boat was also fitted with larger engines and they can plane at 16knts to 18knts, is it worth splashing out £2.5k on a 60hp engine to do that

 

[jbweston]

The builders of my boat, a Sargo 31, produced a sheet giving fuel burn at different speeds for the different engine options. I have two Volvo D4-300s and, according to the sheet, my most economical cruising speed (in litres per nautical mile or mpg) is 24.1 knots / 2,200 rpm. At 10 knots the consumption per mile is around 50 percent more.

Of course the consumption per hour is much less at 10 than at 24 knots, but that doesn't nearly compensate for the reduced distance covered per hour.

Your figures will be different, but this is a good illustration of how a planing hull can be more fuel efficient on the plane than at displacement speeds.

 

[Rappey]

Not that this helps you but others may find it interesting.
16ft chubby centre consol with 50hp mercury.
At 20 knts, around 3500 rpm, 2.1 us gallons/hr
At 5 knts, around 1500 rpm, 0.6 us g/h.
It will take 4 hrs to do 20 miles so 2.4 us gallons.

If i do the same for a 75hp outboard the 20 knts and 5 knts is the same fuel usage but the engine would use 2.8 us g/h on the same boat.

 

[Adetheheat]

So when the manual says Cruising speed is 20knots it doesn't mean you are using the least fuel for the distance travelled?

 

[jlavery]

So when the manual says Cruising speed is 20knots it doesn't mean you are using the least fuel for the distance travelled?

It really depends on what the manufacturer has defined as 'cruising speed'. In aviation, aircraft have a 'max range speed' (for different weights too, IIRC, many moons since my degree in Aeronautical Engineering). Pilots on here will correct me I'm sure.

So for a motor boat there will be a 'sweet spot' speed where your burn per mile is best. Unfortunately, unless the manufacturer/manual says whether this is what 'cruising speed' is, it's impossible to know. However, it's probably a good guide!

For a planing boat, my gut feel is that max range speed will be the speed at which, once established on the plane, one can throttle back to and remain planing.

 

[jbweston]

So when the manual says Cruising speed is 20knots it doesn't mean you are using the least fuel for the distance travelled?

I agree with the last post.

Cruising speed could mean the engine manufacturers' maximum continuous rpm, or more likely their recommended lower rpm that they say is 'economical'. For example, Volvo say full throttle for my engines is 3,400-3,600 rpm. The say:

'We recommend a cruising speed which is at least 10% below the maximum engine speed at full speed (full throttle)'.

So that would give an engine speed of 3,000 to 3,240 or less. Compare that to the boatbuilders' most economic miles per litre of 2,200 rpm (see my post above) which is the same concept as jlavery's aircraft max range speed, because the most miles per litre give the most range on a given quantity of fuel.

In the absence of builders' data you might have to experiment with different speeds and measure how much fuel is used from top-up to top-up for the distance covered. To be completely scientific you would have to fit a fuel flow meter (assuming you haven't already got one).

 

[Momac]

20 miles at 5 kts would use less fuel than 20 miles at 20kts.
What length is the boat in question or is it just a general enquiry?

 

[ylop]

So when the manual says Cruising speed is 20knots it doesn't mean you are using the least fuel for the distance travelled?

I don't think there is any formal / official / standard definition of Cruising speed - so it means whatever the manufacturer wants it to mean. That could be most fuel efficient, but it could also be "the same as our competitors", or "what feels right"

 

[jbweston]

20 miles at 5 kts would use less fuel than 20 miles at 20kts.
What length is the boat in question or is it just a general enquiry?

That's not correct for many boats. For example, see my post above (#4) about my boat. Many planing boats are more economical in terms of fuel per mile at planing speed than at displacement speed.

It depends on the specifics of the boat.