What affect does sea state have on fuel burn?

[burgundyben]

My boat averages 3.03 mpg at 20 knots. Most of our trips are in fine weather on calm water.

Recently on an 80 mile run with a big long swell there was little affect. I guess what you lose going up you gain going down, so net loss isn't so much.

On short chop or rougher weather consumption goes up, but Ive never got as far as gauging by how much.

 

[rafiki_]

The physics say that if the boat has to work harder, it will need more fuel.

 

[Deleted User YDKXO]

I've seen a figure mentioned in one of the boaty mags mentioned of up to a 30% increase in fuel consumption in moderate head sea conditions. I don't know how true that is but some energy must be lost displacing water as the boat ploughs through the waves

 

[Grampus]

Going round Rattray Head in difficult conditions we were consuming 2.25 litres per kilometre at 2800 rpm and down to 20 knots SOG.

Coming down through the North Channel with the swell, wind and tide chasing us home, we were consuming 1.7 litres per kilometre at 2980 rpm and achieving 28.5 knot SOG.

These are actual records from the log.

galps

 

[oceanfroggie]

The physics say that if the boat has to work harder, it will need more fuel.

+1

Can't defy the laws of physics. The greater the sea state the greater the fuel burn.

 

[A_8]

In difficult conditions and as long as I can stay on the plane I've found that fuel consumption increases by 15-20%.

 

[DAKA]

Not attempting to argue with anyones experience or reasoning above but would like to add my experience of two boats

Fairline cornich on outdrives
experience of 5-6 years covering 1500-2000 nm per year
calm sea state 25-26 knots = 1.6mpg
moderate rough sea state 12-17 knots (reduced speed due to comfort) = 1.6 mpg

Princess 360

Calm seastate cruise 24-25 knots
Moderate 18-20 knots for comfort
again no noticeable change in MPG.

Thankfully with 3g and ecwmf giving accurate 10 day weather I havent been far enough in a rough seastate to comment.


I can understand a semi displacement boat that cruises @ 14 knots has to use more grunt to maintain 14 knots but burgundyben has a lightish 20knot boat, extra power isnt required in a rough seastate, reduced power is used to continue @ 12-14 knots.

 

[Dave_Snelson]

I've seen a figure mentioned in one of the boaty mags mentioned of up to a 30% increase in fuel consumption in moderate head sea conditions. I don't know how true that is but some energy must be lost displacing water as the boat ploughs through the waves

Maybe about right as a head sea almost always means a head wind as well. Mind you, on the other hand, in nice calm weather, I'm tempted (more than tempted) to "open her up", so my fuel consumption rockets

 

[oGaryo]

I had a fuel flow monitor in my previous boat that showed the best planing fuel economy was at 23.5knts so in that case, if the sea state dictated I could go on the plane but not reach 23.5knts comfortably, then it stands that sea state does affect fuel economy

 

[Deleted User YDKXO]

calm sea state 25-26 knots = 1.6mpg
moderate rough sea state 12-17 knots (reduced speed due to comfort) = 1.6 mpg

That doesn't prove anything because you haven't told us what the mpg would have been in calm conditions at 12-17kts. Its a bit of a generalisation but lower speeds normally produce better mpg so maybe at 12-17kts in calm conditions, your boat would have achieved better than 1.6mpg in which case, sea state has an effect on fuel consumption

 

[DAKA]

That doesn't prove anything because you haven't told us what the mpg would have been in calm conditions at 12-17kts. Its a bit of a generalisation but lower speeds normally produce better mpg so maybe at 12-17kts in calm conditions, your boat would have achieved better than 1.6mpg in which case, sea state has an effect on fuel consumption

Accepted Mike BUT SIGNIFICANTLY it tells us that we do not need to worry about running out of fuel mid channel because it is rough.

I had some awful trips up the East coast in my corniche, F6-7, moderate to rough in the North (where we were heading into), on passage from Lowerstoft to Brigg (130 nm ish) I can recall seeing one coaster inside us pounding the waves and his elevated bridge set well back getting drenched in wave after wave, I didnt worry about range. We still returned 1.6 mpg

 

[Deleted User YDKXO]

Accepted Mike BUT SIGNIFICANTLY it tells us that we do not need to worry about running out of fuel mid channel because it is rough.

OK we're getting all colourful here

But ONLY if you slow down so that's not actually answering the OP's question which was what effect sea state has on fuel consumption. In fact your figures tend to support the notion that sea state does affect fuel consumption because generally planing boats achieve better fuel consumption at slower speeds than higher speeds. In your case, you say you would have to slow down to 12-17kts in order to maintain the SAME fuel consumption that you could achieve at 25-26kts which tends to suggest that at 25-26kts in rough seas, you would not achieve 1.6mpg

 

[DAKA]

OK we're getting all colourful here

But ONLY if you slow down so that's not actually answering the OP's question which was what effect sea state has on fuel consumption. In fact your figures tend to support the notion that sea state does affect fuel consumption because generally planing boats achieve better fuel consumption at slower speeds than higher speeds. In your case, you say you would have to slow down to 12-17kts in order to maintain the SAME fuel consumption that you could achieve at 25-26kts which tends to suggest that at 25-26kts in rough seas, you would not achieve 1.6mpg

now look here mike

I have as usual assumed a lot here...........

Burgundyben has a 30ft ish planing boat on legs that usually cruises @ 20knots
that sort of covers 75% of forum posters

I assume my 31ft corniche on legs returns similar results to the above.

I assume no one on this forum is capable of helming a corniche @ 26 knots through the wash in a N 6-7 moderate to rough seastate.

I further assume even if some clever dick tried the corniche would break up before we had chance to refuel it.

I suspect that the results are similar for boats even the size of yours but stop short of that, it appears my shaft drive boat also returns consistent mpg but I accept it is possible that a delivery skipper could get behind your controls and thrash it through heavy seas but a bit of extra fuel used would be insignificant compared to the constant wave impact damage.

I believe I am talking about the reality of the situation in that we all slow down in a rough sea and we use more or less the same fuel to get from a-b although extra fuel will be used if the tide turns against us as extra miles will be covered.

hypothetically the rest of the posts are accurate but I had already agreed with that earlier , however from a practical view point it doesnt make a lot of sense.


Excludes semi-dis and plodders who dont need to slow down in order to avoid injury / damage.

 

[Deleted User YDKXO]

now look here mike

I have as usual assumed a lot here...........

Burgundyben has a 30ft ish planing boat on legs that usually cruises @ 20knots
that sort of covers 75% of forum posters

I assume my 31ft corniche on legs returns similar results to the above.

I assume no one on this forum is capable of helming a corniche @ 26 knots through the wash in a N 6-7 moderate to rough seastate.

I further assume even if some clever dick tried the corniche would break up before we had chance to refuel it.

I suspect that the results are similar for boats even the size of yours but stop short of that, it appears my shaft drive boat also returns consistent mpg but I accept it is possible that a delivery skipper could get behind your controls and thrash it through heavy seas but a bit of extra fuel used would be insignificant compared to the constant wave impact damage.

I believe I am talking about the reality of the situation in that we all slow down in a rough sea and we use more or less the same fuel to get from a-b although extra fuel will be used if the tide turns against us as extra miles will be covered.

hypothetically the rest of the posts are accurate but I had already agreed with that earlier , however from a practical view point it doesnt make a lot of sense.


Excludes semi-dis and plodders who dont need to slow down in order to avoid injury / damage.

So you agree then. Sea state affects fuel consumption. In order to maintain fuel consumption, you have to slow down which, in any case, you'd want to do to save your fillings and the contents of the fridge

 

[Bandit]

My boat is a heavy diesel semi displacement 19 knots cruise, sea state appears to make little difference of burn per nm through the water, a good following swell and a bit of surfing to 25 knots reduces fuel burn.

I assume it makes a reasonable difference to sports boats that due to sea state can't plane.

I assume that for petrol powered sports boats that can't plane due to sea state it makes a massive difference to fuel burn per nm through the water.

I really heavy weather my fuel burn per nm reduces substantially because I run at 10 to14 knots in a heavy sea say f 6 to f7.

 

[oGaryo]

my eyes hurt trying to read the colorful stuff, get back to good old B&W please

 

[burgundyben]

Burgundyben has a 30ft ish planing boat on legs that usually cruises @ 20knots
that sort of covers 75% of forum posters

Shafts.

My question is, for any given speed, how much does sea state affect consumption.

I cruise at 20 knots in calm weather and I cruise at 20 knots in choppier stuff, how much more fuel does choppy use?

Its no biggie, I've still not managed to burn more than 1000ltrs in a year, despite trying.

 

[omega2]

As far as we are concerned we do not want to know, if we were "informed" we would never let the ropes go. LOL

 

[PEJ]

I have only had my new boat a few weeks but it has fuel sensors. Coming back from Poole in about a F5 or F6 (much rougher than I prefer to be in) I noticed the fuel burn was around 5.0 when I would have expected it to be about 3.8 to 4.0.

So on that basis about a 25% increase in fuel burn. I also noticed that one engine was using more than the other but I don't know if this was a true reading or the gauges are not meant to be that accurate. One was using about 10% more than the other.

 

[DAKA]

I cruise at 20 knots in calm weather and I cruise at 20 knots in choppier stuff,

Oh , I was talking about waves you cant see over the top of !

Mike 1 DAKA sulks off