Difference in fuel consumption: Displacement v Planing

[Deleted User YDKXO]

We've just returned from a short break on our boat and because we only planned some local cruising, I decided to try cruising at displacement speed rather then blatting about everywhere at 20-25kts. I expected there to be a significant difference in fuel consumption but actually, I was quite shocked at how much of a difference there actually was. First though a small caveat at this point; my fuel consumption figures are taken from the engine electronic display system and I dont know how accurate these figures are.
At planing speeds, our boat normally consumes fuel at a rate of 0.60 - 0.65 nmpg. At minimum in gear idle speed of 700rpm, at which we're doing about 7.3kts, the consumption decreases to 3.5nmpg. In other words, my boat consumes about 5 times as much fuel per mile at 20-25kts than it does at 7kts What was surprising though was that how just a small increase in displacement speed dramatically increases the fuel consumption. On my boat, if I increased speed to 900rpm, at which the boat is doing about 9kts, the fuel consumption increases by more than double to about 1.5nmpg. I guess this is because at 9kts my boat is approaching its max theoretical hull speed and has to displace a lot of water. I also tried running on one engine only and although the speed drops to just over 5kts, the fuel comsumption also drops to about 4.5nmpg.
Even if my figures are not entirely accurate, I'm confident that they demonstrate just how much fuel you could save by going at minimum speed. My boat carries 2500 litres of fuel and at 7kts, I would have a range to empty of over 1800nm compared to 350nm at planing speed. On one engine, I'd have a range to empty of nearly 2500nm, which would almost get us across the Atlantic, in theory! Spending a few days at displacement speed does throw up a few other issues though, apart from the obvious one that it takes forever to get anywhere. First, it took a superhuman effort of discipline on my behalf not to push the throttles further down and I think if the weather hadn't been so sunny and the sea so flat, I would have given in to temptation. Second, going at minimum speed highlights other issues with the boat. At planing speed, you mostly hear wind and wave noise but at displacement speed, exhaust noise and vibration become the dominant irritations. In addition, if youre going to spend several hours at the controls rather than 1 or 2, then the comfort of the helm seat is a big issue.
All in all though, this experiment was a bit of an eye opener for me and I may well be slowing down in the future. It was certainly immensely satifying to arrive back in the marina after 4 days and about 100nm of cruising and see that the fuel guage hadn't discernably moved

 

[PowerYachtBlog]

I have started doing slow speed displacement short runs quite a long time ago. Since 2006, and with more accurcy since 07.
I usually do it with runs under the ten nm distance. One of the longest was with my previous boat on 07 when did from Riposto Etna port to Taormina's Isola Bella.
In 2009 with my current boat I cruised one third of a Gozo round up on displacement 6 knot speed.
Yes you save a lot of fuel and your figures are very similar to myn in percentage terms but for long distances I would become a bit crazy.
I also think that doing this in Croatia is easier as all the place more or less have a nice scenery, especially when cruising in between the thousands of islands. Doing it in open sea is a bit more trickery I guess...

 

[MapisM]

At planing speed, you mostly hear wind and wave noise but at displacement speed, exhaust noise and vibration become the dominant irritations. In addition, if youre going to spend several hours at the controls rather than 1 or 2, then the comfort of the helm seat is a big issue.

I'm not surprised at all by your findings.
And I'd expect those numbers reported to be reasonably accurate, with electronically controlled engines.
Just a couple of comments on the 2 points above:
The first is obviously affected by the fact that you're dragging around a couple of huge blocks of iron which, awfully oversized for D speeds. You wouldn't have such irritation on a boat designed for slow cruising.
The second actually is not an issue at all, if you think about it. Cruising at D speed means that you spend close to none of your time seated at the helm, 'cause you can cook, have a shower, a nap, work on your laptop, or whatever. All you need is an occasional look around, when cruising in crowded areas or where you might encounter some lobster pots or nets.
It's really very different from "driving" at high(ish) speed.
Though I admit that it took a while also for myself to get used to that, when I jumped on slow speed cruising one decade ago, coming from a previous experience of 36 to 38 kts cruising speed...

 

[MapisM]

Doing it in open sea is a bit more trickery I guess...

Yep, that's where P boats used at D speed might be a bit disappointing - even if stabilised.
Otoh, it's exactly in open sea that, with the right vessel, you can really appreciate what slow speed cruising is all about.

 

[mjf]

Thats interesting as the electronics measure accurately I would guess - or why have them?

Like others we started this a few years ago on a fine calm day when SHMBO asked why I was going so fast when she wanted to enjoy the trip. I said 'cos it will be rolly polly and uncomfortable but hey ho lets slow down.

Blimey - it was nice on auto and able to roam about and watch the world go by at sailing speed without any effort. now do this for 80% of the open water stuff if its nice.

Wonderful too to say this 'aint nice lets get home and pull up the skirts and get home at 20-25 knots or whatever. I agree your gauge comment too as quite satisfying to spend time on the boat and moor back at base with needles where they were before or there or there abouts!

 

[miket]

compromise

The irritations of noise and vibration and to some degree ride, would probably not be issues with a "designed" displacement hull.
The Dutch boats (eg Sturier range) are designed to max' at around 9 knots and cruise all day at 7.5/8.0, with little noise or vibration and with stabilizers a good ride even in inclement conditions.
I sympathise with the restraint required not to open up. On the few occasions that we go to sea each year I have never achieved that restraint!

 

[jfm]

Yup, those figures look right to me mike. I see a big mpg difference between D and P, and also between slow D and fast D (6 versus 11 knots, say)

With full tanks and engines in gear at idle I get 6 knots and >4000nm range on the Garmin screens!

But to get a decent stabiliser effect in big seas you need 10-12kts of speed to get enough flow over the fins, and in reality 6kt crusing long term isn't feasible for that reason imho

I personally do not think there is a material seakeeping difference between a P hull and a D hull if you have stabs, but of course the P hull gives you the choce to go faster if you wish. Hence the stabilised P wins hands down, very much imho, and i think that would be your perfect next boat

You're right that helm seat comfort becomes relevant but you could easily retrofit to the Freti upper helm a pair of the marinised Recaros that Fairline use, here, for about a grand each. These are very nice seats in most normal cruising weathers, imho. You might remember I retrofitted three of them on my last Sq58

 

[DAKA]

We've just returned from a short break on our boat and because we only planned some local cruising, I decided to try cruising at displacement speed rather then blatting about everywhere at 20-25kts. I expected there to be a significant difference in fuel consumption but actually, I was quite shocked at how much of a difference there actually was. First though a small caveat at this point; my fuel consumption figures are taken from the engine electronic display system and I dont know how accurate these figures are.
At planing speeds, our boat normally consumes fuel at a rate of 0.60 - 0.65 nmpg. At minimum in gear idle speed of 700rpm, at which we're doing about 7.3kts, the consumption decreases to 3.5nmpg. In other words, my boat consumes about 5 times as much fuel per mile at 20-25kts than it does at 7kts What was surprising though was that how just a small increase in displacement speed dramatically increases the fuel consumption. On my boat, if I increased speed to 900rpm, at which the boat is doing about 9kts, the fuel consumption increases by more than double to about 1.5nmpg. I guess this is because at 9kts my boat is approaching its max theoretical hull speed and has to displace a lot of water. I also tried running on one engine only and although the speed drops to just over 5kts, the fuel comsumption also drops to about 4.5nmpg.
Even if my figures are not entirely accurate, I'm confident that they demonstrate just how much fuel you could save by going at minimum speed. My boat carries 2500 litres of fuel and at 7kts, I would have a range to empty of over 1800nm compared to 350nm at planing speed. On one engine, I'd have a range to empty of nearly 2500nm, which would almost get us across the Atlantic, in theory! Spending a few days at displacement speed does throw up a few other issues though, apart from the obvious one that it takes forever to get anywhere. First, it took a superhuman effort of discipline on my behalf not to push the throttles further down and I think if the weather hadn't been so sunny and the sea so flat, I would have given in to temptation. Second, going at minimum speed highlights other issues with the boat. At planing speed, you mostly hear wind and wave noise but at displacement speed, exhaust noise and vibration become the dominant irritations. In addition, if youre going to spend several hours at the controls rather than 1 or 2, then the comfort of the helm seat is a big issue.
All in all though, this experiment was a bit of an eye opener for me and I may well be slowing down in the future. It was certainly immensely satifying to arrive back in the marina after 4 days and about 100nm of cruising and see that the fuel guage hadn't discernably moved

Great post Mike and I am confident we could all save wads on fuel @ 5 knots, but its not as much fun is it , you could save even more by staying in the marina








..............you cant take it with you

 

[MapisM]

I personally do not think there is a material seakeeping difference between a P hull and a D hull if you have stabs

I have to agree with that J. It isn't a material difference, it's rather a huge one!

 

[tamarind]

I am taking a new princess 64 down to the algarve from Plymouth next week, working out the fuel consumption is obviously very important. At 900rpm doing 9 knots she will have a safe range of 945nm, going up to 1100rpm giving 11knots the range halves to 445nm and increasing to 1500rpm, 15knots the range reduces to 285nm. The total fuel capacity is 4500 litres.

 

[Andrew_Fanner]

The effect of the tide is a lot more significant at displacement speeds as well. As a river based user displacement is pretty much the norm. Steering isn't an issue but comfort, as you say, definately is. I'm prone to standing at the wheel, which is my own fault/bad habit

 

[Hurricane]

Welcome, Mike, to the world of POOTLING

Note thats NOT "Poodleing" - which sounds a bit illegal!!

Yep, I agree with with these figures as well - and have long passages at those speeds to prove the figures.

We dont go down to the 7 knots and single engine etc because on the "big picture" scale of things the difference between 9.5 knots and 7 isnt a significant enough saving.

But why not consider a passage plan leaving late evening after dinner and POOTLING through the night (the Med is often flat at night as well) - you could get 100 miles or so done and still have the fuel gauges reading full.

Your comment about the consumption going up if you push just a little faster
than your max displacement speed is very understandable. Anything over displacement speeds mean that you are actually using energy to lift the boat rather than push it through the water.

I've always found the electronic consumption displays to be accurate even though they are "theoritical".

 

[Hurricane]

I am taking a new princess 64 down to the algarve from Plymouth next week, working out the fuel consumption is obviously very important. At 900rpm doing 9 knots she will have a safe range of 945nm, going up to 1100rpm giving 11knots the range halves to 445nm and increasing to 1500rpm, 15knots the range reduces to 285nm. The total fuel capacity is 4500 litres.

I'm not sure you should rely on the range being 945nm.
Have you got extra fuel on board?
4500 litres seemed a bit big for a P64 so I got out the recent MBY report on the Princess 64 - it shows 3409 litres which sounds correct.
Our P67 holds just over 4000 and I wouldnt rely on a range like the one you've stated - especially if you run into bad weather.

Are you sure about your figures?

 

[mjf]

Welcome, Mike, to the world of POOTLING

Note thats NOT "Poodleing" - which sounds a bit illegal!!

Yep, I agree with with these figures as well - and have long passages at those speeds to prove the figures.

We dont go down to the 7 knots and single engine etc because on the "big picture" scale of things the difference between 9.5 knots and 7 isnt a significant enough saving.

But why not consider a passage plan leaving late evening after dinner and POOTLING through the night (the Med is often flat at night as well) - you could get 100 miles or so done and still have the fuel gauges reading full.

Your comment about the consumption going up if you push just a little faster
than your max displacement speed is very understandable. Anything over displacement speeds mean that you are actually using energy to lift the boat rather than push it through the water.

I've always found the electronic consumption displays to be accurate even though they are "theoritical".

Does this Pootling malarkey cause any wear issues or other engine concerns?


This Pootling is clearly a girly thing as SHMBO calls it that too........

 

[DAKA]

just a thought ...........

could pootling be reduced if your next boat was 1 m shorter ?

buy a row of terraced houses up north with the saving and then blast around to your hearts content on the rents

 

[mjf]

I'm not sure I particularly want to reduce the Pootling - I like the getting there too now at slower speed. Easy to walk about, watch the world move past sit at the F/B table and look throu bino's without the eyecups banging your eyebrows - I'm sold!

Back to saily boat next I think

 

[Hurricane]

Does this Pootling malarkey cause any wear issues or other engine concerns?


This Pootling is clearly a girly thing as SHMBO calls it that too........

Always give it a bit of a blast at the end of the passage.

 

[Hurricane]

just a thought ...........

could pootling be reduced if your next boat was 1 m shorter ?

buy a row of terraced houses up north with the saving and then blast around to your hearts content on the rents

I think it might need to be a whole town

 

[mjf]

Oh! Good. I usually get a move on when clear of the harbour anyway for say a mile or two to check all is ok once the engines are warm etc - and funnily enough I return to this mode on 'finals' too as it sort makes the arrival a bit more exciting.

 

[DAKA]

Always give it a bit of a blast at the end of the passage.

I'm lost now , are you talking about pootling or have you gone back to poodleing