Difference in fuel consumption: Displacement v Planing

[David of Essex]

Fuel usage gauge

With either a SD or Planning boat it's nice to have the choice to either cruise at 8knots or 20knots.

I have a single engine volvo fitted with 260hp engine. Looking to fit a fuel usage gauge. Which make would you recommend?
David

 

[Nautical]

A topic that seems to be gathering more and more discussion these days, reflective of changing times and in times of austerity a worthy issue to be considered if we are to do our bit as it were, perhaps even more importantly to be seen to take the issue of fuel consumption seriously and not just burn it in copious amounts for not much of a reason than for purely leisure purposes.

From a biased viewpoint (for obvious reasons) I would whole heartedly accept that Planing hulls with stabs are a very viable proposition for those of us that do most of our cruising in reasonably fair weather and with ranges less than 500NM, perfectly respectable and in most instances all you need for the vast majority of cruising senarios around the Med and coastal North Europe.

I would argue though that if you cruise outside of this scope and intend long periods offshore or Ocean cruising or indeed anticipate a goodly percentage in less than favourable conditions then one needs to look more towards semi D or full D. While sea keeping down the weather can be comparable between all three P,SD and D it is when you have to make course into the weather is where D and SD really makes a difference (assuming in the case of SD you have a hull designed with this in mind and nice fine entry forward) . It is very hard to describe here the sense of security an extra ten or twenty tons makes length for length when it does get tougher offshore. It is not the weight itself that makes the difference so much it is the make up of that weight, generally the layup of the hull structure is heavier and then everything attached to the hull is just so much heavier and built for purpose, you can measurably feel that difference when pounding hour after hour into heavy weather. A production planing hull by nature has to be lighter if you want to achieve those high speed numbers (25 knots +) and not require a tanker following you, those design issues alone will show up weak links in what in truth was not a vessel designed for continuous heavy weather sailing year in year out.

As far as engines are concerned, it would seem monumental over kill to have twin 1000 hp engines in a planing hull if a goodly % of intended cruising would be at D speeds, while modern common rail, whizz bang electronically controlled diesels can cope with hours of running at tick over it is really not the most efficient way to run them. I am no engineer but I am reliably told that max efficiency comes when engines are running at optimum temps and pressures rather than ten hours and upwards at little more than tick over.

Horses for courses etc, for ad hoc displacement cruising as and when in reasonable weather (with the odd roughty toughty bit thrown in) then P with stabs is a great option, if you intend lots of long passages in less than favourable conditions offshore then I would (naturally ) argue that SD or even D (if you can stand the 7 knots speed) is more applicable.

 

[penfold]

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

Oilrig standby ships usually have it written into the standing orders that when on station the main engines must be run up to temperature for 1/2hr every 12-24hrs if there is prolonged idling; long periods at idle can cause valve and injector fouling(particularly with older mechanically controlled injection systems), and in extreme cases injector failure and exhaust fires. With electronically controlled commonrail diesels it is much less likely that any of these things will occur, but even if the operators manual doesn't specifically recommend it I would follow the above procedure anyway, just to 'clean the pipes' as it were.

 

[asteven221]

We poottle along at D speed all the time unless the sea state makes it uncomfortable. Recently we were doing our usual 7.5knots, however this time with some tide on the nose. My mind drifted and I started wondering about the implications on fuel consumption when the tide is on the nose.

For example if we're going along at 7.5 knots SOG at D speed at 1000rpm then we will be doing "x" mpg, which I reckon is about 2.75mpg. That's without any tide. Now let's say the tide is directly against us at 2 knots, thus 5.5knots SOG. Consequently we will be out at sea correspondingly longer and will burn more fuel as a result.

Is it better in that situation to accept the extra time at sea and the extra fuel cost, or hit the throttles go to P speed at say 20 knots and get a relative fuel benefit?

I think what I am trying to say is this. At full on P speed, does the boat sufficiently "fly" on top of the water to the extent that the influence of the tide on the boat is relatively inconsequential, thus fuel consuption is relatively speaking better in these circumstances. Maybe the faster the tide - the better the relative fuel benefit?

Please forgive me if that sounds like a thicko question but I have a nagging suspicion that P speed can overcome the influence of tide to such an extent that in these circumstances P speed becomes more efficient and cost effective. Have I got this totally wrong?

Of course the obvious thing is to plan the trip better and have the tide on the stern! LOL.

 

[Hurricane]

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

 

[Deleted User YDKXO]

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.

Maybe thats the case in open water but actually I found the opposite in the coastal waters we were in. Normally if I see a bunch of sailing boats and I'm at p speed, I just steer around them and give them plenty of space. But at d speed, steering around them takes a lot of extra time so I was tending to hold my course and pass quite close and that takes more concentration. And a couple of times I nearly ran over lobster pots because I wasn't concentrating! I certainly never felt that I could leave the helm for any length of time

 

[Deleted User YDKXO]

The irritations of noise and vibration and to some degree ride, would probably not be issues with a "designed" displacement hull.

Yup thats true. 1 small engine certainly makes less noise and vibes than 2 large ones. I dont know whether the hull design makes a difference though. I guess exhaust design, soundproofing and engine mountings/drive make more difference.

 

[Deleted User YDKXO]

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

Surprised to hear you say that about your stabs. I guess that begs the question as to how effective stabs are on d boats which simply can't do 10-12kts? Yup, in principle I agree with you on the p v d debate. A stabilised p boat should give you the best of all worlds but in extreme conditions, you'd still want to be in a heavy d hull IMHO. Interesting to compare fuel consumption for d boats and p boats at d speed. A Nordhavn 55 is supposed to do less than 2nmpg at 7.5kts but at a similar speed I'm doing 3.5nmpg. My feeling is that p boats are going to be much more fuel efficient at d speeds than purpose designed d boats simply because of the weight difference but, of course, as I say, that weight is going to be useful in extreme conditions.
Thanks for the link to the Recaro seat. I dont know how I could fit that to the seat moulding I've got on the F53 and I've got a useful locker underneath which I dont want to lose. I have to give this issue some serious consideration because the f/b helm seat on my boat is truly awful

 

[Deleted User YDKXO]

Welcome, Mike, to the world of POOTLING

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

Yeah we do poodling as well. In fact we've got 2 of them, well only one now since the other ate the rabbit

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

.

Do you do that when there's just yourself and your SWMBO on board? My SWMBO is a bit of a night owl and would be up for that but I like my sleep! How do you manage the watches?

 

[Deleted User YDKXO]

I think what I am trying to say is this. At full on P speed, does the boat sufficiently "fly" on top of the water to the extent that the influence of the tide on the boat is relatively inconsequential, thus fuel consuption is relatively speaking better in these circumstances. Maybe the faster the tide - the better the relative fuel benefit?

Please forgive me if that sounds like a thicko question but I have a nagging suspicion that P speed can overcome the influence of tide to such an extent that in these circumstances P speed becomes more efficient and cost effective. Have I got this totally wrong?

No absolutely not! The net effect of the tide on your boat is the same whether youre doing 5kts or 50kts. If there's 2kts of tide against you, you are going to lose 2kts of speed over the ground, whatever speed through the water you are doing.
As to which is better, stay at d speed or go at p speed to outrun the change of tide, I suspect staying at d speed would still be more economical. Of course, the most economical course of action is to anchor to wait out the foul tide

 

[Andrew38]

I think the point of displacement speeds & tides is that at the lower speeds you're exposed to them for much longer and therefore planning is more critical.

A Barrus engineer told me that on my previous Yanmar 440's fuel consumption doubled for every 300 rpm increase over 2000.

I've just done fuel calcs on the new boat (twin D6 370's driving through sterndrives) and since commissioning in March I've achieved an average 2 nmpg over 540 nm at an average speed of just under 10kts. Slow speed cruising is more enjoyable but being able to outrun bad weather is a great advantage.

 

[Hurricane]

Do you do that when there's just yourself and your SWMBO on board? My SWMBO is a bit of a night owl and would be up for that but I like my sleep! How do you manage the watches?

The last time we did an overnight at D speed, we had the kids on board (well they are grown up kids but you know what I mean).
We all stayed up until about midnight and then SWMBO and I went to bed leaving them in charge. We had a pre-arranged 2 hours on and 2 off. Our turn came up at 2 o'clock but instead of handing over and waking them at 4 o'clock, we let them sleep on.
I never stick to strict "watches" - our passages are in a mobo have never been long enough to warrant formal watches.
On this occasion, the kids woke up at about 8 o’clock – we all had breakfast and SWMBO and I then had a few hours sleep before arriving at our destination at about midday.

You can only do this sort of thing if the weathet is good enough though.

With our base in SC, we can reach the islands easily with an overnight run like this. My thoughts are to have dinner under way as well - easier to plan because you know what the conditions are like when you leave.

After about 10 miles out on our run to the islands, the water is too deep even for the echo sounder to work so ther aren't any pots to worry about - just ships!!

 

[jfm]

Surprised to hear you say that about your stabs. I guess that begs the question as to how effective stabs are on d boats which simply can't do 10-12kts?

Well they can have bigger fins or just have less effective stabilisation. Some compromise has to be made there. There is no doubting that a stab can exert more force on the hull if the boat moves faster, AOTBE. That is one advantage of a P boat over a D: you can speed up in huge seas to make the stabs more powerful in a fast boat

in extreme conditions, you'd still want to be in a heavy d hull IMHO.

Agreed. But as an all rounder, and assuming you can only buy one boat, the stabbed P boat is a better buy imho

A Nordhavn 55 is supposed to do less than 2nmpg at 7.5kts but at a similar speed I'm doing 3.5nmpg. My feeling is that p boats are going to be much more fuel efficient at d speeds than purpose designed d boats simply because of the weight difference

Yes, all agreed

 

[jfm]

The net effect of the tide on your boat is the same whether youre doing 5kts or 50kts. If there's 2kts of tide against you, you are going to lose 2kts of speed over the ground, whatever speed through the water you are doing.

I don't think that's the whole picture mike. On those numbers, the 50kt boat will burn 104% of its no-tide fuel on any given trip, whereas the slow boat will burn 167% of its no-tide fuel burn

At infinite speed the tide has no effect,. At the same speed at the tide, it has infinite effect. Just like airport travelators - the faster you walk/jog the less they help you, whereas if you stand still they help you 100%

There is a speed of foul tide at which it makes more sense to plane at 20knots than pootle. (Not in the Med of course!)

 

[rwoofer]

I don't think that's the whole picture mike. On those numbers, the 50kt boat will burn 104% of its no-tide fuel on any given trip, whereas the slow boat will burn 167% of its no-tide fuel burn

At infinite speed the tide has no effect,. At the same speed at the tide, it has infinite effect. Just like airport travelators - the faster you walk/jog the less they help you, whereas if you stand still they help you 100%

There is a speed of foul tide at which it makes more sense to plane at 20knots than pootle. (Not in the Med of course!)

I think you are on the right track there, as you naturally need to be able to connect the two extremes you've illustrated.

In a similar vein, saily boat handicaps are often adjusted at clubs with big tides eg. Hayling Island Sailing Club. The faster boats are penalised a lot more than usual otherwise they would win all the races.

 

[MapisM]

That is one advantage of a P boat over a D: you can speed up in huge seas to make the stabs more powerful in a fast boat

Mmm.... A weird advantage, 'fiuaskme.
I've been very rarely in sea conditions where I had to reduce my normal cruising speed (8 to 9 knots).
But when I did, it was in nasty beam sea with short and steep breaking waves, where at anything above 5 kts the boat was smashing against the water, in spite of her full D hull.
And boys, was I glad that the stabs were still effective!
Trust me, you wouldn't have wanted to speed up in those conditions...

 

[jfm]

Mmm.... A weird advantage, 'fiuaskme.
I've been very rarely in sea conditions where I had to reduce my normal cruising speed (8 to 9 knots).
But when I did, it was in nasty beam sea with short and steep breaking waves, where at anything above 5 kts the boat was smashing against the water, in spite of her full D hull.
And boys, was I glad that the stabs were still effective!
Trust me, you wouldn't have wanted to speed up in those conditions...

I'm not sure what point you're making. Your stabs might have been ok at 5kts but there is no way they were as effective as at 8/9 knots. It is indisputable that stabs produce more anti-roll force as you speed up, which most definitely is an advantage of a faster boat in many conditions, even if not in the rare conditions you encountered that day. Unless you have a whole different set of laws of physics?

 

[oceanfroggie]

Great thread Deleted User. We discovered the advantages of D speeds on offshore passages on nice days. 5 to 10 hr passages are not unusual for us, so obviously the 10hr ones tend to be planing most the time (ie 100-150 miles), but shorter runs (ie <75miles) we are often happy to spend say 9hrs at D speeds and one at P to keep engines happy. The whole experience on the boat at D speeds is so much nicer. Let the AP do the hard work, leave the helm, walk about the quite boat, having relaxed meals in the cockpit dinning area, conversation, listening to music, using iPad, just socialising, and it's just so quiet and relaxing. I love it. Only need to glance forward the odd time, especially if we keep well off to avoid pots. I often wander up to the bow on really nice days and just sit or lay there gazing at the mountainous scenery, cliffs, jagged island formations, sun and breeze in my face, only sound I can hear is the gentle ripple of water against hull, no engine noise at all. Jeapers I'm drifting peerlessly towards a rag! For us D speeds on nice days offer a far more enjoyable cruising experience. If there is no time imperative what's the rush, being out there is what it's all about. The odd run it was so nice we'd give up on reaching a destination, instead pull in somewhere sheltered weather permitting and sling out the hook for the night. Bliss.

Ps. We don't have any vibration at D speeds and 6 cyl engines almost silent when atop, or anywhere outside, only sound is water.

 

[tamarind]

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?

You are right the tank size us 3400 litres. The figures for the range at various rpm is from Princess spec sheet. Crossing the bay of biscay combining speeds of 9 knots and 15 knots which will give the range allowing for a safe margin.

 

[DAKA]

The last time we did an overnight at D speed, we had the kids on board (well they are grown up kids but you know what I mean).
We all stayed up until about midnight and then SWMBO and I went to bed leaving them in charge. We had a pre-arranged 2 hours on and 2 off. Our turn came up at 2 o'clock but instead of handing over and waking them at 4 o'clock, we let them sleep on.
I never stick to strict "watches" - our passages are in a mobo have never been long enough to warrant formal watches.
On this occasion, the kids woke up at about 8 o’clock – we all had breakfast and SWMBO and I then had a few hours sleep before arriving at our destination at about midday.

You can only do this sort of thing if the weathet is good enough though.

With our base in SC, we can reach the islands easily with an overnight run like this. My thoughts are to have dinner under way as well - easier to plan because you know what the conditions are like when you leave.

After about 10 miles out on our run to the islands, the water is too deep even for the echo sounder to work so ther aren't any pots to worry about - just ships!!

As a one off in order to dodge a storm or make a party I can see the merits of a night passage but this thread appears more concerned economy wise and I cant agree it makes any sense at all.

If you calculate your annual running costs and then divide them by the number of 'sunny days' you have on board you will find the daily cost staggering , an 80 nm night passage might save a few euros in fuel but at the cost of writing off the following valuable day as you either sleep through or have an argument with tired kids.
I use my boat for family holidays and the importance of my familys enjoyment is import to me as I want them to join me next year, I suppose if I was on charter it may be different .