Fuel Consumption - The Theory!

[RichTS]

Leisurely Fuel Consumption?

Hi all,

I'm looking to buy a 35-45 ft, old Princess. Something between a P35 and P45. Budgeting for fuel consumption is proving a tricky task. I've read hundreds of forum posts and articles but haven't found the answer I need. I'm hoping this forum may deliver!

It seems that with planing boats of this style and age, 1 MPG is a reasonable estimate at 'cruising' speed. My question is: what about at 'pottering about' speed?

I plan to moor it on the south coast of the UK and take it out only for day trips - not to get from A to B, but to enjoy being on the water. So MPG is less relevant to me as the distance covered isn't important. Hours Per Gallon is much more important. Sure, I'll enjoy going WOT for 10 minutes at a time regardless of the cost, but the rest of the time I'll be happy going at whatever speed is most economical.

I appreciate that in terms of distance, 'cruising' speed is perhaps most efficient, but what kind of fuel efficiency could I expect during an hour of 'gentle' cruising? I.e. At the slowest speed that you can still maintain control and forward movement in the Channel in fair conditions.

From other posts and comments on the forums the general consensus seems that it's so economical it's "not worth measuring". But that probably depends on your budget!

I appreciate there's so many variables and every boat / day is unique, but I'd be very grateful for any estimates you have for minimum consumption / cost for an hour's leisurely boating in the Channel in a 20-30 yr old Princess with twin Volvo engines. Am I looking at £30, £70, £130, £180 p/h? I really have no idea!

Thanks in advance!

 

[Murv]

Re: Leisurely Fuel Consumption?

Our boat is a 1989 Fairline turbo 36, length overall is 42.5'
Engines: Volvo TAMD61a's 306hp 5.6 litre turbo diesels.

Being so poor as to make the average church mouse seem positively affluent, I was a bit concerned about the running costs of our first larger boat and, so, keep detailed records of hours run against fuel added.

We're based on a tidal river with a 6 knot speed limit, so most of our boating is pottering around with the odd faster blast once further down river.
Over the first Year, we used 8.84 litres per hour.
The fuel curve figures I have for the boat show that it should achieve 8 knots @ 1000rpm burning 18.18 litres per hour, but we usually have some tidal assistance so probably run closer to 800 rpm.
With a coastal crossing, and some of the journey at a slow planing speed, some at displacement, we burned between 30 & 54 litres per hour.

We have 1100 litre fuel tanks, so you really don't see the gauges moving much at all when pottering around hence the usual comments that fuel burn isn't worth measuring.
It's only with sustained, fast running that the fuel gauge needles start heading south with alarming speed, at which point I usually start feeling a bit feint and throttle back!

EDIT: Just to add, our fuel curve figures show 24 litres per hour at 1200 rpm, which is about as high as I like to go at displacement speed otherwise it gets very inefficient.
So, worst case, displacement speed in a bit of a chop where you want to forge ahead and keep good steerage would be in the region of 15-25 l/p/h.

Hope this is of some help,

 

[jrudge]

Re: Leisurely Fuel Consumption?

Fuel consumption at displacement speed ( think below about 9-10 knots ) will be very low as per the prior post, but do bear in mind planning boats are designed to be comfortable on the plane. Unless the water is very flat ( a river or a windless / flatcsea day ) it will tend to roll and be uncomfortable. There are plenty on here that “pootle” as hurricane calls it but for it to be tenable you do need flat calm seas.

 

[Surfacesub]

Re: Leisurely Fuel Consumption?

Probably not the answer you require, and you may of considered the alternative vessel.

Surely if you are going to be spending most of your time at displacement speed then I would try to match that to the correct hull form and therefore engine size.

It will be kinder for the engines running closer to the optimum design speed. See SPECIFIC fuel curve supplied by the manufacturer. Unfortunately at low speed large engine exhaust gas temperature is low which gives rise to unburnt fuel and incomplete combustion, all of which pollute the atmosphere and obviously hurt your back pocket.

Sorry never an easy answer.

 

[oldgit]

Re: Leisurely Fuel Consumption?

A cat among pigeons ?

Have recently swapped from a semi displacement hull, Broom 37, to planing hull, Princess 35.
Spend most of my 150 or so hours a year creeping around using the tide at 6 -8 knots and when above locks 4 or 5 knots when pushed.

First observation. was that in the short chop of the Thames estuary the Princess 35 gave far more comfortable ride than the roly poly floppy old Broom.
Second observation.
The upper Thames is full of flybridges and other craft ( 8 thousand in total ) many with pointlessly powerful engines all creeping around at 8 Kilometers a hour for most of their lives.
Are there dozens of companies lining the river making a fortune rebuilding prematurely conked out engines ? ...Nope.
As for fuel consumption chugging around in P35 at a few knots in tidal waters will probably cost you £15.00 -£20.00 or so an hour. We have pair of 200HP 41b.

The downside, slap slap on the bow when the tide ebbs at 3am.
Simple answer, turn the boat round when you moor up.
As time goes on, suspect that when it comes to running costs like many of us, fuel costs will fade from being your number one primary worry.

 

[RichTS]

Re: Leisurely Fuel Consumption?

Thanks so much for the replies - really helpful and exactly what I was after.

The main things I'm taking from them is that:

- There is an economical way to enjoy this style of boat in terms of fuel consumption, but unless the sea is very calm it may not be particularly comfortable!

- Mooring somewhere with river, estuary and sea cruising available nearby would give the most options and enable me to optimise fuel consumption (and enjoyment) based on conditions, budget and tides on any particular day.

- I need to carefully consider the type of boating I'll be doing most of the time and get a hull that suits.

I am looking at displacement hulls too and it may be the better option for me, but I love the style and design of the high performance planing yachts and of course, money isn't the only consideration or I wouldn't be buying a boat in the first place! But nonetheless, I don't want to be reluctant to take it out or to start charging friends and family when they come along for the day!

I look forward to any further comments people have to offer. I really appreciate any advice as I'm new to offshore boating so trying to fill my many knowledge gaps before I take the plunge!

Cheers.

 

[sprocker]

Re: Leisurely Fuel Consumption?

I recently became the owner of a P40 of 1998 vintage, and these are the figures that I was given at the time.....unproven by me at the moment.

Princess 40 - TAMD63P - Fuel Consumption
Clean Hull
rpm knots gph lph mpg
1400 9.3 5.9 26 1.58
1800 12.5 12.5 57 1.00
2000 15.1 15.2 69 0.99
2200 18.3 18.9 86 0.97
2400 22.3 22.2 101 1.04
2600 24.9 26.8 122 0.93
2800 27.2 33.0 150 0.83

Dirty Hull
rpm knots gph lph mpg
1400 9.3 5.9 27 1.58
1800 11.2 13.2 60 0.85
2000 13.8 16.7 76 0.83
2200 16.9 20.2 92 0.84
2400 19.0 24.4 111 0.79
2600 23.5 29.9 136 0.79
2800 24.7 33.9 154 0.73

 

[maby]

Re: Leisurely Fuel Consumption?

If you really are going to be happy pottering at a few knots in a mobo, have you considered a sailing boat? They are generally cheaper than mobos for a given age and size, they are usually cheaper to drive on diesel than a mobo of similar size and if the weather is right, you have the prospect of pottering completely free of charge.

 

[jrudge]

Re: Leisurely Fuel Consumption?

Bear in mind the only thing about fuel is that the cost is controllable, it is not likely to be your biggest cost. Mooring fees followed by maintainance will probably get that gold star.

A tank of fuel is usually good for 250miles ish on a planning boat and on the size you are looking at I guess the tank is. Say 700l.

Short trips. 2 tanks a season ? £1500 ? I don’t want to dismiss it, just bear in mind it is not really the prime cost.

 

[dpb]

Re: Leisurely Fuel Consumption?

Would it be worthwhile re engining such a boat with say 50hp engines for such use?
I have come across a few older river based twenty something foot sportscruisers that have had the original big petrols removed and replaced with very small diesels.

 

[RichTS]

Re: Leisurely Fuel Consumption?

I recently became the owner of a P40 of 1998 vintage, and these are the figures that I was given at the time.....unproven by me at the moment.

Thanks Sprocker, so helpful to see actual figures that include both MPG an LPH. Your numbers really highlight the difference between measuring fuel efficiency by distance or time (MPG or LPH). The MPG doesn't seem to vary much at different speeds but LPH increases more than 5 fold! Whatever my monthly budget ends up being for fuel, it's the number of hours it gives me that matters most, not the distance it enables me to travel. So it's very reassuring to know that whilst lower speeds won't necessarily be any more fuel efficient, they could significantly improve the £££s per pleasure ratio!

If you really are going to be happy pottering at a few knots in a mobo, have you considered a sailing boat?

I have considered sailing boats but I've grown up with mobos (inland waterways only though) and I'm not a current or wannabe sailor. I also want the power and speed of the more high performance mobos and happy to pay for the high fuel consumption at top speeds, but I'll only be running fast for maybe 20% of my time on the water so I want to know it won't always be costing me £150+ p/h to take her out.

Bear in mind the only thing about fuel is that the cost is controllable, it is not likely to be your biggest cost. Mooring fees followed by maintanance will probably get that gold star.

Absolutely! Maintenance on a 25 yr old Princess is really a rolling of the dice! I accept the uncertainty of maintenance costs but it's helped if I can calculate the more predictable and controllable costs like mooring and fuel consumption. I want to get out on the water very regularly all year round, not just a few weekends in the summer, so fuel could well end up being my biggest expenditure over the year. To be honest I hadn't even considered fuel consumption as a significant cost when I started my research, especially as my family's boats have always been river/canal cruisers and use hardly anything. I stumbled across some posts quoting 1 MPG, did some quick maths and didn't like the answers! I've worked out that I can afford to buy, moor, insure, etc, and hopefully maintain, but I'm now trying to answer whether after all that I'll actually be able to afford to use her!

Would it be worthwhile re engining such a boat with say 50hp engines for such use?

I had wondered about that, but as I will be doing a lot of coastal cruising I think I'll need the full power range. I'm not averse to getting a river cruiser in which case it would be a displacement hull with smaller engine/s anyway, but ideally I'd like the best of both worlds - a boat that is comfortable at sea or on rivers but that won't completely bankrupt me!

It'll be interesting to see if anyone else has comments about re-engining. I had wondered whether buying a boat with engines that need replacing for a cheap price and fitting new engines could end up balancing out on the purchase costs and result in lower maintenance costs and better performance and fuel consumption longer term? I'm not a mechanic, I'm not looking for a project and I'm new to coastal cruising, so I've concluded the chances of me judging that one right are very low! I expect it could easily end up being a fools errand!

 

[BruceK]

Re: Leisurely Fuel Consumption?

mmm. Do not under estimate the rolling maintenance of an old boat. I did and you will be taken aback how much extra you end up paying. Even though my boat is a going / working concern and nothing was "critical" the 4 years I have had her have exceeded the purchase cost by 150% . Fair enough, I elected to do that and had I left her to purely essential maintenance repairs the cost may have been only 50% of her purchase cost. But just be aware going old is not going cheap, it just may be that you belay the initial capital expenditure.

 

[RichTS]

Re: Leisurely Fuel Consumption?

mmm. Do not under estimate the rolling maintenance of an old boat. I did and you will be taken aback how much extra you end up paying. Even though my boat is a going / working concern and nothing was "critical" the 4 years I have had her have exceeded the purchase cost by 150% . Fair enough, I elected to do that and had I left her to purely essential maintenance repairs the cost may have been only 50% of her purchase cost. But just be aware going old is not going cheap, it just may be that you belay the initial capital expenditure.

Wow. That's pretty scary. I guess 50% over 4 years for the 'essential' repairs is bang on the often quoted 10-15% per year. 150% isn't though! Thanks for sharing, always good to hear first hand experiences instead of general rules of thumb.

Could I buy you're boat?! Sounds like she's been well looked after!

Do you anticipate that kind of cost continuing long-term or do you expect it to be less in the coming years after your investment of the last four?

It's a tricky one eh? The balance between what you lose on depreciation on a newer boat vs the maintenance costs of an older one. Is there a general consensus that you should buy the youngest boat your purchase budget can afford, or is it really a 50:50 thing?

Clearly the upshot is that you never 'win'! It's certainly a daunting endeavour and whilst the pleasure a boat can give is priceless, there can come a point when you start resenting it and I certainly don't want to be there.

All this info is so valuable in helping inform my decision of exactly how big a financial mistake I'm prepared to make!

 

[CLB]

Re: Leisurely Fuel Consumption?

I recently became the owner of a P40 of 1998 vintage, and these are the figures that I was given at the time.....unproven by me at the moment.

Princess 40 - TAMD63P - Fuel Consumption
Clean Hull
rpm knots gph lph mpg
1400 9.3 5.9 26 1.58
1800 12.5 12.5 57 1.00
2000 15.1 15.2 69 0.99
2200 18.3 18.9 86 0.97
2400 22.3 22.2 101 1.04
2600 24.9 26.8 122 0.93
2800 27.2 33.0 150 0.83

Dirty Hull
rpm knots gph lph mpg
1400 9.3 5.9 27 1.58
1800 11.2 13.2 60 0.85
2000 13.8 16.7 76 0.83
2200 16.9 20.2 92 0.84
2400 19.0 24.4 111 0.79
2600 23.5 29.9 136 0.79
2800 24.7 33.9 154 0.73

Those figures are good, and pretty much what I would expect, but you will get even better MPG at real displacement speeds. This boat will be pushing water at 9.3 knots, and I would expect to see well over 2mpg, possibly 3 at full displacement. .

 

[BruceK]

Re: Leisurely Fuel Consumption?

Do you anticipate that kind of cost continuing long-term or do you expect it to be less in the coming years after your investment of the last four?

It's a tricky one eh? The balance between what you lose on depreciation on a newer boat vs the maintenance costs of an older one. Is there a general consensus that you should buy the youngest boat your purchase budget can afford, or is it really a 50:50 thing?

A: I hope not. I expect a reasonable service life from that which was replaced.
B: I think that depends. I would probably go for a old boat if it's had many owners and in good condition...new broom and all that pumping fresh money in. Otherwise I think the newest you can afford. But in the end it really boils down to each boat you view.

I think it's also important to note the distinction I made. I voluntarily pumped more money in to try and bring it back as close as possible to original spec. Remember when you buy an old / cheap boat something simple as a heat exchanger replacement is already a significant percentage. In my case 2 x superchargers and some serious outdrive work was mandatory. Total spend there was ~15k (50%) but replacing canvass, vinyls, windlass etc etc. You'd be amazed at how quickly these projects can add up, although because they are projects rather than unexpected hits there isn't that feeling of resentment, rather satisfaction and pleasure when done. But it's money. Now that I have the boat I am very happy with it. Had I known what I know now before buying (without knowing the extent of the pleasure I'd get from her) I'd have run a mile. Bit like kids in that respect.

 

[RichTS]

Re: Leisurely Fuel Consumption?

A: I hope not. I expect a reasonable service life from that which was replaced.
B: I think that depends. I would probably go for a old boat if it's had many owners and in good condition...new broom and all that pumping fresh money in. Otherwise I think the newest you can afford. But in the end it really boils down to each boat you view.

I think it's also important to note the distinction I made. I voluntarily pumped more money in to try and bring it back as close as possible to original spec. Remember when you buy an old / cheap boat something simple as a heat exchanger replacement is already a significant percentage. In my case 2 x superchargers and some serious outdrive work was mandatory. Total spend there was ~15k (50%) but replacing canvass, vinyls, windlass etc etc. You'd be amazed at how quickly these projects can add up, although because they are projects rather than unexpected hits there isn't that feeling of resentment, rather satisfaction and pleasure when done. But it's money. Now that I have the boat I am very happy with it. Had I known what I know now before buying (without knowing the extent of the pleasure I'd get from her) I'd have run a mile. Bit like kids in that respect.

Interesting. I hadn't considered that many owners would be a good thing, but now you point it out it makes sense. Thanks for the distinction between proactive and reactive stuff. Very different I guess. Spending time/money improving the boat can be very rewarding, time/money spent simply to keep her afloat less so!

 

[oldgit]

Re: Leisurely Fuel Consumption?

"Had I known what I know now before buying (without knowing the extent of the pleasure I'd get from her) I'd have run a mile. " Bit like kids in that respect. .


Kids and outdrives.
both on initial inspection appear relatively harmless.....what could possibly go wrong.

Let me count the ways ? Sonnet 43.

 

[Andy Bav]

Re: Leisurely Fuel Consumption?

When we were in Torquay we had a Bavaria with twin D4 260s and it had a total fuel burn display.

Cruising at around 20 knots and 2300 ish revs she averaged 60lph. Freshly serviced and antifouled that dropped to the low 50s. WOT would nudge low 30 knots and over 100 lph. A dirty hull and the numbers worsened dramatically.

However, once we got to the Dart, an amble up river returned around 4lph and around 6 knots. We now have D6 370s and cruising at 25 returns around 75lph, which in simple maths for 2 more cylinders than a D4 seems about right. Again a potter at 6 knots is around 6lph .

 

[CLB]

Re: Leisurely Fuel Consumption?

When we were in Torquay we had a Bavaria with twin D4 260s and it had a total fuel burn display.

Cruising at around 20 knots and 2300 ish revs she averaged 60lph. Freshly serviced and antifouled that dropped to the low 50s. WOT would nudge low 30 knots and over 100 lph. A dirty hull and the numbers worsened dramatically.

However, once we got to the Dart, an amble up river returned around 4lph and around 6 knots. We now have D6 370s and cruising at 25 returns around 75lph, which in simple maths for 2 more cylinders than a D4 seems about right. Again a potter at 6 knots is around 6lph .

Shaft drives or stern drives? At displacement speed probably not much difference, but quite significant at faster speeds.

 

[Andy Bav]

Re: Leisurely Fuel Consumption?

Stern drives.