Nelson 40

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Fuel cost can be calculated at approx 200 grammes per HP/hr @ 0.86 kg / L, so if you take out 150 HP, your fuel costs should be calculated to approx. 35 L /hr on each engine.

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This seems very high for an SD boat running at sensible speed. 15gph seems more like planing boat consumption...[ QUOTE ]


2 x 35 L /hr = 70L/hr
70L / 4.54609 = 15.3978474 GPH .... so was 0.397847 GPH out ... Sorry /forums/images/graemlins/wink.gif /forums/images/graemlins/grin.gif

Assuming that 1 Imperial Gallon is 4,54609 litres

But consumption per hour is, as mentioned in different post, calculated based upon what you take out of the enginese... and not how efficient the hull is travelling through water... You could do 20 knots and drawing this ..... or you could do 15 with the same engines (different hulls of course).... but as long as you take out 300 hp ... that would be your approximate figure per hour when the engines are loaded to 300 hp.
 
Sorry - I am confused.

I can understand taking load - I can understand a consumption figure based on a simple formulae for a planing boat where the consumption once past tne hump is reasonably level but with an SD hull where you may reasonably travel at 10 knost or 18 knots, the consumption in mpg must be less for the 10 knots. So on an SD boat the choice of speeds must be significant ... or am I missing something?
 
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Quote:
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Fuel cost can be calculated at approx 200 grammes per HP/hr @ 0.86 kg / L, so if you take out 150 HP, your fuel costs should be calculated to approx. 35 L /hr on each engine.


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This seems very high for an SD boat running at sensible speed. 15gph seems more like planing boat consumption...


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the calculation is for whatever speed you will get from the boat when requiring 300hp so

(1) it's going to be above 'displacement speed' Gludy!
(2) it's going to be below WOT/maximum speed
(3) at 75% rated output it should be a good working speed/fast cruise speed for the engines

In practice I would have thought that's going to be around 16 knots +/- for loading, sea conditions and hull condition.

8/9 knots will be the crossover point (even on an SD) when you start having to use significantly more power

4/5 knots will deliver the mpg

What are you planning to use the boat for and where? Certainly a working classic - I am sure you will enjoy it a lot.
 
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Sorry - I am confused...... or am I missing something?

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Don't think you are missing anything ....
look at a propellor load curve vs engine rating.....mine as an example (two top curves) ... hence time getting back....
load_curve.jpg
Sorry for poor image, but reduced size somewhat ....
@ WOT engine rating is 325 SHP and Propellor load is 325 SHP... however, as one reduces the engine RPM the propellor load reduces faster than the engine rating (ie you take out less than the engine can give out at the given RPM)..

So that at 2000 RPM engine is rated up to 290 hp, but the propellor load is only 210, and so it continues down the way @ 1800 the figures are 280 and 160, @ 1400 I have 230 and 80. And it is the propellor load fuel consumption is calculated upon.... and it is therefore easy for me to calculate what RPM / Speed combination which gives me the best range... and hence fuel efficiency for my specific case.

In my previous quote i stated 150 hp load... which is approximately 75% of max... which for my engines would be @ 2100 RPM, which is higher than I normally cruise as I get better results between 1900 and 2000, where the load is approximately 60%....

More confused now I hope..... /forums/images/graemlins/wink.gif
 
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but with an SD hull where you may reasonably travel at 10 knost or 18 knots, the consumption in mpg must be less for the 10 knots

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not sure what you mean by reasonably travel at........comfort? hull attitude?
there will still be a resistance hump for every hull shape, although for some models it doesn't exhibit the extremes of associated factors that, for example, a small planning speedboat would exhibit.

Dealing purely with mpg you would if you plotted speed v mpg, as you indicate, expect 2 straight lines and a load of wiggles joining them up - say 1000 rpm up to about 0.7 x hull speed and 1.4 x hull speed up to 85% engine max revs after which the characteristics for the engines power delivery will dictate wheter the line continues pretty straight or curves badly.
you seem to be suggesting that the same engines placed in an SD hull will deliver mpg to a single line, but slightly curved. In a good balanced installation this may be pretty fair for an underlying plot but there will be a hump in there somewhere!
In the example here, where you use 10knots, it is just possible that (say) 12 knots could be a better mpg but I would agree that 18 is extremely unlikely to be more efficient for mpg.

As already stated you need to run the figures accurately for each installation - for petrol engines where rpm is a direct (but not necessarily linear) relationship to fuel usage it's relatively easy but for diesels where fuel usage at any given revs will relate to power used it's not so easy without a whole lot of hassle.
 
he knows this stuff backwards Alf - not sure what's going on!

re your reference to the prop curves these are still the maximums available and you need to overlay the resistance characterists of the hull installation to get precise mpg figures surely.
An SD hull installation will normally give a line closer to the prop curve than a large planning hull and this in turn will be closer than a small planning one (because the resistance hump occurs so much lower down in the engines power delivery curves)
 
Interesting.

I agree with all you say .... although I do not think there would be a straight line with an SD hull, I agree totally with you that there must be a hump in there somewhere.

I also agree that say 12 knots may be more economic than say 10 knots for a particular hull/engine combination.

What my point is about is that if you chose to use say this Neslon at lower speeds near to displacment, say 10 knots, then there would be less fuel consumption than using it at say 18 knots and the difference could be considerable. Hence the quoting of a fixed fuel consumption must be based on a usage type, in your case about 75% load.

I think over time this fuel difference, if red goes, and the option to take it on boats like the neslon may become significant.

Do we agree on all that?
 
"he knows this stuff backwards Alf - not sure what's going on!"

No I don't ... I really am seeking info in a genuine fashion and trying to establish what assumptions there are behind a quoted per hour fuel consumption on the basis that this must presuppose a mode of operation, which as it turns out is about 75% load in this case.
 
Everything that others have said about Nelsons is fair comment. I have always loved them.
But ......
Especially if this is "Lady Emma", many at this sort of price are ex commercial, pilot boats or fishery protection, and have been converted inside to provide better "yacht like" comforts. Some will have been done very well but others will be amateurish.
The 4 cylinder 200hp Mermaids sound a bit lightweight but probably OK. The fuel consumption figures quoted at 16 gph sound reasonable to me for a heavy boat with engines working hard, whatever the theories suggest.

If you must have a Nelson and this is your budget, fine. If not I strongly suggest you look at some of the "plastic fantastics". If you are married and wish your wife to enthuse, she will be a rare beast indeed if she enthuses about a 30 year old ex commercial Nelson.
Show her a Princess or Fairline, etc of much more recent vintage for the same money and you are more likely to enjoy your boating because she will enjoy it too.

Having just bought a V8 Morgan, I know what I'm talking about.
Anyone want a Morgan?!!
 
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What my point is about is that if you chose to use say this Neslon at lower speeds near to displacment, say 10 knots, then there would be less fuel consumption than using it at say 18 knots and the difference could be considerable.......... Hence the quoting of a fixed fuel consumption must be based on a usage type, in your case about 75% load

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The quoted figure was at a given load on engine .... and the load is not linear, as one would expect... hence when you travel at 10 knot (or max displacement speed for hull), your engine load will be at significant % less than at 18 knots.
Once again taking my engines.... at 20 knots (1900 RPM), I take out approx 200 hk on each engine and burn a short 100 L/hr on both engines... However, reducing the speed to 10 knots and 1300 RPM (where I happen to get engines up to an acceptable operating temp... which is important for the combusion), I draw out 70 hp and burn a short 35 L /hr ... there is no doubt that it is more economical to run at 10 knots ... even for a large..ish planning hull like mine (Deep V), as I get to go 20 NM per 100 L @ 20 knot and get to go 28 NM at 10 knots ... that is nearly 50% further.. giving me a range of 300 NM @ 20 knots or 428 NM @ 10 knots.
 
As usual you have e readng my own posts in dismay, then relief...............


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I agree with all you say .... although I do not think there would be a straight line

[/ QUOTE ] - I didn't say a straight line, or infer that you thought it would be straight from your earlier post
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a single line, but slightly curved

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so we seem to completely agree!

moving on to
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What my point is about is that if you chose to use say this Neslon at lower speeds near to displacment, say 10 knots, then there would be less fuel consumption than using it at say 18 knots and the difference could be considerable. Hence the quoting of a fixed fuel consumption must be based on a usage type, in your case about 75% load.

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lets fisrt get rid of the 75% load figure. I think Alf, correctly in my opinion, was quoting a fuel consumption figure for 75% of the engines rated max as an indicative fast cruise consumption. Most installations are designed around 75-80% for constant running so this makes sense. Once over about 110% of hull speed the prop power curve will equate power being demanded for a good SD hull so accurate relationships between revs and fuel consumption/power can be worked out. As it's a curve you would expect the most efficient fuel consumption to be at the lower end, as you 'feel' - instinctively around 12/13knots. Exactly where will be a function of the hull engine combination as discussed.
Equally I would expect there to be another area on the mpg plot, between 3 and 8 knots for this boat where it would be even more efficient.
Between 8 - 13 knots there may be speeds to avoid and others that more closely match the underlying 'curve'.

Is this significant - yes I agree with you it is. I don't however think that any future increase in fuel price matters for that significance, only it's relative impact on an individuals finances.
I am not alone in having avoided travelling at poor consumption speeds in all my craft already so don't anticipate changing my practices at all.
For some though what they want to do and what they can afford to do may well move into conflict - personally I already find discussions or comments on fuel consumption on a par with politics, religion and the monachy when out to enjoy the water and I am afraid I have been quite rude to individuals who wish to engage me in their angst at their fuel bills.
 
I have no dispute with any of that.

I think it is more realistic to choose to go at say 10 knots on a Nelson than do the same on a deep V planing boat. The planing boat at these speeds is just not a comfortable ride in any sort of reasonable sized sea.

If red goes then a see a lot of folks opting to want to travel at 10 knots and it is nice to have that as a realistic option. Hence the consumption at this lower load is an important factor for some.
 
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I have no dispute with any of that......

I think it is more realistic to choose to go at say 10 knots on a Nelson than do the same on a deep V planing boat. The planing boat at these speeds is just not a comfortable ride in any sort of reasonable sized sea.

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Now this is where it gets subjective then ... what is comfortable to you may not be comfortable to me ..... and the type of boat etc. selected as "fit for purpose" is therefore also subjective.... Now, we both have planing hulls currently (your Fairline and My deep V). Failine is modified V with tunnels perhaps ?? and my guess is that your, near 60 ft draws just over 1 metre @ 25 something tonnes...
Mine is constant deep V, not modified, 45 foot and draws 1.5 metres + some for the props @ 21 Tonnes and sits very well in the water even at 10 knots.... a bit roly in side sea perhaps, but so is a SD at this speed and side sea... My bet is therefore tha the two boats, although both planning does have very different seakeeping charateristics.... Seakeeping varies for each type / make of boat, and one cannnot say that all SD is "better" than all Planning ... that is too simple. Agree that the average SD can handle more than the average planning, and that most RNLI's are SD for the same reason .... but boy do they roll!!! Which to some novice's are very scary.... Not disputing the fuel economy part at all .... but if I was too worried about that, I'd have one of the boats with a "pointy sticky up" thingie or two.... cause two of those ought to be better than one ... or was that hulls perhaps?? /forums/images/graemlins/laugh.gif ... which may be thee way to go in the future ... and there are no dispute that a sailboat, on average, can handle rougher seas than a MOBO.... or is there?? /forums/images/graemlins/grin.gif
 
agreed to your subjective comments and no to your question but (again) the last part is of course also subjective; at one extreme a water ballast sailer such as a mcgregor and at the other a full displacement, full keel such as those designed/built by Victoria Marine and many others. So to qualify my 'yes' - yes if a sailing craft designed to sail within the constraints of it's natural hull speed (ie displacement craft).
 
"Seakeeping varies for each type / make of boat, and one cannnot say that all SD is "better" than all Planning ... that is too simple"

I totally agree with all you stated up to and including that point.

"Agree that the average SD can handle more than the average planning, and that most RNLI's are SD for the same reason .... but boy do they roll!!! "

Mostly agreed, I hope that rolling charactistics vary greatly between boats but you can say they generally roll a lot - that is why I have stabilisers fitted to my new boat.


"Not disputing the fuel economy part at all .... but if I was too worried about that, I'd have one of the boats with a "pointy sticky up" thingie or two"

I cannot dispute what you would have but for most MoBo oweners they would prefer to get somewhere at 10 or 12 knots than nowhere in a raggie boat :-) You have gone to extremes there and your comment does not apply to most..... a subjective judgement of mine.

"... cause two of those ought to be better than one ... or was that hulls perhaps?? ... which may be thee way to go in the future"

Yes it may be the way.

".. and there are no dispute that a sailboat, on average, can handle rougher seas than a MOBO.... or is there?? "

Not with me is there any dispute with that.

We are very close in our opinions - you are just a little more extreme perhaps? :-)
 
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Everything that others have said about Nelsons is fair comment. I have always loved them.
But ......
Especially if this is "Lady Emma",

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It is, but everyone seems to know her which makes me a little nervous that there is something wrong with her...

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many at this sort of price are ex commercial, pilot boats or fishery protection, and have been converted inside to provide better "yacht like" comforts. Some will have been done very well but others will be amateurish.

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Yes, she is an ex-scottish fisheries enforcement vessel, and has been owned by a private owner for a few years. I understand that there will be some, possibly a fair bit of work to turn her into a second home. It all depends on how much, and I'm waiting for the prelim piccies from the broker.

I'm really after a good very seaworthy boat first, the polish second. From what I can gather, the boat is in a very seaworthy condition, recent hull epoxy, new engines and the required basic nav electronics. Everything required to go out to sea and teach me how to sail a motor boat.

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The 4 cylinder 200hp Mermaids sound a bit lightweight but probably OK. The fuel consumption figures quoted at 16 gph sound reasonable to me for a heavy boat with engines working hard, whatever the theories suggest.

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Broker quotes speed as 18kn max, 16kn cruising @ ~15gph. Plenty of speed there. I suspect I will be sailing at ~9kns at whatever I buy - It's just I want the extra speed in reserve just in case.

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If you must have a Nelson and this is your budget, fine.

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It's not that I must have a Nelson. I am looking at other options, it's just that Nelsons have a good reputation as a very seaworthy boat.

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If not I strongly suggest you look at some of the "plastic fantastics". If you are married and wish your wife to enthuse, she will be a rare beast indeed if she enthuses about a 30 year old ex commercial Nelson.
Show her a Princess or Fairline, etc of much more recent vintage for the same money and you are more likely to enjoy your boating because she will enjoy it too.

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They are all very nice, however they are a little too plasticy for me. I like the sort of boat with all of the real wood, the feeling of being on a boat rather than a floating camper van.

I'm sort of leaning towards the Trawler style SD boats, and the Nelson/aquastar style. I love trader 41's and there is a lovely trader 38 down in kent but at 89k she is quite expensive.

The missus? She is in the same mind as me, as long as it's comfortable, dry, has shower & loo then she will be happy.

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Having just bought a V8 Morgan, I know what I'm talking about.
Anyone want a Morgan?!!

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Why? is it going free? /forums/images/graemlins/smile.gif

Steve
 
Yes you are absolutely right saftey gear first.
However we are getting off the thread a bit with all this fuel consumption stuff.
You originally asked about a Nelson 40 and I replied to you that a 1973 boat is going to sap your finances.
I have first hand experience of this.
I bought a 1982 Birchwood in 2002 that had been neglected over the years, although the hull and engines were basically sound.
I estimated that it would take me 18 months and about 10 grand to re-furbish the vessel.
Well, I'm still at it 3 years and 20 grand later.
I am now going "negative", in as much that I cant hope to get back money that I now spend on her.
I still have windows to replace at a cost of nearly £3000 and cosmetic work on the gel-coat stress fractures which will probably be about £1500.
However she is now much better equipped than most boats of her age.

You will be very suprised at what you actually have to replace even on a 1982 boat.
I have had to replace both fuel tanks which were mild steel and weeping...replaced with vetus 335 litre plastic tanks...one of which has started to leak after 18 months..cause unknown until I get it out in a couple of weeks...just another knockback.
All seacocks replaced, new electric toilet, complete interior refurb, including completely new galley and heads compartment, new forward cabin, new floors, upholstery etc etc.
My mate Chris stripped the engines and did a top end refurb for me, and the single most expensive item on that was a pair of S/S exhaust elbows which had to be imported from the states at a cost of over £1700 with import duty and tax.

I had to have a new shaft in 2004, which cost a packet, and kept me out of the water at the height of the season.
Lots of new instruments, almost complete electronics, comms and Nav replacement, new batteries throughout, bow thruster, (stern thruster next this year), new lifejackets and all safety gear, refurbished water tank, new mains and 24v electrical wiring...I could go on and on and on and on.

I am lucky that I can do a lot of the work myself (assited by SWMBO), but it has taken nearly every weekend and every holiday of the last three years to get this far.
It is easy to buy an older boat and look at it with rose coloured glasses, but the reality is very different.
There have been highs and lows and times when I would have happily taken her out into Swansea bay and scuttled her.
What I am really trying to say, is that you need to be very sure before you take on an older boat, that you can stand the pace and the workload, and have the finances to drip feed the project.
I have to say that if I were to go back to 2002, knowing what I know now, I'm not so sure I would have taken the same course.

I think you need to consider very carefully a 1973 boat or any boat older than about the mid eighties, before taking the plunge.

Steve.
 
Divemaster1 said:
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Fuel cost can be calculated at approx 200 grammes per HP/hr @ 0.86 kg / L, so if you take out 150 HP, your fuel costs should be calculated to approx. 35 L /hr on each engine.

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This seems very high for an SD boat running at sensible speed. 15gph seems more like planing boat consumption...[ QUOTE ]


2 x 35 L /hr = 70L/hr
70L / 4.54609 = 15.3978474 GPH .... so was 0.397847 GPH out ... Sorry /forums/images/graemlins/wink.gif /forums/images/graemlins/grin.gif

Assuming that 1 Imperial Gallon is 4,54609 litres

But consumption per hour is, as mentioned in different post, calculated based upon what you take out of the enginese... and not how efficient the hull is travelling through water... You could do 20 knots and drawing this ..... or you could do 15 with the same engines (different hulls of course).... but as long as you take out 300 hp ... that would be your approximate figure per hour when the engines are loaded to 300 hp.
Click to expand...
Hi, all..... I've just joined as it seems some of you know what your talking about? and I would like a bit of help please in easy terms a Nelson 40 ex patrol boat with 2000lts tanks, I'm just wondering how much fuel it would use at half speed in miles per gallon if anyone can help with an answer I would very much appreciate it thank you
 
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