Fuel tanks and usability

[PCUK]

Just been looking at the Nordhavn 47 test in MBM and like most tests the safe cruising range is decided by allowing a 20% margin on fuel stock. I assume this is to prevent air entering the fuel system in rough conditions when the fuel level is below this 20% figure.
If so, my fuel tank system allows the fuel to drain down to the last 8 gallons before there is any dager of air entering the system. While this would not be an ideal situation to be in it does mean that I have far greater usable range than other boats with the same fuel capacity just through good system design.
It is also a cheap way of increasing range on any boat just by installing this simple system.

 

[Deleted User YDKXO]

Pershilla, I dont think the 20% reserve figure has anything to do with air entering the system but is an arbritrary figure below which there is the danger that debris is drawn into the fuel system from the bottom of the tank

 

[Sixpence]

I thought it was an element of emergency use, as in, get out of unforeseen circumstances, probably wrong though /forums/images/graemlins/smile.gif

 

[Deleted User YDKXO]

Yup, there is that to it as well but I think thats a passage planning issue in terms of not planning to use the last 20% in case of unforeseen circumstances

 

[Sixpence]

sactly so, at least if everything goes wrong after you plan so carefully, you still have a bit of go juice to get to safety

 

[burgundyben]

"that debris is drawn into the fuel system from the bottom of the tank "

when tank is full, fuel is drawn from bottom of tank,

when tank is empty, fuel is drawn from bottom of tank

 

[Deleted User YDKXO]

Good point but the draw off pipe is usually not quite at the bottom of the tank to avoid drawing in debris at the bottom of the tank and the lower the level of fuel, the higher the concentration of debris and the greater the likelihood of debris being drawn in. Thats my theory and I'm sticking to it /forums/images/graemlins/grin.gif

 

[Nick_H]

Agreed, the 20% is contingency, the most likely cause being heavy weather requiring an innefficient "just off the plane" speed, but could also be many other causes like having to go to the assistance of another yacht, hull fouling affecting the mpg etc.

 

[PCUK]

OK, lots of good points here.
The problem with keeping the last 20% for emergencies is that it may be unusable due to debris in the tank.
Answer - keep the tanks clean.
Problem - boat builders can't be bothered to provide access hatches or even access to the tanks in many examples.
There is also my original claim that air will enter the system when the tank is low and the fuel is slopping backwards and forwards, allowing the fuel pick-up to be in clear air momentarily.
So if you can't rely on the last 20% it isn't much use.
Better to be able to rely on the entire tankful down to the last 5%.
Trouble is, this also requires some occasional maintenance, and tank maintenance is something that is very seldom mentioned probably because on most boats it is impossible.

 

[thefatlady]

Surely, a well designed tank has

(a) the take-off fairly central so that under long-term heel the take-off remains submerged and

(b) baffles so that transient slop is reduced to acceptable levels.

Air should not enter the system until the tank is virtually empty under any reasonably expected conditions.

Another reason for baffles: a ton or several of fuel slopping about does not do much good for tank mountings or anything else.

I have, under unforeseen conditions, actually run one tank dry and limped into port on one engine and a thimblefull left in the other tank. Apart from having to bleed the fuel system (and getting showered with diesel when SWMBO didn't know how to stop the engine!), it caused no problems.

 

[fireball]

question of range

Ok - so your car has a fuel guage and possibly a low level warning light .... how low do you take the level before filling up again?

giving a "range" for a boat that uses up 80% of the fuel is a bit arbitory - on a nice day you could get much further and bad day much less ... so it is a guide - how far you actually go on the tank is largely up to you - if it is nice you might push it to use 90%, and if it is rough you'd probably like the break to fill up after 70% .. ?? So isn't the stated "range" of boat just guidance to base your planning on rather than a strict cutoff beyond which the boat spontaneously explodes?

 

[PCUK]

Everything you say is true, but it is in the unreasonable conditions when fuel is getting low that the problem arises.

 

[thefatlady]

I think my point is that, with proper tank design, air should not get into the system, even in very rough weather, until the tank is effectively empty.

I work on the basis that the 20% is emergency reserve. I have made up to 170nm open sea crossings with a nominal 220nm range. I wouldn't like to do much more.

 

[hlb]

In reality you need far more than 20% safe range. There are no fuel stations out there and no way of knowing how much fuel you will use. No on else knows either. Safe reconing is a third there, a third back if problem and a third for other eventualities. I've been there just once. Where a 40 mile trip turned into a 120 mile trip when the weather changed from forcast. I get a bit iffy over 50% useage. Though I do cheat and go passed it a bit.

 

[Sixpence]

Re: question of range

[ QUOTE ]
Ok - so your car has a fuel guage and possibly a low level warning light .... how low do you take the level before filling up again?

Filling up ? /forums/images/graemlins/grin.gif My van never gets more than half a tank and doesn't get that till the warning lights been constant for an hour /forums/images/graemlins/grin.gif I work on the theory that if it gets nicked I know how far they could get

 

[PhilF]

Doh

why not just fit a tracker

 

[Sixpence]

Re: Doh

Now to be perfectly honest,,, I hadn't even thought of that ,, me being a sparky you would have thought I'd know better ,,, any idea where I can get one (save me hunting) /forums/images/graemlins/blush.gif

 

[tazzle]

Agree

I was always taught 'one third out, one third back and a third for reserve'

Or passage plan if coast hopping for refill ports every 65% of tank capacity.

 

[duncan]

Re: question of range

you should be working to 80% of the range based on the conditions you anticipate when you set out. If you are planning tocruise at a fast cruise speed this may be lower than a leisurely optimum cruise speed - if it's looking choppy even less.......etc
your 20% remains contingency (but not the same a consultant fee project contingency which always get used!)

 

[thefatlady]

Re: question of range

Agreed.

Assuming the worst case where the only shelter is at the end points of a journey, using the 80% of range rule means you are never more than 40% of range from shelter. This gives a 50% safety margin over the maximum distance from shelter. In emergency, you head for the easiest shelter to get to, even if that means turning back.