P42 fuel usage

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can you please advise me the correct way to calculate fuel usage estimation etc. for example going on a trip with fuel level indicators showing tanks as half full. So at say 22 knots what safe distance can you travel or how many hours should they be good for and how much in reserve?
 
If your fuel gauges are as accurate as mine then you could find yourself in deep trouble miles from anywhere. Personally I always start with tanks brim full and log useage and mileage and have a pretty accurate understanding of how much fuel is in the tank at any time.

The difference in range depending upon sea state and wind is very significant, could be up to 50% difference from a still calm day with the tide and a horrid day battling the weather and tide.

The only real way of knowing the range on your boat in different conditions is to use it and log everything but always err on the very safe side and assume that the worse will happen because it often can or does.

I also has VP EVC which measures fuel used very accurately but I would never contemplate a journey that I did not have very much more than twice as much fuel as my worse estimates would give assuming maximum fuel consumption.

If in doubt, fill up the tanks, peace of mind is worth a lot.
 
I take your point and of course one does take more that you could possibly need but there must be a calculation to use something like the pilots do on flights?
 
The typical assumption is that a boat of that sort of size does about 1 mpg.

For my own boat I assume an average consumption of 2 litres per kilometre at 24 knots (those are the units that the EVC is calibrated and 1nm is 1.8 km for ref).

The other way to look at it that it takes 200 grammes of diesel per hour to supply 1 horsepower. So if you have 2 engines of say 400 hp, thats 800 in total, then you will consume 800 x 0.2 kg of diesel per hour, ie 160 kg or thereabouts, running at full power. Obviously, lower power uses less fuel and that you need to know for your particular boat.

I tend to think that most people rely on their actual experience rather than a theoretical calculation, as the reality is about arriving safely not the precise mathematical evaluation which will never accommodate the variabilties, uncertainties and unknowables that real travel on the sea encompasses.
 
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Sorry to slightly hijack the thread but I've been thinking about doing some runs at various revs and reading off the electronic consumption gauge. By running in two directions and recording the log as well it's obviously possible to get an average. I agree with all the comments above that this wouldn't take into account varying sea states and wind and in a sense would give a best case scenario, but in a new (to me) boat it would surely give a starting point. As far as I can see the regular logging that one does anyway might give me a better insight having fairly easily established a sort of baseline. It would also mean for the first few 'proper' trips I'd have a ball park to start with.

Taking as read that one starts any trip with full tanks and applies a very wide margin of error have I missed anything? Or perhaps people feel it's of no value (I've not done it on previous boats). There's a load of boating experience on this forum so interested to get any views.

Paul
 
Loads of variables not least of which how clean is your hull?

That boat does around 1mpg. It will run for around 10 h+44-1895252019ours at cruising speed on the plane (all Princess boats pretty much do). If you really want to maximise range put it in tick over and use a bit of tide. You'll get the thick end of 1,000 NM out of it.

Fuel gauges should not be taken literally they are a guide.

Working out you actual numbers is easy enough. Fill the tanks log use (inc Genny hours) and re-fill

Henry :)
 
henryf said:
Loads of variables not least of which how clean is your hull?

That boat does around 1mpg. It will run for around 10 h+44-1895252019ours at cruising speed on the plane (all Princess boats pretty much do). If you really want to maximise range put it in tick over and use a bit of tide. You'll get the thick end of 1,000 NM out of

Working out you actual numbers is easy enough. Fill the tanks log use (inc Genny hours) and re-fill

Henry :)
Click to expand...

Ok Henry so your p42 would do 5 hours on half a tank. Factor in 20 percent for error and u have the figure I guess
 
Except I wouldn't take 1/2 tank on the gauge as 1/2 tank in the tank.

Are you reading the gauge at rest or on the plane?

If you're doing short hops with ample opportunity to refuel then run the tanks down a bit, otherwise top up and see how much it takes relative to your tank size 1,250 Ltrs from memory ?

Henry :)
 
Ok, half tanks whilst at rest. But your right how accurate are they. Going to do 50 miles trip to run it down a bit then will fill up and check how much goes in.
 
Do consider looking at the manufacturers spec sheet for the engine. In my experience the consumption figures they give at certain rev ranges are not a million miles out. How fast you go of course for given revs is a different issue which is related to the boat, fouling, weather etc. This will give you a half decent starting point.

Re aircraft that you mention the technique is identical. Fill it up, fly for x hours. fill it up and work our the consumption, making some small allowance for time to top of climb. However bear in mind that aircraft tend to cruise for long periods at the same throttle / pitch / mixture settings to the consumption for a given time is highly predictable (I was always within 10 litres based on an in my brain calculation at fill up). However, just like tides etc for a boat all this gives is time to empty tank. How far you could fly is down to head / tail winds and so on.

On my boat I note the tank gauge indications at fill up then how much it takes. This gives you some half idea of what the gauge says vs what is roughly in the tank. Note roughly!
 
If you are not sure make a worst case table. Measure your fuel consumption at atum with full fueltanks and load.
Calculate fuel burn vs distance. When you know the fuel capasity you can calculate the range at different levels.

I often makes long trips with a small boat. My range i 200 nm. If the tank is half full its 100 nm. If the boat is light and clean i make at optimum 20 % longer range. i alvays have 10 to 15% margin
 
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