Originally Posted by Ribochet
Do I remember that you said that you had done an Intermediate powerboat course as this should have been one of the key areas covered under Passage planning.
You need to calculate the fuel burn for your rig with the loading you expect on the day - then get the charts out and work out your projected total distance to travel and then using your realistic fuel burn figure and the passage distance - calculate the amount of fuel required and then add at least 1/3 more in case of rough weather or the unexpected.
Running out of fuel is the most common cause of call-outs to leisure boat
It was covered on my PB2. Not sure if that is standard or not - but its a remarkably common question on Ribnet, and one that worries me a little. if your passage planning isn't up to working that out, you know so little about your boat, or you need to cut it so fine that it might really matter then perhaps a trip of this scale is not appropriate yet. I appreciate everyone has to learn, and we were all beginners at some point but there is enough background information around for those who bother to look (RYA "logbooks", "textbooks" and this forum would all give a good starting point).
However, I will once again reiterate some common "rules of thumb":
- An "old tech" 2 stroke engine will burn approximately 1 US Gal (3.8L) for every 10 HP of engine size when running at full throttle.
- A modern direct injection 2 stroke or 4 stroke might run 20-25% more efficiently (say 3L for every 10HP).
So your 40HP engine is going to need between 12-15L for every hour you run it at full throttle. Your 35 L tank will give you an endurance of less than 3 hours.
I'd GUESS (i.e. don't rely on this number) that your rib will run at low 30's knots at full throttle on a nice day. So you could
be getting a range of 70 miles ish... ...heavier laden in choppy weather you might only manage 20 knots and find your range is slashed to less than 50 miles.
Most ribs report cruising (i.e. not full throttle) at 1 L/NM (+/- 20% depending on boat size, engine, load, weather and how throttle happy the skipper is!). Your 35L tank will give you a range of 28-42 NM depending on the various factors... ...now because your boat and engine are small you will probably even get better than this - but its best you convince yourself of this from real data rather than supposition. I can do ~0.5 L/NM in mine on a flat calm day sitting at about 80% of maximum speed. I can burn double that and get half the speed in horrible weather when a lot of the time is spent going "up hill".
If you search the forums you will find other people reporting how much fuel an engine uses at different throttle / speed settings.
If I was taking your boat on a 60 NM trip, I would want 60L of fuel readily accessible (i.e. in proper tanks that just need a quick connection change). I would then also be taking spare fuel with me. If at "lunchtime" (or any other convenient sheltered stop) it looked like I might need the extra fuel I would top up the tanks, rather than find myself in the middle of a shipping lane or choppy sea trying to transfer fuel from cans.
If you are doing a "there and back" trip then the general suggestion is you use no more than 1/3rd of your fuel for the "outbound" leg, which leaves you a third for the return and a third spare. For a circular trip I would want at least 30% more fuel than I planned to use. I.e. I'd have about 20L spare on board for that trip. If you do start to use it then either keep the last 5L (one standard fuel can) back or if your tank has a reserve built in (many portables do, which you then have to tilt to access) then have a plan for what to do with only 5L left. It is probably get to shore as quickly as possible (that should give you about 5 NM to find a safe haven - if you are 5 or 6 miles from "home" - don't aim for home, sods law you will run out 1/2 mile short). Obviously if you are cruising in company it may simply be to get to a safe place to transfer fuel from one of the better prepared skippers, promising to repay in equal quantities of fuel and beer!