Heavy Weather Fuel

[Guest]

Are there any rules of thumb about the extent to which the fuel consumption on a boat goes up in rough seas. In a force six on the Irish sea, I found that my boat virtually doubled her fuel consumption labouring up waves and toppling off the top. This can be a bit of a problem if you think you have a safe range of X and it turns out to be 1/2X as a result of weather, especially if you are not paying too much attention to the fuel gauges whilst trying to stay alive!

In my case it meant having to find fuel half way back to Bangor whereas I set out from Howth thinking I had plenty.

Does this also mean that fuel consumption figures given in boat tests have to be taken with regard to the test conditions? How come this is seldom mentioned anywhere?

Nick

 

[Guest]

Nick I,ve experienced the same thing,came out from Lymington only going to Poole but thinking half a tank would do the trip 4 times or more.

By the time I got to Poole I was running on fumes, its amazing how big a difference it makes, I'm now in the habit of refuelling at the earliest possible opportunity and keeping a spare 12 gallons on board.

 

[hlb]

Ahh! well Nick I see that you've had a few Whiskey's by now.
But my rule of thumb is like this.
My boat has 150 gallon of fuel. now forgetting manufacturers bumph you can recon on 1 mile to gallon. But thats OK on a calm sea and maybe on a good day I can do much better.
Safe rule of thumb is same as for airoplanes.
One third of tank to get there one third incase its rough and you need some more and one third for safety if it all go's wrong.
Cource that gives me a safe range of 50 miles or so.

Mind you the theory worked when I was crossing Lyme bay. The gearbox broke on the way back to Plymouth. I discovered that I was'nt going to make Plymouth. But luckily Salcolmbe was much nearer.
But forget boat tests. Mines supposed to do 28 knots. It once did 25 but as it gets late in the season it struggles to make 18. Also the dry weight is 6 tons but on the crane mine weighs 8 tons.
In reality I reckon on a range of 100 miles with a reasonable amount of safety. After that your pushing it.

Moral of the story.
Dont believe all you read in fairy tales!!

Haydn

 

[duncan]

It is frequently mentioned - just never quantified!

 

[Deleted User YDKXO]

Nick, I think you do have to take test consumption figures as an optimum and, often, the tests do actually state the sea state and load conditions under which the figures were taken. Once you've loaded your boat with beer, stuck a tender on the back and got 6 months fouling, the mpg suffers accordingly. I've had boats in the past which have been tested by MBM/MBY and never acheived either the top speed or mpg figures in the test
Another way of calculating mpg is by using the engine manufacturers own fuel consumption figures for given rpm (look in the manual) which are probably quite accurate and then relating rpm to speed which gives you a mpg figure
Having said this I'm surprised you doubled your fuel consumption. Were you operating at hump speed or throttling on/off as each wave passed? I guess this would increase fuel consumption quite significantly compared to constant throttle running

 

[Scubadoo]

I tend to refuel when down to half a tank just for safety. I notice my fuel consumption goes up dramatically when fully loaded with dive gear. You can imagine four divers with 15kg cylinders and lead weights etc etc and basically the more weight seems to make not a small difference but a dramatic change in consumption.

Mr Max mentions he carries 12gal of spare diesel, I often thought about doing the same, but would 12gal make much difference, you've added more weight - and refueling at sea!.

I guess in the end you get to know your boat in most conditions and estimate consumption. I tend to plot a graph (in Excel) to see it consumption dramatically changes over the season - which could mean a problem somewhere. One last thing perhaps you should try coating your props/outdrives etc with Veridan, I notice my fuel consumption much more stable this year (to date), where as normally fouling reduce speed and increase consumption.

RM.

 

[Guest]

Only 150 gals? My cheapo Birchwood has 200 Gals for circa same size and same engines (with original gearboxes) + 100gals water. As I have intimated before, I tend to cruise about 3100/3200 revs which has the turbos whistling nicely and gives a very comfortable, unstressed 17/18knts. On the particular trip I refer to, we went down to Dublin from Bangor in flat calm balmy conditions. I filled up at Bangor and had just more than half full on arrival at Howth, a distance of around 100 mls so 1 mpg+ is about right. On the way back I filled up again and having been storm bound for three days we just had to get back. We left in force six North Easterlies which luckily put the sea in just the right place off the bow and were able to maintain about 12 knts without the boat feeling like she was going to break up. As someone pointed out this is quite close to our hump speed although, the boat has a near semi displacement hull which does not have a particularly noticeable hump. It is also just below the speed where the turbos cut in so that may also have had something to do with it. We put into Carlingford Loch about 45 miles North of Howth after three hours of torture and my tanks were showing about 1/4.

The last part of the trip, due to the motion of the boat, the gauges were all over the place and it was hard to get a feel for what was happening fuel wise and various thoughts go through your head like what happens if she sucks air or crap stirred up from the bottom of the tanks.

However, she did not miss a beat and it does prove once again that the boat generally can take more than the crew.

One final point - the waters of the channel must be warmer/more weedy as I anti-foul at the start of each season and by the time she is due for the next anti-foul my top speed will only have dropped from 26knts to around 24.5knts and she carries roughly the same complement of whisky the year round.

I had to put into Ardglass for fuel and at some point I will relate the tale of fueling where the marina has no fuel, Irish fishermen and my whisky.

Nick

 

[hlb]

Your right Nick. The only way I can fault my boat is the size of the tank. Which makes 100 miles a bit iffy.
I can see now though, yer problem.
12 knots is about the most expencive speed there is.
8's ok and so is 16. But in between just dosent realy exist.

Thing is, you've got just about the same amount of power on whether yer doing 12 or 16. Its just that the waves are knocking you back to 12.

No wonder theres so much sea round Scotland.
Its you yer silly bugger, Y've pushed the whole bloody Irish sea up there!!
Swap it fer a bike Nick!!


Haydn<P ID="edit"><FONT SIZE=-1>Edited by hlb on Sat Jul 21 09:06:11 2001 (server time).</FONT></P>

 

[rich]

try unloading the boat of all gear and all bits and rubbish put on scales,, then do MPH,

 

[jfm]

Re: value of electronics

Agree all your observations about big decrease on mpg in weather. Sorry to witter on about electronics, but this illustrates why one of the best bits of kit you can ever get is electronic fuel monitoring, which measures fuel flow rate interfaced to gps so you get all consumption data like mpg and range over the ground etc etc. It's a shame that hardly any boats have this

JFM

 

[Guest]

Re: value of electronics

I don't disagree and my own boat has pretty well everything you would normally expect but this does not include a sophisticated trip computer such as I have in my car. I have a seven series and the onboard computer is uncanny and seems to calculate the fuel consumption/range, etc. with unerring accuracy, irrespective of driving conditions. I can confirm this as I am one of those boring people who reset the trip meter every time I fill up and calculate the actual fuel consumption by dividing miles by gals - well it fills the time from Wemyss Bay to Glasgow!

Such a device would make a lot of sense in a boat and if the car manufacturers have sorted them, presumably it would not be outrageously expensive to include in a boat costing lots and lots of thousands.

On the other hand, I know how to use a chart and read my mag compass!

Don't lose too much money at Caesars Palace!

Nick