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Old 17 March 2016, 08:15   #1
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SIB fuel use?

Getting a bit geeky but I know some people like to log these things (me included) and it would be interesting to see how much others use for various engines? SIB's are cheaper to run compare to RIB's but fuel is still used at a fair rate I've found.

I kept the figures for the last eight trips last season. A real mix of longer coastal trips, playing in the waves, kneeboarding and long pootles up rivers.

For my Mercury 15hp 2 smoke the average nautical miles per litre per trip (generally trips of 2 - 4 hours actual moving time) was:

2.6 nm/l
1.5
1.28
1.8
2.3
2.18
2.1
1.9

Average = 1.75 nm/litre

The most accurate average though was a week's use in the Dartmouth area to include a real general mix of river/sea use.

Total distance 122nm, used 64 litres, av speed 5.8 kts/hr, used 3.04 litres hour and achieved 1.9 nm per litre.

If others have figures please post.
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Old 17 March 2016, 20:58   #2
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Getting a bit geeky but I know some people like to log these things (me included) and it would be interesting to see how much others use for various engines? SIB's are cheaper to run compare to RIB's but fuel is still used at a fair rate I've found.

I kept the figures for the last eight trips last season. A real mix of longer coastal trips, playing in the waves, kneeboarding and long pootles up rivers.

For my Mercury 15hp 2 smoke the average nautical miles per litre per trip (generally trips of 2 - 4 hours actual moving time) was:

2.6 nm/l
1.5
1.28
1.8
2.3
2.18
2.1
1.9

Average = 1.75 nm/litre

The most accurate average though was a week's use in the Dartmouth area to include a real general mix of river/sea use.

Total distance 122nm, used 64 litres, av speed 5.8 kts/hr, used 3.04 litres hour and achieved 1.9 nm per litre.

If others have figures please post.
By rights we should get the same figures as you. But I've not worked it out to be honest, I have only ever worked on the estimate of 10% of hp, in gallons, at WOT per hour. Always carry twice that amount. Even though we're very rarely always on full throttle and the consumption definitely drops off a lot if we keep it at 2 thirds throttle. So for instance I have a 3 hour round trip at sea planned, that's 4.5 gallons or 20 litres. I'll carry an extra 20, unmixed, then add some of that and mix it at the destination. If I don't use it I put it in the car.
Luckily our local fuel station doesn't seem to care what size jerry can we fill so I'm currently using a 20 litre quicksilver tank and two 10 litre petrol jerry cans. On the other hand our local supermarket refuse to allow delivery into anything other than a 5 litre can...
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Old 18 March 2016, 08:00   #3
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Hi Simon, yes I have used the 10% thing but have calculated it's a bit of an underestimate of fuel at WOT for us - not sure why, maybe the way we drive like hooligans sometimes and thrash these little engines.

It would give 6.75 litres per hour (1.5 x 4.5 litres) but I think 8 might be more accurate at WOT.

To gauge fuel used easiest thing is to weigh your empty fuel container and then weigh the fuel used before/after. 1 litre petrol weighs approx 0.74 kg.

Had 175 views but no other contributions?? Nobody else use any fuel??
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Old 18 March 2016, 08:09   #4
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Max

never done a real trial myself on the SIB but i watched a you-tube video from an Italian supplier for a 3.8 m boat with a 20hp Suzuki lean burn 4st, normal boating as you would going on any trip he got 2lts /hour fuel usage.
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Old 18 March 2016, 09:43   #5
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>>>Nobody else use any fuel??

Well so little I'm not counting!

We tend to do around 20Nm or perhaps up to 25Nm most day out trips, 30Nm would be a long run for us.

So on a basic trip we are 10Nm out and 10Nm back. That tends to use half a 20lit tank so broadly speaking we are around 2Nm/lit which is very close to your figure.

If we are on the potential 30Nm trip I take a 5l can or second 12l tank as spare.

The new Suzuki (perhaps at the dealers today!) only has a 12l tank but I'm damn sure it won't use half the fuel so I'll probably buy a bigger tank.
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Old 18 March 2016, 09:55   #6
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Blimey David, I thought if anyone logged these things it would be you...

Will be interested to see how you get on consumption wise with the Suzuki.
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Old 18 March 2016, 10:13   #7
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>>>thought if anyone logged these things it would be you.

I'm inconsistent!

I have obsessively collected every Motoring Which of the 50/60/70s... I have a drawer with instruction books for every item we have going back 40yrs... all my nails and screws in the garage are sorted in Chinese take away boxes & labelled with type/size...

But my CDs/Albums are not in artist order and I never log fuel consumption.

Selective OCD I reckon.
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Old 18 March 2016, 12:37   #8
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>>> interested to see how you get on consumption wise with the Suzuki.

Yes I'm interested to see how it does. Mind you I might give the results in how much of a tank per day.

Bit like the sort of guy when asked what his car's fuel consumption is will reply "It's excellent... I put £20 worth in on Saturday and it got me to work and back all week with some left over".
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Old 18 March 2016, 12:46   #9
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SIB's are cheaper to run compare to RIB's but fuel is still used at a fair rate I've found
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Had 175 views but no other contributions?? Nobody else use any fuel??
I WAS going to reply to the first observation - but modesty stayed my hand. The second poke broke my resolve!

If my "sums" are correct - you're paying about 54p/Nm at 16kts? and I'm paying about 47p/Nm at 30kts.

Granted, it's commercial diesel but:
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Old 18 March 2016, 12:50   #10
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I WAS going to reply to the first observation - but modesty stayed my hand. The second poke broke my resolve!

If my "sums" are correct - you're paying about 54p/Nm at 16kts? and I'm paying about 47p/Nm at 30kts.

Granted, it's commercial diesel but:
No, on average I'm paying 54p/nm at under 6kts/hr

At least if my maths is correct.

Over a typical weeks use of all sorts (that Devon trip) we did an average speed (moving time only, not floating/anchored and fiddling about etc) of 5.8 kts/hr and for each litre burnt travelled 1.9nm.

And I thought my old Landys' were fuel in-efficient...

I know your typical RIB burns through the old juice pretty quick but gets my goat a little when people say you can "...go a whole week..." on that little (SIB) tank etc and it sips fuel, rollocks they do...
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Old 18 March 2016, 14:46   #11
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As a guide I've always used the Tohatsu chart.

Tohatsu Outboards - Fuel Consumption

If you look at the 15hp 2-stroke their 7.3lit/hr at WOT or thereabouts equates to around 2.3Nm/lit at say 17Kts so they expect a little better than your figures Max. My tanks per 20Nm day out figure is very approximate but give or take 10%-15% equates to the Tohatsu figure.

My only experience of larger craft is displacement diesels of my Father's and I think like Wilk's comments they used less than a fairly modest outboard on a dayboat.

But if you compare a bit more like for like that Tohatsu chart shows an old tech 115hp 2-stroke would be burning 50lit/hr compared to the 7.3lit/hr of the 15hp. Of course it would be faster so going further but relating consumption to hours is legitimate for a pleasure craft.
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Old 18 March 2016, 19:48   #12
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I think it's all relative surely. If I can spend all day boating for £20 of fuel in a SIB, distance is irrelevant, it's a full days boating for the price of a round in the pub and hugely pleasurable.

On the other hand I could take out a chunky 150hp RIB and spend all day blasting around, I'm guessing I'd not have change from a Ton for fuel...

Can you really compare the two?

If it's purely miles per gallon or ltr/hr then knock yourself out on comparison but in my own little world SIBing is a more economical option and ain't no one going to tell me different ;-)
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Old 18 March 2016, 20:39   #13
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SPG Smiles Per Gallon??
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Old 18 March 2016, 20:54   #14
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2014-en Greece:


HONWAVE T38IE2 + HONDA BF20.
5 persons. Me and wife 150 kg, 3 kids total 100 kg, 24l tank + 12l spare, some equipment - about 300 kg in total.
Average consumption was 4-5 liters (per hr - more realistic 5l), average speed was below 20 km/h so we can say that per 100 km it was 5*5=25 liters.
We did max trips 20 km one way if I remember.

2015 Greece again (5,4RIB 115 HP engine):


Again the same family but all kids had 170 cm height and kids weight in total was more than 150 kg, me and wife again 150 kg, equipment (snorkell equipment for 5 people include neoprene short suits, fishing equipment for 5 people, some food, spare shoes, many things - I think in total 450 kg include petrol (100l + 20l spare)

Average speed was between 40 km/h and 50 km/h (mostly near 50 km/h)
Usual trips 60-80 km one way
Average consumption 2km per 1liter so about 50 liters per 100 km

So:

- on the same distance consumption was 2 times greater use RIB
- speed average was 2 times more than SIB
- average range was more than 3 times

I used to love my SIB but:

- I do prefer jockey seats or stand when drive -> 0 problems with my back (I had such problems uses SIB)
- Kids are much bigger every year (now boys have 175 cm)
--- I doubt if be able to handle family on HONWAVE 3,8 again + equipment
- We do like very much to explore coats, islands by boat so minimum range one way is 50 km usually max 100 km so use RIB is out of question now

>>>>>>

OK, to be honest boat total cost almost 3-4 times more ...
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Old 19 March 2016, 00:56   #15
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I don't measure too precisely (20 HP 2 str) as I passage plan based on 1 US Gal / 10 HP / HR basis and know that is conservative (especially when applying the 1/3rd extra safety net). BUT, from the rough calculations I have done - lightly loaded in flat conditions I can get close to 2 NM/L, but in normal conditions and loading its more like 1.5 NM/L.

Max is quite right, over the course of a week with lots of boating that is certainly not an insignificant cost, however I probably spend more on Tea and Cakes in cafes than I do on fuel! If ever I start to think I'm using a lot of fuel I recall a friend with a jetski and teenage sons... I can get a weekend long trip out of what they burned in 35 minutes.
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Old 19 March 2016, 10:27   #16
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You're all correct chaps - relatively speaking it is not much money for the occasional use most of us have a SIB for. When the old man had his Ski-boat in Majorca he worked out each ski session (1-2 hours for all of us) was about £100 or so a time each morning (all running costs considered) and that was 20 years ago.
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Old 19 March 2016, 10:45   #17
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>>>relatively speaking it is not much money for the occasional use most of us have a SIB for.

Yep and another bit of my man maths... there is a fast ferry ride to Jura from the village where we'll holiday this year. For 4 of us to have a day out there the trip each way is £160 just for the ferry. The Mrs has just been away to an event where the modest level entrance ticket cost is £360 for 4 folks for 5hrs entertainment.

So £10 in fuel for 4 to enjoy a beach day out in the SIB is peanuts.
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Old 19 March 2016, 20:31   #18
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I haven't worked it out specifically for distance etc. But Aerotec 380 with Tohatsu 9.8, and I used about 7l of fuel on a January day out exploring Poole harbour, out to Old Harry and back, much of which was spent at WOT.
Bargain boating in my book!
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Old 19 March 2016, 21:07   #19
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Edit re my post #17...

Don't want to mislead... the Jura ferry is £160 for the four of us as a to/fro trip day out.... not £160 one way.
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