Largest fuel tank in your RIB/SIB? Distance challenges

[Jlomb436]

I'm curious, as I've never owned a SIB but looking at purchasing very soon... so how large of a tank do you usually carry?

I'm interested in going out to the islands that are 20-40 miles off shore, but have serious fuel concerns. I've done some quick math, and on a 12'6" with a 15hp motor:

80 miles travel + 10 to explore.
90 miles at 1.5 GPH at 15mph avg
Total of 9 gallons.

So a 6.6 wouldn't do, but a 12 is HUGE. What do you guys normally run if you do this length of exploring? I think a bladder would be awesome for space, but those things are just way too expensive. This is all new to me so I apologize for any ignorance.

Thanks for your input and help.

[Jlomb436]

And an added question:

Lets say I run the throttle 3/4, does efficiency go up quite a bit on an outboard? If slower means better fuel range, even if it's 10-12 mph, I'd be happy to do it. Once I get the boat I will be doing some testing, however, my engine didn't come with a tank so need to figure out which setup I want to run.

Thanks!

[simonafloat]

I would give serious thought to whether a 20 to 40 mile offshore trip in a sib is a wise move.

I have a 16 mile trip offshore trip planned but this has involved months of preparation and research. Maybe if you are new to sibbing try some short coastal journeys first to get an idea of how you can cope. I am sure the boat will cope but sibs can be a very rough ride, how will your own body cope.

I will be carrying 45litres fuel for my return trip in a standard tank plus two 10 litre tanks.

simon

[Max...]

You might want to re-visit 1.5 gph? Is this the small US gallons too?

I have logged most of my usage and with a 15 2 smoke Merc over all use as an average it's around 1.75nm a litre or 7.8 nm per UK gallon. However, some trips it has gone down to more like 1.4nm litre. There are a few topics on here about SIB fuel usage.

Our last trip was a pootle down the river at 5 kts for about 1.5 hrs followed by a blast along the coast about 10 miles out and then back. We used about 14 litres I think.

SIB + small 2 stroke + lots of WOT = a very un-economical way to travel.

Are you serious about going up to 40 miles offshore in a SIB and back?? Even on perfect flat water you won't average 15 mph. 30 mins in a SIB at WOT is enough to need a break to recover.

Your trip is like doing Land's End to John O'Groats on dirt roads in one day in a Renault Twizy...

We have a 32 litre bladder but fill it to around 25 litres.

For your trip you'd want to carry maybe 3 x 25 litre tanks.....

[Jlomb436]

Hi Simon.

I appreciate the advice and will take it. I definitely plan to take many coastal trips before embarking on a long journey out to sea. The islands are actually 11 miles out, but they are wide, so after 11 miles I'll be right on the coast. But they go for awhile and I plan to go around them, so that starts to add up to the 90.

I was asking as I need to buy a new tank or tanks, and wanted to figure out what sizes everyone was using. I understand that varies, but I thought I would paint my picture so others had an idea. So maybe a 12 and a 6 would be the safest and best bet to ensure I could go 120 miles.

[Jlomb436]

Originally Posted by Max...

You might want to re-visit 1.5 gph? Is this the small US gallons too?

I have logged most of my usage and with a 15 2 smoke Merc over all use as an average it's around 1.75nm a litre or 7.8 nm per UK gallon. However, some trips it has gone down to more like 1.4nm litre. There are a few topics on here about SIB fuel usage.

Our last trip was a pootle down the river at 5 kts for about 1.5 hrs followed by a blast along the coast about 10 miles out and then back. We used about 14 litres I think.

SIB + small 2 stroke + lots of WOT = a very un-economical way to travel.

Are you serious about going up to 40 miles offshore in a SIB and back??

That's like doing Land's End to John O'Groats on dirt roads in one day in a Renault Twizy...

Hey Max.

Thanks for the numbers. Yea I've never use an outboard before, so I don't know their sweet spot for economics. If say throttle at 3/4 yields the best mileage, that's what I'll do. May I ask what your top speed is with the merc? I think with 2 adults and out gear, we're probably talking 850 lbs. Aiming for 15 mph but we'll see. Just want to to plane...

[Max...]

Top speed depends so much on boat and load and water.

With a (draggy) Aerotec and a 15 with 150kg load we hit about 17kts max, 15 - 16 kts cruise. Last trip with another person (+ 85 kg) max was about 16 kts. A flat bottom SIB will go better than this.

[Poly]

Originally Posted by Jlomb436

Hey Max.

Thanks for the numbers. Yea I've never use an outboard before, so I don't know their sweet spot for economics. If say throttle at 3/4 yields the best mileage, that's what I'll do. May I ask what your top speed is with the merc? I think with 2 adults and out gear, we're probably talking 850 lbs. Aiming for 15 mph but we'll see. Just want to to plane...

If you've never used an o/board before don't rely on anyone else to help you estimate fuel use. You'll find UK people here talk about statute miles per UK gallon for cars but usually nautical miles per litre for boats! Beware the confusion, and some people talk litres per NM.

Even 11 miles is a fair trip for a sib beginner, build up enough experience that you know what your fuel usage will be and understand how bad weather will affect it etc.

[two stroke mick]

Note uk gallon is 4.54 lt. A USA Gallon is circa 3.78 lt.

[jonp]

My Yamaha f 70 burns gives me 3.5km per litre cruising around 38-40kmh.A safe 300km range, but I struggle to do more than 120 in a big day.

Take a look here to get an idea on different outboards burn rate at different rpm, to get these figures an engine needs to reach its maximum rpm (be correctly propped).Liters | Mercury - Evinrude - Yamaha - Suzuki - Honda - Tohatsu - Outboard Fuel Consumption per Hour LPH you can select litres or gallons etc.

Jon

[Max...]

Originally Posted by jonp

Take a look here to get an idea on different outboards burn rate at different rpm, to get these figures an engine needs to reach its maximum rpm (be correctly propped).Liters | Mercury - Evinrude - Yamaha - Suzuki - Honda - Tohatsu - Outboard Fuel Consumption per Hour LPH you can select litres or gallons etc.

Jon

With small SIBs and small 2 strokes IMHO tables are all very well but you're probably better off going with real world, mixed use, actual figures from owners on here.

[SixtyNorth]

I would recommend doing longer trips offshore in the company of other boats, it is far safer and you can learn a lot from others, get a tow if required etc.

Regarding the SIB and engine, I would probably consider a min. 4m boat with a modern 4 stroke outboard. They are very fuel efficient and quieter for longer trips. A SIB is a very seaworthy boat but you will not be able to maintain a high speed comfortably and safely in anything other than calm seas. Long rolling swells are better than short, steep chop.

I have done some very long coastal trips in 4m+ SIBs packed with a lot of camping gear. Everything must be stowed securely... we used a cargo net combined with waterproof bags which worked very well. We carried separate 25l fuel tanks to extend the range.

Going further offshore is another matter and it needs proper preparation and planning. Keep in mind your distance from a safe haven should your engine fail, the weather get worse than forecast etc.