Million dollar question....!

citygent111 · 29 August 2021, 19:46

Ok, as a brand new rib owner dare anyone take a guess at an answer for me.....

3.8m rib with 2 adults and 1 teenager aboard and a Tohatsu 30hp engine powering it.

What kind of range will we get from the (25 litre?) fuel tank?!!

I know its a difficult one, but we're going out on it for the first time this coming weekend and I'm in 2 minds about taking spare fuel.

We aren't planning on going too far, its more of a "getting to know the boat" weekend, but I have no idea if we'll get 10 miles or 50 miles out of it!

Any help as always greatly appreciated 😀

jeffstevens763@g · 29 August 2021, 19:53

Lots of variants to consider but I get 140 miles out of 35 Ltrs ish 435 m boat me big lad and lots of kit so roughly the same as you interestingly my 3.65 m boat not much better if at all. For me p for plenty I all ways take too much fuel quoting PD on rather be looking at it than for it

rik_elliott · 29 August 2021, 20:33

Distance with boats can be hard as varies with conditions. Against a strong tide and wind or in chop your range will drop. Rule of thumb I use is you'll probably use your motor hp in litres of petrol in a day out if you are mostly at full throttle. No science to it, but has never left me low on fuel.

A 30hp should run for most of a day with mixed use on 20l of fuel (assuming it is a four stroke motor of course). If you are constantly at full throttle you will get through the tank much quicker though. Where it is your first voyage it probably makes sense to take an excess of fuel so you can safely experiment with how much you burn and get to know it without fear of running out

Pikey Dave · 29 August 2021, 20:35

My 3.9 Heavy SIB with a 20hp suzi does c. 4nm/l, I’d say you should be getting 3nm/l

Poly · 30 August 2021, 00:56

Quote:

Originally Posted by citygent111

Ok, as a brand new rib owner dare anyone take a guess at an answer for me.....

3.8m rib with 2 adults and 1 teenager aboard and a Tohatsu 30hp engine powering it.

What kind of range will we get from the (25 litre?) fuel tank?!!

I know its a difficult one, but we're going out on it for the first time this coming weekend and I'm in 2 minds about taking spare fuel.

We aren't planning on going too far, its more of a "getting to know the boat" weekend, but I have no idea if we'll get 10 miles or 50 miles out of it!

Any help as always greatly appreciated 😀

Rule of thumb taught on my RYA PB2 course was:

2 stroke engines will burn about 1 US Gal (3.8L) per hour for every 10HP at full throttle. To keep the mental arithmetic simple I treat it as 4L per 10HP per hour.

4 stroke engines are roughly about 25% more efficient - ie. 3L per 10HP per hr.

So your boat will use about 9L per hr at full throttle (assuming its a modern engine). I'd guess you'll be getting something around 25 knots with that set up - maybe a smidge more if you are brave enough - at full throttle. So your 25L tank would get you (25 (L) / 9 (L/hr)) * (25 NM/hr) = ~ 67 N.M. at full throttle.

Next rule they taught was 1/3rd for way out, 1/3rd for way back and 1/3rd spare. So don't go further than ~ 22 N.M. from slipway (if going back to where you launched) with your 25L tank. One thing worth remembering is you'll only manage 25 knots on a nice day - so those quick sums change in rough weather (although you probably back off so burn less - but you may be accelerating up the face of waves).

In reality, you'll probably do better than this because you won't use 100% throttle all the time (especially with that combination) - but better to err on the side of caution till you work it out for yourself. Personally I would always carry a separate spare can (5L is fine - it will get you >10NM flat out in good conditions) because otherwise, the first time you realise that you've miscalculated is when there's none left!

If you've not done some RYA training, I'd strongly recommend it as it covers these sort of basic things that avoid you ending up calling for help. If you have - I'm sure it was in the course notes so you might want to quickly refresh your memory what else you've lost to the depths of time.

Finally, some (all?) 25L tanks have an "in built" reserve - this means you get less than 25L before the engine splitters to a stop but you can access the reserve by tilting the tank... if you get that low I'd say your planning has failed - but it would be embarrassing if you called for help and the RNLI turn up tilt your tank and you drive home!

jonp · 30 August 2021, 01:01

What prop is being used and does the engine reach max rpm when when loaded ?

Poly · 30 August 2021, 01:03

Quote:

Originally Posted by jonp

What prop is being used and does the engine reach max rpm when when loaded ?

I'm guessing he doesn't know that yet:

Quote:

We aren't planning on going too far, its more of a "getting to know the boat" weekend,

citygent111 · 30 August 2021, 08:21

Thanks everyone, some great advice.

I did the PB2 course 2 years ago, but don't remember the calculation being taught - although I do remember the advice about 1/3 out and 1/3 back etc....

Anyway, for the sake of this weekend I don't think we'll have a problem, but I'm off to Screwfix today for a 5l jerry can....just in case!!

Fenlander · 30 August 2021, 10:15

Tohatsu's own figures for their 30hp models are 13lit/hr for a 2-stroke and 9.8lit/hr for the 4-stroke. These figures are for running at 5500rpm which is close to WOT.

In general I've found the Tohatsu figures to be in the right ball park for mixed use on/off the throttle at speed.

jeffstevens763@g · 30 August 2021, 18:11

All advice I agree with but for all the years I’ve been boating I know the weather can change and you might have a long trip home at displacement speed or have to constantly keep getting back on the plane so on most trips half my fuel goes in the wife’s car when I get home.

citygent111 · 04 September 2021, 21:25

Brief update on this subject today - turns out it only has a 12 litre fuel tank!

I wondered why we used 3/4 tank after only a couple of hours of messing about.....

Would like to track down a Tohatsu 25 litre tank at some point, but unsurprisingly - everywhere is out of stock or asking for £125 for a plastic box!!!

Poly · 04 September 2021, 22:54

Quote:

Originally Posted by citygent111

Would like to track down a Tohatsu 25 litre tank at some point, but unsurprisingly - everywhere is out of stock or asking for £125 for a plastic box!!!

There’s no need to use. Tohatsu specific tank. All tanks are expensive for a plastic box because they are bit more sophisticated than that, the manufacturer has to pay a lot of money to certify them and shipping air around the world is expensive - but it should be more like £40-50. There are at least 3 different conenctors available at the tank end but you can easily get the matching part and swap on your fuel line (or get a whole new fuel line) or make a short piece of adapter to fit both connectors if you end up buying a different style of tank.

Fenlander · 04 September 2021, 23:58

Certainly need a 25l tank for a 30hp. They have gone up over the last year or so. As Poly says any brand will do. Barnet Marine have the excellent Quicksilver one at £112 and in stock...

https://www.ebay.co.uk/itm/172855337...8AAOSwT4lWRFI6

Cheapest I can see is a Yamaha one at Marine Tech of Norwich... last one left for £81...

https://www.ebay.co.uk/itm/271576243...cAAOSw1rBbUIz8

Matt · 05 September 2021, 22:38

Streuth. I need to downsize, I am loving the idea of only burning 20-30 litres of fuel in a day. I reckon I'd have just as much fun for less aggro and less ££.

citygent111 · 12 September 2021, 18:18

I mentioned to one of the guys at Dulas Boats about changing to a 25l tank but he said we might struggle as the fuel tank sits under the double jockey seat/centre console so the access hole may not be wide enough.

Of course at this point the boat was back in storage so can't measure the gap or the size of the available storage space.

2 weeks till we're back there, then I will measure everything and probably be on the lookout for a tall thin fuel tank and relevant connectors! 😅

Fenlander · 12 September 2021, 18:41

Just as an example there is this type...

https://www.nauticexpo.com/prod/rivi...27-207876.html

citygent111 · 13 September 2021, 12:34

Quote:

Originally Posted by Fenlander

Just as an example there is this type...

https://www.nauticexpo.com/prod/rivi...27-207876.html

Interesting - thanks Fenlander. I think I need to measure the available space inside the centre console. Right now the 12l fuel tank and battery are in there. I'm not sure how big the actual access hole is - I know it was tight getting the 12l tank out, as it had to be dropped in on it's side.
The tape measure is already packed!

Limecc · 14 September 2021, 12:16

It was tight but I managed to wiggle a 25L Mercury tank under the jockey seat of our Ocean Runner 420 RIB. When full I used luggage straps to lower it sideways because it was difficult to grip but normally I just filled it up in-situ. The battery was further forward under the console. Shame I didn't take a photo and now sold the boat.

Btw my 4.7m/50hp boat did 100 miles on 46L recently. Small engine had to work hard to keep good progress, 20 to 25kn. That's 9.88 mpg or 0.53L/nm or 1.87nm/L. $64K question is how much this drops loaded with passengers?

citygent111 · 21 September 2021, 15:29

Quote:

Originally Posted by Limecc

It was tight but I managed to wiggle a 25L Mercury tank under the jockey seat of our Ocean Runner 420 RIB. When full I used luggage straps to lower it sideways because it was difficult to grip but normally I just filled it up in-situ. The battery was further forward under the console. Shame I didn't take a photo and now sold the boat.

Btw my 4.7m/50hp boat did 100 miles on 46L recently. Small engine had to work hard to keep good progress, 20 to 25kn. That's 9.88 mpg or 0.53L/nm or 1.87nm/L. $64K question is how much this drops loaded with passengers?

That's really interesting Limecc, thanks. It just makes me more determined to find a larger tank solution. 12 litres is rubbish!!
We're back down there at the end of the month, so I think I'll run some tests on range etc. Fill the 12l tank, and take a spare 15l in jerry cans. That way, no matter where we run out, I know I'll have enough to get back!!

Quick question on that, being a 1st outboard owner - if the tank runs dry, I presume the engine will just stop? And if so do I simply refill the tank, squeeze up the priming bulb, and re-start the engine?

Thanks!

Limecc · 22 September 2021, 09:00

Quote:

Originally Posted by citygent111

That's really interesting Limecc, thanks. It just makes me more determined to find a larger tank solution. 12 litres is rubbish!!
We're back down there at the end of the month, so I think I'll run some tests on range etc. Fill the 12l tank, and take a spare 15l in jerry cans. That way, no matter where we run out, I know I'll have enough to get back!!

Quick question on that, being a 1st outboard owner - if the tank runs dry, I presume the engine will just stop? And if so do I simply refill the tank, squeeze up the priming bulb, and re-start the engine?

Thanks!

That's what I do. 25L tank in the bilge, 25L tank in the front foredeck storage with two 5L green cans as reserve. Swop them over and head home when it runs out, it just stops dead and loses prime. A few squeezes of the bulb and I'm underway again.

I carry a spare primer bulb and fuel line but only because it came with the tank and doesn't take any space to store. The Mercury tank was brand new from my old boat so I made up a 8" long converter, Mercury fitting to Suzuki. The tank that came with the current boat is Suzuki and a better design, but fyi I don't think it would have fitted under the jockey seat of the old boat. The seat opening was the problem.

Fuel consumption increases dramatically at displacement and when loaded with passengers. It's a small boat , hence not much footprint to spread the load. As an aside, Doel fin helps tremendously in these conditions and I've not noticed any bad behavior from it at speed. Also I'm sure I wouldn't get away with running the engine as high as I do (AV +40mm above keel) without it and this must help fuel economy like it does performance. Running a 14" four blade SS prop.