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Old 15 March 2019, 15:09   #1
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Real life running/fuel costs

Leaving aside the initial purchase price, perceived sea keeping and berthing costs, what are the likely fuel consumption differences between a 6m with a 115 Mercury and a 6.5 with a Suzuki 150? Not worried about top speed, interested in range and economy. Thinking on a cruising speed in the mid to high 20 knots. Both ribs are by XS if that helps in anyway.

Any thoughts gratefully received.
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Old 15 March 2019, 15:31   #2
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Not a Suzuki but a 150hp Mariner (Mercury) on a 6.5mtr Coastline.

Most economical cruising speed is 24 knots. Consumption around 0.8ltr/nm.

The Suzuki might be marginally better but there won't be a huge difference

Actual daily figures from a week last year......

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Old 15 March 2019, 15:37   #3
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Range is my thing.

Oddly enough, I suspect that range will be similarish. Obviously we'd need to know what the tank capacities are!
In VERY general terms, I'd say 115hp is a bit light for 6m and likely requiring a heavy throttle hand, whereas 150hp is very balanced for your required cruising speeds. L/Nm may work out very close.

As regards costs - in that spectrum, your fuel mileage costs will be the least of your worries. Fixed annual costs per mile will be the big influence, the more miles you do, the cheaper it is!
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Old 15 March 2019, 15:53   #4
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Thanks Last Tango, exactly the sort of answer I was hoping for.

Agree with your thinking Wilk, but the purchase and annual cost I can justify and live with. The plan is to be out as often as I can and to do some long distance trips [being retired has some benefits] so the actual cost per outing has a bearing on choice of size. That and keeping the wife on side!
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Old 15 March 2019, 17:38   #5
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Ive found this calculator to be pretty accurate, it can be set in metric or imperial Fuel consumption calculator - MerCruiser - Volvo Penta - Crusader - Mercury - Evinrude - Honda Marine - Yamaha Marine - Suzuki Marine - Tohatsu - Johnson - Outboards | GPH MPG LPH KPL
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Old 16 March 2019, 03:33   #6
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Quote:
Originally Posted by 36valley View Post
Leaving aside the initial purchase price, perceived sea keeping and berthing costs, what are the likely fuel consumption differences between a 6m with a 115 Mercury and a 6.5 with a Suzuki 150? Not worried about top speed, interested in range and economy. Thinking on a cruising speed in the mid to high 20 knots. Both ribs are by XS if that helps in anyway.

Any thoughts gratefully received.
All this burn data in my view is misleading as in real terms people don't use their boat that much to notice the fuel usage difference. My rule of thumb is on a normal half - 1 day out in the rib I use £30 - £40 of petrol and that is on a 60 hp old 2 stroke. Regarding range always carry about 20 - 40 ltrs in cans unless you have a huge tank
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Old 16 March 2019, 03:39   #7
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Passage planning?
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Old 16 March 2019, 03:44   #8
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Passage planning?
Perhaps one might like to expand
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Old 16 March 2019, 05:26   #9
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OP - If you are interested in range because you want to make long trips then focusing on cost per mile may not serve you well. Clearly it's important to buy an economical engine, but it's also vital (IMO) to consider other factors as well. It's a piece of string really...
  • Every engine/boat combo has a sweet spot for speed v economy.
  • Different boats handle waves differently.
  • Your boat needs to be able to achieve the engine's sweet spot in the weather you will travel in.
  • Long cruises tend to mean encountering mixed conditions.
  • Larger boats generally have larger tanks.
  • Loads will effect the range considerably
  • Because the engine will be run harder
IMO take the bigger boat/engine combo and forget about the "fuel economy".
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Old 16 March 2019, 06:19   #10
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On a 6m I'd think a 115HP will be a bit underpowered for economic cruising in the high 20kt band.

The extra 0.5m will also made a ride difference in any kind of seas.

If budget allows - as you say it will, I'd go for the larger engine every time. Not only will it be generally running at lower revs for the same speed as a smaller one, but if ever you need extra power, you have it.

I think that many people who opt for a smaller engine often spend a lot of time thinking "what might have been with the larger one" and that can spoil the enjoyment for a long time: few think I should have gone for the smaller unit.

For these reasons I'd go for the 6.5 with the 150 given the choice, and everything else being equal

LT
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Old 16 March 2019, 07:10   #11
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The figures in the video taken from my log were all based on "Smartcraft" and from refilling I know they're accurate to around 3-4%.
It was a good week and I'd rate the conditions 90% "calm" and 10% slight to moderate with two up.
As others have said, the conditions, load and speed will make a a noticeable difference and that difference will be greater the smaller the boat. eg if I go three up, push the speed nearer 30kn my burn for the day up around 15% (at full throttle I'm nearer 1.2l/nm.)

One of the main reasons I went for the Mercury was the lower cost and simpler servicing routine and the difference in something like that can easily be more than the annual difference in fuel cost from one brand to another.....
This might become significant if, like me, you're doing over a hundred hours a year meaning the engine needs serviced more than once a year.
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Old 16 March 2019, 07:14   #12
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Quote:
Originally Posted by lakelandterrier View Post
On a 6m I'd think a 115HP will be a bit underpowered for economic cruising in the high 20kt band.



The extra 0.5m will also made a ride difference in any kind of seas.



If budget allows - as you say it will, I'd go for the larger engine every time. Not only will it be generally running at lower revs for the same speed as a smaller one, but if ever you need extra power, you have it.



I think that many people who opt for a smaller engine often spend a lot of time thinking "what might have been with the larger one" and that can spoil the enjoyment for a long time: few think I should have gone for the smaller unit.



For these reasons I'd go for the 6.5 with the 150 given the choice, and everything else being equal



LT


^^^^^^^^^
Wot he said
I run a RC 545 with a 140suzuki, some would say that this is an overpowered setup. It isn’t. Now that I have it setup with the correct prop/engine height etc, it cruises at 27kn @4000rpm, right in the middle of the “lean burn” sweet spot. I get 1.2nm/litre at that. A smaller engine would be revving harder & using considerably more fuel. Holeshot is still “average” & she’s never going to win a drag race, but that’s the trade off.
I think a 6m with a 115 on the back will be disappointing for long cruises. When it turns lumpy, you need the power to avoid/get you out of trouble.
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Old 16 March 2019, 07:20   #13
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Quote:
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^^^^^^^^^
Wot he said
I run a RC 545 with a 140suzuki, some would say that this is an overpowered setup. It isn’t. Now that I have it setup with the correct prop/engine height etc, it cruises at 27kn @4000rpm, right in the middle of the “lean burn” sweet spot. I get 1.2nm/litre at that. A smaller engine would be revving harder & using considerably more fuel. Holeshot is still “average” & she’s never going to win a drag race, but that’s the trade off.
I think a 6m with a 115 on the back will be disappointing for long cruises. When it turns lumpy, you need the power to avoid/get you out of trouble.


Agree. We had a 140 Suzuki on a 6.3m Humber, not underpowered, certainly not overpowered, but there was always that niggle that a little more power when friends are on board making the boat heavier would be nice - we sold the 140 and now have a 175 opti. On a 6m boat the 140 would be ideal. Perhaps a 150 4s if the transom weight etc allows.
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Old 16 March 2019, 07:39   #14
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Last Tango - Great video. Thanks for sharing. Roll on summer.
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Old 16 March 2019, 09:52   #15
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last year we had a 5.3 humber with a 115mariner on it 4stroke so a reasonably modern engine and we were averaging 1.3 n/m per litre we sold that as it was getting a bit small for us we now have a north craft 6.3 with a honda 150 on it and now get the same if not slightly better with this set up its a bigger and heavier boat with a canopy on it and a bigger motor
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Old 16 March 2019, 10:32   #16
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Well the thread got busy while I was at the dealer making the deal.

Looks like I made the right choice even if I couldn't wait around to hear the advice I asked for.

I've gone for the 6.5m with the 150 Suzuki coupled with a 180 litre tank. This should give me all the range I think I'll need in the future.

I'm out a couple of times a week at the moment on the rib I have now, well I was before this current weather came in, and the plan is to carry on like that when I move to Devon in a couple of weeks time. The one off costs I can live with but the daily fuel costs are the ones I get to think about the most and for me have an impact on whether I enjoy or feel a little guilty about what is for me a bit of a selfish pleasure given my wife's limited use of the boat.

Again, thanks for all the views and thoughts.

I've now got a few weeks to decide what options and gear to fit to her
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Old 16 March 2019, 10:36   #17
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good choice
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Old 16 March 2019, 11:11   #18
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Look forward to seeing photos! Sounds nice!
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Old 16 March 2019, 12:22   #19
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Where are you basing her in Devon?

Looking forward o seeing her.

LT
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Old 16 March 2019, 12:38   #20
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Well done that man
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