Big single vs twin engines fuel consumption comparison

[Sr SQ]

Is that a planing hull? What's your top speed and burn?

Thanks for the answer.
Many boat & O/B mfgrs have "prop test info on their web sites. Most boat mfgrs bundle with Yamaha power, so info on Yamaha is the most widely available. I just repowered with twin 300 G2 ETEC's. The shop owner had twin 250's on his 28' well craft as a free promotional deal where he had to sell them every year. This deal ended when BRP stopped making OB's & he replaced them with twin 300 Yamaha. Dropped 5mph in top end & suffered a 20% drop in fuel consumption. I am going to venture that loss go torque (2 stroke versus 4 stroke) on a heavy boat is at play here, so a motor with larger displacement & longer piston stroke would win here (not all 150hp engines have equal power curves & good luck trying to get this info UNLESS it make that mfgr look better than the rest.
I have a 33' chriscraft that probably weighs 13000#, and before the OB's I was leaning towards twin diesels. I spent a lot of time using the prop calculator on www.boatdiesel.com where you can model boat speed/mpg/etc. The biggest factor is weight, followed by hull design (deep/shallow V). achieving the same HP with twins versus a single didn't matter a lot; more like 5%. If you run in rough water, having twin props will result in less RPM drop than a single prop, so economy with twins might be better. If you are considering a boat with around 300 HP which SHOULD be at least 25' long, twins with an autopilot & trolling on 1 (or 2) would be IMO the absolute best combo. My chris hits 40MPH & idles at 3 MPH in dead calm water on 2 engines; 7 MPG at 8mph on twins. Furuno makes auto pilots with a handheld remote & Sabiki mode (keeps you headed into the seas when in reverse still fishing).

As said, it all depends on the hull.........
Did the mfg design the hull for the weight of twins, weight, style, beam, length, typical sea conditions?
It will cost you more to run twins........how much more, that depends on the hull and its use.
I prefer the twins for west coast ocean fishing but on a bass boat on a calm lake a single would be a better fit IMO
I am a twins fanboy after running one and 1/2 dozen singles, I will never deal with a kicker and all its shortfalls again, but that's me.
With todays mid size 4cyl OBs troll fuel burn vs a kicker is negligible and new kickers with trim/remote/etc are 1/2 the price of a new 150.......
Its hard to explain the handling characteristics, , but the're very noticeable. In rough seas the twin props feel like a 4 wheel drive in the snow similar to trolling on the 150 vs the 15.

 

[ericl]

It is a planing hull of 12' dead rise . Right now top speed is 40mph @6100RPM with so-so AL props, fuel tanks 25% full 3 passengers. Bottom paint is in poor condition; very rough but as the boat is on dry-stack I will remove the bottom paint. I haven't run the boat much as the shop made some interesting wiring choices including cutting running light wires when they removed the arch to get the boat into the shop, & lights are a legal requirement. I expect that with 4 blade SS props full tanks & a slick bottom top speed will remain similar.

 

[daddystoy]

Thanks for all the info everyone.

 

[myles]

In theory I think they would be very close. This chart would appear to bear that out:
https://www.boat-fuel-economy.com/yamaha-outboard-fuel-consumption-liters
with the twins burning a little more.

I have heard that you should expect a 20% HP loss from twins. Think about it you are putting 2 lower units in the water. Depending on some factors such as the lower unit size you could be adding double the drag which contributes to lower fuel economy. Larger single will always beat twins all else being the same. So many factors for a fair comparison.

 

[profisher]

I personally think the most important factor is engine longevity. These things whether a single 300 or twin 150's are not cheap to buy. You want them to last as long as possible and buy as few of them as possible over your boating seasons. I've seen twin 150 Yami's and a single 300 Yami used in camp on Grady charter boats over several seasons. The Grady with the twin 150's had way less mechanical issues related to the work that was demanded of it in big water and heavier loads. To me it just makes sense....12 pistons and 2 crankshafts and two props/gearcases sharing the work instead of 6 pistons, 1 crank and 1 prop/gearcase. Yes you have to buy 2 motors to get started so higher outlay initially and likely if you don't put many hours or tough hours on your equipment each season you may never recover the difference spent on the extra motor. But if you do lots of running with a heavy boat I think you will save enough in repairs to justify that 2nd engine outlay.

 

[Gong Show]

I have heard that you should expect a 20% HP loss from twins. Think about it you are putting 2 lower units in the water. Depending on some factors such as the lower unit size you could be adding double the drag which contributes to lower fuel economy. Larger single will always beat twins all else being the same. So many factors for a fair comparison.

Drag is key.
Increases exponentially to velocity. I appreciate the other factors, but drag cannot be overlooked.

 

[Derby]

I personally think the most important factor is engine longevity. These things whether a single 300 or twin 150's are not cheap to buy. You want them to last as long as possible and buy as few of them as possible over your boating seasons. I've seen twin 150 Yami's and a single 300 Yami used in camp on Grady charter boats over several seasons. The Grady with the twin 150's had way less mechanical issues related to the work that was demanded of it in big water and heavier loads. To me it just makes sense....12 pistons and 2 crankshafts and two props/gearcases sharing the work instead of 6 pistons, 1 crank and 1 prop/gearcase. Yes you have to buy 2 motors to get started so higher outlay initially and likely if you don't put many hours or tough hours on your equipment each season you may never recover the difference spent on the extra motor. But if you do lots of running with a heavy boat I think you will save enough in repairs to justify that 2nd engine outlay.

The one thing as a previous Yami owner of a 300HP... they sucked as a motor and so did there 350 Hp... that being said I have heard great things about the there 150 hp and they have had some success there. I think it maybe which motor was a better engineered

 

[uclueletcharters]

The 150's yamis are a flag ship motor. I think you got the friday motor when you mention 300 yamis being no good. Have run 4 different ones from 2016 - 2018's all have been rockstar engines as far as reliability.

 

[Aces]

I get what you mean about drag but the lift off counter rotating twins it pretty impressive

As far as the 150 Yamahas I saw lots of them providing good service. Did good in a Mercury town

 

[sly_karma]

Drag is key.
Increases exponentially to velocity. I appreciate the other factors, but drag cannot be overlooked.

It might be key to you, but only one of many factors raised in this thread. Drag affects fuel burn, but is not a factor in longevity. Both those are components in a complex calculation of overall ownership costs which will come out differently for every hull and usage.

Interesting discussion that has been respectful and considered. Much food for thought for the OP.

 

[Gong Show]

Original question is fuel consumption.
I would think twins have higher mass and drag.
I appreciate the discussion concerning one powerhead vs two, there are some differences in efficiencies.
The thread may have drifted to longevity, I was commenting on fuel consumption.

 

[Sr SQ]

There are dozens if not hundreds of Mfg boat tests published with the single/twin comparison on the same hull.
Mercury has pages of them on their website.
I have only seen a couple where twins on the same hull netted better milage and speed than the single.
From all the tests I have seen most fuel burn #s show the twins burning a little bit more.
As mentioned once your hull reaches a certain size a single prop in the water just won't perform well enough to compete against 2 props unless water conditions are ideal.
So its' a personal decision on what's most important to you, fuel consumption or performance, there are trade offs for everything in life.
My boat with the twins will plane down to 22mph with some tab, same boat with a single will be 26-7mph, on a rough day let's see who burns more fuel running out to the fishing grounds and back, not to mention who gets the hell beat out of them doing it.