Fuel consumption different when sailing at same speed using different engines?

[Mike_31]

Hello,
Newbie questions here..
I'm not a native English speaker so thx for your understanding.

A 15ft boat with 40hp is sailing next to a 15ft boat with 90hp and both are sailing at 20 mph.
1. Which boat will have a better fuel consumption?
2. If I am not looking for more than a speed of 20mph, which engine would you suggest I'll use for the boat in the attached photos in order to get the best possible fuel consumption?
Will a 30hp be too weak? is there any difference in the fuel consumption between 30 and 50hp?
The boat already has a 90hp engine so I need to decide if I replace or keep it.

Thank you very much in advance!

 

[jfm]

Hi and welcome. Great English! :encouragement:

1. The 40hp will use less fuel. Not because of the nominal engine size, because AOTBE both the 40 and 90 will be burning identical fuel. But the 90 will be bigger, have more internal friction, be dragging a bigger gearbox (4 inch dia on a 90; 3 inch or 2.5 inch dia on a 40) and bigger prop thru the water, be heavier, etc, and these things will make it burn more fuel. Perhaps 10% more

2. Your problem is that to get a small boat over the hump, especially with people on board, you need much more power than you need to maintain a steady 20mph. 30hp is fine for maintaining 20mph, but you need say 50hp for 10 seconds to get over the hump. So keep the 90, I think. 50 minimum on that boat

Ref question 1, I said AOTBE. Of course if you are swapping an old 90 2T for a modern 50 4T, you will see significant fuel savings

Unless you do BIG mileage, or are swapping an old 2T, the fuel savings you are discussing here are going to be pretty small anyway. Just save fuel by cruising at 20mph with your 90hp

Which 90 engine do you have?

 

[Mike_31]

Hi and thx for your reply, very much appreciated!
I am attaching a photo of the old 90hp Mercury engine.

 

[jfm]

Ah ok. That's a late 80s/early 90s 3 cyl 90hp Merc 2 stroke. 1.2 litres iirc, basically half of their V6 2.4 litre motor. A 2 stroke from that era is pretty fuel inefficient by 2015 stnadards, so swapping it for a modern 4 stroke or an uber modern 2 stroke like an ETec will probably cut the fuel burn 25% or so on a like for like basis. At a significant capital cost, of course! But that fuel saving will be because of the more modern engine, not because you're running 20mph with a smaller engine

 

[Mike_31]

Very helpful info, thank you.
In that case, which engine should I buy in your opinion? which manufacturer how much hp? a recommendation from an expert such as yourself will help alot and very much appreciated!

 

[jfm]

Very helpful info, thank you.
In that case, which engine should I buy in your opinion? which manufacturer how much hp? a recommendation from an expert such as yourself will help alot and very much appreciated!

For what you describe, a 50hp Yamaha is lovely and the 50hp Evinrude E-Tec too. The Yam is quieter - you will be amazed how quiet it is at idle after the Merc 90. But they're 100x the €€€ cost of the fuel you will save in fuel in several years!

If you really want 40hp get the Evinrude eTec because the Yam 40 will struggle to get the boat over the hump - it lacks torque/power until past 3000rpm, which might not happen if boat will not go over the hump.

I've owned new Yam 40,50,70 in the last 5 years and like them very much

 

[MapisM]

Hi and welcome.
If I can ask, is there anything wrong with your current engine? I mean, does it need any major repair/maintenance?
I'm asking not because I disagree with anything jfm suggested - quite the opposite, in fact - but because if your engine still runs well, in your boots I would do some maths before buying a new outboard.
By heart, I would think that your break even point based on lower fuel consumption is well into the next decade - by which time, you might as well have changed the whole boat...
Of course, that depends on how many hours/year you make with your boat, but imho the investment could make sense only if that number is VEEERY high.
Just a thought.

 

[Mike_31]

Hi and welcome.
If I can ask, is there anything wrong with your current engine? I mean, does it need any major repair/maintenance?
I'm asking not because I disagree with anything jfm suggested - quite the opposite, in fact - but because if your engine still runs well, in your boots I would do some maths before buying a new outboard.
By heart, I would think that your break even point based on lower fuel consumption is well into the next decade - by which time, you might as well have changed the whole boat...
Of course, that depends on how many hours/year you make with your boat, but imho the investment could make sense only if that number is VEEERY high.
Just a thought.

Hi and you all for the help.
Sorry that I didn't explained the situation properly, I am about to buy this boat which comes with 90hp Mercury engine which by the seller's claims has no problems.
I will be soon renting a house next to the beach so I plan to use the boat 2-3 times/week so I'll be using it alot.
As buying a brand new engine is not cheap (but it is something I consider), how about trading the used 90hp with a used 50hp from the same period (or maybe slightly newer)?
What is the gph difference when using old 90hp vs 50hp cruising at the same mentioned speed +/-?
Again, thx alot, your help is very much appreciated!

 

[jfm]

Hi and you all for the help.
Sorry that I didn't explained the situation properly, I am about to buy this boat which comes with 90hp Mercury engine which by the seller's claims has no problems.
I will be soon renting a house next to the beach so I plan to use the boat 2-3 times/week so I'll be using it alot.
As buying a brand new engine is not cheap (but it is something I consider), how about trading the used 90hp with a used 50hp from the same period (or maybe slightly newer)?
What is the gph difference when using old 90hp vs 50hp cruising at the same mentioned speed +/-?
Again, thx alot, your help is very much appreciated!

OK then you're back to the ~10% fuel saving in my post #2 if you swap from 90 to 50/40. Remember a swap isn't necessarily completely straightforward. You need to buy the same make, Mercury/Mariner, else the fuel connection and gear/throttle cable ends will be different. You'll have to deal with wiring harness - don't assume plug and play. You'll have to deal with the old bolt holes in the transom. The steering will likely be ok on both engines. All of that is do-able, but you could find the cost of these peripherals to be a few hundred € even if you do the work yourself. And €500+ if a boatyard does it. That's a lot of fuel you have to save. 3GPH vs 2.7 GPH say, at 18 knots (UK gallons, =4.54 litres)

I still think keeping the 90 might be best. Those clamshell 3 cyl 1.2 litre Mercs are nothing hugely special in the history of outboards but they're perfectly good

Your EN is excellent, by the way!

 

[Mike_31]

OK then you're back to the ~10% fuel saving in my post #2 if you swap from 90 to 50/40. Remember a swap isn't necessarily completely straightforward. You need to buy the same make, Mercury/Mariner, else the fuel connection and gear/throttle cable ends will be different. You'll have to deal with wiring harness - don't assume plug and play. You'll have to deal with the old bolt holes in the transom. The steering will likely be ok on both engines. All of that is do-able, but you could find the cost of these peripherals to be a few hundred € even if you do the work yourself. And €500+ if a boatyard does it. That's a lot of fuel you have to save. 3GPH vs 2.7 GPH say, at 18 knots (UK gallons, =4.54 litres)

I still think keeping the 90 might be best. Those clamshell 3 cyl 1.2 litre Mercs are nothing hugely special in the history of outboards but they're perfectly good

Your EN is excellent, by the way!

Thank you so much for the time you invested in helping me.
Same goes for your nice comment on my English.
Your logic and vast experience just saved me alot of hassle, time and money!
I wasn't at all aware of the complexity of adjusting a different manufacturer's engine vs the actual minor benefits of getting different engine so I will ofcours keep the 90hp until it dies and once it will, I will know which type to buy thx to you
All the best to you.