Outboard Engine Fuel consumption/HP Graph

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C08

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I am thinking of buying a Tohatsu 30hp fuel injected engine to replace my 20hp Normal Aspiration engine and I am pondering about fuel consumption. The Tohatsu websites show WOT figures but do not show the different consumptions for the 20hp injected and 20hp carburetor engines or have any indication of the power curve or the consumption at different rpm,s. There is an American website called Boat Fuel economy which gives the consumption at different rpm,s which is some help.

What I am trying to understand is how the fuel consumption will change with the larger engine having a greater capacity to drink fuel or if I use the same amount of power cruising with the 30hp engine as with the 20hp engine the fuel consumption will be similar. I.E. if I do 4000rpm with the 20hp which gives me 5kn and is this 4000/6000rpm = say 14hp? and 3000rpm with the 30hp engine and is this 3000/6000rpm = say 15hp.
Any enlightenment on this would be helpful. Tohatsu have nothing specific other than WOT figures!
 
I would say that simplifying things, it takes a certain amount of HP to move a boat at a certain speed. If you used all 20hp of your old engine, and use 20hp of your new engine, you will burn roughly the same amount of fuel. A couple of caveats, the fuel injected engine will use fuel more efficiently, so may use slightly less to produce the same power. The 30hp will be heavier, so will use slightly more power to get the same speed and having an extra 10hp on tap will be hard to ignore, so you will use some or all of it and burn more fuel.
 
CLB said:
I would say that simplifying things, it takes a certain amount of HP to move a boat at a certain speed. If you used all 20hp of your old engine, and use 20hp of your new engine, you will burn roughly the same amount of fuel. A couple of caveats, the fuel injected engine will use fuel more efficiently, so may use slightly less to produce the same power. The 30hp will be heavier, so will use slightly more power to get the same speed and having an extra 10hp on tap will be hard to ignore, so you will use some or all of it and burn more fuel.
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That is what my instinct tells me - it is disappointing that an enormous company like Tohatsu do not do fuel consumption figures for the 20hp FI engine compared to the traditionally aspirated one!
 
C08 said:
That is what my instinct tells me - it is disappointing that an enormous company like Tohatsu do not do fuel consumption figures for the 20hp FI engine compared to the traditionally aspirated one!
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I guess fuel consumption for engines of this size is quite negligible. Maybe they just don't use enough to worry about.
 
Most of these figures are produced to give guidance only as any engine will be affected by numerous factors and a newer engine may only have a slight (if any) increase in cylinder capacity but using modern designs they can change many aspects of the engine to give different characteristics, an improved induction system, fuel injection, cooling, and exhaust are all simple changes to improve power and economy and give a better power graph.

Many other things can reduce the performance.
 
Assassin said:
Most of these figures are produced to give guidance only as any engine will be affected by numerous factors and a newer engine may only have a slight (if any) increase in cylinder capacity but using modern designs they can change many aspects of the engine to give different characteristics, an improved induction system, fuel injection, cooling, and exhaust are all simple changes to improve power and economy and give a better power graph.

Many other things can reduce the performance.
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All true but I am still surprised that TOhatsu do not emphasise the improved fuel consumption due to the fuel injection!
My seasonal average (about 70 hours) is about 0.7galls per hour at 5knots pushing a 3T catamaran. So about 50 gallons per season, not a lot but not negligable.
 
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If this for a planing boat or a displacement boat?

If for a planing boat then I would assume that new 30hp fuel injected engine would use less fuel than an older carb engine, for a given speed.

If, for example, the boat can do 20 kts WOT with the 20hp carb but the usual cruise speed is say 15 kts, then we can assume that the engine is delivering say 15hp ish to give you the 15 kts. The 30 hp fuel injected engine will still only use 15 hp to give you the 15 kts (maybe a bit more, if the weight is more). So, the fuel consumption will be largely similar. The WOT consumption will be probably be more with the 30h, but it is giving you 50% more power at WOT.

As a real life comparison, our Avon 5.4m searider with a 90hp Mariner two-stroke, carbed burnt about the same as the current Scorpion 6.5m with a Mercury 150hp Pro-XS, two-stroke, fuel injected. The Scorpion probably weighs twice as much....

So, 1.3 litres carbed burns about the same as 2.5 litres fuel injected, in our experience.

A main Volvo Penta dealer round our way claims that carbs are 'nothing more than buckets with holes in the bottom!'

When Volvo switched the V6 4.3 from 205hp carbed to 225hp fuel injected I believe the fuel consumption went down by around 30%.
 
Hi
Worth noting that they are both naturally aspirated, but more importantly the difference is that a carb fuelled engine has some wastage, a direct fuel injection usually only uses what it needs to.
Not sure but one may also be 2 stroke and the other 4stroke. You didn’t say. If they are both 2 stroke...one carb and one DI the difference in fuel economy is unreal. Same goes for 4 stroke.
 
Both 4 stroke-word of moth from ob dealer is that FI engine is 20% better on fuel but no fuel consumption from Tohatsu.
 
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