'Detuning' to save fuel?

alystra · 08 April 2007, 11:05

My year 2000 carbed Mercury 4st. 90 hp specifications seem identical to the 75 hp version. Presumably the extra power is found by pouring more fuel into the engine? (bigger jets?) My usual cruising revs are between 3000 and 3500. - around 21-23 knots. WOT is just short of 40knots and the fuel vanishes in minutes

. I rarely want to go that fast and can't afford it anyway.

Just wondered if I could 'detune' the engine to the 75 hp spec., would I get better fuel consumption at my 23 knots? Presumably, with less power one would need more revs to keep the speed.

I just wondered if the engine would be more efficient running at higher revs and therefore burn less than it does at the moment at my cruising revs.

Mind you, there's going to be more stress at higher revs. Mmm! Perhaps I've answered my own question?

Any experts? Any thoughts?

BruceB · 08 April 2007, 11:08

AFAIK the only differences between a Yam 60 and 70 hp are the carbs and the exhaust. Presumably yours is much the same as the Yam

Hightower · 08 April 2007, 12:24

IMHO you would be wasting your money doing this as it's the weight, length and shape of the boat that dictates fuel efficiency more, not the engine (at cruising speeds). At WOT yes there are savings to be made, but you have control over this by traveling everywhere at the most economical speed.

I'll give you an example of this.

Kohaku which is a 5.3m RIB with a Merc F90 (same as yours) did 2-1.5nm/l at cruising (4000rpm) on a cruise where WOT was used most of the time she was drinking it at 1nm/l.

Now compare this with some other members RIBs with a 6m RIB and bigger engines. I would expect a 150 Optimax to give a very comparable cruising economy but the WOT to be significantly worse.

Certainly not worth the differance betwen a 75 and a 90

jwalker · 08 April 2007, 12:25

You're going to need x amount of power to sustain y amount of speed and, for a given efficiency, you'll use z amount of fuel. So it's going to be down to using the engine at its most efficient speed. Just reducing its maximum possible power is not likely to do it unless, by doing this, you also improve efficiency. It is often the case that carbs with a smaller venturi are more efficient and responsive at mid range revs so this could give an improvement but, of course, also a reducion in maximum power and it's not likely to be a great improvement.

Stoo · 08 April 2007, 13:33

I have found that the weight in the boat makes a significant difference. I used to routinely leave my dive gear in the boat... including two sets of double tanks, plus another O2 bottle, plus all of the other stuff.... My guess is that it weighed 250 pounds. Also, I was in the habit of topping up the fuel every time I went out, even if I was just heading out for a short trip. With a total fuel capacity of 180 litres, the weight of this fuel is close to 300 pounds. So between dive gear and fuel, the "potential" is there to reduce weigh significantly on a lot of the short trips I take. Between this and watching my speed, I have been able to make some significant changes in my fuel consumption.

I read an article a while back which explained "how" fuel was consummed in an outboard, and the reality was that a great deal of the fuel is wasted just producing heat. Water conditions also makes a huge difference as we all know, as do driving habits and crap growing on the hull.

My guess is that detuning your engine as you describe will have less effect on fuel consumption than other factors... but that's just my opinion!

wavelength · 08 April 2007, 15:41

I think you have the answer in your own hands-just dont drive flat out! We have a 90carbed merc 4 stroke on a 5.5 Ocean Pro. More economical than our Suzuki 90 injection 4 stroke and have never had the situation where the fuel "just disappears" at any speed. Need to drive a 2 stroke V4 to see fuel vanish

Stoo · 09 April 2007, 02:58

Quote:

Originally Posted by wavelength

Need to drive a 2 stroke V4 to see fuel vanish

LOL.. Try a 2-stroke V6! It burns through money faster than my ex!

pathalla · 09 April 2007, 03:37

Try two v 6's sucking it down. Hey look at it this way, if you don't burn it someone else will.. Still love the sound of those motors when you nail it coming out of the hole... Sometimes you just have to ante up for the good things in life!

Hightower · 09 April 2007, 08:25

Quote:

Originally Posted by pathalla

Try two v 6's sucking it down. Hey look at it this way, if you don't burn it someone else will.. Still love the sound of those motors when you nail it coming out of the hole... Sometimes you just have to ante up for the good things in life!

Not surprised at 30 pence a litre

BogMonster · 09 April 2007, 10:38

Quote:

Originally Posted by wavelength

Need to drive a 2 stroke V4 to see fuel vanish

Tell me about it

alystra · 09 April 2007, 17:58

Quote:

Originally Posted by BruceB

AFAIK the only differences between a Yam 60 and 70 hp are the carbs and the exhaust. Presumably yours is much the same as the Yam

Yes, it's a Yamaha powerhead.

Quote:

Originally Posted by Hightower

Kohaku which is a 5.3m RIB with a Merc F90 (same as yours) did 2-1.5nm/l at cruising (4000rpm) on a cruise where WOT was used most of the time she was drinking it at 1nm/l.

Interesting, same sized boat and engine. I seem to get an average of 1.7nm/l at 3250-3500rpm. Guess it would be 1.5 at 4000.

Quote:

Originally Posted by jwalker

You're going to need x amount of power to sustain y amount of speed and, for a given efficiency, you'll use z amount of fuel. So it's going to be down to using the engine at its most efficient speed. Just reducing its maximum possible power is not likely to do it unless, by doing this, you also improve efficiency

Yes, as I thought. Guess I was wondering whether running at higher revs would improve efficiency. I'm thinking not.

Quote:

Originally Posted by wavelength

I think you have the answer in your own hands-just dont drive flat out! We have a 90carbed merc 4 stroke on a 5.5 Ocean Pro. More economical than our Suzuki 90 injection 4 stroke and have never had the situation where the fuel "just disappears" at any speed. Need to drive a 2 stroke V4 to see fuel vanish

I think you're right. You've made me feel so much better - I had the illusion that the modern injected engines were much more economical. Guess 'disappearing fuel' is all in the mind. I'm still getting used to engine power after many years using 'free' (apparently) wind. You've reminded me. I did have a part share in a Glastron speedboat in the late 60's, with a big Evinrude on the back. Buried it as an unpleasant memory, I think.

Thanks all for thoughts, ideas and advice. I'll leave things as they are, keep the throttle arm short, lie back and think of the poor chaps in the Falklands.