60hp 4-S Fuel Consumption

[Gimme5]

Does anyone know how the fuel consumption of the larger gear case variants of 60hp 4-Strokes (Mercury CT or Yamaha HT) compare with the standard models? I'm not after detailed numbers, just in general.

[Noelm]

Can't imagine there would be much (if any) difference between them.

[gazza2006au]

The motor will be running at the same RPM if propped correctly so it should use the same amount of fuel its when u run one heavy rig and one light rig say a fibreglass boat to a alloy boat u see the difference because of the boats over all weight and throttle position

when u change from normal to larger gear boxes that motor is still going to achieve the same RPM just u get a different output ratio at the propeller using the same throttle position all that will change is your speed from pushing a boat to pushing a barge or party pontoon

[stevej]

its not that simple
the gearbox ratios are different , so can swing a substantially bigger proper for controlling large cumbersome craft, house boats barges pontoons etc
the gear box casing is bigger but these dont generally see the same speeds as their normal gearbox models so drag doesnt matter as much

go up in horsepower before considering one as a overweight fishing boat isnt their real intended use

[gazza2006au]

Steve the gearbox is not what burns the fuel tho the crankshaft would be designed to output so many NM of torque in either configuration at a set rpm, the gearbox just has a different output

[BigE]

Having owned both versions , i can confirm

1. both can obtain excellent economy
2 the engine with the larger gearbox is substantially more sensitive to proping correctly.

I have managed 3km per litre with both engine variants both on glass boats but the larger gear case required several props to get it on the money.

BigE

[stevej]

mate dont know if you reply just for the sake of replying but i didn't mention fuel consumption

i mentioned the two gearbox designs are for different purposes and if youve got a conventional planning hull go up in hp first if you can

gear boxes are a torque multiplier ie can swing a bigger prop with more load on it
in this care you will travel less distance per turn of the prop and burn more fuel to travel the same distance

but int he end doesnt matter as they are for different purposes

[gazza2006au]

mate dont know if you reply just for the sake of replying but i didn't mention fuel consumption

i mentioned the two gearbox designs are for different purposes and if youve got a conventional planning hull go up in hp first if you can

gear boxes are a torque multiplier ie can swing a bigger prop with more load on it
in this care you will travel less distance per turn of the prop and burn more fuel to travel the same distance

but int he end doesnt matter as they are for different purposes

I donno if u read the OP's post but this thread was about fuel consumption so it was fuel related

[stevej]

mate your like a rash, yes i read his post

a bigger gear case will always burn more fuel as has more drag, may be negligible but will burn more

thats before even looking at the gear ratio issue

[gazza2006au]

Your of ya chops mate.

Bigger gearbox has more torque to push the forces easier which in return it will be easier to deal with any extra drag but whos to say a bigger gearbox going slower is anymore draggier than a smaller gearbox going faster?

outboards have a rated RPM they must be ran within if your within this area than u will see normal fuel economy now if u run the bigger gearbox its torqued accordingly by the propeller its called a load u use the right propeller to achieve this correct load and this will put u within the RPM range limit

when u run a engine at the correct RPM how on earth is the fuel economy going to change you have the same power head on both engines your have a different gear ratio however your still achieving the same "load" on the engine otherwise your RPM would be compromised

[stevej]

Some words to replace the content

[stevej]

not worth the effort of a reply

[gazza2006au]

An easier way of seeing it, You have 2 outboard engines both running at 5000RPM you now have two different gearboxes and propellers, Your still going to use the same amount of fuel per hour run time just your distance is going to be different due to the gearbox output and propeller ratios

Your still running the powerhead at 5000RPM on both engines which burns the fuel not the gearbox

[swof63]

I think the point that Stevej is trying to get across is that the high thrust gearbox’s not only have a higher ratio and thus greater torque multiplication, but they also are often physically bigger, thus have a greater “frontal area” in the water and thus higher drag at any given speed. You can prop them both differently to get them to similar speeds at similar revs, but the bigger gearbox will always have more drag, thus a cost in power.


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[gazza2006au]

Swof63 yeah but what produces more drag a bigger gearbox doing 40kph or a smaller one going 70kph? gearboxes are hydro dynamically engineered to use the lease drag possible so i think there would be hardly any difference

[swof63]

Moot point. The op will not be placing high thrust and normal motors on different boats, the question will relate to performance on the same hull. For any given hull, you would prop the motors to reach max wot as close as you can. This will result in similar top speeds, not 40-70 km/hr difference. Under these circumstances the the xtra drag of the larger gear case will result in a slightly higher l/nm consumption and possibly a slightly lower top speed. This of course assumes the magic fairy is having a holiday. There will be other differences also - the high thrust gearcase will be turning a higher pitch prop thus creating more stern lift, yet the extra drag of the larger gearcase will result in a more nose-down running attitude.


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[BigE]

Hi Gimme
been away (holidays) so i haven't responded

not sure if any of the replies have helped for what it is worth if i was doing it again i would be going for the standard gear case over the big foot or CT gearbox.
BigE