Logic Behind Selecting Motors with Different RPM Range

[Splash]

Team,

What advantages exist by selecting a motor with Max RPM at Full Throttle at say 5800-6400 RPM or a motor at 5000 - 6000 RPM?

Reasons?

Splash

[geoffmck]

Probably none. Just choose a prop. with a pitch that matches the max. recomended rev range. Theoretically faster revs might equal faster speed, but also increased engine wear.

[tunaticer]

I can see no real benefit apart from maybe a differing sales pitch.

After all we should be using our vessels in a safe manner and driving to suit conditions, which are seldom ideal where we can open to full throttle.

Is there a reason why we should always try to remain in the top section of the rev range? Does it actually prolong engine life?

If we should be operating theoretically in a certain rev range then should not the engines be devised with multiple gear ratios? (2 speed or 4 speed)

Even in good conditions these days the traffic on the water will not let us operate at full throttle unless you escape the majority.

Jack.

[Spaniard_King]

Non splash,

Max RPM or the ideal rpm is what the manufacturer recomends the boat/engine setup should be able to acheive for best performance. If the boat/motor setup cannot acheive the recommended rpm range it concidered to be overloaded.

Also it is quite OK to have the boat/motor setup to acheive more rpm than the recommended limit however it's not a good idea to spend prolonged periods over the maximum recomended RPM. Just about any engine produced in the last 20 years has an RPM limiter which comes into affect a few 100rpm above the recommended RPM range.

[tunaticer]

Max RPM or the ideal rpm is what the manufacturer recomends the boat/engine setup should be able to acheive for best performance. If the boat/motor setup cannot acheive the recommended rpm range it concidered to be overloaded.

In saying that Garry, would you suggest that maybe there is a need for multiple speed gearboxes on outboards to be able to stay within a rev range?

Often a bad trip home will be a quite a few hours at much lower revs.

Jack.

[Spaniard_King]

Jack,

If the boat/motor setup is correct then the boat should be at it's best performance through out the entire rev range.

Yes IMO you could acheive beter performance if you had a multiple of gears to select. You could basically select a gear that keeps the engine in its best economy RPM range for the desired speed but IMO the manufacturers struggle enough with single speed gear boxes. Be a whole new kettle of fish just trying to get a 2 speed reliable.....size/shape of the gearbox plays a big part in performance so more gears would have an effect on that as well.

[BM]

Jack,

Its not about actually staying in the max rpm range all of the time. Thats simply not possible.

The importance of the max rpm range is that a correctly propped boat when at full throttle will be in the max rpm range, preferably near the upper end.

This doesn't mean the boat has to operate at that rpm but it means if the boat can achieve this condition then it is correctly propped so no matter what rpm you drive it at you are not overloading the engine and engine life is prolonged.

So hours on end at say 3000rpm is of no concern when correctly propped.

Cheers

[Splash]

Interesting responses guys - luv it!

Splash

[disorderly]

Sounds like somebody is finally looking at a new motor?.

[Splash]

lol..........I'll keep u guys guessing :-)

[Splash]

BTW -

What is an acceptable burn rate (litres/hour) for a new 4 stroke 140HP and 115HP with optimally pitched prop fitted?

What is the expected burn rate for a 20 yo 110 HP 2-stroke outboard with an optimally pitched prop fitted ?

Splash

[FNQCairns]

Just buy the 115 zuk and be done with it perfromance down on yours a little but economy way up, 140zuk performance near comparable with a fuel use more or less dividing the two.

cheers fnq

[ozbee]

hull weight has a lot to do with it also just because you have pitched it right doesnt mean a will burn the same fuel as b engine cubes will give you the lift on heavy boats while high powered canoes will fly on lower cubic capacity motors . hows that for not getting into a brand fight.

[Blackened]

high powered canoes will fly on lower cubic capacity motors . hows that for not getting into a brand fight.

G'day

What brand of canoe are you referring too?

hahaha

Dave

[disorderly]

Just buy the 115 zuk and be done with it
cheers fnq

Ahhh...but what if he really does buy a motor that actually runs properly .....what will become of that overactive mind...with splash it doesn't seem to be the fishing that is important...more like the journey to get there..