Changing gears for more speed?

[Paradise David]

More speed?

Prop:10" 16p
engine: 40hp Mercury 2 stoke
Max rated rpm: 5500
Gear ratio: 2 to 1

......................................
engine rpm = 5500
prop rpm = 2750


1/2= .5
.5(5500)=2750

...........................................
Now let change the gear set:

1.5 to 1
same prop
Same engine and spes's

1/1.5 =.66
.66(5500)=3630

3630 - 2750 = 880 rpm gain.
.......................................
With little torque loss.
Now, back in the water, this prop will over rev oblvisouly.
Now get a bigger pitch prop to get the rpm where it is needed.


On paper, this works great. But I don't run my boat on paper!

Will this work? I am missing something here?

 

[CATransplant]

Re: Changing gears for more speed?

Won't work. The engine in your outboard can only produce a certain amount of power. If it is set up right, you reach max RPMs with a specific prop, and that's it. All the power is going to drive that pitch prop at that speed.

If you lower the gear ratio, the engine won't have the power needed to drive that prop to the higher RPM. The engine won't run at its max RPM, and will produce less power, so you'll actually go quite a bit slower.

It's all about the power of the engine.

If you're just reaching max RPM with the prop you have, that's it. If the engine could rev higher, try a prop with a higher pitch. That would give you more speed. I'm betting, though, that you're turning max RPMs with your current prop. That's all you get.

 

[Woodnaut]

Re: Changing gears for more speed?

...The engine in your outboard can only produce a certain amount of power. If it is set up right, you reach max RPMs with a specific prop... All the power is going to drive that pitch prop at that speed.

That's pretty much it. If, however, you can somehow reduce drag (changing the boat's trim, make sure the outboard is positioned properly, etc.) you might gain a little extra speed. Of course, make sure the engine is properly tuned up.

 

[Philster]

Re: Changing gears for more speed?

+1 w/ CATransplant

If your engine pushes itself to redline with a given prop, that is the top speed. If you change gears to a taller gear (lower numerically), you are out of power before redline. You will lose top end.

What you can do is boost acceleration by going to a shorter gear (higher numerically), but then the engine is setup to blow redline (which would happen at a lower top speed).

If you have a given engine reached redline at full throttle, that is ALL she has to offer for top speed. Your only PRACTICAL option is a labbed, SS prop. For a thousand bucks, you could milk another 2 MPH (give or take an MPH) out of the setup. On a 40 horse motor? Notsomuch.

Important: SHEDDING WEIGHT and raising the drive or doing other things to make the boat more hydrodynamic will NOT typically boost top speed if the engine is running at 100% redline at wide open throttle in basic and modded formats. If you indeed find more top end, you must be blowing the redline and now need a different prop to knock RPMs down. However... it is unlikely you created so many more RPMs that you expect to run a bigger pitch prop.

 

[CATransplant]

Re: Changing gears for more speed?

That's true. Fine tuning can get you a few MPH. But, the OP was thinking about something that defies the physics of the thing.

Sorta like putting an old VW engine in a Porsche, back when they were both air-cooled and could actually be bolted up. I've seen it done.

The result was a vehicle that looked like a Porsche, but with an engine that couldn't drive it to its former top speed. The final gear ratio was just too low, so the VW engine lugged in top gear.

Not a good bargain.

 

[Philster]

Re: Changing gears for more speed?

We definitely agree.

I know the story all too well. Every yahoo I know who owns a performance boat is always wanting to go 70 mph. They can do 63-65 all day and night, but they want 70 MPH.

Time and time again, they find they need to drop about 15 grand to make enough extra power to go 70 mph. That is a very, very expensive 5-7 MPH. You have to pour on something like 65-100 H.P.

15 grand gets you the chance to tell your buddy the GPS said 70. Moral to the story: You need more horsepower [and bigger prop(s) of course].

 

[cr2k]

Re: Changing gears for more speed?

I have corrected your formula as below in the 2nd half of the equation.


More speed?

Prop:10" 16p
engine: 40hp Mercury 2 stoke
Max rated rpm: 5500
Gear ratio: 2 to 1

......................................
engine rpm = 5500
prop rpm = 2750


1/2= .5
.5(5500)=2750

...........................................
Now let change the HP set:

1.5 to 1
same prop
50HP engine and spes's

1/1.5 =.66
.66(5500)=3630

3630 - 2750 = 880 rpm gain.

Seems there is just NO substitution for HP.

 

[NSBCraig]

Re: Changing gears for more speed?

15 grand gets you the chance to tell your buddy the GPS said 70.

I think I'd just run with a current and video my gps. Might be cheating but it don't matter if you don't get caught!

 

[smokeonthewater]

Re: Changing gears for more speed?

3630 - 2750 = 880 rpm =24% gain
expect the same 24% loss of torque
With little torque loss.
Now, back in the water, this prop will under rev (lug) obviously.
Now get a smaller pitch prop to get the rpm where it is needed and theoretic speed returns to the same figure but actual sped is likely to suffer because of reduced efficiency from turning the prop at the higher rpm

I have edited the part of your theory where it fell apart... this should help you understand the 'why' part of this discussion

 

[Paradise David]

Re: Changing gears for more speed?

Thanks for all the feedback on this.
I had I thought after reading all the replies here. I did a search (a long one)
Just to see if I could find a 40 hp outboard 2 stoke, 2 cyl. (didnt matter on the brand name) with a gear ratio less than 2 to 1.

Results:
I couldn't find one. Closest I came was a 40 hp 3 cyl. with 1.83 to 1. Most were more than 2 to 1 meaning 2.08 to 1 and bigger. So this tells me, not that I didn't believe anybody opinion here, that this wouldn't work.
Actually, I should have done the search before I post here.
I think I'll just find me a lightweight 60 hp.
Thanks