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Final Gear Drive Ratio Larger Prop

mr_johnson

Regular Contributor
Suzi says you can swing a larg

Suzi says you can swing a larger prop because of it's 2.59:1 final gear ratio.
Johno has 2.38:1 final gear ratio.

Q1 - WHy are these ratios different when they are supposed to be same donks?
Q2 - Can you still put a larger prop on the 2.38:1 final gear ratio?

How critical is having minimal exhaust fumes when trolling and livebaiting?

Are 4 strokes better than 2 strokes for minimising exhaust fumes?

Cam
 
"Cameron, I'll take a stab

"Cameron, I'll take a stab at this one for you as well (mine is but one opinion).

Those gear ratio's are for "high thrust" motors.

To start with, you should always match the motor and the prop to the rig you are putting it on.

A standard gear ratio on say a 50 horse may be in the 1.8:1 range (one prop revolution for 1.8 turns at the flywheel). That with a 17 pitch prop may push a 16 foot alumimum fishing boat along at 30 mph just fine.

You stick that same 50 horse of a 24 foot pontoon boat, that weighs more and has hydrodynamic characteristics more in line with a rock than a boat, and that 50 is going to gasp, gag and bog down trying to get it moving.

By moving to a lower gear ratio and lower pitch prop, you can push it along at a reasonable speed, while you and your dozen guests sip your drinks. The motor isn't working as hard and it's pushing "less" water initially to get forward motion. Totally different application though....

You could use the high thrust on the fishing boat, with a rediculously high pitch (if you could find one) and achieve the same results as a "standard" set-up, but my question would be "why"? (unless you got the motor for nothing).

At 5000 rpm (at the crank), 1.8:1, would equal about 2778 rpm at the prop shaft. A 17 pitch prop should travel (shaft rpm x inches of pitch - slippage) about 3935 feet (minus slippage) per minute.

At 5000 rpm, 2.59:1, would equal 1930 rpm at the prop shaft. So to cover the same distance with the high thrust you would have to up your pitch from 17 to about 24.5. (1930 x 24.5 = 3940 feet), just to attain the same speed as the convential set-up.

The value of the low gear, is for moving more weight with a lower pitch - where you simply want to "putter" along....

Less exhaust fumes on a 4 than a 2 - not sure if the fish mind either way
"
 
"THanks Graham,

So, when a


"THanks Graham,

So, when a final gear ratio is stated as 2.59:1 - this is not very good then??

Is this final gear ratio regarded as low gear ratio or am I missing somehting?

Cam"
 
"What 4 stroke engine should I

"What 4 stroke engine should I then acquire for my rig (1989 5.3m Haines HUnter SOlid Glass boat about 750kg hull weight alone plus fuel, etc. total around 1000kg max)?

What prop. pitch should also be used on this rig?

Rig will be towed to and from ramp and trolling will be a significant activity.

Cam"
 
"Where are you running this bo

"Where are you running this boat? Offshore? If you are on a freshwater lake, I might recommend a 90 horse - if you are worried about getting back in when the Pacific gets nasty, that could well change what you want to hang on the back...."
 
"THanks Graham,

No fresh wa


"THanks Graham,

No fresh water - All out wide 30-50 miles off coast open water (top of Australia actually).

I am also looking at the Honda BF90.

Thoughts?

Cameron"
 
"Sorry for interupting, but fe

"Sorry for interupting, but feel for a small comment.
Most often Diameter and Torque goes together. The higher torque - the more diameter You may pull.
On the other hand -the more diameter - the more 'wet suface' the more resistance. At high rpm this will encrease drastically and take the often wanted pitch-power (top end speed) off as pitch has to be reduced.
Next comes the caracteristics of 2-strokes and 4-strokes. The modern 2-stroke injection engines are superior in torque at lower rpm compared to similar 4-strokes, not to forget the power/wheight ratio.
Evinrude (Bombardier) took full advantage of this when the 40/50/60 2 cyl E-tech came on the marked. The encreased torque at lower rpm's made it possible to change the gear-housing to the same used on the 3/4-cylinder (not same ratio) and by that encreasing the prop dia.
Try then to compare the prop dia (area) on the 40/50 4-strokes (Suzuki) or even Honda with the E-tech 40/50.
Technical fuel consumption between modern injected 2-strokes and 4-strokes is almost the same. Trolling consumption mesured by 'trolling mile' will probably be in favour of the engine with the biggest blade area. (You paddle with the blade or the handle in water). Cruising economy the same, and her comes even the less wheight of the 2-stroke at play as less wheight at transom will normally mean lower planning speed.
I might keep on, but I think You get my picture: technically I would preferr an injected 2-stroke for Your kind of use.
But it all comes down to the kind of service and parts availability You have in Your area which brand I would recomend to buy!"
 
Thansk Moreton.

Very consis


Thansk Moreton.

Very consise informaiton here - alot better than other forums I have participated in - Really appreciate everybodys' thoughts thus far!!

THe etec is very appealling isnt it.

Cam
 
"I was atill 'active'

"I was atill 'active' when they were introduced and sold quite a number of the 50 and 70 versions. I even ran quite a lot of camparison tests versus 'old' 2-strokes and the Suzuki 4-strokes. performance wise in all aspects they did beat everything. The only slightly negative was perhaps the slight smell of oil in some siyuations, but not a problem.
But all engines will need service sooner or later,they claim E-tech needs it later, check the local market!"
 
"Cameron, completely agree wit

"Cameron, completely agree with Morten. I got to see the end of a year long test on a 60 ETEC, run in Canada (harsh conditions).

It really sold me on the DFI two strokes. If that is the way you are leaning, and you have the sales/service support, you could take a look at the Tohatsu's as well, they have built in a variable/adjustable idle speed (right into the key switch) to give you a 200 rpm range at idle.

Initial cost (here at least) is somewhat lower than the Evinrudes - technology is similar, although the ETEC and Yamaha Direct injects are high pressure systems, the Tohatsu and Merc's are low pressure.

So far though, regardless of the brand, have heard some impressive comments from both owners and techs...."
 
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