Gear Ratio Debate 1.62 vs 1.75

[2.5-21]

If propped properly shouldn't they both achieve the same speed? If not then why?

[davemvegas]

All depends on the boat and setup. Simple answer is if you are running a 300xs with a boat that you plan on topping out around 95 mph. 1.75 is the gear. to run a 1.62 to try for a few mph is not worth the throttle response you give up. If you single engine boat is light enough that you are thinking 103 or faster. You wont get there with 1.75 gear and a 6400 rpm motor with reasonable priced prop. When my boat had a single 300xs i ran a 1.75 with 26p 4 blade prop best of 78 mph. Tried a 1.62 a few times and even with a 24p my heavy 22ft Eliminator Daytona would not get on plane. Converted it to twins 4 years ago. Now it runs 1.62 gear with 32p Bravo props cut and pitched to 33p. Best speed has been 109 mph. always will run 105 at 6300 rpm. A single with 1.75 is a sports car feel and light hull will run mid 90s. Same boat propped for top end with 1.62 will feel like you are towing another boat when you hit the throttle at any speed.

[engineermike]

It’s an interesting subject. I think what you’re asking is, if any prop pitch were available and the geometry was otherwise the same, then why wouldn’t, say, a 25” prop and 1.62 gear run exactly the same as a 27” prop and 1.75 gear.

I believe the 1.62/25” combo would have a better holeshot because of the blade angle of attack against stationary water would be better. As for acceleration, it should be close to a wash. On top end, one of the 2 props will have a small efficiency and speed advantage because the propeller “specific speed” (adapted language from axial flow pumps) will be closer to optimal than the other.

[HStream1]

Sense the Original question was only about both ratios achieving the same speed and nothing else. Lets look at it in this theory. 1.62 vs 1.75 should look like this .

26 Pitch Prop X 5500 RPM = 143000 Therefore @ 0% slip = 83.59 MPH
1.62 x 1056 = 1710.72

26 Pitch Prop x 5500 RPM = 143000 Therefore @ 0% slip = 77.38 MPH
1.75 x 1056 = 1848

In The above equations the Theoretical speed shows a difference of 6.2 MPH

Now lets look at the same equation using the 1.75 gear ratio except by adding 2" to Prop Pitch.

28 Pitch Prop x 5500 RPM = 154000 Therefore @ 0% slip = 83.33 MPH
1.75 x 1056 = 1848

In conclusion we can say that the two different ratios with Identical Props, Boat, Weight and conditions in Theory the 1.75 ratio should Run 6.2 MPH slower.

And If one adds 2" of Pitch to the 1.75 ratio The Theoretical speed is almost the same between the two Ratios or within .26 tenths MPH. On a Measured race course that small difference could result in a Win Or Loss eliminating Boats handling, Driver Skills, Ever changing water conditions etc.

[LakeFever]

Those last few percent, thats where it all happens. Chasing ounces not pounds. Chasing fractions of seconds not minutes. A lot of R&D goes into those small gains but over time, they sure add up

[2.5-21]

Thanks for all the Input......
Here is the real life scenario.
2 equal boats possibly a little weight difference but same hull design, One boat has a 300XS 6200 RPM other boat has a 300X 7000 RPM

300XS Runs a 1.62 Ratio at 6250 rpm 7% slip = 95mph

300X with same 1.62 Ratio 7000 rpm Requires much smaller pitch prop and creates much higher slip= ?

Will a 1.75 Ratio allow a larger prop and lower slip?

Are larger pitch props more efficient than something in the lower 20" range?

[FORBESAUTO]

It’s quite possible. I ran into a similar scenario on a high strung 2.5 on a heavier bass boat. Found that going from 1.87 to 2.0 gear the take off and acceleration is no comparison between dropping two prop pitches versus gear change with gear change being far superior. Also was able run higher top end speeds because before couldn’t really run much more than 25 pitch props and still have any kind of take off, to being able to run 27-29 pitch props and still have satisfactory take off. Gearing has lots of factors that usually get overlooked, it’s really conditional depending on engine characteristics/power curves and the way a boat performs and it’s needs as to which will perform better.

[engineermike]

Just to add more numbers to your situation:

6250 rpm, 1.62, 28”, 7% = 95.6 mph
7000 rpm, 1.62, 25”, 7% = 95.6 mph
7000 rpm, 1.75, 27”, 7% = 95.6 mph

[2.5-21]

Just to add more numbers to your situation:

6250 rpm, 1.62, 28”, 7% = 95.6 mph
7000 rpm, 1.62, 25”, 7% = 95.6 mph
7000 rpm, 1.75, 27”, 7% = 95.6 mph

Understood but what reduces slip?

[HStream1]

A Good Prop Guy!!!!!!! I said it Many Many Years in Racing. Id much rather have a Good Prop Guy in my Pocket at a race than a Dyno happy Motor any Day. I just had one of my Drag props worked again by the Same guy thats been working all my Props. And this last Tune he did I gained 4 tenths and better MPH at just under 4% slip.

Understood but what reduces slip?

[HStream1]

Im saying there is a Prop issue. To prove this swap props and see it the issue follows.

Thanks for all the Input......
Here is the real life scenario.
2 equal boats possibly a little weight difference but same hull design, One boat has a 300XS 6200 RPM other boat has a 300X 7000 RPM

300XS Runs a 1.62 Ratio at 6250 rpm 7% slip = 95mph

300X with same 1.62 Ratio 7000 rpm Requires much smaller pitch prop and creates much higher slip= ?

Will a 1.75 Ratio allow a larger prop and lower slip?

Are larger pitch props more efficient than something in the lower 20" range?

[NICE PAIR]

Thanks for all the Input......
Here is the real life scenario.
2 equal boats possibly a little weight difference but same hull design, One boat has a 300XS 6200 RPM other boat has a 300X 7000 RPM

300XS Runs a 1.62 Ratio at 6250 rpm 7% slip = 95mph

300X with same 1.62 Ratio 7000 rpm Requires much smaller pitch prop and creates much higher slip= ?

Will a 1.75 Ratio allow a larger prop and lower slip?

Are larger pitch props more efficient than something in the lower 20" range?

What are the 2 boats and what is the weight of each?

If air entrapment, is the C.G. correct, and setup correctly?

What are the propellers, brand, amount of blades, pitch, diameter, stock or worked?

Close, is not the same.

[xb03fs]

All depends on the boat and setup. Simple answer is if you are running a 300xs with a boat that you plan on topping out around 95 mph. 1.75 is the gear. to run a 1.62 to try for a few mph is not worth the throttle response you give up. If you single engine boat is light enough that you are thinking 103 or faster. You wont get there with 1.75 gear and a 6400 rpm motor with reasonable priced prop. When my boat had a single 300xs i ran a 1.75 with 26p 4 blade prop best of 78 mph. Tried a 1.62 a few times and even with a 24p my heavy 22ft Eliminator Daytona would not get on plane. Converted it to twins 4 years ago. Now it runs 1.62 gear with 32p Bravo props cut and pitched to 33p. Best speed has been 109 mph. always will run 105 at 6300 rpm. A single with 1.75 is a sports car feel and light hull will run mid 90s. Same boat propped for top end with 1.62 will feel like you are towing another boat when you hit the throttle at any speed.

Said perfectly

[davemvegas]

26 Pitch Prop X 5500 RPM = 143000 Therefore @ 0% slip = 83.59 MPH
1.62 x 1056 = 1710.72

26 Pitch Prop x 5500 RPM = 143000 Therefore @ 0% slip = 77.38 MPH
1.75 x 1056 = 1848

In The above equations the Theoretical speed shows a difference of 6.2 MPH

Now lets look at the same equation using the 1.75 gear ratio except by adding 2" to Prop Pitch.

28 Pitch Prop x 5500 RPM = 154000 Therefore @ 0% slip = 83.33 MPH
1.75 x 1056 = 1848



And If one adds 2" of Pitch to the 1.75 ratio The Theoretical speed is almost the same between the two Ratios or within .26 tenths MPH. On a Measured race course that small difference could result in a Win Or Loss eliminating Boats handling, Driver Skills, Ever changing water conditions etc.

[/QUOTE]

Looks good on paper. Not close to describing the difference of acceleration and throttle response at all speeds. That formula on paper does not understand torque multiplication/leverage. Same real world testing on a a car set up for drag racing. You can achieve the same final drive ratio with a 3.08 rear gear and a small diameter tire. As a car with a 4.11 rear gear and a large diameter tire. Even though both the 3.08 and 4.11 will run down the road at 70 mph at 3000 rpm with the appropriate diameter tire to achieve same final drive ratio. The torque multiplication with the 4.11 and tall tire will out accelerate the 3.08 with short tire. The feel to driver will be totally different.

[HStream1]

I responded to his question Directly based on Theory. In Lieu of trying to describing the difference of acceleration and throttle response at all speeds using a car as example. A car doesnt have Prop Slip, The Drag coefficient is totally different in a Boat and a car, Horsepower to weight ratio etc etc etc As far as torque in any equation. Torque has nothing to do with Achieving the same speed with Same Prop Same Boat and same ratio.. Torque and Horse Power Cross at 5252 RPM at that point The Torque starts to Drop while Horse Power continues to Increase. So Yes A Outboards Gear ratio in relation to prop Pitch are similar in Nature to a Cars Gear ratio and Tire diameter. But Its still like trying to compare Apples and Pamagranets.

Looks good on paper. Not close to describing the difference of acceleration and throttle response at all speeds. That formula on paper does not understand torque multiplication/leverage. Same real world testing on a a car set up for drag racing. You can achieve the same final drive ratio with a 3.08 rear gear and a small diameter tire. As a car with a 4.11 rear gear and a large diameter tire. Even though both the 3.08 and 4.11 will run down the road at 70 mph at 3000 rpm with the appropriate diameter tire to achieve same final drive ratio. The torque multiplication with the 4.11 and tall tire will out accelerate the 3.08 with short tire. The feel to driver will be totally different.[/QUOTE]