What are the advantages/disadvantages of changing from 2.38 to 2.68? I have a 3blade 80" R series set on mark 2 turning 5400rpm with 2.38. Do I have enough prop for a 2.68, will I gain that much more as far as push on the ground? Happy with what I got but always looking for a little more, not speed but push on the ground. Thanks
-
If you log in, the ads disappear in the forum and gallery. If you need help logging in or getting registered, send request to: webmaster@southernairboat.com
2.38 or 2.68
- Thread starter JWH
- Start date
Bob in Canada
Well-known member
I don't have the HP you do but I'm curious about this too . I think I remember Dave saying it will raise the cruise RPM enough to drop the gas mileage a bit. I'd like to have him chime in on this topic .
I run a big block and when I broke my 2.3 gear box I decided to jump on the 2.68 band wagon. It raised my cruise rpm and lowered my fuel mileage. I run S blades currently,and it did give me some more pitch in the prop,but I didn't get stuck with the 2.3 either. If I had it to do over again I would have stuck with the 2.3. And yes your prop would be enough for a 2.68
Wild Bill
Well-known member
I had my 489 2.68 with S blades set up for 4900 rpm, it would cruise nice 2300 rpm deep and great fuel consumption but had to work getting off sand. It would do it but same thing no poly and lots of bottom coat peeled off from rocks so.. Reduced pitch for 5600 rpm and even lousy bottom could go where I needed and bottom cruise in deep was 2700. Fuel consumption up a bit but ran nice. With R blades I think you might just have the optimum setup, with S blades I would go with more gear.
No gear box on the next.. :mrgreen:
air-roberts
Well-known member
what you need to do first is see the dyno or if you don't have look at the cam card and see where you h.p. range is most bb are best at lower in which case I advise the 2.38 and most sb are higher which I go with the 2.68. so it's important to know your h.p. & torq ranges.
to answer quickly your basic average motor not knowing much and had to dicide quick i'd go with a 2.37 for a bb and a 2.68 for sb
to answer quickly your basic average motor not knowing much and had to dicide quick i'd go with a 2.37 for a bb and a 2.68 for sb
junglecat
Well-known member
Having a 2.68 doesn't automatically mean you get bad fuel mileage. When I had the 383 on boat we ran the Loop here, 128 miles and boat got 3.86 mpg. That was two people on boat and full 40 gallons with 3 extra 5 gallon cans full for my buddy's boats with smaller tanks. I used 6 gallons more than my brother in law's 0540 on a 12' sled. Like Air said you need to know your motor . I had the 383 with 3 blade 82" R set at 3 and boat would all around perform better and get best mileage , turning 5400 rpms. I could turn pitch down and turn motor harder , boat was faster in 400' but mileage went down and ground running was hurt.
Now I have the same prop at same setting on my new 427 sbc. I can turn it 6100 rpms , I'm going to play with the pitch till I hit the right setting , around 5500rpms . With the added torque of the 427 it will do everything the 383 would do at a lot less rpms. I'm going to move pitch to 3.5 for weekend to see the difference.
Now I have the same prop at same setting on my new 427 sbc. I can turn it 6100 rpms , I'm going to play with the pitch till I hit the right setting , around 5500rpms . With the added torque of the 427 it will do everything the 383 would do at a lot less rpms. I'm going to move pitch to 3.5 for weekend to see the difference.
keep your 2.38 i made the jump from a 2.38 to a 2.55 my motor spun the same prop at around the same speed. I spun mine to 6000 on the water at the 2 mark went to the 2.55 added pitch and still spin to the 6000 mark i think boat doesnt run as good on the ground and lost LOTS of top speed 8mph to me it is not worth it and was a dumb move on my part. However i am no expert only an idiot who made a bad decision IMHO just saying
OneBFC
Well-known member
Every prop has an RPM that it works best at and anything over that RPM will net you additional thrust but the amount of torque required to get it will be much more per lb of thrust than what it takes when operating in the design RPM of the prop.
Easy rough formula:
Engine torque peak RPM/prop design RPM = required gear ratio
Example:
Prop design: 2000
Engine torque peak: 4000
4000/2000= 2.0
Now before someone says it is not that simple, they are right it isnt.
It is actually a function of your engines torque curve vs its torque peak.
If your engine can maintain enough torque at a higher RPM then moving to a higher gear ratio works out.
Some engines fall off on torque steeply and so you must be careful not to end up with less usable prop shaft torque when changing ratios.
I would explain better but I am on my phone away from civilization right now.
Do the math and it will become apparent to you which ratio is best for your engine.
Easy rough formula:
Engine torque peak RPM/prop design RPM = required gear ratio
Example:
Prop design: 2000
Engine torque peak: 4000
4000/2000= 2.0
Now before someone says it is not that simple, they are right it isnt.
It is actually a function of your engines torque curve vs its torque peak.
If your engine can maintain enough torque at a higher RPM then moving to a higher gear ratio works out.
Some engines fall off on torque steeply and so you must be careful not to end up with less usable prop shaft torque when changing ratios.
I would explain better but I am on my phone away from civilization right now.
Do the math and it will become apparent to you which ratio is best for your engine.