prop changes from 1.5 to 1.36
 

Is there a general rule of thumb for how much pitch to drop when going from 1.5 to 1.36 gears. If I run a labbed 30 now what should I try with a imco shorty 1.36

28

mathematically, 1.5 ratio with a 30 pitch prop is equivalent to a 1.36 with a 27 pitch prop. you said your running a lab 30 but is it labbed with a recipe that adds pitch or lower pitch also there is a slip factor to take into consideration. generally a higher pitch prop will have a higher slip percentage, so if the gear ratio change results in a lower slip percentage you will need to go even lower in pitch.
this is based on the power of the engine remaining constant and not having the torque greatly affected by ratio change.
Ex. 1.5 ratio 30 pitch 5500 rpm 16% slip 88 mph
1.36 27 5500 16% 88 mph
1.36 26 5500 13% 88 mph

88mph? Let me grab my flux capacitor and delorean. I'll be right back.

On a serious note, I've done a back to back comparison on a twin engine deep V hull with 1.36 and 24 bravos and the same boat 1.5 and 26s. It actually ran alittle better with the 24s and 1.36.

Cruised 59 at 4,000 while with the 26s it was 56 at 4000 the top ends were 73 at 5000 with the 24s and 72 with the 26s. Was surprised how much better the boat liked the smaller props and 1.36.


So as said above you're going to be somewhere between a 26 to 28p prop depending on slip and rpm. If you have room to gain rpm with your engine setup then the smaller prop should give you some better acceleration if you can turn Afew more r's. I heard a rumor bblades does prop test program (in my best Brett voice)

The boat may be a little lazy pulling out with the 1.36 vs 1.5

I run 1.36 and the cat is definitely not lazy out of the hole . I see the op has a 25' Daytona so our boats are similar . My bud with 25' Daytona and 1.5 ran a 28 while I run a labbed 26 on mine .

1.5 to 1.36 is right about 3" in pitch with the identical props.

[QUOTE=the deep;4187916]I run 1.36 and the cat is definitely not lazy out of the hole . I see the op has a 25' Daytona so our boats are similar . My bud with 25' Daytona and 1.5 ran a 28 while I run a labbed 26 on mine .

That's where the May comes in, having no clue as to what he's running for power, he may or may not have an issue.

650hp with plans to increase

Changing gear ratio changes the torque multiplier, at some point with big power you run out of pitch and have no choice but to change ratio.

Interesting thread. I just blew my drive and I have a 1.5 ratio with a 28 pitch prop that is too small I've tried a 30 that ran really well, I was going to try a buddies 32 to see how that ran. What I'm seeing here is that I should go to a 1.36 and keep my prop. It appears that what I've heard about loss of acceleration with the 1.36 is a wives tale. The 1.36 should be stronger as well which I need as much as anything else.

OK, so lets throw this question into the scenario. 32' hydra Cat running 2- 700hp Bandit engines Imco SC's on Stellings ext boxes with -2 shorties. Prop depth is 1 3/4" above the bottom. Running 36P Bravo 1's and I'm running up on the rev limiters at 6K at about 115mph. I have the 1:36 gears for the lowers to put in and want to go to a 5 or 6 blade but not sure where to start? Guesses? I tried a set of 5 blade Hydros but boat hated them. They where not P5 X's tho Ass of boat got real loose at 100, VERY UNCOMFORTABLE feeling.

P5x is a bow lifter round ear I believe you want the clever style. Have you used a prop slip calculator ? You can trim the diffuser ring for less stern lift as long as you have to trouble getting on plane

Clever gives you transom lift, Bravo gives you bow lift, guessing you're running 1.5 now, I would try the props you have with the 1.36 then make your decision.

The hydros I tried where clever type blades, they didn't work very well. I have a pretty bad porpoise issue going on as well so not sure if I'm bow or stern heavy, something else to work on I guess. This dialing in process is giving me a headache.

Was the porpoise better or worse without the shorties ? I didn't like the Hydromotive on my DCB, felt like it was trying to drive the bow under water.

I don't think the props are causing the porpoise. I have changed so many dynamics on this boat from stock. Major HP increase, extension boxes, moved engines forward about 2ft and lowered them about 6in. Shorty lowers which at first prop shafts where 2 7/8" above bottom then spaced down 1 1/2" to current location. Boat hooks up well and gets up on plane easily. Haven't done any slip numbers yet but I think the porpoise is a ballast issue that needs to be worked out along with several other things. I'm very pleased with the progress so far, it's heading in the right direction. I know the boat has more in it and at this point every mph is harder to get. I figure I need to get the gear and pitch issues worked out first then work on the trim and balance issues. I think I can coax another 5-10 mph out of it putting it around 125-130 mph boat.