Strip Poker 388

iTrader: (6)

I was told by a few people that if ya can run a 1.50 with the it proped right it would be faster than like a 1.36 proped right ,

Hustler told me a 1.50 30p bravo 4bl at 5250 84mph would be faster than a 388 boat with a 1.36 gear 26 p 4bl.


I know now that it like spining the props faster, the 30 turn around 6000 and the 32 with turn the same speed with a lower rpm but it doesent handle as good when running the 30's. I am guessing its getting up on the prop?

Prob if the 32 was labbed let the motor buz more it might handle better.

Rambunctious

I am wondering for my application too

when my boat was build ( 1995) what pitch props were available?

i have 525sc's and formula installed 1.36 gear bravos
and 25 mirage plus props
boat now has labbed 24 bravo props ( 24.4")

are there props now days that would make my baot faster with 1.5 gears ?

Ramb

hoozeyurdaddy

check out this site with diferent #, every boat and every prop has a different slip#, pick the rpm you are looking to run and the gear ratio, you will not know the actual slip # until you run the prop and put in the #, pick a slip # to use with all your figures. but most boats run between 10 & 15 percent slip, you will see that changing ratio doesn't mean more speed, getting the slip # down and rpm up is where the difference comes in.

http://www.rbbi.com/folders/prop/propcalc.htm

Brad Zastrow

I just went through this very issue on my 32 Skater.
I bought a boat that used to have big power (1200's) and number sixes with 1.24 gears. I installed a pair of 525's and chose to keep the gears and run smaller props. The boat was a slug but would still pull to 112 mph at the rev limiiter. I just installed 1.5 gears and bigger props. DAY AND NIGHT difference. Same top speed but pulls so much harder.

Rambunctious

right. those are all inputs .
knowing the actual slip for a low and high pitch prop is the key

the question is:
at top speed, does a higher pitch prop have more or less slip to offset the drag losses of a lower pitch prop spinning more revolutions in the same distance traveled......

the other question is:
during the accelleration to get to that top speed, how efficient/inefficient is a larger pitch vs. a smaller pitch

i would love to see power and efficiency curves for various props at given thrust. that woudl allow one to calcualte the better choice, and then verify with testing

Ramb

JIMG

what you are forgetting is the prop turning faster takes the same surface area through the water more times. this increases the total friction seen by the drivetrain at the water.
the other thing is the steeper prop pitch hits the water differently and will carry the boat differently. you must experiment to really know for sure. most of the time you can get good advice about a particular combo from a top notch propeller person, or the boat builder, who has already played with combinations for your hull. or a reputable dealer who has alot of experience.

hoozeyurdaddy

I see what you are looking for, but until every boat has the same weight and hull design I don't think that question can be answered, I'm running a 26p B1 witch you would consider to be a small prop, but I run it with 1.34 gears and 5800 rpm, my prop slip is less than 7 percent running right at 100mph,
if I put on my 30p B1 it runs 5300 rpm and same speed but slip jumps to over 10 percent and its a dog out of the hole.
as to the 2nd question, I think the smaller prop is quicker to top speed but I don't know how the slip # would be.
It would be cool to see someone come out with a chart on props with the information you are looking for.
good questions, makes one think a little.
Larry

Pismo10

It's all about torque multiplication. Torque gives that hard pull feel. A 1.5 puts more torque at the prop than a 1.24.

Rambunctious

Larry
exactly

the interesting thing to see would be your 30 pitch behind a 1.5

ie. if you got your engine back to 5800 with a 1.5 gear set and "appripriate" pitch prop of greater pitch than the 26... would you have more top end ?.... and would you have a lazy accelleration getting there......?
the other thing is with slip calculation. a very slight change in the cup of the prop affect it's equivelent pitch and the slip numbers jump accordingly ( just numbers)

ie. is that 26, really a 25.8 or a 26.2 ........
don't really care other than understanding that a low pitch prop will loose energy turning the blades throught water, the higher pitch may loose energy through more slip......
which one performes better and ... my question.... is there a generic answer ( for deep drive v bottoms liek mine say)

seems to me that the new boats have 30 pitch on them for example instead of a 1.35 gearset and 26 pitches ( in my speed category of 75 mpg)

someday if I go to imco shorties which is suppoese to work well on my formula. I will prolly go to a 1.5 and higher pitch prop vs. my current 1.36

jsut looking for confirmation before spending $6k on new/used lowers some day

Ramb

bwiencek

Just reading all this info and it seems that there is a lot of information floating around... I was just going to swap to a higher pitched prop to hit my target RPM once I get the engine broken in...

I was more curious than anything just in case I break the alpha outdrive with the new combo - should I gear it the same or drop the gearing and prop pitch...

Keep the discussion going - it's very interesting... with any luck the new engine combo will be started tomorrow night...