Prop pitch .VS. outdrive gearing questions
 

OK - I've done some basic searching here and haven't seen any real discussion about this - what's the relationship / benefits/drawbacks between prop gearing and performance - not the lower (higher numerically) outdrive gearing slows prop rotation thus lowers speed so increase pitch to compensate, change prop pitch for best performance at a target RPM, etc.

What I'm wondering is what's better - to spin a prop faster at a lower pitch or to have a higher pitched prop spinning slower and why ? I'm thinking that there's got to be some magical balance in there between slip/pitch/gearing, etc....

Anyone care to take a stab at it??

often wondered the same thing myself, somebody's gotta know

good question! im curious to see what people say!

I'm no expert on this but have been told it's better to turn a larger prop slower than a smaller prop faster, the larger prop spinning slower is more effecient giving you lower slip#'s.

A 1.5 drive will mutiply the torque to the prop more than say a 1.36 drive. So go with a slower prop needing higher pitch. Once you get to huge HP then you need a lower ratio, you will run out of pitch.

There is a discussion similar to this on the Scream and fly forum. I would think bigger ptch in an I/O application where you dont want to rev the motor too high, but they are saying the oppsite on an outboard, since you dont want to load the 2 stroke down. seems they like RPM over lugging the motor, I agree for the outboard, but in an I/O, I would run as much pitch as ya can and still reach your lower end of the max RPM Band.

The effect on the motor is the same... not an issue of lugging vs. spinning the motor. Your prop would have to be changed to accomodate the gear pitch.

What it's going to come down to is prop hydrodynamics.

I don't really know what would be better, but I do know that the discussions regarding diesels on high performance boats have led me to believe that a lower pitched prop spinning at a higher RPM, comparable to a gas boat, is preferable. There is going to be a sweet spot, though, and it's going to depend on the resistance of the water to the prop blades turning vs. the efficiency of the prop blade at a specific angle. The intricacies of prop design will play a big part as well.

I think what they were referring to is that on a 4 stroke you dont wanna spin it up too high due to the valvetrain, whereas on a 2 stroke they like to rev, and dont like to be loaded down with the big pitch props, at least not so much to where they wont come up to the reccomended RPM at WOT, for instance, it is OK to run it at 4000 RPM(at part throttle), but not to where it is loaded down by the big prop, so that it only reaches 4000ish at WOT.

Think the Question being asked is why change gear ratios. We change ratios depending on length of run and speed wanted. More midrange lower ratio.Tall ratios speeds are normaly higher but takes forever to get there. You can actually spin a prop to fast in which prop slip will rise and handling will go away. The idea is to find a compromise at your motors torque and hp. You want the motor to be able to use all of its potetial.

Also big props load the drives alot so most try not to run over 32 pitch on a bravo style drive. Brake alot of drive this way.

I believe the 'sweet spot' or ideal combination varies by application. Hull design, prop type (3, 4 or 5 blade), max engine rpm where peak horsepower occurs, etc, and the combination of all these play a part.

I have read a couple threads related to this discussion and the consensous is for peak performance you are better off turning a taller prop before lowering your gear ratio (as long as the prop pitch is available). I think of it kind of like drag racing. My car ran a lot faster with a 5.13:1 gear and a 32" tall tire compared to 4:11's and a shorter tire. Probably doesn't relate to boats though!:p

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.

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

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

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.

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

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.

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

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.

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

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...

you stated a higher pitch to reach your target rpm, are you over reving it now or do you mean lower your pitch. what prop and gears are you running, what engine?
Larry

OK ME AND MY BUDDY HAVE 24 BAJA 136DRIVE THE OTHER 150 3000 RPM 40MPH THE 150 RAN 3600 TO DO 40 BOTH HAD 23MIRAGE PROP 136 DRIVE GOOD FOR MY BAJA BUT NOT ALL BOATS ARE SAME WE WERE LUCKYTHE BOAT CAME WITH 136 SO WE TRY ALOT OF PROPS AND COMPARE THEM TURBO/MIRAGE/CLEVER THEY WORK THE BEST ON MY BOAT 3 blade props were the fast at top end 4 blade midrange good top lost speed:boat::boat:

I was figuring that with the new engine combo I'm going to be producing about 40-50 more HP than before and my target RPM will be around 55-5600 RPM according to engine analyzer pro (wish I had a 'real' dyno to test it on to get target) - engine was built with better parts that have withstood costant 7-7500 RPM runs on a circle track so I'm figuring 6K in a marine engine should be real safe.

Those changes in HP and RPM should need to re-prop to get the targets reached and I was hitting the old target pretty easy with the current prop and had to back out a little if the water was nice.