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topwater
12-17-2002, 04:32 PM
I was told if I posted this question here, I might get some good response..

What effect does dia make in RPM and Boat Speed.

Ie 13 1/4'' X 19p or 13" X 19 p

Which may make a faster boat at the same RPM

Ie 13 1/4" X 19p or 13" X 21p

Mark75H
12-17-2002, 05:24 PM
Actually the "Technical" forum would be better, but here is the answer anyway: it depends on the boat and set up. You have to try them, no one can tell you from their desk or armchair.

Higher pitch will go faster at the same RPM if the motor has the power to push the boat. (In theory with no boat, higher pitch would always go faster, but in practical application there is a limit to how much pitch your motor can swing before RPM will decrease.)

My Dad's old Glastron V-156 can swing either a 20 or 22, but both props push the boat the same speed, one at 6,000 RPM the other prop closer to 5,400 RPM.

Diameter can have more effect on accelleration than top speed.

1/4" on a 13" diameter is not going to make a noticable difference.

topwater
12-17-2002, 05:52 PM
Well thank you for repling, I am at this site because if you want info go to the guys that think speed, get the knowledge and then detrime how it applies to you..

You stated : "Higher pitch will go faster at the same RPM if the motor has the power to push the boat"
and I got confussed If it can reach the RPM range you want doesn't that mean the motor has the power..

Lets say that a 13 1/4" X 19 on the same boat runs at the Same Tach RPM as a 13" by 21 would it then be the 21 p SHOULD and I use that word be a Faster boat.. than the lager dia prop.

What advantage/disadvantage is a Larger dia prop.. Would you alway want a Higher Pitch and small Dia..

Jay R.
12-17-2002, 06:05 PM
I hope i get this right and understandable. You only want enought diameter to push the boat without to much drag. this is part of the whole prop slip factor. to much diameter and you have to much drag, to little diameter and you have to much slip. slip is like spinning your tires in your car. but in a boat it not as noticable and a fact of boating. most small speed boats like V-pads and tunnels are somewhere around 7 percent slip. and it goes up from there depending on size and weight of the boat. if boat props you mentioned are running the same speed at the same RPM then the 21 is slipping more and the 19 is more effcent. most of us will start at 13-3/4 or 14 diameter. but it depends on the boat. tell us what you have and what you running, give us all the details and we can get you in the right dierection. like Mark75 said, no one can say for sure you just have to try a bunch of props, but we can put you at a close starting point. you want to keep you engine in it's RPM range to make it make the most HP and you have to prop it right to do that. by the way once you got the right diameter. usually its 250 rpm between inches of pitch. in theroy a 19 should spin 500 RPMs faster than a 21. I hope this helps.

topwater
12-17-2002, 06:24 PM
Thank you Jay R.
I don't think you could wrote it any better.. I understand much more now... The Drag VS Slip is what I been missing .. I knew slip was there between pitchs but the Dia was driving me looney..

I came here because I knew someone who ran like you guys do would have the knowledge I seek although I do not run anything like you guys do.. the principal/theory applies to all boats. and is what is important.. ...

I am just a guy that fishes and has a 1990 17ft SeaNymph with a 70 hp Johnson that I would like to get the set up as best as possible.. So knowledge is what I was seeking I am a smarter man now... thank to you..
I will post again with my boat stroy and picture but you have taken me leaps and bounds with your note Thanks again..

topwater
12-17-2002, 06:30 PM
Story Told
I have two SS props 13 1/4" X 19 ( which I just had reworked to a 17P) and a 13"X 21p Both seemed to run within 50 RPM on my Tach.. 5500 to 5550... Now comes the story .. For reasons we need not discuss I had to have my engine rebuilt (something to do with Carbon)))).. 1990 70hp Johnson.. It was bored .020.. OK when set up I found out that the tach on the boat was appox.. 500 RPM off... So I installed a new one .. Now the boat ran 4950 with the SS 13X21 and 49 to 4925 with the 13 1/4" X 19 the speed ... Well hard to say... Boat speedo a joke.. Ran one season Ok next comes the rework of a prop and the guy chose to rework the 13 1/4"X 19 Solas to a 17p.. In the mean time I get a GPS Cool thing.. The 13 1/4" X17P turns 5300 RPM Still not the mean for the engine.. 5000 to 6000 on a 1990 70hp Johnson.. So I am looking for 5500 to 5600.. The engine was attached to the boat in the motor highest hole putting the engine in it lowest posistion.. I raised the engine two holes and the RPM came up 200 to 5500RPM.. and with GPS Runs a whopping 33.3mph.. Just off Idle for most of you guys LOL>> but here I am in the middle of winter wondering if I had a 13 X 19 prop would it help and should I try to trade my 13 X21 for one or Maybe I should raise my engine more and see if the 13X21 would give me more.. Questions Questions Questions.. but knowing now aout Drag vs Slip and Dia Vs Pitch and the end result Speed I will have somethings to try when spring Comes.. Thanks all Topwater

Jay R.
12-17-2002, 06:33 PM
Which is what this is all about. I learned alot of this prop stuff at OMC school. I needed help with set up and learned alot of that from other racers and from this board!!

Jay R.
12-17-2002, 07:12 PM
sounds like your on the right track now. there are alot of prop calcs on the web that you can use to help figure out slip. and let you know where you stand. just type in "prop calc ona search and it should get you going. byu boys and BAM boath have good ones. I have one that i downloaded and you can get from me on kazaa but its not as good as the bam and byu boys. Bam has a lot of differnt calcs here is there web address:

http://www.go-fast.com/boat_speed_predictions.htm

good luck

topwater
12-17-2002, 07:16 PM
Thanks Jay R
I have looked at prop Cal. but none of them have Dia in the equation.. Only pitch.. I would think there would be a slip difference between a 14 1/2 X 21 and a 13X 21 picth slip..

delawarerick
12-17-2002, 07:31 PM
Welcome to the board. Your rig looks like a mighty fine fishing machine. Here everything is saltwater ie center consoles heavy fiberglass. Your speed is good for you rig. Welcome again. Rick

topwater
12-17-2002, 07:38 PM
Thank you delawarerick

It gets the job done for me.. I catch a lot of Small mouth bass here in Michigan on Lake St. Clair.. The world greatest lake for Smallmouth. and soon to be if not now Muskie.. LOL..

But it good to know all you can, and I think there is 2 or 3 Mph out there for me just in making it right..

Topwater

Jay R.
12-17-2002, 08:24 PM
you have to enter all you info about both props, it will tell you therotical speed and slip. for both. you should see a differnce in slip. don't asume anything or just change the pitch. enter everything you have each time you change the set up.

Jay R.
12-17-2002, 08:28 PM
you have to enter all you info about both props, it will tell you therotical speed and slip. for both. you should see a differnce in slip. don't asume anything or just change the pitch. enter everything you have each time you change the set up.
sorry i just looked i thought that was a link to all the calcs on that site, here is the prop slip calc:

http://www.go-fast.com/Prop_Slip_Calculator.htm

David_L6
12-17-2002, 09:46 PM
Here's the address for the other site: www.byuboyz.com

I believe Hill propellers site also has a prop calculator. You may not need any of those calculators once Ron Hill gets around to reading this thread!

David_L6
12-17-2002, 09:53 PM
Originally posted by Mark75H
Actually the "Technical" forum would be better

Sam,

Forgive me please, but I recommended this forum to topwater because I know this is where all you gurus hang out. Y'all can quote theory at him and also tell him why the theory is correct/works.

topwater
12-17-2002, 09:54 PM
Thanks David_L6

I have been looking at Prop Cal now but having trouble figuring out what my Grar Ratio is I have my manual out and is says

Gear Ratio 12:29 (0.414)

but none of those numbers seem to work.. even the .414 so I don't know


LOL You make is sound like Ron Hill is the master at this.. Well for sure he has your respect.. Good for him..


Topwater

topwater
12-17-2002, 10:35 PM
Well look like I may have the answer the OMC Service manual must have a type-o I believe it should read (2.414) 29 divided by 12...

David_L6
12-17-2002, 10:37 PM
He's very knowledgeable. Helluva boat racer! Just hang around for a while, you'll see.

Try 2.41 or 2.41:1 for gear ratio in the calculator. .414 would be an overdrive ratio and your prop would be smaller than a trolling motor prop.

David_L6
12-17-2002, 10:45 PM
I see you found the answer while I was TRYING to post a reply. Doesn't matter how you got the answer. You're on your way now.

topwater
12-17-2002, 10:49 PM
Yea I actually figured it out on my own.. LOL another first.. the Service Manual is Wrong.. Not another first LOL.. but from what I put into the cal.. I'm not sure if I am not already at the best I going to get .. shows about 12% slip with what I have..

Jay R.
12-18-2002, 05:58 PM
12% is pretty respectible. if you start working on your set up you'll get it lower. was tere a change in slip from the 21 to the 19?
some of the race boats your size get around 7-9 % aired out. I can get about 11% aired out in my check. roughly anyways. I don't have a pad.

topwater
12-18-2002, 06:28 PM
Well at this point I am logging in Data that is from memory.. from the end of last season.. I now have all this new and great info and can't wait to apply but alas it must wait to spring.. False Data only leads to more speculation.. I will do it right when spring comes and go out with GPS in hand, Props and play to include raising and lower the engine to achive the best from both props and then figure if there is something in between that would be better or If I am as close as I am going to get now.. The Soga will continue and now I have to write down everything then do the math..