Hunting Fishing Discussions



Forums Home > Off Topic Discussions
Share on Facebook subscribe feed
10 Replies | Page 1 of 11 | Top of Page | Bottom of Page

Prop / Speed Equation

by walynut , Posted to Fishing Buddy on 03/18/2010 1:18 PM | "Quote" | "Quick Reply" |
walynut

Joined: 01/08/2003
Location: ND
Found this and thought that I would share it with you. In order to find out the rough speed that a prop will give you.

Pitch x RPM / Gear Ratio x .000946 equals speed

I ran my numbers and I found it to be a 1 MPH high

19x5200 / 1.86 x .000946 equals 50.2 MPH and I am running 49 to 49.6

Kinda cool! Does it work out for anyone else?

SUPPOSABLY, if your numbers are way off you could have a vent or cavitation problem, or trim / motor mounting to high or low.
 

Good Luck and Good Fishin'

Eric T
Re: Prop / Speed Equation
by sweeney on 03/18/2010 1:56 PM | Reply #1 | "Quote" | "Quick Reply" |
sweeney
Joined: 03/07/2002
Location: ND
probably off because your amount slip is not corrected into the equation
Re: Prop / Speed Equation
by YarcraftStorm on 03/18/2010 4:25 PM | Reply #2 | "Quote" | "Quick Reply" |
YarcraftStorm
Joined: 07/14/2003
Location: ND

It off by 7 mph on my boat and I know that my boat runs right where it should.

The real key is to determine the Slip %. A well tuned walleye boat will probably run around 12% while the bass rigs can get down below 8%. It really comes down to effiecency. FYI, my rig runs around 13% depending on the prop. There are a pile of prop calculators that will do this stuff for you if you do a little searching. Mercury has an excellent propeller site that has all kinds of good info.

Here is some technical reading, have fun!

http://sites.mercurymarine.com/portal/page?_pageid=126,48572,126_49291:126_49315&_dad=portal&_schema=PORTAL

This will get you to the prop selector, which of course is going to reccomend a mercury prop but you can find similar prop styles from other manufacturers.

http://sites.mercurymarine.com/portal/page?_pageid=126,53050,126_53052&_dad=portal&_schema=PORTAL

Here is a speed / pitch selector / MPH / Slip calculator.

http://www.csgnetwork.com/marinepropcalc.html

Remember when using high end cuped stainless props to add 1 degree of pitch over the actuall pitch to make up for the agressive cupping and rake. Same goes for props with more than 3 blades.
Re: Prop / Speed Equation
by YarcraftStorm on 03/18/2010 4:31 PM | Reply #3 | "Quote" | "Quick Reply" |
YarcraftStorm
Joined: 07/14/2003
Location: ND
Here's my example:

21 x 5700 / 1.75 x .000946 = 64.71 mph (theoretically)

My rig runs between 56 to 59 mph depending on conditions. It makes it worse when I add 1 degree for cupping!

22 x 5700 / 1.75 x .000946 = 67.78!

I only wish it would run like this. Now, if you take 13% of slip into acount that brings it back down to .............................................58.97mph, go figure! (0.13 x 67.78 = 8.81 mph so 67.78-8.81 = 58.97)
Re: Prop / Speed Equation
by walynut on 03/18/2010 4:35 PM | Reply #4 | "Quote" | "Quick Reply" |
walynut
Joined: 01/08/2003
Location: ND
Odd, it shows my slippage at 0.1527298245614035.

Something must be wiggy with my numbers cause I dont even belive that.
 

Good Luck and Good Fishin'

Eric T
Re: Prop / Speed Equation
by dwmaster on 03/18/2010 4:56 PM | Reply #5 | "Quote" | "Quick Reply" |
dwmaster
Joined: 01/05/2009
Location: ND
You can't put this stuff out there now when people tell me they go 65 or 70 mph and I ask them what prop they're running, they will know better than say 19 or 21 pitch.  And now I am going to  have less occasions to be smile and be amused. :-(
Re: Prop / Speed Equation
by YarcraftStorm on 03/18/2010 5:17 PM | Reply #6 | "Quote" | "Quick Reply" |
YarcraftStorm
Joined: 07/14/2003
Location: ND
When I put your numbers in I get 1.5% to 2.3% slip. Boy would I love to own a boat that can run those kind of numbers, ahh just puliin your leg. You must have atleast one of you're numbers wrong. Don't worry though, soft water is on it's way! 
Re: Prop / Speed Equation
by luveyes on 03/18/2010 5:59 PM | Reply #7 | "Quote" | "Quick Reply" |
luveyes
Joined: 08/16/2004
Location: ND
Wow, Im supposed to get 74.59.....   Hit 61 last year; but prop work, weight transfer, and motor height changes I should see 64-68 fully loaded.  I must have alot of prop slip!!

Walynut......  we gonna use my boat for the Chamber!!!!!   Hope you have $$ for fuel??

YES, I am that foolish!
Re: Prop / Speed Equation
by Captain Ahab on 03/18/2010 7:18 PM | Reply #8 | "Quote" | "Quick Reply" |
Captain Ahab
Joined: 12/01/2008
Location: ND
Mr. Obvious says......is there any variable for boat size and weight?

"Diligence is the mother of good luck."
Benjamin Franklin
Re: Prop / Speed Equation
by luveyes on 03/18/2010 7:37 PM | Reply #9 | "Quote" | "Quick Reply" |
luveyes
Joined: 08/16/2004
Location: ND
nope...  they dont matter at all.

YES, I am that foolish!
Re: Prop / Speed Equation
by walynut on 03/19/2010 07:33 AM | Reply #10 | "Quote" | "Quick Reply" |
walynut
Joined: 01/08/2003
Location: ND
luveyes Said:
Walynut......  we gonna use my boat for the Chamber!!!!!   Hope you have $$ for fuel??
Just say the word and I will send you some cash; as for gas, i'm not scared because when we win it, we'll still be ahead!!! lol
 

Good Luck and Good Fishin'

Eric T
10 Replies | Page 1 of 11 | Top of Page | Bottom of Page
Posted By: walynut
Posted On: 03/18/2010 1:18 PM
356 Views, 10 Comments
Rate This ForumTopic
  • Currently 0/5
  • 1
  • 2
  • 3
  • 4
  • 5

0/5 (0 votes cast)

Share and Save This ForumTopic

You must be signed in to comment on this topic




Forums Home > Off Topic Discussions
Start a New Topic in "Off Topic Discussions




Total Outdoor Network
TON Sites | Contact | Advertise | Privacy Policy | Terms of Use



North Dakota Fishing and Hunting | Minnesota Fishing and Hunting | Wisconsin Fishing and Hunting | Nebraska Fishing and Hunting | Walleye Fishing | South Dakota Fishing and Hunting | Alaska Fishing and Hunting |

submit feedback / bug report