prop calculation question

[david_r]

i have a buddy that is looking into buying a used boat and it doesnt have a prop.

i told him i thought i had a link on how to calculate and get in the ballpark, but now i cant find it. can anyone help me out? i know there are a lot of variables and the boat doesnt even have a tach so looks like hell be borrowing mine (tach) to fine tune everything.

the owner doesnt remember what the diameter and pitch was on the old one, and apparently someone else thought they needed it worse than he did.


i know it has to do with weight, hp, etc., but i dont remember the exact formula.

thanks for the help guys!

 

[Silvertip]

Re: prop calculation question

Here is just one the many prop calculators on the web.

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

When starting from scratch, you need to first specify what engine engine you have so you can at least get the correct hub and general diameter from (for example) the iBoats propeller listings which is here:

http://boatpropellers.iboats.com/

Once you have the year make & model of the motor identified you will be presented with a listing of props. I chose to look up a 1997 Johnson 150 HP engine. The list of prop pitches is like this:

http://boatpropellers.iboats.com/Jo...r=1997&**********=312492184&*******=247090407

You will note that the pitch range is 10P to 28P. Therefore with nothing to go on, one would likely start in the middle of the range. Middle of that range of props is in the 18 - 19P range. At this point you don't need to worry about diameter because that's figured in the design of the prop. Prop diameters are matched to a particular series of engines from any manufacturer.

You now need to determine which manufacturer you want to go with and whether you want aluminum, stainless, three blade, four blade, etc. That list looks like this:

http://boatpropellers.iboats.com/Jo...andard&**********=312492184&*******=247090407

Obviously with no tach you will have no way to determine if this is 100% correct so I strongly suggest a tach be added. You really don't need to know speed because speed will be whatever it is when you have a prop that allows the engine to rev at the top of its recommended rpm band at wide open throttle with an average load.

 

[steelespike]

Re: prop calculation question

I think silvertip understated the speed requirement a little.
Its true a props speed will be what it is;but::
Without some sort of speed you can't determine if the application is on the nose.Speed can vary for a number of reasons motor height,trim, weight distribution, tune ,prop selection,etc.A setup that developes the best speed will usually deliver the best efficiency.

 

[david_r]

Re: prop calculation question

thanks guys,

ill see if now he is a little more comfortable starting around 17 or 19.

it seems like i also have seen a calculator that took the boat length and weight along with hp and had some kind of formula like that, now i need some more info from the boat to find the gear ratio.


thanks a million guys.

 

[Silvertip]

Re: prop calculation question

You might check the Mercury web site. They used to have a prop selection section.