prop advice

[tuskar]

Hello folks,
Have a 15hp johnson two stroke year 2002. The original prop 9 1/4 x 8 is getting past its best and I am thinking of replacing it shortly. My SIB is a 12ft achilles.
Wondering if the original prop was ever matched correctly it seems to accelerate the boat very quick and then the engine seems to rev very high , top best speed is 17mph with two adults.
Wondering if I should chance getting a 9 or even 10" idea is to reduce the engine revs a little . An 8" seems a bit low for planing
where is a good source to buy in the UK ?
Thanks

[boristhebold]

Props are a science, if you want to save yourself some hassle then just check the WOT rev range from manufacturers info and replace prop with what they suggest. If you want hassle then try some other props, borrow or buy, but remember it depends what you want to achieve as different props will give different results, some will have no change, some will have greater top speed but lower exceleration and the reverse, some better for hole shot for water sports etc etc.

Its a pain getting a perfect prop, used to be a company that rented for a small fee props so you could try but I think they are long gone now.

[dave3235]

Hi

Digital tach hour metre from ebay for around £10 easy to set up.

[tuskar]

Thanks for the good advice guys. I looked at the Johnson range of props for my engine 7 to 12" and I am at the bottom 8" range , perhaps previous owner of the outboard had a different purpose to me. It is neck breaking when off to a start and and then sort of sits there at high revs has all the signs of being under geared a tiny bit ,and seen that the original prop has seen better days. ...... Will get on ebay for tach meter . Who are good dealers for props on net ?

[Roflhat]

getting a tacho is worthwhile if you're wanting to fine tune your setup or optimise speed and economy. You want to choose what you want from your boat and prop it to suit. That or get multiple propellers for various applications.
Steel Developements list a prop that sounds like it would work - 3 Blade Stainless Steel A series - 8 -20 hp PROPELLERS approx 8.5" to 9.5" diameter with 2.1/2" gearcase

SOLAS make decent props too, (don't know about this website but the props are OK)
Solas Prop Saturn A3 Johnson/Evinrude/BRP SS 9.9-15HP 10 #2121-093-10 - Boat Parts -10 pitch

Solas Prop Saturn A3 Johnson/Evinrude/BRP SS 9.9-15HP 11 #2121-093-11 - Boat Parts -11 pitch

As said above, props are a science. The manufacturer props are usually very good, but they obviously aren't optimised for every application.

[tuskar]

Only a newbie guys. Once I take the rev reading at wot how do i calculate the prop pitch from there ? Is there a chart to consult or what ?
Thanks

[Roflhat]

Originally Posted by tuskar

Only a newbie guys. Once I take the rev reading at wot how do i calculate the prop pitch from there ? Is there a chart to consult or what ?
Thanks

You can prop it to suit your needs, the maximum revs the manufacturer recommends will be on the engine somewhere, usually around 5100-5900. For a light boat you should be aiming for the upper end.

[willk]

Originally Posted by tuskar

Only a newbie guys. Once I take the rev reading at wot how do i calculate the prop pitch from there ? Is there a chart to consult or what ?
Thanks

There is a general formula but I forget it Something like an extra inch of pitch will lose 200RPM and gain a knot? That kinda thing - lol. In empirical testing, I've found it to be not so hard and fast. With your rig it's not going to matter so much. First move is to find out how much you are overrevving (against recommended WOT revs) and then the route will probably be fairly obvious. Steel Developments in the UK are a handy source of props, but more importantly, can rebuild and tweek your existing unit if required for modest bucks.

My Granny was a Wiksfjord woman BTW. I'll not hold that against you

[jonp]

I find this very helpful continuousWave.com: Whaler: Reference: Propeller Calculator

Jon

[tuskar]

Thanks everyone for the good advice. Expecting the tach meter to arrive in next day or so, I will let you know the result of course.

[Locozodiac]

Factory engines with delivered props gives very good performance on light to medium light applications, usually at near full wot factory stated rpm. Install the induction tach, go for a wot spin you alone with lightly loaded boat on ideal flat calm, no wind water cond. and check achieved max wot rpm, compare them to engine's factory wot parameters to check if inside safe ones.

That engine revs 5-6K rpm, check if in that range, can play with prop pitch to be at or very near 6K. One -, + prop pitch is around 150-200 rpm difference,

Happy Boating

[tuskar]

Hi folks,
Got the tach meter during the week and got out on the river today for a run myself and a passanger 14st each .The engine clocked 6400 rpm. ie with the 8 ' pitch prop. So am I correct in calculating an 11 or even 12' prop would be suitable ? The recommended rpm is 5500rpm