Props

[daledog]

Whats the deal with 'aftermarket' props for getting higher revs/speed? Can anyone explain it it 'dumb' terms for me?

[saurian]

Dale, props , very complicated to explain......
Ok; "wot" is the wide open throttle revs for a given motor.
Every motor has a manufacturers "wot" revs, normally between 5500/6500rpm.
If your motor can only get 5100rpm flat stick and the manufacturers say you should be getting 5500rpm flat stick then you are probably overpropped.
overpropped; props pitch or size to big to spin fast enough
underpropped; props pitch to small or size to small (spins to easy)
pitch; 17 inch prop refers to amount of cup or bite allowing forward motion.
5100 rpm x 17 inch slower (overpropped)
5500rpm x17 inch faster ( good depending on motor)
6000rpm x 17 inch fast but to fast ( motor over revving)
Overpropped; uses more fuel, less speed, damage to motor likely
Underpropped; more speed, uses more fuel sometimes, damage to motor from overrevving
Sweet propping; optimum speed, less fuel used , motor/boat combo at it's best,
no damage to motor , probable gains in speed etc....
Props come in many different sizes and pitches , lots of 3 bladers, 4 bladers, 5 bladers, ali, stainless, plastic , 2stroke motors/ 4 stoke motors.
To achieve the optimum prop/motor/boat /sea/load conditions can take a while
with lots of swearing ( like me , trying to explain it) trial and error , swapping props etc ....
Do not go out there and expect lots more speed just because you put xxxx prop on, thats not how it works . If you want lots more speed buy a bigger motor/smaller boat.
You will need to detail what revs /speed/motor/boat combo you get and ring up
solas or someone similar and let them choose the obvious choice for you, I tell you it probably won't be the right choice until you are on your 4/5 th prop if your fussy like me.
Hope I havn't over dumbed it for you ...I hope I havn't over dumbed myself by trying to put props into words.
Sure some others can eat my words but to your gain dale.
Ta

[trueblue]

Thats a pretty good start for the fella

[Seahorse]

blown me away. all very interesting. great explanation. i think will just leave mine as is. sounds all to hard. i finally got chance on sat to give bit of stick. 75hp merc on 5 met stacer runabout, 5500revs hit 60kmph. may have been fly shit left. think that sounds ok

cheers
greg

[saurian]

Greg , I'm not sure if a flyshit is overpropping or under ?? lol...
Ta

[Seahorse]

i dont know either. neither i think.
cheers
greg

[blaze]

only thing I will add is the 17" in the above post refers to the theretic measurement the prop will move through the water in 1 rev, theretic because there is prop slipage to factor into the equation. The 17" has nothing to do with cupping IMO, cupping has more to do with grip in the water to eleminate slipping which in turn can alter the WOT of an outboard. Before playing with props the first thing to do is have the engine set at the correct transom height.
cheers
blaze

[Kerry]

Props are not a simple thing and at times after market dealers go on with a whole load of hu ha. Just because a prop fits on doens't mean zit. One sometimes needs to take a long hard look at the standard prop compared to the so called after market "right" prop.

An interesting thing is with Mercury and Yamaha props, they are basically interchangable and many do this in that they will fit and even look proper but if anybody ever gets a chance take a beep at the thru hub exhaust area!

Restrictive exhausts one would think would have an affect on back pressure and engine design? How about the other way round?

[Cheech]

I took mine to solas and we water tested 3 different props. Great way to do it. Ring them and talk it over and they will be very happy to do the same for you so that you can find the right combo.

[FNQCairns]

Kerry the hub internal size due to 2 stroke principles mainly, will not make a difference, more important is tuned length of exhaust for standard engines and that always finishes up inside the leg somewhere I cannot speak for the 4 strokes because they are already pretty strung out, 2 stroke O/Bs until into the modified engine rpm ranges (some run 12k rpm) they don't have much call to be worried about overall exhaust backpressure as it relates to the hubs.

Before the need to worry about off the shelf hubs sizes causing too much back pressure one would first consider dry stacking + tuner mods but to anyone running sub 7k (so anything pretty stock) the hub size found and purchased in any (that I know of) off the shelf props makes no difference (apart from exhaust gas handling in the immediate region of the prop blades).

cheers fnq

[whiteman]

I have a 19" prop on my 115 Yammie 2s. This is probably oversized as it doesn't like to rev past 5400. A 17" would be closer to the sweet spot (assuming correct design of blades) but I am not willing to pay $600 to gain a knot at the top end and mayby a fuel gain of 1or2%. It still goes like a scalded cat fully loaded.

[Kerry]

You are not paying for an extra knot at the top end but those extra few hundred revs that will "please" your motor. Yes you want a 17.

But if anybody is in the market for a pair of 19K SS yammie props and matching alloy then I just happen to know where a set is

[newchum]

while the pitch question is on the board, i have two ss props one marked 17m the other just has 17 stamped on the outside at the nut end of the prop , they both look similar, can any tell me if they are the same