9.9 Johnson prop swap

[jaybrown]

hey guys not sure on how the prop pitch and diameter works but I just picked up a solar 9 1/4" 9pitch stainless prop for my 9.9 Johnson ,would this work on the motor well? Couldn't pass it up brand new in box$50

 

[ondarvr]

The correct prop is determined by the motor amd boat combo, plus how you plan to use the boat. We don't know if it's a good prop for you without more info.

 

[Sea Rider]

Will needd an induction tach to know if inside wot range for that model engine powering whatever you need to push with any given load.

Happy Boating

 

[jaybrown]

It's a 1981 9.9hp Johnson on a 14' spectrum aluminum boat. Not sure on the rpm there is no tach. Probably not going to buy one for it as it's just a tin can just want to know if the prop will work better then stock. Is the pitch higher better for top end or smaller better for top end, and out the water to plane better smaller or bigger, I guess I just want to know how the pitch works I'm new to props? I just use the boat for ripping around the lake.

 

[jbcurt00]

From a stock 9.5X10 prop on a 10hp on a 14ftr to a 9.25X9 prop is likely to have little improvement on top end or 'hole shot', IMO. Esp if you dont know what RPM its turning at WOT w the current prop.

 

[ondarvr]

You need to use a tach to dial in the correct prop. Since we don't know what prop is on there now, there's no way to know what this prop may do in comparison. Higher pitch means it should move the boat further with each revolution, but that's only if the motor has the HP to spin it, if it doesn't your performance will be ruduced and it will put more stress on the motor, which can cause problems and shorten its life.

 

[jaybrown]

Yes it has stock 9.5x10 aluminum prop on it now not looking to fly out the water just don't want to blow it up

 

[jbcurt00]

Thats why you need the tach.

It MIGHT be maxed out now at optimum RPMs at WOT. Again a 14ftr w a 9.9hp, its equally likely to be that its underpowered for what you want to improve.

Like already mentioned over-rev'd and lugging the motor are both not good.

 

[pecheux]

You'll get approx 500 rpm extra with a 1 inch drop in pitch. (been there done that on my 9.9) Positively it will push your boat easier when loaded, Nega, trottle back a little at WOT when you are alone in the boat. Like others mentioned only a tach will give you a 100% answer. I also would have bought it for that price ...

 

[Sea Rider]

If wanting better performance pass that 9.9 into a 15 HP, will push much better while being more loaded. will need to buy 15 HP parts such as carb, reed valve assy and rv gaskets. Will still need a induction tach to check if inside wot range for the 15 HP. Could wot test both props and stay with the one that gives middle to max wot rpm as usually loaded.

Pecheux you're another prime example confirming that 1 inch drop does not equals + 150-200 rpm. A friend of mine went two pitch down as he wanted 400 revs more, told him to go just one less, became def, now in repent over revving + 950 rpm by following the idiotic rule of thumb.

Happy Boating

 

[steelespike]

Another "rule of thumb" going to ss from al, lower pitch 1". I think little motors will gain or lose more rpm than say.. a 50 with a 1" change.
The 9.9 and 15 use the same prop the 15 turns 5500 to 7000 the 9.9 5000-6000 prior to 89. 89 >5500 to 6500.

 

[Sea Rider]

Gee those HP engines have evolved lowering high wot rpm through the years. My ex 96 Evi 15 run 5000-6000 rpm at wot. A 50 HP OB will overrev more than stated + 150-200 revs if powering solo a bare bone combo with minimum fuel on its tank when going 1 pitch less from its current factory delivered medium "alum" size pitch. Is fantastic for faster hole shot and move more load better than with previous prop. A fast to plane boat with low hull drag coeficient adds more points to that equation.

Has someone "confirmed" with healthy tach if you'll achieve same RPM when solo boating lightly loaded when going 1 pitch less from say a 16 alum prop to a 15 SS prop while boating with medium sized OB's ?

Happy Boating

 

[pecheux]

My ex 115hp Merc 6 cyl tower mounted on a light boat was over-reving at 7000rmp + at WOT with a 19 pitch. Replaced with a 21 pitch and got down to a confortable 6000. That experimentation ended up being my rule of thumb.
Had a permanent tach installed ... turned out to be an onboard doctor.

 

[Sea Rider]

Quite interesting, so 1K revs less going 2 more pitch size, assume same 3 blade prop, right ? that's 500 rpm less rpm per one more pitch. Be carefull with "rule of thumb" while going both sides of wot range, you could pull a rod or blow your motor if not counting with a healthy tach.

Happy Boating