Another re-proping question.

[Cal]

I still need my outboard to rev 500 rpm faster to hit the top of its rated RPM range with the load it usualy carrys. With one guy its pretty close, with two adults which is its usual load its about 500 rpm too slow, and with 3 guys its working hard. The prop that is on the motor is 11 3/4" with a 17 pitch, will going down to a 15 pitch get me the extra 500 rpm or should I go lower? The prop that it has is aluminum, I want to know if going to a stainless prop will affect the amount of RPM gain I can expect and how so. Thanks for any help.

[billie]

The 15" will change things about 13%. You don't provide any RPM numbers to calculate but it should be a good boost in performance.

A SS prop maintains its form under stress and transfers more power to the water from the engine, if you have excess or adequate HP for the size of boat. It is also a bit tougher when it clips something in the water. I don't think you will gain anything based on the limited info you provide about you boat.

An aluminum prop will flex a bit (theoretically reducing the pitch) on hole start allowing the motor to rev a bit faster than a SS. This is a probably a good thing for your application.

A SS is not a bad idea but I would go to the 15 alum and buy a spare, after testing and confirming adequate performance. Maybe a shop will let you try one with an option to trade to a 13" if you had to to get the performance you want.

[Cal]

Originally Posted by billie

The 15" will change things about 13%. You don't provide any RPM numbers to calculate but it should be a good boost in performance.

A SS prop maintains its form under stress and transfers more power to the water from the engine, if you have excess or adequate HP for the size of boat. It is also a bit tougher when it clips something in the water. I don't think you will gain anything based on the limited info you provide about you boat.

An aluminum prop will flex a bit (theoretically reducing the pitch) on hole start allowing the motor to rev a bit faster than a SS. This is a probably a good thing for your application.

A SS is not a bad idea but I would go to the 15 alum and buy a spare, after testing and confirming adequate performance. Maybe a shop will let you try one with an option to trade to a 13" if you had to to get the performance you want.

Thanks for the info Billie, the motor is a 50 hp 2 stroke Johnson. Its on a boat rated for 60 hp max, the engine is supposed to make 5500 rpm with the throttle wide open. With two people I am getting around 5000 and with three around 4500. I want to be able to make 5500 with two people which should do a decent job with three people and I'll just have to watch my RPM when I go alone. If nobody thinks I'll be gaining much with a SS prop then I'll stick to aluminum.

high. i bought a new spitfire prop, havent put it on yet but its slightly smaller diameter 4 blade. better hole shot and no top end loss.
might look into testing it to see if it helps.
what your talking about is fine tuuning your prop so you have the right pitch but that small motor just cant overcome the weight. its like running a chevy sprint car into a headwind. could always underprop for those heavy load days.

good luck. but most of us agree we cant hit red line with a loaded boat.

[JohninAB]

On a 50 hp motor I would stay with aluminum as the flex in it won't be much. Big hp motors you notice it more. If going to stainless on big motors general rule is 2" less pitch in stainless versus aluminum.

[billie]

Originally Posted by JohninAB

On a 50 hp motor I would stay with aluminum as the flex in it won't be much. Big hp motors you notice it more. If going to stainless on big motors general rule is 2" less pitch in stainless versus aluminum.

That "general rule" has sold more SS props than anything else. When you drop 2" of pitch the performance will no doubt improve significantly (but with a sacrifice of top end speed) so many people believe the SS is god's gift to boating. Fact is an aluminum will do the same thing if you drop 2". Proper prop match to expected performance is the key. SS will defnitely define the hole-shot "snap" much better.

Not that SS are not good props for a lot of reasons, it's just that salesmen use that line to ensure the customer gets a "WOW" factor, and that sells even more SS props to their friends.

I've run a bunch of them on a Carlson CVX but my opinion is worth what you paid for it

[Kim473]

Go with the aluminum. You will gain the 500 rpm but your top end will probly slow by 5mph with either prop, or so. Your hole shot will increase greatly. On my 115 when I change the props I go from beeing able to pull a 250 lb guy out of the water with alot of effort to allmost pulling his arms off if I pin it. Gain over 500 rpm but lose about 5mph top end.

[JohninAB]

Originally Posted by billie

That "general rule" has sold more SS props than anything else. When you drop 2" of pitch the performance will no doubt improve significantly (but with a sacrifice of top end speed) so many people believe the SS is god's gift to boating. Fact is an aluminum will do the same thing if you drop 2". Proper prop match to expected performance is the key. SS will defnitely define the hole-shot "snap" much better.

Not that SS are not good props for a lot of reasons, it's just that salesmen use that line to ensure the customer gets a "WOW" factor, and that sells even more SS props to their friends.

I've run a bunch of them on a Carlson CVX but my opinion is worth what you paid for it

Drop in pitch of 2" is to compensate for the loss of flex you do not get with stainless versus aluminum. Try running same pitch props, stainless and aluminum, on the boat and tell me there is no difference in performance. Been there tried it. 150 Johnson on a 16' Carlson CVX.

[billie]

Originally Posted by JohninAB

Drop in pitch of 2" is to compensate for the loss of flex you do not get with stainless versus aluminum. Try running same pitch props, stainless and aluminum, on the boat and tell me there is no difference in performance. Been there tried it. 150 Johnson on a 16' Carlson CVX.

Agreed, no question, but dropping 2" aluminum to aluminum will also improve performance. If you have the right size aluminum prop to start with, you don't have to drop pitch for a SS. I was talking more of salesmen than the products.

So the question becomes, why use SS?

IF you have the HP, SS is like better tires on a performance car. Putting 50 series pirellis on a civic is a waste of cash. IMO. A SS on a 12' aluminum would also be folly.

Anyway, didn't intend to create an arguement, same boats and likely similar testing. The CVX's are a sweet boat aren't they?

[Shmag]

Personally i would say leave it alone, its probably where you should be. Just a guess but thinking there is a range on where you should be, you mentioned your high of 5500 at WOT, you are getting close to that by yourself. 5000 at WOT with 2 guys, nothing wrong with that.

What is it saying your low should be at @ WOT, my guess is 47-4800 and your getting 4500 with three guys, close enough.....Go Fish !!!