prop survey

[reelapeelin]

Mine's a SS...I hear what you're sayin' re aluminum, but I like SS...If it were me, I'd go w/ the latter... ;) 8)...

[phester]

Step, IMO, the noticable difference between the s.s. and aluminium, is price and thats it. You're not going to see a dramatic MPH difference, you may SEE a rpm difference because the s.s. blade is thinner and will turn a little free-er. Also, if you did bounce the s.s. off a rock or something you risk more possible damage to your drive system...with the aluminium you just bend or break a blade[or two].If it were a raceboat of sorts,then you gotta go w/ S.S.I've got a 15 1/4x15,which on my 86 hull , 150 Johnson is good all around. as a spare I'll get a 15 1/4 x17, just for a little more on the top end.BTW,aluminium.

[Skools Out]

The biggest difference in the stainless and aluminum is Stainless props are usually more aggressive due to they are Cupped and Aluminum props aren't. The cup gets you out on plane faster and helps when in rough seas or pulling skiers to get a good hard low end thrust. The stainless is used more for salt water use due to sand bars which kill the aluminum props but dosn't hurt a stainless plus salt eats up and away an aluminum prop. personaly i only use stainless on all my boats but keep a cheap aluminum for a spare in case were to spin a prop.

[reelapeelin]

This is an old thread, but here's the bottom-line for me on props...if your choice of propellers will not allow your outboard to wind up to the max RPMS the mfgr specs, then you're over-working your motor every time you run it and forshortening it's life...on the other hand, if your prop allows your motor to OVER-REV, then it shouldn't take a rocket-scienist to tell you you're brewin' up a big problem there... :o...you gonna BLOW CHUNKS twice...first, chunks of aluminum all over the water, then blow your own chunks when you see the repair bill!!!... :o :P...

Of course all the above is only MHO... ;)...

[rb437]

I never really paid much attention to my prop. It seems to be working fine. But after reading this thread I decided to figure out what I have. Currently I have a stainless prop with no markings anywhere that I can find mounted to a 1989 165hp, 4cyl. w/ alpha 1 outdrive. The old aluminum prop has the no. 48-78122-21 in the center of the hub and the no. 48 78122 A40 21P on the side of the hub. Holding the two props side by side, they appear to be the same size and pitch. Anyone know if the diameter and pitch can be determined from either of these numbers? Also, according to the Clymer manual the full throttle spec on this engine is 4200 - 4600 rpm. The most I can get is 3600 rpm. I this enough of a difference to be concerned with? The previous owner had the outdrive replaced after the original one was stolen so now I'm wondering if the current prop is different than the original and possibly wrong for this engine / outdrive / boat. Any thoughts?

[chumbucket]

You've got a 21 pitch prop on there and that's probably why you're not able to get the recommended RPM's. You should need about a 17 pitch to bring it where it needs to be. You're really working your motor hard by running that prop. I would also wonder if you've got the correct gear ratio in that outdrive that the previous owner replaced. Assuming that the gear ratio is correct, just try a lower pitch prop.

[Skools Out]

Rick i'd try a Cupped 17 pitch stainless prop.

[reelapeelin]

Rick...yeah...big loss in rpms...I'd second all CB and Skool said above...only addition would be to recomend a large diameter prop...15" to 15.5"...good luck w/it!!... ;) ;D...let us know how it works out...

Just remember...depends on what you want your boat to do...your "hole-shot" may be pretty good...Great for skiin'...if you change your prop, you may lose some of that and gain top-end...but what CB said about strainin' your motor w/that prop is true, so I'd tell ya to get down off the 21 just because of that...

[jaysea]

RICK.IM RUNNEN A 4CYL MERC 170 I/O IM PLEASED WITH THE PROFORMENCE WITH A 19 P PROP AT A 3500 RPM CRUSE IM AT 24-25 KNOTS AN WOT 29-31KNOTS ALUNINUM PROP#48 78120 4A 19P ,IWAS USEN A *17"SS PROP FOR YEARS IT WAS A GOOD PUSHER PROP GAVE A GOOD PUSH FROM A STOP BUT LOST TOP END SPEED THAT PROP GOT 19 KNOTS AT 3500RPM AN TOP SPEED AT 25 KNOTS #48 888438 17 ,IM STICKEN WITH THE 19 FOR NOW TILL I TEAR IT OP INTHA SAND.

[rb437]

Thanks,

Now a couple more questions -
1. How do I determine outdrive gear ratio?
2. Where's a good place to find a prop?

Rick