95 Sea Ray factory spec prop question

[sharkeydude]

I'm running a factory prop, 48 78120 a40 19p, on a 95 Sea Ray 175 with a Mercruiser Alpha One 3.0. I assume this is the original factory prop considering the serial number, basically a 14x19 prop. The Sea Ray specs for this year call for a 13.75x21 prop. If I were to go to the 21 pitch, would that improve my top end? As of last boating season, I topped out at about 4200 rpm with this propeller. I usually ride with just 2 people. I'd like to get a bit more speed if I could. Any suggestions? Thanks!

 

[Bondo]

Re: 95 Sea Ray factory spec prop question

Ayuh,.... If yer only pullin' 4200 rpms with a 19", ya don't have the horsepower to spin a 21"....

The rpms are Already at the lower end of barely acceptable....

Actually, a 17" prop might be Better suited...

 

[sharkeydude]

Re: 95 Sea Ray factory spec prop question

Thanks for the info. I'm going to run this prop a few more times when I get the boat back in the water in a few weeks. I could be wrong on the rpm assessment since I was going from memory of last season. However if the rpm rating is say WOT 4500-4700, would you still recommend the 17 pitch? I'm still learning about this boat and just don't want to screw anything up. Thanks again!

 

[Bondo]

Re: 95 Sea Ray factory spec prop question

Thanks for the info. I'm going to run this prop a few more times when I get the boat back in the water in a few weeks. I could be wrong on the rpm assessment since I was going from memory of last season. However if the rpm rating is say WOT 4500-4700, would you still recommend the 17 pitch? I'm still learning about this boat and just don't want to screw anything up. Thanks again!

Ayuh,.... I believe yer motor calls for 4200/ 4800 rpms at Wot,...

Ya want to be inside that window,...
Below it causes the motor to lug, 'n then detonation sets in, destroyin' the motor....

Luggin' the motor is Worse than an occasional Over-rev...

 

[sharkeydude]

Re: 95 Sea Ray factory spec prop question

So, as you mentioned before, the 17 pitch is still recommended? If so, I'll order a new prop. I'm wondering why the factory would recommend such a higher pitch. Is there something I'm missing here?

 

[Bondo]

Re: 95 Sea Ray factory spec prop question

So, as you mentioned before, the 17 pitch is still recommended? If so, I'll order a new prop. I'm wondering why the factory would recommend such a higher pitch. Is there something I'm missing here?

Ayuh,... 2" of prop pitches equates 'bout 400 rpms,... So Ya, a 17" should be 'bout Right...

Donno why Searay said that, maybe yer boat has gained weight, 'n needs more help now...
Or, maybe yer motor is down on Power...

 

[sharkeydude]

Re: 95 Sea Ray factory spec prop question

I'll have to get it back on the water and get some better facts. You've given me plenty to go on and I appreciate it.

 

[steelespike]

Re: 95 Sea Ray factory spec prop question

Your wot rpm and gps speed would be helpful for determining your performance and if the numbers make sense.
Your gear ratio may be on the outdrive or in your manual.
The 21" prop is very likely for the best speed. They may qualify that suggestion either by suggesting a different prop for heavy loads or water sports. Or indicating less pitch may be in order with more load.
At 18 years old your motor may be a little tired or in need of a major tune-up with a compression test.
It's also possible the throttle may not be opening all the away.
If the timing is mechanical sometimes they don't advance all the way as they age.

 

[sharkeydude]

Re: 95 Sea Ray factory spec prop question

Sorry it took so long to get back here with more info. My WOT is right at 4800 rpm with a top speed of 35-37 mph on the prop mentioned above. Bear in mind, those numbers are with a Stingray Clasic hydrofoil installed. (I didn't remove it because I've been worried about the holes in the cavitation plate causing other problems, but, I know that issue is a new thread entirely.) Should I still consider the 17P prop considering the numbers given or can I turn a 21P prop to gain some more power?

 

[steelespike]

Re: 95 Sea Ray factory spec prop question

Your present setup is working fairly well, the calculator indicates about 14% slip at 37 mph. On the high side but about normal.
Don't worry about the foil holes remove the foil and see what happens.Probably slow the hole shot but may help speed.
There are better ways of improving hole shot.Test with the 19" prop its ok to over rev long enough to get some numbers.
Cavitation plate is old terminology. "Anti ventilation plate" is more accurate.You don't feel cavitation its a different animal.
You do know to trim down for hole shot and gradually trim up for ideal rpm and speed?Speed by gps is usually more accurate
and you should try to figure the most accurate speed,35-37 will produce different results at 35 your slip is probably on the high side where 37 barely falls in the normal area.Your smallish boat would tend to produce lower slip numbers than a biggedr heavier boat.
Of course there are exceptions.

 

[sharkeydude]

Re: 95 Sea Ray factory spec prop question

Thanks for the info! Not having one of these type of boats before, I have nothing to compare to, so, I pick up what info I can here. I learned the trim lesson long ago after cruising the lake nearly vertical all day, lol.