Over revving, and choosing a prop.

[Thejhall72]

Hello,
so Im at a loss of brain cells from banging my head on this boat. I have a very limited knowledge of the whole boat scene, but I'm very good with engines, and electrical.(professionally in the field)

So I have a 93 glastron ssv195. (19 foot fiberglass)
It has the 4.3 mercrusier sterndrive
alpha 1 gen 2 (from what I have heard from people I have talked to, and what I can gather... if anyone can help me determine gear ratio, or has a way to tell otherwise, PLEASE let me know.)

So into the issues I'm having. When i got the boat. It has a no-name 14x21 prop on it. (From what i have gathered looking at other props side by side a friend had)
the prop is unmarked.
The motor will go way over 5k RPM if I were to allow it.
that prop would do 37mph trimmed up until right before cavitation. At 4800 RPM
so I tried a black diamond 14.25x17
It would still go over 5k. Top speed at 4800rpm was 34 trimmed up.
yesterday I tried a black diamond 4.25x21 it would still over go over 5k easy
it went 42mph at 4800rpm trimmed up with that prop.

All props are 3 blade

So my question is, what else can I learn about the boat to determine what it should have, and where should I go from here?
I read online that I can fit a 15" diameter prop. Should I go that big and see what happens?
Should I go 4 blade?

I ultimately want to have a prop I can get good speed out of, and not go over the reccomended RPM range. But I don't want to destroy the outdrive/motor.

pulling is of no concern with the prop I'm looking for. I intend on purchasing another prop for that purpose

 

[Scott Danforth]

first, welcome aboard

second, read thru the stickies in the prop forum. good infor there

going from a 21 pitch to a 17 pitch was a step backwards. the 17 pitch will probably shoot you out the hole much faster, however if you were over-reving with the 21", you will over-rev about 400 RPM higher with the 17p

diameter has less to do with it than pitch.

first and foremost. verify your tach. if the tach is set to 8 cylinder, you are reading 33% high on RPM
secondly, use GPS for speed, you could be moving faster than you think.

you may need to go to a 23P

 

[WesNewell]

Online chart says it's either 1.62 or 1.81 GR. Should be on the outdrives tag. You might also check for the correct tach setting for your engine. Check the back of the hub for prop pitch/number.

 

[Thejhall72]

So I was just thinking of the tach today at work. I'm going to use a tach on my multimeter for accuracy. And the speed is verified via GPS. I will try a 23 tonight, and verify my tach as well. Thanks for the insight on the gear ratio as well!

 

[Thejhall72]

So last night I verified the engine RPM. The tach is correct.
and the gear ratio is 1.81
my prop shop didnt have another prop for me to try last nigh . But they should get a shipment in today. He said he isnt sure if they are getting any 23p but said that he may have a 4 blade 21 coming in. And he mentioned that could help drop the RPM a bit.

So my question now is, since I have a steeper gear ratio, is that possibly why I'm needing such a bigger pitch to drop the RPM?
or is something not seeming right here?

 

[WesNewell]

Yes, the higher the GR, the prop turns less with every engine rev, thus to move the same distance with each rev, the pitch has to be higher. With a 1.81gr the engine has to turn 1.81 revs for the prop to turn 1 rev.

 

[QBhoy]

I’d make sure the tach is accurate...but I’d naturally imagine a 23” might be a little much for it to be all round useable. Might get away with it if it’s a small diameter ally prop.
I’d take a guess that a good big diameter stainless 21” like a tempest plus would see her spot on and quick too. But it’s a bit of a financial gamble if you can’t get one to try first. I’ve recently seen a 19” tempest show more top end (by 5mph) and lower rpm (went from being on the limiter at 5000 rpm down to 4850 approx) than a 4 blade spitfire 21” pitch. Who would have thought that would be the outcome ! Goes to show again...you think you know props...then they throw results like that at you. Black art of a thing really.

 

[Thejhall72]

So, I found a deal on a stainless 14.5 diameter, 25" pitch that i can't refuse. New in box quicksilver for $140.
the only reason I'm going that far, is because I spend a lot of time on lake superior during the week, and like to get to where I'm going. Then i have a less aggressive prop for pulling.
As long as my RPM isnt to far over, or under. Can I cause damage going to far in pitch? I was told as long as it doesn't over rev, I'm good. But then someone told me that's to much stress on the outdrive. But I'm assuming that over revving is worse for it? (I always stop the throttle anyways) but I'm just finding a lot of mixed opinions on it.