4.3 over revving?

[Merc_man77]

Hey all, first time boat owner and poster here. I recently bought a 96 sylvan 17 ft with a 4.3 and alpha out drive. On the test drive the speedometer said we reached about 46 at 6000rpm. I read the recommended peak rpm at wot should be around 4800. Is it even possible that engine revs to 6000 rpm or is it more likely the tach is off? The motor did sound to be screaming some as my dad has a 18ft with the same motor and it doesn't sound like it runs as high. If it matters it has a blade prop aluminum pitch, idk the pitch but it is pretty chewed up. Would a 4 blade prop with maybe a 20 be about right? Any suggestions would be appreciated as I'm new to boating and internet research is only getting me so far

 

[Merc_man77]

Sorry for the typo, it's a 3 blade aluminum prop

 

[Sea Rider]

What's the min-max wot rpm range of that motor factory stated ? Is the combo a second, third hand one ? probably previous owner propped it way wrong, tach is OFF, prop is on its way to become fully spum. Only a wot run as loaded with a 4 blade 20 pitch prop will determine so...

Happy Boating

 

[QBhoy]

You’re likely way underpropped by the sounds of it. Hard to judge and not sure if you are saying that speed is GPS. Your rpm range on that engine is 4400-4800.
A boat like that and a 4.3 well propped would be at 50 gps or if you’re lucky, just over.
Depends on the drive ratio but a normal set up would be 21 or rarely a 23” 3 blade, depending on the ratio.

 

[Merc_man77]

Sea rider what do you mean by fully spun? I dont know the lingo yet

 

[Merc_man77]

Speed was according to the speedometer, didnt even think to use GPS on my phone. I'm assuming the prop is too damaged to get a reliable baseline even if I do know the pitch. From what qbhoy said and other things I've read I should just do a 21 and go from there. I plan to buy stainless does that make a difference in the pitch I should get?

 

[Scott Danforth]

Check your tach

 

[QBhoy]

You really need to make sure that tachometer is working and you know your gear ratio and gps speed. Problem with guessing. Can only assume what is normal. Sometimes older boats have been messed with and can have miss matched drives. Can easily throw a curve ball.

 

[Merc_man77]

Hopefully I'll be able to so some testing by Friday, thanks for the help

 

[Sea Rider]

Sea rider what do you mean by fully spun? I dont know the lingo yet

Means that the prop hub no longer remains vulcanized inside the prop body and spins freely, that accounts for more revs too. What's the current prop pitch, you said it was a 3 blade prop, how about pitch ? it's very important to know so to go from there..

Happy Boating

 

[Starcraft5834]

What Scott said.. you have to make sure your tac is accurate... cant know how to respond until thats known... 6000 rpm is screaming.... for an engine thats suppose to be in the 4500-4800 rpm range... if your tac is correct, you need more prop... quite a bit more... or as said, prob hub spun out?

 

[Merc_man77]

Ok so I I figured out it was a 12x19, which after doing some research i realized 12" diamater is way too small and probably accounts for the ridiculously high rpm's. I bought a 14x19 to start with and get a baseline. Been non stop rain since I bought the boat but as soon as I get a dry day I'll get out and post some results. Thanks for all the help

 

[Merc_man77]

It's possible the hub was spun and it's possible the tac is off but once I run it with this new prop that's more in line with what's required for my boat I'll see if I get more realistic rpm readings before I go buy the stuff to test my tach.

 

[porscheguy]

It sounds like you figured out a big part of your problem. That small diameter prop wasn’t doing you any favors. I will say that if you’ve got the standard 1.84 gear ratio that’s on most carbed 4.3s you’ll still overrev and have a top speed of 40-42mph with a 19P aluminum. That’s what I got when I put a 19p aluminum on mine last year for a few minutes. If it’s not too late, you ought to try to change your order to a 21p and don’t waste your time with the 19.

 

[Merc_man77]

Porscheguy, yea I'd imagine props are like gears, small diametee=acceleration and big=top end. I wanted to try the 19 because a fair amount of time I'll have 4 or 5 adults and I'll be pulling skiis and tubes. Maybe another option could be trip tabs and a 21?

 

[Sea Rider]

When you said motor was over revving was while having 4-5 adults onboard ? Useless going from a 19 pitch to other 19 pitch prop with a different diameter. Pitch is all, diam is secondary and what prop manufacturer wants to play with.

For the record report max wot rpm with new prop, would be interesting to know how many more or less wot rpm were achieved playing only with prop diameters.

Happy Boating

 

[Merc_man77]

No, overreving was during the test drive with a pretty light load. 2 185ish pound passengers, no gear besides vests and a quarter tank of fuel. I read somewhere that prop diameter can increase/decrease engine rpm by 500rpm per inch which would put me down around the 5000ish at wot range and much closer to what's recommended and loaded down I figure I'd be right where I should be at 4700rpm. Maybe that's not correct but that's my rational behind it. Either way the prop that came on it was trashed beyond repair, I bought a relatively inexpensive Michigan wheel aluminum prop so even if I end up going to the 21 at least I'll have a spare lol

 

[Merc_man77]

Maybe the 4.3 on a 178 is enough motor to do what I want to do with a 21 pitch, you guys would know better than me

 

[Merc_man77]

Ok so I got to boat out today. Tach is definitely not right. The guage was pinned saying 6500 rpm. I dont think that motor would go to 6500 without throwing a rod or pistons smacking spark plugs. Hit 45 mph on choppy water with the 19 pitch prop, 2 adults and a half tank and the motor sounded a little underloaded but the boat popped right out of the water, I think its going to be just right loaded down pulling a tube or ski. I care more about low end power than top speed as I'm usually on small inland lakes, but my last question is realisticly how hard on the motor is it to run a little lighter? I wont be wot very often, mostly cruising speed and pulling. I'm not overly concerned about maximum fuel efficieny either, I was prepared to buy alot of gas when I bought a boat

 

[Scott Danforth]

Second time...check the tach