Wrong prop?

[Fishallot]

I have a 1993 115hp 4cyl. on a 17' Aquasport. I think it may be propped wrong but would like some advice before I try to change anything.<br /><br />At full WOT I can turn 6300 rpm. I know this is too high and am fearfull of doing some damage. In the past I have just been carefull to not open her all the way up but would like to correct this situation.<br /><br />I don't know the specs of the prop I'm running now. The only numbers I could find on it were Michigan 031043. I'm hoping this number will be enough to identify this prop? If it does would anyone recomend making a change to a different prop?

 

[Clams Canino]

Re: Wrong prop?

Assumin gyour tach is set right (for the number of stator poles), you NEED to reprop that puppy right away. Those things are known to have a harmonic that SNAPS CRANKSHAFTS over 5500 rpm. <br /><br />I don't have a Michigan wheel cross reference, but you gotta go up a few sizes in ptch to get your WOT down below 5500 for sure.<br /><br />Someone else here might know the number.<br /><br />-W

 

[Fishallot]

Re: Wrong prop?

Thanks for the info Clams. The tach is set correctly on 4.<br /><br />Does anyone have the specs for the prop I have? Do I need to look for more numbers to identify it?

 

[rickdb1boat]

Re: Wrong prop?

It's a 13" X 19" Prop. A 12-3/4 X 21" pitch is recommended for your size boat. It all depends on the top end RPM's of course.. But that would get you down to about 5900 RPM's...

 

[Fishallot]

Re: Wrong prop?

Thanks for the info RB. So in thoery I'd need a prop pitched more than 21" to bring the RPM's down to a good level? Do they make a prop with that much pitch?

 

[Clams Canino]

Re: Wrong prop?

First of all, I'd recheck the stator vs tach thing. I do indeed smell a rat.<br /><br />I *know* that the predessor to your motor has a TWELVE pole stator. Most tachs that I've wired to it are set to "6" representing positive peaks. I'm pretty sure yours is also a 12 pole stator - I'll check ASAP.<br /><br />The tach settings typically do NOT go by cylinder count if that's what you meant by "4".<br /><br />The only way "4" is correct is if "position 4" translates to the setting for a 12 pole stator.<br /><br />The tach runs off the charging stator - not the cylinder count or firings.<br /><br />-W

 

[Fishallot]

Re: Wrong prop?

Thanks for the info Clams. It appears to me that the tach is the original on the 1993 Aquasport. A few months ago I pulled the tach to check the setting but I can't remember the numbers that were on the back of it. I thought it was a 4 and a 6 and that it was set on the 4. I'll repull the tach tommorrow and see exactly what's stamped on it and what its set on. In all honesty, from an untrained ear, the motor really doesn't sound like its turning 6300 rpm.

 

[Clams Canino]

Re: Wrong prop?

OK, best info I get is that you indeed have a 12 pole stator.<br /><br />Typicaly your tach would set to 6 - check the documentaion from the tach maker. (I assume we're talking about an in-dash tach wired to your harness?)<br /><br />If all this is right, your tach (set to 4) is reading 1/3 too high, meaning you're only really making 4200 at WOT.<br /><br />While this doesn't sound quite right either, it makes a little more sence than 6300. I don't think those fours will hold together at 6300 for very long at all, IF at all. <br /><br />To be SURE of your true RPM's - borrow a hand held plug wire fired tach and set it to ONE pulse per rev (two-cycle). If you can only get a hold of a 4 cycle tach, divide the reading by 2.<br /><br />-W

 

[Clams Canino]

Re: Wrong prop?

For giggles describe a '93 17' aquasport for me? What's it made of? Weight? Hull type? etc.<br /><br />My older 115 on a 17.5 foot (loaded) fiberglass bowrider makes 5800 WOT with a 17 pitch prop and breaks 6000 with a 15P. But my boat is fairly heavy. Both props stainless.<br /><br />-W

 

[Fishallot]

Re: Wrong prop?

Here's a couple pics of the boat. She weighs in right at 1,800 lbs. with motor. The tach is the console mounted gauge on the far left. BTW... these are pics are from before I bought her, she's alot cleaner now and has several key additions. <br /><br />

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<br /><br />

 

[quantumleap]

Re: Wrong prop?

I would be suprised if you were turning 6300 rpms with a 19 pitch. I have a 125 that turns 5600 with a 19 and 5300 with a 21. my boat is a little heavier but not by much.

 

[Fishallot]

Re: Wrong prop?

I pulled the tach and now I'm really confused. The switch has 6 settings, 1 - 6. Right now it's set on 4. There's a chart beside the switch that reads. # poles = 4 set on 1, 6 set on 2, 8 set on 3 and 12 set on 4. There's no mention of settings 5 or 6. Is it possible my tach is bad?<br /><br />I guess I need to check the tach against a plug fired one to see if they match up. I'll see if I can locate one.

 

[rickdb1boat]

Re: Wrong prop?

So, you have it set on the correct setting for that tach. I would still see if you can borrow one and double check it out. Although, your RPM readings at WOT could just be right with that prop. Just have to check it out and if you get the same WOT readings, change or repitch the prop.

 

[Clams Canino]

Re: Wrong prop?

Yes - position four seems to be correct for a 12 pole stator. But those readings still sound suspicious. Check it against a plug fired tach before you go any further.<br /><br />-W

 

[rickdb1boat]

Re: Wrong prop?

Fishallot <br /><br />Is it just me, from that picture, does the motor look like it's mounted pretty high or is it just an illusion? The Cav. plate look awful high compared to the bottom of the hull. Maybe it's just the angle of the pic. Just wondering....

 

[Fishallot]

Re: Wrong prop?

Motor height is something I never thought of. The motor is original to the boat and as far as I know has always been at the same height. There are 5 mounting holes on the bracket where the motor mounts to the transom, mine is using the middle holes so if needed I could raise or lower the motor 2 holes.<br /><br />Here's a better pic to show how the motor is mounted relative to the boat.<br /><br />

 

[Fishallot]

Re: Wrong prop?

Does this motor setup appear to be too high on the hull? I have no idea what the proper setup is.

 

[Fishallot]

Re: Wrong prop?

Hi gang. Just thought i'd bring this topic back up as I haven't done anything to the boat yet. I've had such a hectic year that the boat has been used very little. <br /><br />Last time out I was still pushing 6300 rpm at wot. I'm searching for the tool to borrow to get an accurate rpm reading to rule out a bad tach. <br /><br />I'm really curious as to any opinions on the height of my motor realitive to the bottom of the boat. The last pic I posted shows it well.

 

[Fishallot]

Re: Wrong prop?

anyone?

 

[twag]

Re: Wrong prop?

The boat bottom appears to be aligned with the anti-ventilation plate of the outboard. I would say it's the right height.