Prop Pitch, motor height, how do I get better performance out of my pontoon?

[edsjr01]

First, yes I know its a pontoon and not built for speed. I'll admit that I'm ate up but I want to at least get as much as I can get out of it.

18' Bennington Pontoon
60hp Mercury Bigfoot 4 stroke EFI

Mercury says it needs to run 5500-6000 at WOT

All the props I've tested are 14" diameter

It started with a 9, the tach said it was running 6600 rpm. I didn't trust that tach because it also said the motor was idling at 1000. The tag on the motor says idle speed is 725 so I figured it was actually turning about 6275 which is right around where Mercury says the motor shuts down to avoid over rev. It's been setup like this for 6 years when the motor was bought new.

I replaced the tach and learned at that time all I really needed to do was calibrate it with the little screw on the back. That tach was bought new with the motor and the idiot dealer didn't calibrate it. Since it didn't match the rest of the gauges I went ahead and bought a matching one, turned the little screw on the back to calibrate it with the motor at idle so now at idle it reads about 700.

First I tried a 13, it didn't do well. Got about 3600 out of it.
Next I tried an 11, a little better but only 4000
Next (yesterday) I tried a SS 10. Still only about 4000. Out of the hole it would go to 4200 but would drop to 4000

What would going to a 13 diameter do for me? anything? The motor has a sticker on it saying it requires special props, although I don't see them for sale that say they are specific to a bigfoot. I'm assuming the big difference is the 14" prop. It does handle pretty well, I can literally get up and walk to the cooler to get a beer while under full power.

How about motor height? I've read the cavitation plate should be level with the bottom of the toons. It's about 4" low and it's setting all the way on the transom. Would lifting it up do anything for me or what kind of effect would it have on the way the motor runs?

Thank you in advance for the help

 

[clemsonfor]

A smaller diameter pro won't have as much resistance in the water and should in theory spin faster for the same pitch. Ie. A 13" x9 prop will turn more RPM than a 14" x9 prop.

I have never had or been around a Mercury other than the 115 that was on my father in laws pontoon that has been gone for years. But, I can't see why it would take "special props"? As long as the splines and hub was right it should work. Maybe it's something to make you want to buy genuine Mercury/quicksilver props, but I don't have any idea?

 

[edsjr01]

One thing I forgot to mention. When I took the 9 off of it originally the hub was cracked, the insert not the prop. It also would not just tap out, was like it was cocked in there. Last night using a cold chisel, a punch, a bfh and beer muscles I was able to get it out. I've got another hub which I'm going to put in it and put the 9 back on the boat just to go back to my starting point.....my though is at this point the only things changed from the beginning will be the new tach and the new hub.

 

[ahicks]

I'm struggling to believe your new tach results with the props you have. I would confirm your tach is reading correctly. From the sounds of it, your original tach may be closer.

Generally, the tach manf. will have the correct setting for your engine available.

 

[edsjr01]

I'm struggling to believe your new tach results with the props you have. I would confirm your tach is reading correctly. From the sounds of it, your original tach may be closer.

Generally, the tach manf. will have the correct setting for your engine available.

Thing is, Mercury told me there was no way possible that motor was turning 6600 RPM. I confirmed it was on 6P a year ago and then when I bought the new one noticed the calibration screw. It showed the motor idling at 1000 rpm the same as the old one. The tag on the motor says 725 and at 1000 I would think it would slam hard into gear and it never did. So, I adjusted the tach to 700ish at idle with the calibration screw and it is set on 6P per Mercury instructions

I did ask Mercury if there was a way for the dealer to override the rev limiter through the computer. I have 0 confidence in the dealer the motor came from in terms of competency and honesty (been burned by them on both). They claimed it's not possible, at the same time even if it was possible it wouldn't surprise me if they said it wasn't because they don't want hacks like me in there messing with such stuff.

Yesterday I was wondering if maybe there was something wrong with the tach and for some reason it wouldn't go past the 4000 mark. I pushed the neutral button and "goosed" it. The tach spun over to about 5500 immediately with the motor and came right back down with the throttle so I'm of the opinion the tach is working properly. When I installed it I took a picture of the back of the old one and put everything back in the same place. The two are the same tach, the new one is just a different color

 

[clemsonfor]

Why was the original tach not matching if original? Did they just want it to stand out?

 

[edsjr01]

Why was the original tach not matching if original? Did they just want it to stand out?

From what I understand they said the tach that was in it was correct for the original motor on the boat, Johnson 50hp 2 stroke and wouldn't work with the Mercury 4 stroke. All of the gauges are the same brand, the one the dealer installed was the same brand also just black instead of tan (looked like crap). When I replaced it I ended up with the same thing. The math tells me there was nothing wrong with the original tach and was likely the same as the other two but don't know for sure. Remember when I said I have no faith in this dealer?

 

[edsjr01]

I think tomorrow I'm going to call a dealer I do trust and see if they can get me in real quick for a diagnostic scan, from what I've read there is a ton of data stored in the computer so it's also going to verify my tach reading

 

[edsjr01]

Ok so now I'm at the lake and the 9 that the original tach said she as running 6600 is now reading 4400. Is there any kind of throttle cable adjustment? Could I have missed a wire when hooking up the new tach?

I just thought I was baffled before

 

[edsjr01]

Another thing, the hub on the original 9 was broken

 

[clemsonfor]

Another thing, the hub on the original 9 was broken

If the hub was broken you could have been turning higher RPMS as the motor spun higher and the prop was slipping and not turning as fast...but I would think that motor could turn a 9 pitch prop over 5000 rpms all day long?

 

[ondarvr]

If it was broken and slipping or spinning it would self-destruct in minutes and you wouldn't be moving.

 

[edsjr01]

So as I was in party cove stewing on this I pulled the cover off, put the controls at wot and learned I can move the throttle on the motor another 3/4 - 1" so in reality this motor has never run wfo. So.....more prop testing after I mechanically get it to wot

Still gonna get it hooked to a scan tool at the dealer so I know what's really going on

 

[ahicks]

Sounds like you're making good progress! Just me, but my thought is you're dead in the water as far as getting max performance with your prop/set up until you have a tach you can trust. That would be the first thing I would have the tech checking on when he plugs in.

 

[edsjr01]

Sounds like you're making good progress! Just me, but my thought is you're dead in the water as far as getting max performance with your prop/set up until you have a tach you can trust. That would be the first thing I would have the tech checking on when he plugs in.

My thoughts exactly. Gonna have a dealer pull the data out of it and specifically look at RPM's today and I should have some answers. Regardless I'm going to adjust the throttle cable

 

[edsjr01]

Took the boat to a dealer this morning, they verified with the 9 prop it is running just below the rev limiter (which is why the throttle cable was adjusted out). He told me to put the 11 on it, adjust throttle cable and also to raise the motor 2 notches as it's way low (I've suspected this). Then he went on to mention it would also help if I took the shipping bolts out that the dealer who installed it didn't take out so the motor can be trimmed all the way down. So, this motor has 116 hours on it, has never been trimmed down and never seen WOT

They said if I do these changes I should see 22-23 mph out of it

Remember when I said I have 0 faith the competence of the dealer who installed this motor?

 

[flyingscott]

What kind of boat and motor.

 

[ahicks]

Dealers are where you go to get parts in a pinch. That's it, and I'll just leave it there. No point boring everyone with another rant....

What did you find out about your tachs, so you don't need to proceed blindly as you seek your perfect setup?

 

[edsjr01]

Dealers are where you go to get parts in a pinch. That's it, and I'll just leave it there. No point boring everyone with another rant....

What did you find out about your tachs, so you don't need to proceed blindly as you seek your perfect setup?

Mercury said to set it on 6P but the dealer said all tachs are different and never 100% accurate. He set it on 5 and now it's just short of the rev limiter when it says 6600 (which is wrong). Said to subtract 300 from what it reads and I'll be fine. Weird thing is at idle it's not off by much.

He said if I raise the motor 2 spots (did it tonight), adjusted linkage to get WOT and an 11 prop he thinks I'll get 22-23

 

[ahicks]

22-23mph would be awesome, but don't be too disappointed if it doesn't.

Keep in mind a lot of this set up is all about compromise. You're looking for a solution that will allow you to do everything you want reasonably well. If all you are looking for is top speed, your ability to pull a big guy on a tube is going to suck! Point being raising the engine is a great plan to get some performance, but it could also get you a cavitation problem in a hard turn or accelerating from a stop. If you change prop sizes and raise the engine all at once, if you have an issue you aren't going to know which is causing it! Baby steps......