Any room for improving top speed merc i6 on a 20ft bass boat

[trev]

I had a 200hp 83 mercury but it blew up i found the 88 inline six for a cheap replacement I have a 18 pitch turbo prop on it now running 43mph at wot on the gps at 5200rpm the boat is a 1983 20' bayon bass boat think there is any room for improvement ???

 

[dazk14]

Yes, I'm sure theirs something left in there. We'll need to start with how many HP your tower of power is.

Is the prop blowing out at full trim?

 

[trev]

Its a 1988 i6 115hp i have it 2 1/2 below the pad does not blow out when trimmed out..thanks

 

[steelespike]

Using a 2.07 gear ratio your numbers aren't quite right.Your slip is under 2% highly unlikely.
is the speed by gps?Do you know the gear ratio?If the tach is off low it might acount for the low slip.
If we guess a more reasonable 13% slip we get about 5800 rpm.My info indicates 5000-5500 rpm range.
We need to confirm the tach the gear ratio and get a gps speed.

 

[Texasmark]

You didn't say what is 2 1/2 BELOW the pad. Also is there some setback from the rear of your pad to the transom? Every application is different.

I ran the same engine on a Ranger 17.5 pad hull and mid 40's was tops with a Laser 19 as I recall. I ran up at 6000 rpms at WOT-sweet spot trim, which was trimmed out past max mph and tucked back into max. Did not have any setback from pad to transom and don't remember where my AV pad was with respect to the pad. I think the clamp bracket was on the transom meaning it had no jacking. For the record, I ran this boat at WOT about every chance I got. I had it for 7 years and sold it to my BIL. Never had a wrench on the engine other than to put a new water cover gasket on the rear of the block and do the impeller.

Mark

 

[trev]

No Title

The boat has a 8" factory set back plate speed was on gps 43mph possibly tach could be off .

 

[trev]

The boat is fairly light bayon used the same hull for jet boats and i/o not sure what the boat weighs though but it did do 65mph with the merc 200 that was on it turning a 24 pitch at 6000rpms

 

[steelespike]

Numbers for the 200 are right on works out to 10% slip.
Perhaps the tach is set wrong Both 6 cylinders but stator count may be different on the 115?

 

[Chris1956]

Steele, I believe the gear ratio on that IL6 is 2::1.

I think 43MPH is pretty good. Maybe he will get a 1-2 MPH more with a 17" Pitch prop.

 

[steelespike]

2.00 ratio indicates almost 5799 @ 43, using 13% slip. If we use 10 % its 5606.
While possible; slip less than 10% is unlikely.
If we go down in pitch we likely will gain rpm and hole shot, more power but top end will probably be less.
If we go to a 19 probably loose some rpm,hole shot and power.Speed probably about the same.
If the 18 is old and worn a new prop might help speed a little.
If its pretty good you might have a prop shop play with the pitch/cup.
Maybe add some bow lift.

 

[dazk14]

Its a 1988 i6 115hp i have it 2 1/2 below the pad does not blow out when trimmed out..thanks

You need to raise that motor. With all the setback you have, it may be quite a bit, but I realize the merc design has it's limits.

I didn't see where you answered Texas Mark with current A/V plate height.

Verify your tach reading, but I don't have a big problem with your numbers. Turbo's have always had lot's of progressive pitch and cup, so at 19" pitch, you're at 8%.