83 mercury 80hp

[Coldwave600]

Ok so I have a 83 merc 80hp I'm getting a bout 30mph GPS but only running at 4500-4600 WOT
17 pitch. if I drop to a 15 pitch it should get my WOT RPM around 5000. Will this effect top speed?

 

[WesNewell]

Not much, if any. Engine mounting height plays a major part in performance and max rpm. That's where you should start. Get that right then look at rpm and prop size. If engine isn't mounted right to begin with you're just wasting time.

 

[roscoe]

Dropping in pitch should help the hole shot, and put the engine under the correct load, while maintaining the speed pretty much where it is.

 

[Faztbullet]

Motor setup and do you have trim??

 

[jimmbo]

Don't worry about engine height too much, unless the engine and transom are complete mismatches. Heights where the Anti-ventilation is higher than the boat bottom only matter at speeds above 50 mph

Your engines WOT rpm range is 4800 - 5300. Being that you are below the low end of that range, a drop in pitch may very well increase your top speed.
I prefer props 17" and higher. Reason, below that number, the props for that engine, the inline 4 loopers, and the inline 6s start to take on a design of a work prop(less rake, less cup, more blade area, more dia.)

What condition is the engine in mechanically? It is 35 yrs old. Compression test numbers? Firing on all cylinders?

 

[WesNewell]

Don't worry about engine height too much, unless the engine and transom are complete mismatches. Heights where the Anti-ventilation is higher than the boat bottom only matter at speeds above 50 mph

That's funny, when I raised my engine height just 1.5", I gained about 10% more top speed using the same prop and rpm when up about 100-200rpm. Starting speed was 34-35 mph, 38-39 mph after raising the engine 1.5". And porpoising with the trim up decreased drastically when i trimmed it up to get over 40 mph.

 

[Coldwave600]

Trim is not working... and it sits on the second to last motor stop "trimmed down" hole shots alright with me wife 2 kids and dog. motor seems to run pretty good so I have not compression tested it. Just bought the boat this spring. Was a bow rider but I built a casting deck, added Minn kota fortrexx and two 12v batteries pluss fishing gear. My main concern is that the motor is under to much of a load with the current prop. Thanks for the replies!

 

[jimmbo]

Have you tried the tilt/trim pin in different holes?

 

[Faztbullet]

Fix the trim and pickup speed and RPM!!!!

 

[Coldwave600]

I was going to try moving the tilt/trim pin the other night got out on the water... wouldn't start. Got it back home lost all spark! All the wiring is pretty sad on the motor. So I've decided to repower the boat with 2002 mercury 115 2 stroke!! can't wait!! Thank u all for your input tho!!

 

[jimmbo]

Mercury did seem to have a habit of obtaining wire with insulation that hardened and cracked as it aged. Must have been one cent cheaper per hundred feet