mercury prop questions

[greasemonkeyjmj]

have a 1977 50 hp mercury on a 15 foot bomber fiberglass bassboat.with me (370 lbs) and my dad (205 lbs) my gps showed 25mph. with my engine trimmed out, my tach was showing 6 thousand to 6,100 rpm. correct me if i am wrong, but i think my engine is rated for 5,500 max. i am currently running a stock mercury 12 pitch aluminum prop. i have a stock merc 13 pitch aluminum i can switch to. i have read on the approximately 200 rpm per inch rule of thumb. my question is, should i go up to the 13p or maybe just go on up to a 14 ? i would be going with aluminum. i know prop swapping can get expensive, so i would like to buy the fewest props possible. also what kind of mph change should be expected from a change like this.

 

[steelespike]

Re: mercury prop questions

I had a 77 50 great motor till someone swiped it.Got better mileage than my old 1960 25 Gale.
Seems to me the tach may be reading high.Perhaps you could borrow a shop tach to check it.To get to 5,500 you would need to go to a 15"pitch.
about 200 rpm change per in of pitch.It should increase speed.
You could just go to the 13 and don't go over 5,500.It should give a little more speed and still have good hole shot.Might give you an idea if you should go all the way to a 15.

 

[CaptAhab]

Re: mercury prop questions

Thats a flat bottom boat , for sure that 12 is too low SteelSpike is probably right, 6000 rpm is pretty much screaming... you could tell it was revving too high,btw how is you'r holeshot?
You'r motor looks low also..

 

[CaptAhab]

Re: mercury prop questions

If he's got a good holeshot then 13 might be too low,if he's looking for top speed 2 props might be the way to go to address both situations, load,..no load...

 

[greasemonkeyjmj]

Re: mercury prop questions

Thats a flat bottom boat , for sure that 12 is too low SteelSpike is probably right, 6000 rpm is pretty much screaming... you could tell it was revving too high,btw how is you'r holeshot?
You'r motor looks low also..

actually the boat is a tri hull design, i had it out a couple of weeks ago and the tach showed around 5,100 rpm but i was having ignition problems. being the older motor it is, iand a merc it had the dreaded wire harness rot so i went ahead and changed the stator,trigger, and the switchbox. the rectifier was shot so i changed it also, well today was only the second time the boat and motor has touched water since 1992, and it didnt come out with a tach. i had to install one. it is possible the tach is reading high, but the engine does seem like its turning on up pretty high.holeshot is actually pretty descent even with the weight it was carrying.i had a 15 ft waco aluminum bb with a 50 johnson, and this rig has a lot better holeshot. my anticavitation plate is actually about 1 1/2 to 2 inches below bottom of hull.

 

[CaptAhab]

Re: mercury prop questions

I'm running a 15 pitch 2-blade brass prop on my classic Merc 50 on a 16 footer, adequate hole-shot with a top speed of around 35 mph.