This is going to be lengthy, sorry in advance, just trying to be thorough since I know I've tried a few things likely to be suggested.
I have a 1979 Mercury 20 hp (model 200) short shaft that needed work and did not have a prop when I acquired it. It also has a 1/2" of the skeg tip chipped off (guessing someone hit obstruction, which explains missing prop). Other than that, the motor is in pretty good shape. I have it running well and ordered a 10" 3 blade 11 pitch prop from PropCo. Factory prop I believe is a 9 7/8" 2 blade 11 pitch.
I'm running it on a flat bottom 1448 Jon boat with a 16" transom that weighs about 250 lbs dry. Anti-vent plate is about 1" below bottom of boat. First time on the water the motor was over-revving and either ventilating/prop slip/other gear box issue. I adjusted the tilt/trim pin on the motor to all positions up and down and problem was worse all the way down. Max speed was about 7 mph with high rpm. I was able to slowly get on plane through the slippage one time with some weight shifting and the motor in the third pin.
I opened up the gearbox, resealed it, changed the impeller. The forward gear has some wear were it engages the clutch dog, but I don't believe it to be significant. Motor shifted much easier after cleaning up the gear box and re-seating the shift cam properly. Took it back out and it was slightly better, but still same problem. With weight shifting we could get on plane. Boat was running about 23 mph with 2 people (200 lbs each) and still pushing the bow down after planing.
I decided to try the motor on a stout 1236 flat bottom Jon boat with a 15" transom to see if having the motor lower in the water made a difference in case it was just ventilating. It worked a little better but still same problem. Running up to 24 mph at 5100 rpm after a very gradual acceleration with weight shifting. It was intermittently and quickly revving at WOT like it was losing grip. Max RPM for that motor is 5500 I believe.
Motor slips/revs worse trimmed all the way in, but I am unable to trim the motor out beyond the third pin hole (where it eventually gets on plane) to raise the bow of the boat because it definitely ventilates at that point. I'm not sure why hole shot slips so much. I wouldn't think a spun prop would allow me to run near top speed, plus it's a new prop. Anti-vent plate skims water surface on 1448 and is just under the surface on the 1236.
I have 2 other short shaft motors of similar vintage and hp that work fine on the same 1448 Jon boat, although my 25 hp Evinrude likes to kick the stern up/bow down and ventilation limits proper trim. My 15 Johnson works great. Prop diameter on those motors are 9" and 9.5" respectively, both 10 pitch.
1) Is this a simple prop pitch issue given WOT at 5100 rpm on the lighter 1236? Prop diameter too large and catching air?
2) Is this a common issue with flat bottom Jon boats planing and settling with the bow down with drop in speed?
3) Exhaust gas escaping around instead of through hub? Anyone had issues with PropCo prop fit?
4) I've also read that the bottom bolts on this Merc 200 lower unit above prop can be over-tightened, cracking the housing and exhaust gas can cause ventilation. Mine look fine unless the holes/cracks are tiny. I thought about sealing them anyway with a washer and 3M 847, but will try a different prop first.
5) Broken skeg?
I bought a 3 blade 10" 9 pitch Michigan wheel prop off iBoats. So we'll see how that goes.
Anything I'm missing here? Any thoughts would be greatly appreciated!
I have a 1979 Mercury 20 hp (model 200) short shaft that needed work and did not have a prop when I acquired it. It also has a 1/2" of the skeg tip chipped off (guessing someone hit obstruction, which explains missing prop). Other than that, the motor is in pretty good shape. I have it running well and ordered a 10" 3 blade 11 pitch prop from PropCo. Factory prop I believe is a 9 7/8" 2 blade 11 pitch.
I'm running it on a flat bottom 1448 Jon boat with a 16" transom that weighs about 250 lbs dry. Anti-vent plate is about 1" below bottom of boat. First time on the water the motor was over-revving and either ventilating/prop slip/other gear box issue. I adjusted the tilt/trim pin on the motor to all positions up and down and problem was worse all the way down. Max speed was about 7 mph with high rpm. I was able to slowly get on plane through the slippage one time with some weight shifting and the motor in the third pin.
I opened up the gearbox, resealed it, changed the impeller. The forward gear has some wear were it engages the clutch dog, but I don't believe it to be significant. Motor shifted much easier after cleaning up the gear box and re-seating the shift cam properly. Took it back out and it was slightly better, but still same problem. With weight shifting we could get on plane. Boat was running about 23 mph with 2 people (200 lbs each) and still pushing the bow down after planing.
I decided to try the motor on a stout 1236 flat bottom Jon boat with a 15" transom to see if having the motor lower in the water made a difference in case it was just ventilating. It worked a little better but still same problem. Running up to 24 mph at 5100 rpm after a very gradual acceleration with weight shifting. It was intermittently and quickly revving at WOT like it was losing grip. Max RPM for that motor is 5500 I believe.
Motor slips/revs worse trimmed all the way in, but I am unable to trim the motor out beyond the third pin hole (where it eventually gets on plane) to raise the bow of the boat because it definitely ventilates at that point. I'm not sure why hole shot slips so much. I wouldn't think a spun prop would allow me to run near top speed, plus it's a new prop. Anti-vent plate skims water surface on 1448 and is just under the surface on the 1236.
I have 2 other short shaft motors of similar vintage and hp that work fine on the same 1448 Jon boat, although my 25 hp Evinrude likes to kick the stern up/bow down and ventilation limits proper trim. My 15 Johnson works great. Prop diameter on those motors are 9" and 9.5" respectively, both 10 pitch.
1) Is this a simple prop pitch issue given WOT at 5100 rpm on the lighter 1236? Prop diameter too large and catching air?
2) Is this a common issue with flat bottom Jon boats planing and settling with the bow down with drop in speed?
3) Exhaust gas escaping around instead of through hub? Anyone had issues with PropCo prop fit?
4) I've also read that the bottom bolts on this Merc 200 lower unit above prop can be over-tightened, cracking the housing and exhaust gas can cause ventilation. Mine look fine unless the holes/cracks are tiny. I thought about sealing them anyway with a washer and 3M 847, but will try a different prop first.
5) Broken skeg?
I bought a 3 blade 10" 9 pitch Michigan wheel prop off iBoats. So we'll see how that goes.
Anything I'm missing here? Any thoughts would be greatly appreciated!