Mercury model 200 20 hp on 1448 jon boat - ventilation?

[jpw1488]

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!

 

[Sea Rider]

That's a tiller or central console model OB ? If with manual trim, previously trim OB to sit at 90º on transom. Go for a wot spin on flat calm water cond and have someone check what's going on at back transom. If water flow at speed is passing very close to upper AV plate will need to raise OB accordingly on transom by means of wooden shims, transom mount till prop aeration if fully cancelled out.

Other that that could have already a spun hub if prop is an oldie one. Perform a hub slip test to rule that out.Report your findings. Post a pic of the lower leg to check its current condition, much better than 1K written words..

Happy Boating

 

[jpw1488]

It’s a tiller model so I’m able to see what’s going on back there while operating. The AV plate just above the prop is skimming the surface at WOT on the larger 1448 Jon boat 16” transom and is just under the surface on the 1236 Jon boat with 15” transom. The AV plate is set about 0.5”-1” below hull at the transom on the 1448 with 16” transom.

I have the manual adjust tilt/trim pin set out a little farther than 90 degrees. 90 degrees and inward spins out and never gets the boat up. 100 degrees or so will get the boat on plane by shifting weight around as it gets up, but then the bow drops and loses speed. Trimming out more to level the boat causing the prop to catch air.

I’ll try to post a picture over the weekend.

 

[WesNewell]

From what I found, a 2.25 gear ratio, you should be running a 15p prop on such a light boat, hitting ~30 mph with a 200# person.

 

[Faztbullet]

Is it a true flat bottom or does it have ice runners???

 

[Sea Rider]

As it's a tiller OB much better to look what's going on at back transom. A suggestion, sit a boating mate up further middle or bow area, trim OB to 90º, that's fully vertical as possible. Go for a wot spin, as you have deck weight evenly distributed report if having back transom water splashes, if none, make close sharp turns and report if having some sort of prop aeration. This test must be done in plain flat calm no wind water cond.

While testing both described water situations don't move trim away from 90º, we're trying to make a perfect transom/engine height match for a perfect marriage for both to work spot on at 90º Each boat model and engine brand is a unique installation, let's focus on the boat you're having issues with and correct them.

Happy Boating

 

[jimmbo]

From what I found, a 2.25 gear ratio, you should be running a 15p prop on such a light boat, hitting ~30 mph with a 200# person.

Wrong.
1980 and earlier Merc 20hp had a 1.85:1 gear ratio and a pitch range of 7" - 12",

In 1980 an 18hp was introduced and later relabeled a 20hp, that engine used a 2.25:1 ratio. Nothing in common with the 79 engine

jpw1488: If your engine is a Merc 200, it is older than a 79, 78 was the last year mercury added the extra 0 to their motors.

You are running a prop for the early 20hp(22 cubic inches), not one for the later 20hp?

 

[jpw1488]

jimmbo:
My motor is a 1979 Mercury 20 hp according to serial number. I assumed it was a model 200 since the parts and info I’ve found for it in general cover 1963-1981. But no where on the motor does it say 200 like the 1978 and older. I believe it to be the 22 ci engine.

faztbullet:
It’s a flat bottom standard jon boat with 5 runners. Not much ice where I’m from so never heard them referred to as that. I’m assuming it’s the same.

Sea rider:
I’ll give that a try.

Thanks everyone for the replies.

 

[jpw1488]

It has an 8 spline prop shaft also which is like the model 200.

 

[jpw1488]

Update - I pulled the propeller today and noticed it has pitting and erosion. I thought it was funny that I had missing paint after running it a couple times and chalked it up to poor paint, but this is obvious. So I guess now this appears to be a true cavitation issue. Could it just be a bad or imbalanced propeller? I don’t see any other damage to the LU other than the tip of the skeg. I’m going to run the new Michigan wheel prop this evening.

 

[jimmbo]

Cavitation occurs when a low pressure area develops, low enough for the gases dissolved in the water to reappear as gases, basically the water is boiling. When the low pressure disipates, the gasous bubbles collapse, with enough force to pit metal(cavitation burn) Many things can cause Caviatation, Unsmooth water flow, poor blade design, rough and or bent prop blade edges, bent or damaged skeg. Cavitation burn shows up initially as paint erosion, but it will eventually result in rough pitted metal
Ventilation is often wrongfully called Cavitation, the causes and results are different. Ventilation is when air, or exhaust is introduced to the prop blade causing it to lose it grip on the water.

Check the gear housing carefully for dents, dings, rough surfaces. Also look at the skeg, for twists, bends, flattened edges from impact, curled edges.

 

[jpw1488]

Ran the new 3 blade 10” x 9” pitch Michigan wheel prop. Seems to have fixed the issue. Paint intact on the prop. Ran 20-22 mph at 5100-5250 rpm (both of which I expected to be a little higher). Planes nicely. I adjusted on the throttle linkage because I think my throttle shutter in the carb wasn’t fully opening. Will have to run it again to see if that helps.

The skeg does have an irregular broken tip with maybe more missing that I thought 1-1.5”. I wanted to get the motor running before I spent money on the skeg. Other than that there’s some surface scratches here and there on the LU, but in general looks pretty good. Is having the skeg welded a better option than a bolt on skeg?