1966 Johnson vs newer models with flow through exhaust

[heypawpaw]

I have a 1966 20 hp Johnson on a heavy 1448 welded alum flat. It runs great and pushes boat plus 23 mph and I cruise at 18 mph (Me, 6 gal gas, tool box, troll motor and battery). Problem is that with a second person boat will not plane. I'm looking at used outboards 20 to 35 hp in my area. The 9x10 prop with a shear pin seems to have to develop speed. I have a 9.9 Johnson with a flow through prop and it seems more responsive just too small of a motor. My questions:

1. When OMC changed to flow through exhaust props was that around the time engines were rated at prop ?
2. Will a newer model 20 hp with a flow through prop perform better than my old 20 hp.
3. Difference in 20 to 30 hp? 3 to 5 mph maybe?

 

[hardwater fisherman]

I know what your saying. I have a 1976 40 hp evinrude that will push me and all my gear at around 26 mph. And with a second or third person it still planes and responds well. Just some loss of speed. And I have a 1995 20hp mercury that pushes me at around 22 mph. But if I add a second person it just planes out. And with a third it will not plane. To your question I would think a prop rated motor will be more powerful. I think for the range you seek the 35 was the first to have the thru hub exhaust.That being in the 70s. And the 20 and 25 was the split gearcase until at least the mid 80s. I do not know if the newer thru hub exhaust means more power though. I think the prop rating came in around the mid 80s also. Maybe around 84?

 

[Scott Danforth]

engines were prop rated from 83 on for certain manufacturers, and 85 on for most

yes, a newer 20hp will perform better than your 52 year old crank rated motor. for that mater, you probably should do a compression check on your motor. what you are describing is low compression

difference between a 20hp and 30.....the 30 will most likely keep you on plane when you have a few buddies along

 

[Chinewalker]

Exhaust out the prop has nothing to do with your power issues.

Horsepower was re-rated to propshaft for 1985 model year on OMCs.

A 1980s era 20hp will likely perform a bit better for you, in part because it was based on the 31 cube powerhead, while your old 20 is based on the 22 cube powerhead. Same horsepower, but the more cubes might hold a little better, lug a little better.

Also, I always felt the 31 cube 20 was a bit under rated, on the low end of the 15% variance they're allowed when rating them.

 

[heypawpaw]

Thanks guys, great info. Used "through prop exhaust" as reference to newer models. However I did think the larger prop on newer models might be more responsive than the small prop on the 66 model, but the increase in cubic inches is probably why they seemed more responsive.

 

[jimmbo]

It is comparing apples to oranges

1966
20hp@4500rpm
22 cu in
4000 - 5000 rpm
21:12 gear ratio
9" - 9 14" dia props
snout exhaust

mid 80s onward
20 hp@5000
31.8 cu in
4500 - 5500 rpm
28:13 gear ratio
10" - 11 1/4" dia props
thru hub exhaust

The non thru hub props were better when the engine was running in reverse as there was a reduced tendency for the exhaust to get into the prop.

In the early 80s we had a 1979 25hp and a 1980 35hp, both 31.8 blocks, on identical boats. The 35 had the thru hub exhaust. The 25 idled better and had a slightly better holeshot, once on plane the 35 pulled away and had about 2 - 3 mph advantage.

 

[heypawpaw]

Thanks Jimmbo, that's what I was thinking 2 to 3 mph increase going from 25 to 35 and probably increase of 5 mph or so going from my old 20 to newer 30 or 35.
I'm staying away from newer four strokes due to increase in weight, needing all the performance I can get per hp, amount of maintenance required on four strokes, and sticker shock (especially the new ones). The boat is a commercial hull with max hp suggestion 30 hp so I'm not limited on hp. Now I need to decide if the newer 30 or 35 and controls and steering to get my 300# but off the back of the boat will be enough to carry me another person and gear; or if I should just up size to a 40 or 50 and have more power than needed.

 

[jimmbo]

We used to waterski behind the 35hp, two or three people in the boat. I could do shallow water standing starts on one ski and about 5 - 6 ft of slack. I was about 185.