Johnson 6hp Seahorse - Any idea of top speed?

[jrsat20]

I know this is broad question, but I have a 6R-73 outboard on my 12' v-hull aluminum boat. The engine, despite its age, runs like a champ. I've had it WOT for 40 minutes straight and it never sputtered once. She's a little temperamental first thing in the morning, but starts on first or second pull for the rest of the day.

However, according to my GPS, I'm cruising at 6mph at WOT on a quiet glass-like lake ... alone or with another person. Anyone have an idea what speed I should expect from this model? I've seen YouTube videos of the same engine getting 15mph (according to the owner). Thanks in advance!

 

[nwcove]

if your tinny is light weight and you arent a big guy, it should just plane off with you alone and a gas tank on board. jmo

 

[F_R]

For comparison, a 5hp Gale on my 14' Alumacraft jon boat produces 12.3 mph, GPS. A 7-1/2hp Evinrude does 15.5 on same boat. A 5hp TN Johnson only gets around 9-10 mph because it can barely break over into a plane. All with just me (200 lb) and light load in the boat.

EDIT: Your speed is about right if it doesn't have enough power to get it up- on plane. I suspect that is the problem. A 12' V might be harder to plane than you would expect.

 

[racerone]

It is very possible that the motor is running on one cylinder only.----It has a dual ignition system and one may be defective.--Or a blown head gasket.-----Start with a compression test.----Check to see if there is strong spark ( must jump 1/4" ) on both plug leads.

 

[nwcove]

For comparison, a 5hp Gale on my 14' Alumacraft jon boat produces 12.3 mph, GPS. A 7-1/2hp Evinrude does 15.5 on same boat. A 5hp TN Johnson only gets around 9-10 mph because it can barely break over into a plane. All with just me (200 lb) and light load in the boat.

EDIT: Your speed is about right if it doesn't have enough power to get it up- on plane. I suspect that is the problem. A 12' V might be harder to plane than you would expect.

didnt notice the "V"....that could be an issue with just 6hp on tap. can you borrow a flat bottom 12' for a try?

 

[jrsat20]

Thanks for all the good advice! I'll start with Racerone's suggestion, because I didn't know it was dual-ignition (is that common for two stroke?). I'm certainly not hitting the plane even after adjusting static trim and the load (with only me) is less than half of the max capacity.

I know a 6hp from 1973 won't necessarily perform as a modern 6hp, but I just didn't know what to expect from it. It's such a workhorse, I'd hate to replace it.

Thanks again.

 

[racerone]

The motor that you have has the " universal magneto " used on 2 cylinder Johnson from about 1950 to late 1980's on 2 cylinder motors.---A very good / simple / cheap to maintain system.--Parts are readily available.

 

[Vic.S]

I know a 6hp from 1973 won't necessarily perform as a modern 6hp, but I just didn't know what to expect from it. It's such a workhorse, I'd hate to replace it.

Thanks again.

You are absolutely right there. The system of rating the engine power has changed.

Since about 1983 or '84 the power quoted has been the power measured at the propshaft. Earlier the power quoted was the crankshaft power.
I reckon if and older 6 hp engine was tested to the new standard ( ICOMIA 23/83 ) it would come out as something a bit less than 5hp..