Johnson 15 top speed

I ran my new to me motor today solo and GPS'd the top speed. 1980 Johnson 15hp, stock 9.5x10 prop on a 14' Hewescraft semi-v. Has 3/4" plywood low decks, was carrying 4gal fuel, 2 anchors and not much else. Ran best with the trim pin closest to the transom, no hydrofoil. Cav plate dead nuts even with the hull bottom. Was able to plane @ 12mph and maxed out @ 16.9mph @5800rpm according to the Tinytach. Does this sound about right? I'd like to plane a little lower, which hydrofoil. SE 200 or Stingray Jr. ?

My 89 15 Johnson put me at 22mph on a heavy flat bottom just for reference


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How heavy is the boat and you? Seems slow'ish to me, surprised your best was with the trim all the way in, usually trimmed out 2-4 holes will take you forever to get on plane but keep most of the hull out of the water and give you more speed.

If what your looking for is slow plane then your doing it right.

I've had the stingray junior with good results.


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A little slow, however, the speed you are getting aligns well with your rpms. It just seems like they would be higher.

In my haste to post I gave the figures in mph. The GPS was set on knots. 12kts = 13.2mph, 16.9kts = 18.5mph. I weigh 180lbs and the boat I don't know. Sound better?

I have a 93 Johnson w/10 pitch prop (no hydrofoil) mounted on a 2011 Tracker 1542. I run 21mph with just me, and usually @ 18MPH w/2 people and gear. I also have my trim pin set all the way in, boat seems to ride better IMO.

ProduceMan said:

In my haste to post I gave the figures in mph. The GPS was set on knots. 12kts = 13.2mph, 16.9kts = 18.5mph. I weigh 180lbs and the boat I don't know. Sound better?

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That's better and good enough to not be worth doing much fiddling with the motor, but I would check to see that the carb butterfly is opening fully to the horizontal position. If it is opening fully maybe check the compression and try doing a decarb. And as always check the condition of the spark plugs and their gap.

Update - Took the boat out for another run. Carrying 6 gal fuel, trolling motor (MinnKota endura 40# transom mount), cooler, battery and all the stuff as last run. I was trying to simulate the weight of another person. My tank is up in the bow and battery in the stern. I clamped the TM horizontally , to the middle bench, motor head forward lying down to add weight to the port side. Cooler was in front of the middle bench. Tried different trim pin settings. Boat still likes 1-2 holes out from transom. Tried the 3&4 holes but it sounded like it was lighting the tires once on plane. Topped out at 22.7 mph this time. I put a 1" riser to bring the cav plate as close to even as I can. It's been on for both runs. Might take it off and try 3&4 again. Thoughts?

I think your floors already simulate the weight of another person. I'm about to take my heavy ply floor out and replace with 10 mm coroplast.

Sounds like your doing all the right things. I've had boats that went faster when I dropped the motor on the transom. That was the only way to get enough positive trim to get the speed out them. I've also had luck fooling with timing. If you don't have the tools then crank it a quarter turn and retest speed. I've milked a couple miles an hour out of them that way. Free up the gears with synthetic lube for a few tenths. Hard to find but this stuff is so good that the desert race trucks use it. It may say Amsoil on the truck but it's got Mercruiser racing sterndrive lube in the tranny. https://www.mercuryracing.com/vital-fluids. Open up the cowl for air, file nicks off the prop and skeg. Cut down the trim tab and pound out the dents in the bottom near the transom. Tin boats are get hook in the hull from transom flex. Max sure it's flat then sand out the big gouges in the rest of the hull. Where the bottom meets the transom you want a sharp 90. File straight down on your transom. No rounding. You may see 25 easier than you think. I just went from 28 to 34mph on my Bayrunner with simple tweeks and you already went from 18 to 22. Good job! Run 40:1 penzoil from Walmart and let her rip.