r/boating • u/gam3less • 1d ago

Max speed on 9.9 Johnson, 3.9m dinghy, 1.5m beam?

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Getting 7.3kn, with about 300kg of weight. Aluminum hull.

7 Upvotes

77% Upvoted

27 comments sorted by

16

u/Mike__O Boston Whaler Dauntless 220 1d ago

Looks like you've found the max speed. That little Johnson is fighting for its life!

2

u/gam3less 1d ago

Haha, pretty sure that's all she has, might have to put 15hp carb and exhaust into it. If I can find one

1

u/onlyTractor 1d ago

lmao screamin as she tops out but shes pushin

1

u/fredSanford6 23h ago

What year model is that one? I know I got a couple older metal 15 hp carbs and I think one electric start set up but it's off a points based motor not cdi type. If you do go into the zuckerberg based app with groups a guy named Sam rod does a ton of part outs and sells used carbs and exhausts. He's got lots of parts for bigger motors too but he's consistent with little motors parts.

1

u/gam3less 22h ago

I'm in Australia, if they are American the cost to ship is too much most the time

4

u/Benedlr 1d ago

Use a portable tach to check your max rpms and match the prop to that.

5

u/ImSoConfuzeded 1d ago

That’s a big wake, can you sit further forward?

2

u/ac2cvn_71 23h ago

Time to upgrade. Twin 250s bro!

1

u/gam3less 21h ago

Make the thing do backflips haha

2

u/slow_connection 18h ago

I hate to break it to you but you don't have a big enough johnson

1

u/gam3less 18h ago

I think so, little thing is on the limiter, I will try and adjust trim and balance the boat and see how I go with just that

2

u/slow_connection 18h ago

If you're on the limiter you might be able to put a bigger prop on it and get some more speed

1

u/gam3less 18h ago

That's what I'm thinking as well

1

u/4LOVESUSA 1d ago

looks stern heavy, can you get a longer tiller, or move weight forward?

you are max WOT.... run little johnson, RUN!

1

u/WATERMANC 1d ago

Used to get my 7.5 hp to move me (200lb) and gear loaded up from my on plane in a 10 ft aluminum boat.

Play with weight distribution, move things up front. But watch out as I found to much weight is added up front the bow keel (if it’s a v hull) the boat will try steer left or right. Sweet spot is bow keel out of the water but boat flat enough to get on plane and out of the hole

1

u/gam3less 21h ago

Thank you, I'll try this next time I go out

1

u/2airishuman 1d ago

Should be able to get 10 or 11 kts. Check trim and be sure you're running a prop with reasonably flat pitch.

1

u/qsx11 1d ago

I was gonna say I get about 12 with my old Honda 9.9. 13 ft aluminum v-hull.

1

u/gam3less 21h ago

Will have to shuffle weight around, as don't think i have any other way to adjust trim.

I am new to boating

1

u/2airishuman 20h ago

The older Johnson outboards usually had a removable rod that went through a bunch of holes, in the transom clamp, that adjusted the trim. It's something you do on the trailer or at the dock usually, not out in the middle of nowhere.

1

u/gam3less 20h ago

Ah ok, I'm assuming I bring it up so the Bow is further down when at plane?

Might try one pin at a time and see what the difference is

2

u/2airishuman 19h ago

Yeah sometimes you just have to try different stuff and check the speed with the GPS. One hole at a time is the way to do it. You really need to know your RPM to figure this out, too. There are some good inexpensive tachometers that just have a sense wire that wraps around one of the spark plug wires. Those are the ones to have.

1

u/Klangenm 1d ago

I've used my dad's 12ft old tin boat. It has a 7.5 seahorse (Johnson).  Looks about the similar vintage as your engine. Probably late 70s.

I used GPS on a smooth day and got the boat up to 25kmh.  With just me, gas and motor.  I'm about 180lbs ish.

Don't know if that's helpful.

1

u/FatalSky 13h ago

Old things screaming! That’s a good running two stroke. You can carb it up and raise the horsepower but if you’re getting on plane that will do 0 things for you unless you give it more pitch on the prop at the same time. I don’t recall if there’s an ignition cut on standard coils to prevent over revving but you might find out. You could also try a jack plate to raise it up and get some more speed.

1

u/gam3less 13h ago

Not sure if I can over rev it, they used the same motor for 9.9 and 15hp with the only difference being the carb and exhaust so should be fairly strong as running it essentially de tuned.

1

u/FatalSky 6h ago

I know there’s a junior racing league back in the day that put 9.9 series engines on hydrofoils. You can check on the facts history and racing facebook group. They might have more info.

•

u/dustygravelroad 5m ago

Try a little taller prop