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Discussion Starter · #1 ·
Well looking into I'm getting the new Tohatsu 20hp 4-stroke within the week for my WeldCraft flatbottom 1432 jonboat. It's all-welded and weighs around 210 pounds empty, and at the moment I have been running a Briggs/Stratton 5hp. By myself I can get it up to around 13mph with light gear, but with more than 2 light people on it it can't even plane.

1). Will it still be realistic to plane with 3 or 4 (~145lb.) guys?

I then found this plane speed calculator. I entered all the weight and hp and speed to get a constant of around 135. According to the calculator, I should be able to get around 25-27mph with the 20hp while by myself. Anyone ever test this calculator to be accurate? What are yall's jonboat constants at?

Thanks!:big_smile:
 

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I have the same size boat. I just upgraded from a 5 hp gamefisher to a 20 hp honda. My max speed with just me (205 lbs.) and light gear in the boat is 26.4 mph, so your estimates appear to be right on. I've been out a couple of times fully loaded down with gear and one other person (240 lbs.) and the boat got on plane just fine, although a bit slower. It sounds like you may go faster considering our weight differences.
 

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Well looking into I'm getting the new Tohatsu 20hp 4-stroke within the week for my WeldCraft flatbottom 1432 jonboat. It's all-welded and weighs around 210 pounds empty, and at the moment I have been running a Briggs/Stratton 5hp. By myself I can get it up to around 13mph with light gear, but with more than 2 light people on it it can't even plane.

1). Will it still be realistic to plane with 3 or 4 (~145lb.) guys?

I then found this plane speed calculator. I entered all the weight and hp and speed to get a constant of around 135. According to the calculator, I should be able to get around 25-27mph with the 20hp while by myself. Anyone ever test this calculator to be accurate? What are yall's jonboat constants at?

Thanks!:big_smile:
Where did you find the calculatoe at, on the web or did you buy something? I Thanx
 

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I'd say your numbers are pretty close. I've got a 1448 flatbottom jon that weighs 260# empty, 100# motor (25hp), i'm about 250 and fish with someone that is 200. I figure about 50# of gear give or take 10 or 20#. My boat will go about 26 or 27mph by my gps wide open. I cruise about half throttle 22mph. With just me in the boat it'll do about 28 or 29 mph wide open. Your boat should definately plane with 3 people, 4 might be pushing it, but i'd say it probably will.
 

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I have a friend that has a 1436 lowe with a 15 hp 4 stroke nissan and it will run 20 on gps with 2 people. They load it down with 4 guys (way overload) small end guy 155 top end 210 and it will still plain out. I have a 1448 lowe with an old 18 hp johnson that runs good and the 15 blows mine away! I am planning to buy a 20hp Tohatsu this month with my tax return. I know this is a late post but if you have aready got yours how do you like it and how fast is it running your boat?
 

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Ryan, your just lucky superman was in the boat with you. A cat that big could probably whip a man in 10 foot of water. LOL
 

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Discussion Starter · #10 ·
YES! The predictions were RIGHT ON! 27mph by GPS by myself and it was not even topped-out, it would over-rev after that. Easily 22-25 with 3 (140-150lbs) people in the boat. The Tohatsu 20 is a kicka$$ motor, with lots of low-end torque. BIG step-up from the 5-horse. Almost done breaking it in, can only run WOT for a few seconds at a time, and it's more than I care to. I can cruise with a few people around 18-20 at half throttle. The acceleration is abonoxious.

I'ma get some pictures of my boat next time I'm about to go fishing....it's a really nice looking rig now with a new aluminum/wood deck!:roll_eyes:
 

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Ryan, your just lucky superman was in the boat with you. A cat that big could probably whip a man in 10 foot of water. LOL
Are you kidding I wasn't in that little ass boat with all of them guys. I was in my big boat and the camera. I will say this, that little boat has seen a lot of big flats though!!!!
 
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