'96 Johnson 15HP can you increase hp on it?

arrow_man

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I have a 1989 17ft smoker craft with a 60hp VRO on it. It also has a 1996 15hp Johnson kicker. I have had this motor since new and love it. It ran great on my smaller boats and does well trolling and slowly getting me around on hp limit lakes in PA.

At full throttle, only me (240lbs) and my gear, with a nice breeze pushing me I could only get to 6.9mph on my GPS. Is there a way to get a little more out of it? It takes forever to get down the lake and I am not allowed to start up the 60hp unless I want a big ole fine.

I have heard about boring it out, changing carbs, and props. But will it really get me to 10 or 12 mph or even more? The boat is pretty heavy and I would like a little more power. Any advise would be great.
 

wilde1j

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Re: '96 Johnson 15HP can you increase hp on it?

The little motor @ 6.9 mph is pushing at close to hull speed, which is the fastest you can go w/o planing. Even if you had 25HP, if the motor couldn't plane the boat, ~ 7 mph is the best you can get. True even with much bigger motors as well. My boat had a pair of 150HP Mercs. One had to be shut down BC it was going to grenade. We came back from 75nm offshore at hull speed, a very long trip BC on motor could not plane the boat.

You are hoping for the impossible.
 

kbait

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Re: '96 Johnson 15HP can you increase hp on it?

I'm sure that you're not getting the rpm's for max hp, if it's the standard prop. I run a 10" by 8 pitch prop on my heavy 14' fishing boat (15 johnson). You probably want less pitch (more RPM's). Check iboats props. You'll never find a better price on a new prop! You'll likely gain speed, but as a previous poster mentioned, if you're boat is really heavy, it may not get you to 12hp, but it'll certainly help. Good luck! Ps.. 'standard' is about 10" X 9.5 pitch. I guess you usually gain 200 rpms/unit pitch change, if that helps.
 

wilde1j

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Re: '96 Johnson 15HP can you increase hp on it?

It won't go any faster if it can't plane. The motor RPM and wear will be greater though.
 

arrow_man

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Re: '96 Johnson 15HP can you increase hp on it?

I will check the pitch of the prop. It is not standard but I think I can still go a little lower. Please understand, I am not looking to plane. I know it will not be enough. I was just hoping to gain a little top end speed. I can handle going slowly most of the time. It is a little tough when I want to go a few miles down the lake and it takes me forever. I keep thinking to myself that I could be there if I was permitted to use the 60hp. It's top end is around 26mph which is plenty fast for my purposes. My main concern is when a storm hits the lake, as it gets real rough really quick because it is so shallow. Many people have capsized trying to get in with a small motor and not able to make it back to the landing. I too have had trouble in the past and decided my best bet was to get a little wet and head for the closest shore instead of the landing. With this boat it can handle bigger waves and I feel more comfortable with my daughters. I was hoping to get in a little faster.

I also read on another posting about adjusting the trim. I believe there are four positions for this. If position 1 is closest to the transom and position 2, 3, and 4 further out respectively, I have it in position 2. I should probably experiement a little with the others to see if I can gain a little speed. Which way should I go with it?

I also need to rebuild the flooring this coming winter. I have been thinking about eliminating some of the side storage and some other things that could be weighing it down. As long as it is not a structural component, I should be able to reduce some of my weight by not installing these components again. I guess it would also help if I took out the extra anchor and some other heavier things that are not necessities when I am only running the 15 hp.

Thanks for the information, Jim
 

wilde1j

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Re: '96 Johnson 15HP can you increase hp on it?

Look, I had the same deal once. On a lake w/ 10HP limit (I had a 3HP motor and a 40HP), a Tstorm came up, w/ big waves and wind. Guess what, I started the big motor to get to a protected place quickly by planing the boat and moving at 30mph.

You don't seem to be getting it. No matter what, if you can't plane, hull speed is ALL you're going to get. Hull speed varies very slightly with hull design, but is never more than ~ 7 knots. Your not going to go 7.5 or 8 knots ... 7 knots, period!
 

arrow_man

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Re: '96 Johnson 15HP can you increase hp on it?

Thank you for your input wilde1j. Maybe I am not getting it as I am not as wise and educated in the matter as you. I have come here to ask this as I am wanting to know more about it. I am ignorant in this area. I appreciate your knowledge of the matter. But I would appreciate if you would not be rude. You may have not meant to, but your last replay was taken as insulting. If you are frustrated with my responses and questions, then please do not reply. If you would like to help me to understand, your advice is welcomed.

I am not understanding this part. If I lessen the weight and and trim the boat better, I still will not get any response? I am having trouble getting a grasp on this. If someone could explain why this happens I may be able to better understand.

Thanks for you assistance.
 

wilde1j

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Re: '96 Johnson 15HP can you increase hp on it?

It's clear you do not understand "hull speed". Google it and see what wiki, etc. has to say. You can stand on your head, but it's not going any faster than "hull speed", regardless of all the things you do, short of changing the boat, to say, an 8' pram. My big, heavy boat did ~ 6.5 to 7 knots with a 150HP, and trim, tabs, etc., made no difference, nor did full throttle. Lower power settings achieved exactly the same speed.
 

rolmops

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Re: '96 Johnson 15HP can you increase hp on it?

First of all.there is truth to the concept of hull speed.But I seriously doubt that anybody knows what the hull speed max of your boat is.
If you can,lift the 60 out of the water and stow most of your weight to the front.I sort of imagine this 15 to be a tiller with you,the battery and the fuel tank sitting in the rear and cursing the damn thing for going slow.
If you can, put an easysteer system on the 15, then you can sit upfront thereby balancing the boat. That may give you an extra 5 miles if not more.
I push a 19 foot Sylvan with a 9.5 kicker at 6 mph while it is loaded with gear,fuel and a 75 horse merc down in the water.You should do way better.
Hull speed is a function of water displacement and is only a factor in very deep v boats.A well balanced 17 footer should come up on plane with a 15
 

wilde1j

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Re: '96 Johnson 15HP can you increase hp on it?

Hull speed = 1.34 X (LWL)sq rt
Hull speed = 1.34 X 17sq rt for your boat
Hull speed = 5.52 knots or 6.35 mph , or very close to 6.9 mph your GPS measured.

BTW, hull speed is a function of any boat acting as a displacement hull, which almost all are, until they plane. The exception is very specialized designs like canoes, racing rowing boats, etc.

If your little motor can cause your boat to plane, then higher speeds are possible, but I don't think your going to see it. There may be a 17' boat a 15HP could plane ( a square stern canoe, for example), but I haven't seen a typical fiberglass or wood boat a 15HP do it, especially since most boats need 16 to 18 knots to plane. Why don't you try to borrow someone's 20 or 25HP motor to test the idea, before spending anything on yours.
 

rolmops

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Re: '96 Johnson 15HP can you increase hp on it?

Hull speed = 1.34 X (LWL)sq rt
Hull speed = 1.34 X 17sq rt for your boat
Hull speed = 5.52 knots or 6.35 mph , or very close to 6.9 mph your GPS measured.

BTW, hull speed is a function of any boat acting as a displacement hull, which almost all are, until they plane. The exception is very specialized designs like canoes, racing rowing boats, etc.

If your little motor can cause your boat to plane, then higher speeds are possible, but I don't think your going to see it. There may be a 17' boat a 15HP could plane ( a square stern canoe, for example), but I haven't seen a typical fiberglass or wood boat a 15HP do it, especially since most boats need 16 to 18 knots to plane. Why don't you try to borrow someone's 20 or 25HP motor to test the idea, before spending anything on yours.

My 17 foot 1963 duratech Orion,the hull of which was deemed good enough by the CIA for the bay of pigs attack regularly did 12 to 15 mph with the 9.5.
The proper balancing of the weight will allow for far less initial water resistance thereby allowing for some lift,which in turn allows for more speed and then more lift.Up to a point of course.
 

wilde1j

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Re: '96 Johnson 15HP can you increase hp on it?

Tin boat, and I'll bet it planed? Then I'd believe it.
 

arrow_man

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Re: '96 Johnson 15HP can you increase hp on it?

Thanks for all the information. I appreciate the info. Just to give you some more facts. It is an aluminum boat and I have the ez steer connected to it. So I have a little less weight in the back since I am about midway at the console.

The gas tank and 1 battery are back by the motors. It only has 1 battery.

I am seeing that there are some conflicting opinions if I can gain even a little more out of it.

No matter if I can or cannot, I still need to rebuild the floor and I am planning on reconfiguring some things and eliminating some of the weight. If nothing else, this may get me a little faster with the 60hp and save some fuel. It will also give me a boat configured exactly the way I want.

If I am lucky, maybe I can squeak a little more out of it. I can play with it the next time I am out and see what happens. Unfortunately, that will not be until spring. Thanks
 

arrow_man

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Re: '96 Johnson 15HP can you increase hp on it?

Okay here goes...some more info for you. This may add even more to the discussion. I am going to play devil's advocate here for a minute.

Overall boat length is 17ft. Water line furthest up the hull is 15ft. I have not washed the boat and you can easily see the waterline from this weekend. From my research online, that is the measurement I need not the overall length of the boat.

Hull speed = 1.34 X (LWL)sq rt
Hull speed = 1.34 X (15)sq rt
Hull speed = 1.34 X 3.87
Hull speed = 5.18 knots or 5.9 mph

My GPS got me to 6.9mpg yes it was with a wind but it got me there. Now does this mean that I was actually getting better than hull speed and possible closer to plane than first thought? Without the wind I was averaging6.7 or 6.8 through light to medium chop and into the wind. That is still almost a mhp better than the hull speed calulations.

IF this is the case, tweaking may help me a little. No it won't get me there twice as fast, but it may be worth trying to tweak before putting any money into it.

I could also be dreaming all this up and doing alot of wishful thinking. You thoughts on this are appreciated.
 
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