Yamaha 25hp on 14 foot jon boat

Kenny1970 · Dec 28, 2007

I replaced my 1971 Evinrude 25hp with a 2007 Yamaha 2 stroke tiller handle 25hp.

The power between the two is like night and day.

The boat is made by Alweld and is called a 1448.

I just did the 10 hour break in and it is only pushing the boat 35 MPH at wot 10 mph faster than the old 25hp.

At 3/4 throtle it is running 34mph and at full throtle it reves about 1000 rpm higher but only goes to 35mph.

It came with a 9 and 7/8 by 11 pitch prop.

I think I need more pitch on the prop.

My question is would a 13 pitch prop be to much prop?

Thanks in advance.
Kenny

JB · Dec 28, 2007

Re: Yamaha 25hp on 14 foot jon boat

You seem to be getting an amazing top speed now, Kenny. How are you measuring it?

Without rpm numbers it would be hard to make any recommendations.

In fairness to your Evinrude, it was flywheel rated and probably only produced 18-20hp at the prop, which was on a far higher drag foot.

Get some rpm numbers for WOT and repost your question in the Prop Forum.

Kenny1970 · Dec 28, 2007

Re: Yamaha 25hp on 14 foot jon boat

JB said:
You seem to be getting an amazing top speed now, Kenny. How are you measuring it?

Without rpm numbers it would be hard to make any recommendations.

In fairness to your Evinrude, it was flywheel rated and probably only produced 18-20hp at the prop, which was on a far higher drag foot.

Get some rpm numbers for WOT and repost your question in the Prop Forum.

Wow thanks for the fast reply

I had a new speedo still in the box and mounted the pick up thing on the transom and hooked up the tube to the speedo.
I never evin mounted the speedo in the boat,I just laid it on the floor of the boat in the box it came in and watched the mph go up an up.

It realy kind of freaked me out because my 1971 16 foot ski boat with a 120 Chrysler only goes about 40 mph.

I dont have a tack in the jon boat but was thinking of getting one of those little ones called tiny tach where you just wrap the wire around the plug wire and mount the little gauge on the engines hood.

Are those any good?

JB · Dec 28, 2007

Re: Yamaha 25hp on 14 foot jon boat

Yes. TinyTach will give you usable numbers.

I don't trust pitot speedometers so I am still suspicious of your speed reports. They matter little, of course, compared to rpm numbers.

steelespike · Dec 28, 2007

Re: Yamaha 25hp on 14 foot jon boat

I'm not surprised your new motor out performed a 36 year old motor.That most likely needed at least a major check up.
If possible get the rpm reading and speed with a GPS.
35 mph isn't likely though might be possible after very careful setup and prop selection.

Kenny1970 · Dec 29, 2007

Re: Yamaha 25hp on 14 foot jon boat

JB said:
Yes. TinyTach will give you usable numbers.

I don't trust pitot speedometers so I am still suspicious of your speed reports. They matter little, of course, compared to rpm numbers.

I went to there site and the have 3 of them but none say they are for a outboard engine.

2C for 1x per revolution (typically for Briggs, Honda and Kohler type engines);

1C for cylinders firing 2x per revolution (2 cylinder personal watercraft such as Seadoos typically fire this way); and

4C for cylinders firing every other revolution (typically K series Kohler engines)

I am guessing that I need the one that is marked 1C.

Is that the right one to order?

JB · Dec 29, 2007

Re: Yamaha 25hp on 14 foot jon boat

Yup.

gotboostedvr6 · Jan 9, 2008

Re: Yamaha 25hp on 14 foot jon boat

Im gonna bet your mounted too deep... can you snap a pic like this one?

Also how is your trim? negitive, neutral, or positive? does the bow ride high at full speed?

The mounting of an outboard changes everything

ron7000 · Jan 10, 2008

Re: Yamaha 25hp on 14 foot jon boat

i have a 2006 25hp on a 10' hardbottom raft,
find a regular old tachometer, borrow one out of somebody's boat if you can or "rent" one from west marine.
Set the pole setting to 3 pole.
Bring a 12v battery, hook up the tach + and - leads to the battery, and wire the - battery post to the engine powerhead using an alligator clip. On the ignition box on the right side, sitting in the boat looking aftward at the motor, there will be a handful of wires hanging loose. It's a yellow with red stripe wire. I used that to measure rpm, connect that to the sender post on the tach. Don't get a tiny tach, they are gay.
Mine came with a 9.875x11 prop too, found out i was turning 6800 wide open. I bought a stainless turbo hot shot prop that's 10x14 and now get 6000-6200 rpm wide open at 39mph consistently on handheld gps.
I got my prop from soderbloom.com.

Kenny1970 · Jan 20, 2008

Re: Yamaha 25hp on 14 foot jon boat

JB said:
Yes. TinyTach will give you usable numbers.

I don't trust pitot speedometers so I am still suspicious of your speed reports. They matter little, of course, compared to rpm numbers.

Ok I finley got the numbers 6170 RPM at WOT doing 35 now on the cheep mph gauge.the book said 3/4 throttle was about 4000Rpm so I did that the last 7 hours of break in.Well the tack said 3/4 throttle is 5200RPM.I am glad I did not blow it up.

Kenny1970 · Jan 21, 2008

Re: Yamaha 25hp on 14 foot jon boat

gotboostedvr6 said:
Im gonna bet your mounted too deep... can you snap a pic like this one?

Also how is your trim? negitive, neutral, or positive? does the bow ride high at full speed?

The mounting of an outboard changes everything

The trim is set right I think.At 29 to 30 mph it skims on top of the water and much faster than that it started to hop at around 30 to 35 mph.If I set the trim one more pin down it it plows throw the water with the bow down.
I found that 2 cases of beer in a ice chest in the front of the boat makes the hop goes away.

steelespike · Jan 21, 2008

Re: Yamaha 25hp on 14 foot jon boat

Kenny1970 said:
The trim is set right I think.At 29 to 30 mph it skims on top of the water and much faster than that it started to hop at around 30 to 35 mph.If I set the trim one more pin down it it plows throw the water with the bow down.
I found that 2 cases of beer in a ice chest in the front of the boat makes the hop goes away.

See if you can buy or make some wedges so in combination with your trim you can get or remove 1/2 a notch.

jspringator · Apr 13, 2008

Re: Yamaha 25hp on 14 foot jon boat

I was thinking about switching to a new Yamaha 15 2 stroke from my 1975 Rude 25HP. Similar weight, fewer cubic inches, but technology should make up the difference. Would I notice any difference with my 12' inflatable?

gotboostedvr6 · Apr 13, 2008

Re: Yamaha 25hp on 14 foot jon boat

yes it will be slower

Yamaha 25hp on 14 foot jon boat

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