15 hp Johnson

[Johnson209]

Hey fellows any Inputs appreciated. I have a Johnson late 70"s 15 hp running with original prop at
9 1/2 x10 pitch. I've recently added 3/4 plywood flooring to my boat and now loosing mph. By myself before i was getting approx 20 mph. With 2 person 16 mph and with three I get 13 to 14 mph . What should i do when i carry a heavy load like three person. I tried adjusting the pin to number three slot and sometimes I can hear the prop slipping when carrying on a heavy load. Should i change the prop to a higher pitch or lower it so i can gain some speed back? I dont have a tacho meter so i cant really test my motors rpm running on heavy loads. If i was to go a step higher in prop with heavy load do you guys think i would get my speed back or is it the other way around by going down a pitch.
Thanks for reading and have a nice day..

 

[steelespike]

Re: 15 hp Johnson

What size is your boat what did it weigh before and what does it weigh now?
A healthy carefully setup 15 on a bare aluminum 14 footer should be about 22-25 mph with one aboard.

 

[steelespike]

Re: 15 hp Johnson

What size is your boat what did it weigh before and what does it weigh now?
A healthy carefully setup 15 on a bare aluminum 14 footer should do about 22-25 mph with one aboard.
Is your speed by gps?
For more power you would go with less pitch. A 9" Solas Amita should help.
Selecting a prop could be more accurate if you had a tach and you could tell better about your results and your setup.
A induction tach like the Tinytach is reasonable and easy to install Can be hand held or permanent. Will work on
almost any motor.The 15 hp is capable of 7,000 rpm.
A very rough estimate at 20 mph your motor is only turning about 5400 rpm.
The rpm range is 5500-7000.
It's possible your old props diameter is worn undersize and the technology is 38 years old. A new prop will have the latest technology.

 

[Johnson209]

Re: 15 hp Johnson

What size is your boat what did it weigh before and what does it weigh now?
A healthy carefully setup 15 on a bare aluminum 14 footer should do about 22-25 mph with one aboard.
Is your speed by gps?
For more power you would go with less pitch. A 9" Solas Amita should help.
Selecting a prop could be more accurate if you had a tach and you could tell better about your results and your setup.
A induction tach like the Tinytach is reasonable and easy to install Can be hand held or permanent. Will work on
almost any motor.The 15 hp is capable of 7,000 rpm.
A very rough estimate at 20 mph your motor is only turning about 5400 rpm.
The rpm range is 5500-7000.
It's possible your old props diameter is worn undersize and the technology is 38 years old. A new prop will have the latest technology.

I have a 14 ft Gregor. I didn't have any floor board before i was hitting 20 mph by myself. Now that i replace all boards and add a extra board on the bow it seems slower now.My gps reading was done on my smartphone.
When I'm on water I'm always with another person and sometimes bring along 2 other adults. It drags when I'm taking three and I dont know if I should go up a step or lower .
U say I should go lower to a 9, would that help with all the load I'm carrying and by 9 do you mean the pitch cause I aready have a 9 1/2 x10 prop..
Thks for the input

 

[steelespike]

Re: 15 hp Johnson

Decreasing pitch increases power, kind of like putting a car in a lower gear.
Diameter is a secondary consideration and generally as pitch goes down diameter may go up though there are exceptions.
Your setup should be approaching 25 mph
While you may need a lower pitch I think first you should be sure your setup is right
and the motor is making full power.
Things to check: The anti vent plate(just above the prop) should be about even with the bottom of the transom.
Check that the stator advances as it should and picks up the carb roller correctly as you advance the throttle.Do a compression check. Should be about 100 psi.
Is the bottom clean and smooth?
Did you try different trim settings for best speed with a light load.
Did you try trimming down with a load.(prop closer to the boat)
Is your boat a small 14 perhaps barely making 14' centerline length?
Some small 14s measure something like 13'8" centerline.
What is the transom width and what is the widest point?
What is the hp rating?
Smaller boats have trouble getting heavier loads up on the water.
A bare 18 footer could run faster with a load than your bare 14
with the same motor.

 

[jestor68]

Re: 15 hp Johnson

Isn't the WOT rpm range 5000-6000 rpm for the 9.9 and 15?

What you are experiencing is how a boat works. When you add weight, they slow down unless you add more power.

Since you don't have more power, you must compensate by reducing the propeller pitch so that the motor can again run up to it's required rpm with the added load placed on it.

 

[Johnson209]

Re: 15 hp Johnson

That makes sense now, decreasing the prop to make the engine stronger, I think I?ll just do that. My anti vent plate check out fine its even with the bottom transom. I recently did a link and sync on the carb roller and check the compression last week and they also check out fine. The top cylinder register at 105 and bottom one was 110. I don?t fish alone and Im always with a buddy weighing around 300 so im carrying a big load including my self 180 and all the heavy ? ply wood I recently installed. I did tried trimming down the engine with a heavy load on but it seems to respond better with the trim set higher on hole three. The prop also seems to slip sometimes also when im on number three carrying on a big load like having 2 passenger on board ,maybe im just having to much weight. The boat measures at 14 foot exactly and the transom is 15 inches and thr widest point is 61 inch that?s if im understanding you correctly. I don?t know the horse power rating for the boat, theres nothing on the transom or the on boat describing hp rating. I might just go down a pitch and see if that helps. By going up a pitch what does that do?
Im new into the boating scene and want to understand the concept.
Thanks for trying to help me out, I really appreciated it.

 

[Johnson209]

Re: 15 hp Johnson

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[Johnson209]

Re: 15 hp Johnson

Thanks for the clarification. I been reading through some other forums, not here and thought that if you wanted to go faster then you have to go up a pitch since the prop is bigger it will go faster. Now that you have put it that way, it all makes sense. Thanks alot....I was going to buy a bigger prop. You guy save me time and moneY, I REALLY APPRECIATED GUYS Thanks..

 

[steelespike]

Re: 15 hp Johnson

Your boat is on the small side. A "big" 14 has about a 67" beam and a 900 to 1000 lb. capacity
including about 600 lbs of persons. A dry weight of about 240-300 lbs.
Your boat max capacity is about 700 probably including for 4 persons at about 140lbs per person.
So I think you can see that your probably over weight. Your motor is about 72 lbs 6 gallons of gas and a tank about 40 lbs.
I figure about 100lbs at least for your floor and deck.

 

[Johnson209]

Re: 15 hp Johnson

Your boat is on the small side. A "big" 14 has about a 67" beam and a 900 to 1000 lb. capacity
including about 600 lbs of persons. A dry weight of about 240-300 lbs.
Your boat max capacity is about 700 probably including for 4 persons at about 140lbs per person.
So I think you can see that your probably over weight. Your motor is about 72 lbs 6 gallons of gas and a tank about 40 lbs.
I figure about 100lbs at least for your floor and deck.

Yea I kinda figure I had too much load on board. Im going to have to drop down a pitch and see how much speed I can gain back. I was looking at some props for my motor and they even made smaller pitched then 9. If i was to go with an 8 would that be even better?
Heres a pics of the 9"prop..let me know if Im looking at the right one for my motor. Im just a rookie and this is my first boat and motor so Im learning stuff as i go along. Any help would be great thanks....

 

[Johnson209]

Re: 15 hp Johnson

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[steelespike]

Re: 15 hp Johnson

Isn't the WOT rpm range 5000-6000 rpm for the 9.9 and 15?
My 89 aftermarket repair manual says 5500-7000 I saw one today on youtube on a 10 footer turning 6960 at 28 mph.
She sure was singing!

 

[steelespike]

Re: 15 hp Johnson

That looks like the right prop Would be nice if you could get it through iboats the sponsor that make this forum possible.
A 8" prop would give even more power but you would have to be careful not to over rev with a light load.
As you decrease pitch you lower speed assuming you weren't badly over propped in the first place.
Speaking very roughly a 9" prop at 7000 could make about 22 mph.
an 8" about 20 mph. These results are assuming some very broad guesses the key being 7000 rpm and the ability of the boat to get up on the water.
There no doubt your motor will be happier with the 8 " It probably won't over rev with the 8" and a load but you need to be aware.
A tach would be a very good investment preferably before getting a prop.
Do some runs with a light load and full load and get us the wot rpm and speed.

 

[Johnson209]

Re: 15 hp Johnson

That looks like the right prop Would be nice if you could get it through iboats the sponsor that make this forum possible.
A 8" prop would give even more power but you would have to be careful not to over rev with a light load.
As you decrease pitch you lower speed assuming you weren't badly over propped in the first place.
Speaking very roughly a 9" prop at 7000 could make about 22 mph.
an 8" about 20 mph. These results are assuming some very broad guesses the key being 7000 rpm and the ability of the boat to get up on the water.
There no doubt your motor will be happier with the 8 " It probably won't over rev with the 8" and a load but you need to be aware.
A tach would be a very good investment preferably before getting a prop.
Do some runs with a light load and full load and get us the wot rpm and speed.

Thks for the info steelspike. I'm always buying my motors parts on here anyways, the pics was just a quick description only. I might just get a tach and see we're my rpms at if not I'll probably shoot for the 9 " since it gives me more speed, if it doesn't it's always good to have a backup prop when boating I'll see if I can buy a tach for cheap online since I'm going to use it once or twice only. But I'll keep you guys up to date and see how my speed goes after prop change or if I decide get a tach and give you guys the figures so ya can help me out.
Thks a mill

 

[Johnson209]

Re: 15 hp Johnson

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[Johnson209]

Re: 15 hp Johnson

Isn't the WOT rpm range 5000-6000 rpm for the 9.9 and 15?
My 89 aftermarket repair manual says 5500-7000 I saw one today on youtube on a 10 footer turning 6960 at 28 mph.
She sure was singing!

I" ll be happy just to hit 20 with all my load

 

[steelespike]

Re: 15 hp Johnson

Keep in mind the Tinytach and probably others will work on any motor with spark ignition.
It might require some math on say a 12 or 16 cylinder but any spark ignition from 1 cylinder up
A tach would be a very good purchase we could evaluate your present setup and be able to tell your results with the new prop.
Yesterday I saw what appeared to be a Tinytach strapped to the tiller of a newer Evinrude 15 It's the one I saw doing 28 at 6960 rpm.

 

[Johnson209]

Re: 15 hp Johnson

Keep in mind the Tinytach and probably others will work on any motor with spark ignition.
It might require some math on say a 12 or 16 cylinder but any spark ignition from 1 cylinder up
A tach would be a very good purchase we could evaluate your present setup and be able to tell your results with the new prop.
Yesterday I saw what appeared to be a Tinytach strapped to the tiller of a newer Evinrude 15 It's the one I saw doing 28 at 6960 rpm.

Thks spike... I will be purchasing a tach online. It won't get here until next week or so. Until then I'll get the data and let you guys know. Thks