Need help with which prop I should use? and max rpm on a johnson please help!

[johnson426]

Hey guys, im new on the boats but im gradually learning but I really need help with which prop I should use. I have a 18ft mantee with a 1986 90hp johnson and when im at full throttle its reaching to 5000rpm-5200 max but it seems like its lugging. Couple people mentioned it should be higher. Should i go with a smaller pitch of a prop or is my rpm at a safe level ? thanks!

 

[johnson426]

Re: Need help with which prop I should use? and max rpm on a johnson please help!

Can anyone please help!

 

[hwsiii]

Re: Need help with which prop I should use? and max rpm on a johnson please help!

Johnson, if you will get me your WOT RPM and GPS speed I can help you.



H

 

[surrender]

Re: Need help with which prop I should use? and max rpm on a johnson please help!

Before changing props check motor height and make sure engine is running properly. I had to raise my motor as it was mounted to low from previous owner. Got a little more RPM then dropped down to 15 inch and got 5700 rpm. No i run 4600 and do 25-26 mph cruise. I dont think thats bad for a 17 McKee(heavy tri hull) with an good load on board. 34+ top end. A little more slip then some might like but its a heavy boat and not designed for speed.

 

[johnson426]

Re: Need help with which prop I should use? and max rpm on a johnson please help!

My max rpm is 5200 and I dont have a gps or a speedo..I have manual trim on my boat, how far should I tilt it ? I have it tilted the lowest it can go because thats how my old motor was but the old owner could have it wrong. thanks for the help guys!

 

[johnson426]

Re: Need help with which prop I should use? and max rpm on a johnson please help!

Can I hurt the motor from having the motor tilted all the way down ?

 

[jimmbo]

Re: Need help with which prop I should use? and max rpm on a johnson please help!

If you have the motor on the inner most tilt pin hole you are probably trimmed in too much and the boat is plowing bow down, aldo the steering is most likely pulling to the right. Moving the tilt pin out will raise the bow reducing wetted area and drag. This will allow the boat to run cleaner and faster which will raise your rpm and improve top speed and handling. Most boats run best if the tilt pin is in the 2nd or 3rd hole out, but expermentation is the only way to find out. Too far out and the boat will porpoise or ther prop will ventilate. Power trim is the way to go.

 

[jimmbo]

Re: Need help with which prop I should use? and max rpm on a johnson please help!

duplicate post

 

[Silvertip]

Re: Need help with which prop I should use? and max rpm on a johnson please help!

You have too many variables and not enough information. Without power trim or a way to determine speed you have no way of knowing if the boat is trimmed properly. We also need to know the diameter and pitch of the current prop. If you have the boat trimmed full down it is probably plowing and therefore not running at its optimum. You also need to check the enging height (position of the AV plate in relation to the keel). Too low slows you down, trimmed full down slows you down, a dirty hull slows you down, water soaked foam slows you down, as does an engine that is not tuned properly. You need to buy or borrow a GPS to get some speed numbers with an average load. Check engine height and then play with trim. When all is optimized, then tell us what you have for numbers.

 

[surrender]

Re: Need help with which prop I should use? and max rpm on a johnson please help!

When I said I raised the motor it had nothing to do with trim. It was the height on the transom.

 

[johnson426]

Re: Need help with which prop I should use? and max rpm on a johnson please help!

Ok guys my prop size is 13/75x15 and seems like if there is 4 people on the boat or 2 people, the rpm doesnt really change. It will go to 5200rpm, it only plains faster when there is less people on the boat. I tried playing with the trim but its manual so its harder, I moved it up one tab but didnt seem to raise the rpm. let me know if anyone can help. thanks

 

[johnson426]

Re: Need help with which prop I should use? and max rpm on a johnson please help!

ok guys, I been going thru my paper work of my boat and I found out my boat weights about 2,000 pounds and its a 18,2 ft mantee cuddy cabin boat with a 1986 90hp johnson with a 13/75x15 prop and manual tilt. Im pretty sure thats not the same size motor it came with and the badge on the boat says max 140hp, so is this motor too small for my boat ? Only reason I ask that is because it only goes up to 5200 max on full throttle and thats where I would have to leave it, in order to plain.If I try to back it off couple hundred rpms to release some stress off the motor, the boat will start to stand up.If someone can help me out that would be great because I dont want to ruin the outboard because the boat is too big and reving it to its max. thanks!

 

[Dhadley]

Re: Need help with which prop I should use? and max rpm on a johnson please help!

The motor is fine for that boat. I'd think the 15 might be on the small side as far as pitch. I'd start by making sure the motor is running like it should and if so then look at set up. My guess is that the motor is probably a little low and can be raised. I'd also guess that the prop is an aluminum prop. What shape is it in?

 

[steelespike]

Re: Need help with which prop I should use? and max rpm on a johnson please help!

I think we need a starting point lets be sure the motor is in excellent tune,throttle plates in the carb opening all the way,Prop in excellent condition.Antivent(cavitation)plate just above the prop even with or slightly higher than the bottom of the boat.
When we know the motor is right we need to check the boat next.
Not staying on plane is worrysome.

 

[johnson426]

Re: Need help with which prop I should use? and max rpm on a johnson please help!

The motor is in pretty good condtion. This year I put new wires and plugs and cleaned out the carbs and I did make sure the throttle plates are opening all the way. The cavitation plate looks like its a lil higher from the bottom of the boat but when I go fast around turns the boat revs up and starts to cavitate.Should I lower the trim one pin down ? I dont know much about props or boat but the prop looks good to me but can it be bent or how can i tell damage ?

 

[trendsetter240]

Re: Need help with which prop I should use? and max rpm on a johnson please help!

The motor is in pretty good condtion. This year I put new wires and plugs and cleaned out the carbs and I did make sure the throttle plates are opening all the way. The cavitation plate looks like its a lil higher from the bottom of the boat but when I go fast around turns the boat revs up and starts to cavitate.Should I lower the trim one pin down ? I dont know much about props or boat but the prop looks good to me but can it be bent or how can i tell damage ?

Have you tested the compression on the motor? It may be a little worn and not putting out the 90hp it was in 1984. It should have no problem turning a 15p prop. (Although cuddys are heavy)

If the motor checks out ok then I'd look elsewhere; i.e. water in your bilge, soaked foam under the floor, growth on the hull etc.

It's hard with manual trim to get the best performance. You want it trimmed in for holeshot and out for cruising and top speed. If it's ventilating in turns then you should lower the tilt one notch.

What is the brand of your current prop? You may benefit from a 4 blade 13p solas amita if you are not too concerned with top speed.

 

[johnson426]

Re: Need help with which prop I should use? and max rpm on a johnson please help!

I havent gotten a chance to compression test it yet but bildge is working, the boat takes in no water. Im not concerned about the top speed. Im concerned about hurting the motor,since full throttle i can only get up to 5200rpm max. On glass days it runs good and i can back it off and stuff cruise at a decent speed but on lil choppy days anything less than full throtte the boat will start to stand up and you can feel the motor is getting stressed.

 

[johnson426]

Re: Need help with which prop I should use? and max rpm on a johnson please help!

How do I know if the prop is damaged ? The prop thats on now is from my old 90hp johhson outboard

 

[steelespike]

Re: Need help with which prop I should use? and max rpm on a johnson please help!

If the prop isn't nicked up it probably ok.Props do wear and actually the diameter can be reduced.I doubt yours is to that point.
If your gas and batteries are in the rear you could move something forward.
If your having treouble staying on plane the suggestion of a Solas Amita 4 blade
is a good one.It will plane easier,and stay on plane with less power.It will also resist venting (cavitation)may allow more motor vertical height.

 

[johnson426]

Re: Need help with which prop I should use? and max rpm on a johnson please help!

hey guys, I took my boat out today and had my gps on the boat and looks like im hitting about 30mph at full throttle at 5100 rpm with the motor trimmed all the way down to the last hole