Prop sizing ?

[ChevyReb]

I have a 1989 Nissan NS70A2 on a 24' pontoon boat. It currently has a 14 pitch prop and I have been told it needs to have one in the 8 to 11 pitch range for this application. I would like to try another pitch to see if it will allow the engine to slow speed more smoothly. Can I still buy props for this engine and is there any others that will interchange? I would like to try one before I spend money, so if other brand props will work maybe I can borrow one to test it out.

Thoughts??? Thanks,

Chevyreb

 

[TOHATSU GURU]

Re: Prop sizing ?

Any Nissan or Tohatsu dealer for an OEM replacement or any dealer for a generic...Including iboats.

 

[ChevyReb]

Re: Prop sizing ?

So any thoughts on the sizing for this engine boat combo? Agree with other dealer recommendation or other thoughts?

Thanks

 

[Silvertip]

Re: Prop sizing ?

Read the sticky at the top of the forum that describes the information we need to even "guess" at what you need. Then you need to explain in more detail what you mean by "allow the engine slow speed more smoothly". That makes no sense whatsoever. Perhaps Nissan and Tohatsu props are interchangable.

 

[ChevyReb]

Re: Prop sizing ?

Sorry post was moved to this from another section so I did not see the sticky...

1) Performance issue you are trying to correct.
2) Current prop manufacturer, model, aluminum or stainless as a minimum.
3) Current prop diameter and pitch (required).
4) Wide open throttle RPM and speed with an average load (very helpful)
5) Engine/drive make, model, year, and HP
6) Boat make model, year, length and weight


1. Engine shakes bad at low idle speed in gear, was told too much pitch can cause this
2. Not sure of manufacturer, Aluminum prop with 14 in the middle of a circle
3. Will have to measure diameter over the weekend and post back.
4. Engine WOT makes 46-4700 RMP been told should be 5500
5. 1989 Nissan NS70A2 70 hp Nissan outboard 2 cylinder
6. 1990 24' Scotty Boat Pontoon. Small pontoons. cannot read weight sticker, google search did not turn up anything.

 

[steelespike]

Re: Prop sizing ?

Its hard to believe a healthy prop would cause the motor to shake.
I would look at the idle mixture adjust and carb and carb linkage
also link n sync.Idle should be about 700 rpm.
If you drop to an 11" prop it should put you at "about" 5,500.

 

[ChevyReb]

Re: Prop sizing ?

Well I had a Tohatsu dealer perform the idle adjustment and link and sync and he recommended changing the prop to get the engine to run in the proper RPM range. I too don't really understand why the prop could do that but he says it can so I just wanted to see if people here had a second opinion. It does run better after he worked on it but its still not right. He did not put it in the water only on the ears so maybe it still needs fine tuning? I really dont want to spend money for a prop and have it make no difference.

BTW: Anyone have an idea of what a low budget 11" prop would cost for this engine? I would even consider a good used one.

 

[rutcutter]

Re: Prop sizing ?

Turning Point makes props for that motor and you can order one for about $80 with the hub. They come in 8 to about 15 pitch.

 

[ChevyReb]

Re: Prop sizing ?

ok got back to the lake and measured the prop. It is 11.5" (or very close to that) in diameter and has a circle with the number 14 in it which I am presuming is the pitch. So if this new info sheds any light on the thread please let me know what you guys think.

Thanks!

 

[steelespike]

Re: Prop sizing ?

If your at 4,700 you need at least 800 rpm a 10" pitch should get "about" 800 rpm.
Before making a change you may want to check under the prop nut for a number.
It does appear though that a 14 might perform as yours does.
You might consider a 9" pitch 4 blade prop it will give better control at speed and around the dock.Also will resist venting(cavitation).

 

[smallboatlover]

Re: Prop sizing ?

all you have to do is go to a dealer and tell them what motor you have and the year and they will set you up with the correct motor

 

[Robbabob]

Re: Prop sizing ?

Just seems odd a dealer would saying something about a pitch change to cure an engine shake at slow speed

If everything checks out with the carbs idea, then I'd start looking at the balance (prop/shaft). Are all blades even and clean with no significant damage? As you throttle up from the shake, does it go away or can you notice it's still there but maybe less pronounce due to the feeling of cutting through the water?

As Steele says, a 9P 4-blade will help in many ways, but it won't cure a problem elsewhere.

Good luck with this!