Choosing a new prop

[zopperman]

You're all going to hate me for this post.

I have an 18' open bow '77 starcraft that I'm restoring. I just splashed her last night and I have a 70 HP chrysler on the back that's running smoothly. IDK the current pitch because I can't see it anywhere on the prop. I was hitting about 23 with me, a buddy, 12 gals of gas and some gear. The motor doesn't have a tach since the new ign doesn't have an active tach circuit. The prop LOOKS to be about a 14 P. 3 blade aluminum prop. I've never dealt with a prop before so I know only the basics and I've only taken it off to lube the prop shaft when winterizing last year. so I don't know much about how to change the prop/hub kit or the lower unit in general... any help is appreciated

Zak

 

[craze1cars]

Re: Choosing a new prop

Nobody hates you...just make sure you realize that you've asked questions that are unable to be answered.

Nobody on earth can eyeball a prop and accurately determine its pitch. Not even you. However a prop shop can figure it out for you by laying it out on some of their pitch blocks.

Choosing the correct prop is 100% based on RPM and current propeller pitch. After you provide us with ALL of the following, someone can help you select a prop:

1. Current RPM at your current top speed.
2. Exact pitch of your current prop.
3. Your engine's recommend operating RPM range.

If you skip 1, 2, or 3, your question simply cannot be answered with any degree of accuracy at all....

 

[zopperman]

Re: Choosing a new prop

RPM range I can get and the actual pitch of my prop I can get-- RPM's I cannot... the engine has no way of measuring them and I've heard that the "tiny tach" that goes around the top cyl spark plug wire is grossly innaccurate..

 

[craze1cars]

Re: Choosing a new prop

I have never heard of Tiny Tach's being inaccurate unless installation was incorrect. Properly installed, they will register every single fire of the spark plug inductively...which would be no different from using a timing light, an inductive shop tach, or any other such thing that hooks up the same way.

As far as I'm concerned, they work.

At least they would be far more accurate than just guessing an RPM by sound...

 

[zopperman]

Re: Choosing a new prop

At least they would be far more accurate than just guessing an RPM by sound...

How did you know my secret

Might be a worth while investment then...