Need help with replacement prop

[Mike Franke]

I have a 2004 220hp Startcraft deck boat. I'm currently running 46mph at 3600rpm with a 14.5X 19 3-blade aluminum prop. WOT should be 4400-4800. I want to go to a 4-blade prop. Any recommendation for a replacement prop? Thanks.

 

[BoatDrinksQ5]

Re: Need help with replacement prop

Did you mean 3600 ? That seems awfully fast for that low of an RPM - i would have to question the gauge before going further.

But the prop Gurus could clear that up better then myself (greenhorn).

 

[Mike Franke]

Re: Need help with replacement prop

3600 is right. Wouldn't know how to check if tack is working right or not. Idle is where it should be 600-700. Seems about right when it planes at 2000.

 

[Philster]

Re: Need help with replacement prop

Based on your #'s, something is off.

If we assume 3600 RPM is your wide open throttle RPM (the max RPM she'll run at top speed), then you would need a baby prop to somehow get the RPM up.

Tach needs checking.

 

[QC]

Re: Need help with replacement prop

Agree, sounds like tach. Please post engine and drive combo and we can confirm. Ultimately we're looking for the gear ratio.

Check on the back of the tach. Most marine tachs are adjustable by a switch for 4, 6 or 8 Cyls or OBs. All use a different setting and it coudl be the entire issue.

 

[Mike Franke]

Re: Need help with replacement prop

agree, sounds like tach. Please post engine and drive combo and we can confirm. Ultimately we're looking for the gear ratio.

Check on the back of the tach. Most marine tachs are adjustable by a switch for 4, 6 or 8 cyls or obs. All use a different setting and it coudl be the entire issue.

4.3
220hp
alpha 1 gen 2
1.62

 

[QC]

Re: Need help with replacement prop

Tach's wrong, and the most likely problem is the adjustment I mentioned. VERY common.

If all the data you have provided is correct, this shows a negative 15% slip. If that were true you would be going 15% faster than the propeller blades can push you, we look for 10 - 12% slip which is 10 - 12% slower than the prop pitch can push you (19 inches in one revolution) . . . So, you either have a boat that is faster than laws of physics allow, or at least one piece of the data is wrong. RPM is the most likely. To be 100% clear, it could be the gear ratio is stated wrong, the speed, the pitch, or a combination. I'm betting on Tach.

Edit: BTW, if we plug in 12% slip, this calcs to 4707 RPM as is

 

[Mike Franke]

Re: Need help with replacement prop

Thnx. I'll check tach setting tonight and post.

 

[QC]

Re: Need help with replacement prop

Hey Mike,

I assumed GPS speed here. That is another piece that needs to be right.

 

[Mike Franke]

Re: Need help with replacement prop

Hey Mike,

I assumed GPS speed here. That is another piece that needs to be right.

Yes, it was gps speed of 46mph. Speedometer isn't working this season.

 

[Mike Franke]

Re: Need help with replacement prop

Yes, it was gps speed of 46mph. Speedometer isn't working this season.

Checked tach last night and I'm sure it was set for 8, not 6 (boat 4.3 220hp). Tach options are 4C/2P, 6C/3P, 8C/4P, 5P, and O. I assume C stands for cylinder, but what does P stand for? Anyway, I changed it to 6C/3P and now my tach reads about 200rpm with the ignition on, not running. I'm thinking maybe I can adjust it a little one way or the other and zero-out the tach. Does this make sense? Will take it out for a spin tonight and see if my problem is fixed. Kind of expecting WOT at/near 4707rpm.

 

[QC]

Re: Need help with replacement prop

P stands for "Pole". I would ignore the ignition off reading. There should be no "zeroing" adjustment. If the tach is getting the correct signal, and the switch is set right, it should be accurate. Only real way to confirm is with another tach. Get us the numbers after your test tonight and we can pretty much confirm whether the tach is right though with the method you see above.

We need new speed (GPS) and new RPM. Trim her out for as fast as she will go. If you get 4707, I'm heading to Vegas.

 

[Philster]

Re: Need help with replacement prop

Can I wager 4825?

 

[Mike Franke]

Re: Need help with replacement prop

Can I wager 4825?

Ran it this weekend and got 42mph at about 4500rpm. I ended up being the tach, changed setting from 8c to 6c. Now I'm still looking for help with a 4-blade prop. I'm currently running a 14.5 X 19 3-blade. WOT should be 4400-4800rpm. Should I buy a 19 pitch 4-blade (which should drop the rpms slightly) or drop to an 18 pitch 4-blade to keep the rpms up? Don't want to over rev. Thanks.