prop pitch

[the454assassian]

I think I have the wrong prop on my boat and I need some help figuring out which one to buy. I know nothing about prop pitch or anything like that. I know I am hitting a top speed around 35 and I want it higher than that. Can some one please enlighten me on props so I get the right one?

 

[Bt Doctur]

Need to post the engine condition, engine operation, current prop . wide open throttle RPM.trailered or slipped.

 

[the454assassian]

Need to post the engine condition, engine operation, current prop . wide open throttle RPM.trailered or slipped.

its a 5.0 ford motor thats pretty decent in shape. the boat is mainly used in the lake and from time to time in the river but i am not going to pull a tube on the river. at WOT its running about 35 mph and about 3000 rpms and it is trailered. its a 14 inch diameter prop i got the numbers 10867, 11551 and 46 78122 a40l 21p off the prop

 

[Dick Whetstone]

I did not look it up but WOT RPM on this engine should be around 4400-4800. I had a 21 foot cuddy cabin cruiser with 5.0 Chevy and Alpha 1 drive, but I do not know the gear ratio in the drive. It came with a 19 inch pitch Mercury prop. WOT was 4600 RPM and top speed was around 48 MPH. I tried a 21 inch pitch SS prop, but performance fell off and I went back to the 19. If everything on your boat is "ship shape" you should get similar results with a 19 inch pitch prop.

 

[Half Fast]

I did not look it up but WOT RPM on this engine should be around 4400-4800. I had a 21 foot cuddy cabin cruiser with 5.0 Chevy and Alpha 1 drive, but I do not know the gear ratio in the drive. It came with a 19 inch pitch Mercury prop. WOT was 4600 RPM and top speed was around 48 MPH. I tried a 21 inch pitch SS prop, but performance fell off and I went back to the 19. If everything on your boat is "ship shape" you should get similar results with a 19 inch pitch prop.

I doubt he will see similar results (4600RPM) going to a 19, everything else being equal. Dropping 2 inches in pitch is going to get him around 400 extra RPM tops, not 1600!! RPM.

WOT RPM range for that engine needs to be verified, and knowing the gear ratio of the outdrive would go a long way towards providing a decent recommendation.

 

[SeaRide]

454,

Call MerCruiser to find the WOT range. As I recall the MerCruiser 888, 302 2V, had a wot range of 3,800 - 4,200 rpm. The 898, Chevy 305 2V, was the same. Our 1981 Merc. 260, a 350 4V, had a range of 4,200 - 4,600 rpm. The 228, 305 4V, was 4,200 - 4,600 rpm as well.

 

[Dick Whetstone]

I did not mean to infer than changing from a 14 inch pitch prop (or whatever it was) to a 19 inch pitch prop would increase his WOT RPM by 1600. What I did mean to convey was that I have basically the identical engine (302 Ford vs 305 Chevy), identical Mercruiser stern drive, and nearly identical boat (size and weight). In my case a 19 inch pitch prop puts me right in the middle of the factory recommended WOT RPM.

 

[blaster]

Even an old engine like that ford should be spinning a little faster. Make sure the flame arrester is clean and there is good compression. The gear ratio should be stamped on the drive and with that along with your speed, pitch, and rpm's you can determine prop slip %.

 

[the454assassian]

do those numbers on the prop mean anything?

 

[blaster]

21p means 21" pitch the first numbers are the part number. Maybe google it.

 

[CaptArmstrong]

If you keep the boat out of gear but rev the engine will it get to the higher RPM's you are looking for?

 

[the454assassian]

Yes it will. Sorry for the long response time. I put the boat on the burner for a while and now that I am settled into a new job and home, its time to get back to getting this beast back to its former prime and more.

 

[mistercomputerman]

FYI
The general rule of thumb is that you have to take all steps necessary to attain manufacturer spec WOT rpms. No ifs, ands or buts.

Once the engine is tuned and running soundly, then the prop guy can mess with pitch and cup to attain WOT rpms.