change of pitch question

[diamondlangus]

I have a 1973 18' starcraft aluminum boat and a 1998 120hp Force motor. I am currently getting 4800 rpm at WOT and about 31-32mph. I have a stainless 13.25 dia 18 pitch cupped prop. I was thinking of going to a 17 pitch prop and want to know if there is really any advantages in my application to go with a stainless or aluminum prop? Will I gain or lose speed?

thanks

 

[Sandhammaren05]

Re: change of pitch question

At what transom height are you running the motor? Is the cavitation plate above the boat bottom? Should be at least an inch above to make any use of the cup in performance. How well does the setup carry the load? No big advantage there of stainless over aluminum in performance, but helps if you hit a log. I run a cheap 10x13 OMC al prop, and also an expensive 10x14 (really 10x13) Stilleto stainless with a 35 that runs about 41 mph, only lose a mph with the cheap al prop. Wouldn't be the same story for a high hp/weight ratio faster rig, though.

What's the rated RPM range of that motor? Is it a Mercury, or what?

I have a 1973 18' starcraft aluminum boat and a 1998 120hp Force motor. I am currently getting 4800 rpm at WOT and about 31-32mph. I have a stainless 13.25 dia 18 pitch cupped prop. I was thinking of going to a 17 pitch prop and want to know if there is really any advantages in my application to go with a stainless or aluminum prop? Will I gain or lose speed?

thanks

 

[diamondlangus]

Re: change of pitch question

The cavitation plate is just a little above the bottom of the boat. It is a Force motor by Mercury. the RPM range is 4750 to 5250 so I am just a little above the minimum.

 

[Sandhammaren05]

Re: change of pitch question

Might help if you answer the rest of my questions. Is the prop cupped? If not, cup it and in any case try raising the motor 1/2" with a wooden shim. Use two 1/4" shims so you can drop slightly it if necessary.

The cavitation plate is just a little above the bottom of the boat. It is a Force motor by Mercury. the RPM range is 4750 to 5250 so I am just a little above the minimum.

 

[diamondlangus]

Re: change of pitch question

Might help if you answer the rest of my questions. Is the prop cupped? If not, cup it and in any case try raising the motor 1/2" with a wooden shim. Use two 1/4" shims so you can drop slightly it if necessary.

sorry to sound rude but it might help if you read completelty. In the original post I stated it was a cupped prop.

 

[Sandhammaren05]

Re: change of pitch question

I would try raising the motor first.

sorry to sound rude but it might help if you read completelty. In the original post I stated it was a cupped prop.

 

[GBOverKill]

Re: change of pitch question

I agree, you may want to raise the engine.

I also noticed you're running below mfg reccommended WOT for that engine; I believe it's somewhere in the 5300 range if I remember correctly. I have the exact same engine but a 1994.

If you change the pitch, that will affect your ability to get to plane, your top speed, and hole-shot power vs. top-end power. I've run 17, 19, and just ordered a 21 pitch for my boat. I noticed a big difference in planing time just from 17 to 19 pitch; only about 1 mph (GPS) gain though. I'll let you know how the 21 pitch works out. Bottom line, I've learned...the higher the pitch....the more top-end speed, quicker planing, slower hole-shot power....and visa-versa.

Aluminum vs. Stainless.....I think a personal perference. I would never run a stainless around here in the rivers....you never know where there's rocks or logs when you live in an old logging town.

Just my 2 cents.

 

[GBOverKill]

Re: change of pitch question

Just an update....

Had the boat out all day yesterday with the new 21 pitch prop and gained about 5 mph, 43 mph (gps); compared to 38 mph with the 19 pitch. Also was able to get on plane in no more than 4 seconds! Perfect fit for my needs.

 

[diamondlangus]

Re: change of pitch question

Thanks, I think I will go with 17pitch AL. prop and see how many RPM's I gain.

 

[Frank Acampora]

Re: change of pitch question

The cup will normally decrease RPM by about 400 and two inches of pitch will do the same. Additionally, stainless normally runs about 2-400 RPM faster than the same prop in aluminum. This is for equivalent props of the same brand or design.

SO: switching from a 19 pitch cupped stainless prop to a 17 pitch uncupped aluminum would give an expected increase of : 400 RPM for the 2 pitch decrease, 400 RPM for the lack of cup, MINUS 200-400 RPM Decrease for aluminum instead of stainless for an increase of somewhere around 400-600 RPM with the 17 pitch aluminum prop.

Mind you: These numbers are not absolutes and will vary from rig to rig. They are just approximations to guesstimate expected performance, given no other changes (engine height etc.).

Since stainless USUALLY delivers a slightly higher top speed than aluminum, it is conceivable that with the 17 pitch prop, even given the increased RPM, top speed might be less.