Think I found prop want to check
 

Hi guys,
I have a 21 ft shabah, with the small motor 5.0L. Alpha 1. I have a 14 diameter by 19 pitch 3 blade aluminum on it now. I have found a 4 blade SS 14 diameter by 21 pitch. I want to make sure this isn't going to sky rocket my RPMs or send them way low. Thanks for the help guys as I know it isn't scientist just don't have the time to research!!

The general rule is that you drop the pitch as you go to higher # of blades. Going to a higher pitch will drop your rpms and may lug the motor. Remember, 4400 to 4800 rpm is where you want to be. If I were you, I'd be tempted to try a 4 blade in the same pitch as your current prop and then go from there.

There are a pile of prop threads on here, a search should get you plenty of reading.

That change in props will 'likely' drop your WOT rpm's by about 3-400. I suspect with the 19" pitch prop you are already at, or near, the 4800 rpm max at WOT. You will lose some of that 'jump' out of the hole but likely gain a little top end speed. Again, these assumptions are just 'likelys' cuz of the variances in prop performance but if you can try out different props without being committed to buying, that will be the best way to proceed.

I am at 4500 99.9999999% of the time. Unless the water is FLAT as glass (rarely is always ripples), then i can get it up to 4600. The other thing is you say i will loose the jump. I am going for an average of things. Good planing speed, crusing speed and good top speed. I think the boat has a bit of a planing problem, but that might be cause the prop is old.

Also, anyone have a vibaration if they are trimmed wrong?? I have a pretty good one, then I trim it and you can "feel" the water going under the boat not a vibaration. Thanks for all your guys help. After all I am..

I agree with mikeyt, I think you will drop your RPM way out of range. If you can ever try a 17' rev4 with your set up I think you would be pleased with its performance! The decrease in pitch should gain RPM, the four blade will help hole shot and mid range fuel economy, and the rev4 is fast for a four blade. I think it is best to run at the top end of the RPM range.
Good Luck!

21p is probably to much for your boat.

I have tried a 3 blade stainless 21p and cant get my rpms over 4000 @ 40mph
Drop to a 3 blade 19p puts me at 4500-4600 Rpms @ 45 mph. (good power all around) Current Prop.
I spoke with a guy and he has stated in his exp. The difference from alluminum to stainless is 2*. If im running a 19*p alluminum I need to go with a 17* or 18* stainless due to the flexing of the prop.

If you are at 4500 rpm with the 19" pitch 3 blade then changing to the 21" will drop your rpms well below the recommended range. That said, if you are saying you have a significant vibration at speed & trimmed then it sounds like a problem with the existing prop balance which can in turn affect the prop (& boat) performance. If the prop is 'old' then i am assuming it has some dings etc which will cause a vibration if they are serious enough so it should be repaired & balanced or simply replaced. An 18" SS 4 blade or even a 17" SS 3 or 4 blade prop will lift your rpms up to the maximum area as well as provding a lot more 'jump' and even a 19" 4 blade SS will give slightly better overall performance. As i mentioned elsewhere though, this isn't an exact science as even similar boats will perform differently with the same prop. Finding a friendly prop vendor who will let you try out a couple of different ones is the best way to go.

If you are having a planing issue then i would suggest you have a look at installing a set of Smart tabs. The 80# composites or SS versions (even better given the size of your boat) should eliminate that issue.