Re-pitch a SS Prop??

[Solittle]

I know alumnium props can be repitched but I don't know about SS. I have a 13 1/4" X 17" that needs to go to a 15" pitch I think. I am running 5300 rpm WOT & would like to go to 55-5800. Do able??

 

[DES]

Re: Re-pitch a SS Prop??

Usually 1" in either way is about all that can be done effectively without cutting blades completely off and re-welding, which gets a little costly.

 

[Dhadley]

Re: Re-pitch a SS Prop??

Anytime you decrease "bite" (no matter how you do it - with pitch or diameter or cup or all) you decrease efficiency of the prop. Overall its better to replace them with true 15" props. <br /><br />Going the other way (adding bite) is always better. Thats why we like to go with the smaller pitch when we have no other choice.<br /><br />We're also assuming this is on your Sea Drives so chaniging the X dimension is not realistic.

 

[Solittle]

Re: Re-pitch a SS Prop??

Yep - - It is for my Sea Drives - Good memory DH!!

 

[Dhadley]

Re: Re-pitch a SS Prop??

Well, the memory thing happens once in a while.<br /><br />Are you running twins?

 

[Solittle]

Re: Re-pitch a SS Prop??

Sometimes they walk but yes twins. Props can get expensive with twins real quick.

 

[Dhadley]

Re: Re-pitch a SS Prop??

Is one counter rotating? I'm almost assuming not.

 

[shep70057]

Re: Re-pitch a SS Prop??

Actually SS props can go 2" in either direction without re-welding. I had a 19 turned into a 17 and it cost right at 80 bucks.

 

[Solittle]

Re: Re-pitch a SS Prop??

Dhadley - Nix on the counter rotating.<br /><br />Any ideas where I could do some swappin? I have 3 props in addition to the two on the boat. Two are alumnium and one is a freshly rehubbed Solas SS - never been on the boat.<br /><br />By the way we have friends in your neck of the woods. They moved over there from Miami some years back. They are quite active in their Catholic Church - the name escapes me right now - neat folk.

 

[Dhadley]

Re: Re-pitch a SS Prop??

Oh well, I was hoping on the C/R thing. I guess we're back to looking for 15's.

 

[Solittle]

Re: Re-pitch a SS Prop??

In the "for what it is worth department" - I called one of the major prop shops here in Miami and told them what I had and what I was looking for. They told me that a SS prop could be repitched to a max of 2" in pitch & gave me a price of $102 per prop. They said that would get me an additional 400 rpm.<br /><br />Before I do anything I think I will run out some fuel as I probably have 70 gallons in the tank (weight thing) and play with the trim a bit. I also want to make sure that the throttles are wide open at WOT before I do anything. I also have an old pair of al props to try. <br /><br />I'm going to think on it a bit and may even wait until the marine flea market in Dania in the spring.

 

[walleyehed]

Re: Re-pitch a SS Prop??

by Shep:<br />

I had a 19 turned into a 17 and it cost right at 80 bucks. <br />

Actually, what has been done in this case is you removed the "efficiency" of the 19 and made it act like a flat 17.<br />Rule of thumb: Always error on the low side of pitch and add cup/bite (efficiency) when possible-slip will go down, not up as will be the case with "turning" a prop down in pitch.

 

[shep70057]

Re: Re-pitch a SS Prop??

[/quote]Actually, what has been done in this case is you removed the "efficiency" of the 19 and made it act like a flat 17.<br />Rule of thumb: Always error on the low side of pitch and add cup/bite (efficiency) when possible-slip will go down, not up as will be the case with "turning" a prop down in pitch. [/QB][/quote]<br /><br /><br />You are right. I've always understood that pitching up a prop is better than pitching down. It was mostly an experiment... (a little costly one at that). I now use a 16p Power Tech 4 blade.

 

[Triton II]

Re: Re-pitch a SS Prop??

When you guys say "I've always understood that pitching up a prop is better than pitching down." do you mean that it's better to repitch an existing prop up (i.e from 13 to 15") or are you talking about buying a new one of less diameter and great pitch? Dumb question? Maybe, but I'm still dithering over a new prop and wasn't aware (until now) that you could repitch.

 

[walleyehed]

Re: Re-pitch a SS Prop??

No Dumb questions......<br />What we are refering to in this case is to error on the side of low pitch insted of higher pitch. The reason is very simple, when you remove cup/bite from a prop trying to get more RPM, the slip ratio increases. By taking a prop you get a few more R's than you want, providing the engine set-up is optimum, you can have a shop add a bit of cup/bite, or install a new prop in the lower pitch with a more efficient design-lower the slip ratio.<br />There are a few more items that come into play, but that is a brief explaination of the prop we want in general.

 

[Solittle]

Re: Re-pitch a SS Prop??

Could someone explain slip ratio? I understand that it is better to pitch up rather than pitch down but not why. Maybe you could explain the difference between a 17" with no cup, a 17" with cup and a 19" "down pitched" to a 17".

 

[walleyehed]

Re: Re-pitch a SS Prop??

If we have a 19 pitch, without cup, it will perform very simular to a cupped 17. The cupped 17 "may" run slightly higher RPM, but will have more lift due to the cup, and quite possibly more speed-maybe not much, but due to a more efficient design, it's possible.<br />The slip ratio would be the difference between the theorectical movement of a 17 pitch, of moving 17" fwd in 1 complete revolution. Beings we are dealing with a fluid, this is not possible, and the difference in theoretical and actual would show the percentage of slip, or "slip ratio". A flat, non-cupped prop will always have less efficiency than a cupped prop.<br />Example would be a flat, non-cupped 17 pitch may move fwd only 82% of the pitch value. On the other hand, a cupped 17 pitch may move fwd 90% of the stated pitch value.<br />If you will notice, many of the prop manufacturers say "If you are using an un-cupped prop, use a cupped prop with 2" less pitch".<br />Hope this helps.