cupped vs not

[sabastianunf]

I am currently running a 3 blade 17 pitch non cupped prop and am hitting ideal rpms. Want to swithch to 4 blade for wakeboarding. I am looking at a vortex prop which is cupped. Does the cupping have any effect on rpm's or top speed? rule of thumb is that a 4 blade 16 pitch replaces a 3 blade 17 pitch but was not sure if the cupping affected this rule. Any help is appriciated.

 

[Silvertip]

Re: cupped vs not

Most prop calculators have you add one inch of pitch if using a cupped prop. So -- yes it does affect rpm and speed. How much depends on the setup but 100 - 200 rpm decrease in rpm can be expected (same prop but cupped vs uncupped).

 

[sabastianunf]

Re: cupped vs not

the vortex prop is a 4 blade cupped but I can only find it in a 16 pitch. According to what you say silvertip - I will not hit the top of my wot rpm range with this prop. The only other options that I can find is a Solas Amita 4 in a 15 pitch - says it is a progressive pitch but no info on cupping. Does anyone know how if it is cupped? I am worried that it may overrev my engine if it is not. Also went to mercury prop selector and they suggest the alpha 4 - does anyone have an opinion or any info about those props? Thanks again for any input.

 

[Silvertip]

Re: cupped vs not

Changing props is always a bit of a trial and error exercise. Some props with the same diameter and pitch are more efficient than others so replacing a prop from one manufacturer with the same type from another manufacturer can indeed cause performance differences. Also, a prop that performs great on one boat may not perform equally on another. In your case you are changing four variables: pitch, cupping, manufacturer, and number of blades. It's very difficult to get it right the first time when that many factors change. I would guess that the Vortex four blade is a cupped prop.

 

[sabastianunf]

Re: cupped vs not

You are correct the vortex is cupped and I am playing with a lot of factors. I guess my next question is can you get an aluminum prop repitch, about how much would it cost and does it fatigue the metal or otherwise negatively affect the prop? Thanks again for the input.

 

[Texasmark]

Re: cupped vs not

My experience in going from alum to ss (besides cupping mentioned) is the fact that ss blades are usually thinner and usually are more complex in their shape. So (again discounding the effect of the cup) you could expect to get more mph and more rpm with a ss over an alum. So, as mentioned it is a trial and error thing. Best bet is to pay a little more and buy from a dealer who will work with you for the right prop. You'll be much happier and come out with less overall expense than if you follow the trial and error method............BTDT.

And yes to all the alum questions you asked in the last part of your last response.

Mark

 

[hillca74]

Re: cupped vs not

I'm going through similar issues as well. I'm so glad that I'm not the only one who needs a little help when plunking down hundreds of dollars on one of the most important parts of my boats propulsion system!! Whew...

 

[Tail_Gunner]

Re: cupped vs not

I own a couple of 4 blade stilletto's that are cupped quite a bit Using a prop calc, the slip they indicate is carzy aka 3-4 %. At the suggestion of Walleyehed, i was told to treat them like they were 1 pitch higher than stated,giving a more realistic figure of 8-9%

They do bite the water however, the amount of lift is apparent at speed's above 30 mph,

 

[sabastianunf]

Re: cupped vs not

Iemded up ordering the 4 blade 16 pitch vortex which is cupped. Should be in tommarrow and will hit the water on thursday. I will let yall know the results. Thanks for the help.

 

[Texasmark]

Re: cupped vs not

I use the BAM racing site's information and their calculator which states that 1" of pitch increase for cupping when calculating prop performance including slip. Amazing how adding 1 inch to your calculations makes things work out to a reasonable value.......did for me.

Mark

 

[sabastianunf]

Re: cupped vs not

Okay - I got the prop bolted on tonight. Had to shave a 1/4 inch off the zink peice be hind the prop to allow for clearance - anyone know how much distance is reccomended for clearance at tip of prop?

 

[Texasmark]

Re: cupped vs not

I did the same thing and only have about 1/4 inch clearance. Have had no problems.

What I used in determining the clearance was to see how much movement there was in the propshaft (caused by the shifting setup in Merc) . Once I had that I figured 1/4" was enough and it works fine.

Mark

 

[sabastianunf]

Re: cupped vs not

I can pull back on mine and it move just a little - I assume that is what you are talking about. Even when pulled back I still have a little clearance @ 1/4 inch. I think I am okay unless anyone else thinks I need more clearance. I would hate to mess up my new prop - It sure does look pretty.

 

[Texasmark]

Re: cupped vs not

I guarantee you that the trim tab will go before the prop....Zinc vs SS. No contest. Besides when you do most of your running the prop is forced forward away from the tab.....that little bit you (we) are talking about.

Mark