Changeing Diameter

[MSmith3615]

I recently was shopping for a stainless steel prop, the original prop is a 14.25 X 17 pitch. The boat is a 21ft cuddy with a 260 merc. A friend of mine has a 14.50 X 17 pitch stainless steel merc prop. Does it matter that it is not the same diameter as the 14.25? I tried it out on my boat and the mph went from 42 to 49 mph which was good, but my rpm went down from 4200 to 4100. My wot is 4200-4600, so is this bad for my engine? If so what prop should I go with, diameter, pitch and 3 or 4 blade. thanks

 

[walleyehed]

Re: Changeing Diameter

Diameter is usually slightly less on SS props than the same pitch aluminum.<br />What Model outdrive do you have??

 

[MSmith3615]

Re: Changeing Diameter

Its a Alpha One, so are you saying the 14.50 is too high and I should go to a 14. What about 3 or 4 blade. The boat ran good with the 14.50. From 2500 rpms to 3500 the rpms matched the speed exactly. But once above 3500 the speed increased past the rpms. It topped out at 48 or 49 and 4100 rpms. If I trim all the way up I can get 4200 and about 50 but the ride is horrible.

 

[walleyehed]

Re: Changeing Diameter

Horrible??? are you saying it's porpoising???<br />Don't worry about the 14.25 vs the 14.5 diameter...the SS prop is obviously showing you how much more efficient it is...typical.<br />Do you know what Model the Merc SS prop is??? that would help for comparison sake..

 

[MSmith3615]

Re: Changeing Diameter

yes its porpoising , I do have trim tabs and they due compensate but they also slow the boat down. Its a Vengeance prop.

 

[walleyehed]

Re: Changeing Diameter

The Vengance should be a fair match for your boat...<br />Do you know the tach is in good shape and giving us the correct numbers???<br />GPS for speed or do you have an in-dash unit??<br />I think I found part of our problem...

 

[MSmith3615]

Re: Changeing Diameter

New guages and speedo tube, I wish I had GPS. The wiring and everything on the inside of the boat is brand new. Except engine and outdrive.

 

[walleyehed]

Re: Changeing Diameter

OK, the numbers you give for the Vengance calculate out to a negative slip ratio....impossible.<br />We need to do 1 of 2 things to know for sure...borrow a hand-held GPS and verify the speed or have the tachometer verified.<br />The numbers for your original prop show fairly accurate as far as slip goes..I thought maybe the Vengence was a 19P or maybe a 21P but the numbers still don't jive.<br />If you can get you hands on a GPS that would answer the questions at hand with the Vengence prop.

 

[MSmith3615]

Re: Changeing Diameter

what is negative slip ratio?

 

[MSmith3615]

Re: Changeing Diameter

I due have a extra auto tach that I could hook up and see if accurate. I'm pretty sure tach is right but I'm by no means an expert. The speedo does bounce from 45-49. But never falls below 45 at WOT. The tach always tops out at 4100. I do have a 19pitch merc black max 14.25x19 pitch and rpms were 3900-4000 way to low.

 

[walleyehed]

Re: Changeing Diameter

When a 17" pitch prop is showing that it is moving forward more than 17" in 1 revolution...it can't be done in a fluid, or even a solid for that matter. Most slip ratios on a hull that planes will be from 6% to 18%.<br />Example: A 17" pitch moves 16" in one revolution because we are running in a fluid. The "efficiency" of a prop is based on slip ratio. Think of it like a screw into wood....the screw does not pull in farther than the pitch of the threads...In a fluid such as water, it's always less than the pitch of the prop.

 

[MSmith3615]

Re: Changeing Diameter

Wow great info Kenny, you are the prop guy for sure. What do I due?

 

[walleyehed]

Re: Changeing Diameter

Really, we need to make sure the tach is right. That's the best instrument we can use, but it must be accurate, and "IF" you run the Vengence, make sure it is a true 17 pitch and hasn't been re-worked for some reason.

 

[MSmith3615]

Re: Changeing Diameter

Well I'm currently running the vengeance. It was still in box when my friend gave it to me. Should I still run it? I threw out the old prop Since both the vengence and black max have the flo-thruor the XHS hub or whatever it is called

 

[walleyehed]

Re: Changeing Diameter

Yes, That's fine to run it....as long as we know it's a 17" it'll be fine.

 

[MSmith3615]

Re: Changeing Diameter

What do you think I should run? I've been told 4 blade is better for my boat, but some people tell me 4 is not any better. Should I use auto tach to check boat tach?

 

[walleyehed]

Re: Changeing Diameter

To be Honest, I'm not sure I can answer "what prop to run" until we get an accurate RPM reading.<br />4 blades have better hole-shot and good cruise but suffer slightly on top-end.<br />Basically, in order to know which way to go, we need an accurate base-line, ie, known good tach, GPS verified speed, etc...If we can get a good base number to go on, we can continue from there.

 

[MSmith3615]

Re: Changeing Diameter

Tach is new and all wiring was down by boat shop, I will hook up auto tach in morning and I can get gps from my work. Will test run in morning. I work on the water so I will get some numbers first thing in morning or I should say in about 4 hours

 

[MSmith3615]

Re: Changeing Diameter

walleyehead, here are the new numbers, 46mph at 4200-4300 rpms. Did speed with gps.

 

[walleyehed]

Re: Changeing Diameter

What Is your lower unit gear ratio...???<br />I thought it was 1.5:1, but I'm not sure..