2 differant props, same speed results ! ! !

[U Rest U Rust]

I am starting to loose it. I have been reading all the prop issues and did as much research as I can to find the correct size, pitch prop. As I am sure alot of readers are in the same situation as me, maybe there are hard core answers?? Here is the bottom line:

Bought boat from friend, he has no clue about boats, so he was no help.
1989 Sea Sprite (full cuddy) 23', Mercruiser 5.0 w/ 1.5 ratio. Bennet trim tabs and power tilt, Bottom is clean and freshly painted. Had 3 props on board. 1 SS 15 17, 1 Aluminum 15 17 and an aluminum 20 - 10 - 16 (this one looks like it belongs on a tug boat it's so big.

here are the results at WOT

aluminum 15-17 35 mph (GPS) 4200 rpms
aluminum 20 - 10 - 16 32 mph 4100 rpms

The 15 17 is useless, the boat would not plane for a long time, handling was horrible and it would loose plane on turns and around 20 mph.

The 20 - 10 -16 was excellent planing and handling as well as holding plane at lower speeds.

So why am I confused you might ask, recommended RPM is 4400 - 4800 and speed should be around 45. Why such a huge prop? What prop would give me the same results as the big boy and also add some speed??

Please keep in mind that I have not had the Tach recalibrated...

I thank you all in advance for your help...

 

[tommays]

Re: 2 differant props, same speed results ! ! !

With the combo of trim tabs and motor trim there are a lot of adjustments that could be hurting the boats performance also at this point


The 20 - 10 -16 there is no such prop so we need to try and ID it


It could be a 16 X 20 ?


In general at this point you do not have enough power to push the 15 X 17 to the corrrect WOT of 4400 to 4800 so you need to look at how healthy the motor is or how the trim and tabs are being used




I would be a bit supprised to see 45 MPH











Tommays

 

[U Rest U Rust]

Re: 2 differant props, same speed results ! ! !

That is how it is stamped on the side of the prop. so I do think it is a 16 x 20. 20 pitch.

not to concerned about speed as much as just proping it correctly.

 

[tommays]

Re: 2 differant props, same speed results ! ! !

The 15 X 17 result was close the question is why it stoped at 4200 RPM



What are you doing with drive trim and the tabs at full speed



Tommays

 

[MikDee]

Re: 2 differant props, same speed results ! ! !

Apparently you need a bigger diameter then the 15X17, because as you said it was long, & hard to get on plane, and dropping off plane in the turns, and your boat must be pretty heavy to need this, thus why you can't get it rev any more, or go any faster. I had a 24' SeaRay Cuddy with a 350/260Mercruiser, with thru-hull exhaust, that could do 40 fully trimmed out at 4500rpm, with a 17" pitch prop, (pretty good for a big heavy boat too), and I had to get a prop with a bigger diameter for the same reasons you stated, don't remember the dia.? but I think it was a Michigan wheel.

 

[MikDee]

Re: 2 differant props, same speed results ! ! !

Sorry, double post, not my fault though?

 

[U Rest U Rust]

Re: 2 differant props, same speed results ! ! !

I set the trim tabs at 17 and power tilt all the way down, once I plane i set the trim tabs at 5 or so and til up motor to get the spray equal to my steering wheel.

The larger prop does help the planing out as I mentioned earlier.

 

[tommays]

Re: 2 differant props, same speed results ! ! !

The motor needs to get in the corrrect WOT, if it is not reaching it with a 15 X 17 then it will not get better with a bigger prop


Running fully loaded with a motor that will not reach the correct WOT is bad for the motor



The motor is not makeing full power or the bottom is dirty or it is just not trimed out correctly


If the 15 x 17 was makeing 5000 RPM it would be time to look at a bigger porp




Tommays

 

[U Rest U Rust]

Re: 2 differant props, same speed results ! ! !

Understood, will have marina check out the motor and tach. once thats done I will start over and keep you posted.

thanks all your help guys...