New prop put on and now....

[Chaparralboater]

Ok I have a 1991 chaparral 18 footer. Has a 3.0 mercruiser. I had a 15x 15 pitch prop. I switched to a stainless steel 14x19 AND changed my plugs. My old plugs were trashed. The new plugs gap are smaller then the old ones but they start the boat great. Ok here's the problem I am confused about. My old prop got my boats rpm to top out at 4500 rpm and had a top speed of 33 mph...my new prop gets my rpms to a lower 3800 but the boats speed jumps to 37.. I am confused. I want to get the boats rpms up to the same high range to get maximum speed but is it because of the change in prop or maybe the new plugs?

 

[Oldskool]

Re: New prop put on and now....

You can figure on losing 200 RPM for every additional inch of pitch. Also, SS props of the same pitch will often produce a reduction in RPM (less flex in the blade). You increased 4 inches, thus the loss in RPM. You may wish to consider a 17 inch. Your WOT is now below recommendation, meaning that you are lugging the engine, which will likely lead to early failure.

 

[Chaparralboater]

Re: New prop put on and now....

So should I go back to the original diameter of the prop (15) instead of 14 like my new one has? I like the speed but I want to rpms back up. A 17 sounds good to me..why does some links say to go with a 23 pitch? That's a huggggge difference.

 

[steelespike]

Re: New prop put on and now....

Diameter simply follows the design.As pitch goes up diameter as a rule goes down.
Unfortunately you may have motor problems; As a rule a healthy 3.0 18 footer would have a 21-23" prop for speed. and run 40+ mph. a 17" pitch would be for water sports,etc
and would likely over rev with one aboard.You should check that the throttle plates open all the way. Check the compression,be sure its running on all cylinders.
bottom clean and smooth.I hope its something minor.

 

[Tail_Gunner]

Re: New prop put on and now....

Ok I have a 1991 chaparral 18 footer. Has a 3.0 mercruiser. I had a 15x 15 pitch prop. I switched to a stainless steel 14x19 AND changed my plugs. My old plugs were trashed. The new plugs gap are smaller then the old ones but they start the boat great. Ok here's the problem I am confused about. My old prop got my boats rpm to top out at 4500 rpm and had a top speed of 33 mph...my new prop gets my rpms to a lower 3800 but the boats speed jumps to 37.. I am confused. I want to get the boats rpms up to the same high range to get maximum speed but is it because of the change in prop or maybe the new plugs?

Nothing at all confusing here...your old prop was not working well....and you new prop has good bite...a few things...one verify your tach is on and can you use gps to determine speed

37 mph @ 3800 with a 19 3blade ss???
18 gets 4000
17 gets 4200
16 gets 4400
15 gets 4600
14 gets 4800..........

Thats just a rule of thumb....But you combination should turn a 17-18 p blade...So somethings not right.....

Correct tach readings
True GPS speed
Check your gearing and post back
How many people are with you...

 

[Chaparralboater]

Re: New prop put on and now....

Well before I changed plugs and prop the boats rpms would hit around 4600 then tach out..I put in new plugs and changed to a steeper prop and now I only get to 3800 rpms...would it have anything to do with my plugs? The old plugs gap is father apart then the new plugs gap..should I gap the new plugs to the old ones even tho the boat starts and runs great without the gap?

 

[NoKlu]

Re: New prop put on and now....

What rpm do you get with the old prop and new plugs?

 

[Chaparralboater]

Re: New prop put on and now....

Boat stopes at 4500 and then sometimes would seem like it would miss or bog

 

[steelespike]

Re: New prop put on and now....

Guys;
He's got an 18 ft.with a 3.0sx or lx anyway the hotter one. With a 15 X 15 prop he's getting 4500 rpm and 33 mph A 15 X 15 should be wound so tight it would be sweating.
It would run easily in the low to mid 40s on a 21 -23" pitch.Unless the
Chaparral is a real tub ad then I feel it would still press 40+.
He has the same basic post farther down maybe he thinks I'm kidding him.

 

[Chaparralboater]

Re: New prop put on and now....

No I know your not kidding. I posted this thread in hopes of finding the right prop. Now that I know the 15 is doing the job but really isn't what I Gould have on there I now know I have an engine problem. I started the other thread to find out if anyone knows what's going on w my engine. I have had this boat less than a week. My first boat period. Sorry for being confused and confusing others. Just stick with me guys.

 

[steelespike]

Re: New prop put on and now....

No I know your not kidding. I posted this thread in hopes of finding the right prop. Now that I know the 15 is doing the job but really isn't what I Gould have on there I now know I have an engine problem. I started the other thread to find out if anyone knows what's going on w my engine. I have had this boat less than a week. My first boat period. Sorry for being confused and confusing others. Just stick with me guys.

I think you mention you were pretty good with cars.This is basically a car motor.
Under the circumstances I would start with a comporession check. Keep track
of the plug locations and their condition.You probably know that adding a couple of squirts of oil will tell you if it's rings or valves.

 

[mikecjn]

Re: New prop put on and now....

Maybe I am off the wall here but I would put the original prop back on and see if the rpm's are the same as they were before you changed the plugs.
Once you figure that out you can figure out if you need a different pitch or of the plugs need regapped.
Just take it one step at a time.