Trying to get the right prop

[the6c]

Hi, I'm new to the thread. I have a 21' sea ray walkaround w/cc. It has a 5.0 200 Mercruiser engine. The boat had a 15 1/4 x 15 prop which got between 3800-4000 RPM. I changed to a 14 1/2 19 and am getting 3700 RPM. The boat is supposed to get between 4400-4800. Any recommendations on the correct size?

 

[Matt89GT]

I would check to gear ratio in that outdrive and see what ya got. Something doesnt sound quite right, a 15 pitch should make that outfit rev to the moon and your on the money with what the engine should turn, I'd shoot for 4600 to 4800 though, just sayin

 

[EricW]

I agree with Matt -- a 15" pitch prop would normally have your boat/engine combo overrevving at Wide Open Throttle.

A 19" pitch prop should be about right for your boat/engine. I guess you could try a 14.5 x 17" prop and see where that puts you. But, I think you may have something else going on.

Make sure the boat has a clean bottom, and is not overloaded/water saturated. Check your engine for possible problems. Maybe do a compression check on the engine. Also check for good spark, proper ignition timing, and no fuel restrictions or contamination.

 

[the6c]

Thanks to both of you for responding. I think the problem is water saturation as the stern sits lower than the drain holes which are plugged up and the bottom paint line is higher than it should be, (was wondering why). Looks like I may have a fall or early spring job on my hands to see if the foam under the flooring is soaked with water. Any thoughts on the best way to approach that would be appreciated.

 

[cdodds]

as far as i know all drives put behind little v8s were 1.5 ratio. look on the upper gearcase on the mercruiser sticker. there should be a ratio number like 1.84, 1.5, 1.36. if it is in fact a 1.5 ratio then a 15x15 should be too small (over rev) providing the motor isn't extremely tired or something wrong with it. that size boat with with a little 8 cyl should run about right with a 15x17 maybe even a 15x19 depending on bottom condition or amount of weight onboard.