2005 Cobalt 200BR slow to plane

[Mben]

Hello Everyone,
I am new to all this forum stuff, so bear with me. I recently bought a 2005 Cobalt 200BR with the 5.0 Mercruiser. The boats dry weight is said to be 3900(ish). It currently has the High 5 stainless prop (48-815760-21) on it and it is very slow to plane. The motors WOT rpm label says between 4,600 & 5,000. While I couldn't get a long run at WOT, but the times I could hit it for any distance, it seemed to max at 3,600. I am under the impression I am over propped with the High 5 (21?). Does any one have any experience with this set up? Any help/advice is greatly appreciated.

 

[WesNewell]

Try this.
https://www.mercurymarine.com/en/us/propellers/selector/#!/step-one

 

[QBhoy]

If that set up is running a 1.47 gear ratio I can tell you that it won’t run a 21” well most likely. Apart from that the Hi 5 is a hard prop to compare to others for a base point. They can have funny characteristics on some boats.
but if you have a 1.62 ratio...then something isn’t right at all. I suspect you might with a heavy boat like this.

 

[Mben]

If that set up is running a 1.47 gear ratio I can tell you that it won’t run a 21” well most likely. Apart from that the Hi 5 is a hard prop to compare to others for a base point. They can have funny characteristics on some boats.
but if you have a 1.62 ratio...then something isn’t right at all. I suspect you might with a heavy boat like this.

Im not 100% sure what the ratio is, but from what I have been reading, it sounds like mine is the 1.62. Using Merc's prop selector that WesNewell provided, it seems that I need to be in the 17" area. I have been reading that you gain/lose approx 200rpm per 1" of prop change. That would make since if accurate, if I am at 21" and running 3,600 rpm, then moving down 4" to a 17" would put me at 4400- 4500 rpm which is much closer to the motors label at 4,600-5,000 rpm WOT. Does my reasoning or logic sound accurate on that?

When you say "then something isn't right at all", are you referring to prop size or something wrong with the motor?

Thank you for your knowledge and help!

 

[QBhoy]

You need to know your gear ratio before anything else really. Otherwise you’ll be throwing money away.

 

[WesNewell]

The gear ratio is on the drive somewhere. Or it should be.

 

[Mben]

The gear ratio is on the drive somewhere. Or it should be.

I found it on the sticker, it's the 1.47. Back to the drawing board (or merc site) to figure more out.

 

[Mben]

Is it accurate to assume you can gain/lose 200rpm for every 1" of pitch change? Thats what I have read online anyway. Merc's prop selector is saying that with my setup (2005 Cobalt 200 BR 5.0 Alpha One, 1.47 gear ratio, High elevation {4400ft}, 3910 dry weight,) it is recommending a 14", 15" or 16". If the 200rpm per inch gain/loss is accurate, I wouldn't think I would want to drop down to 14", because that would put me over the WOT 4,600-5,000 rpm range. Or is high elevation playing a key roll as well? I feel like the 15" would be a happy medium for my boat over the 21" that is on it now. Any thoughts or recommendations?

 

[WesNewell]

High elevation causes your engine to lose HP. I'd go for a 16P 3 blade ss myself.

 

[QBhoy]

Try a 17” out. I’d say you’ll be good with that as a starter. No way it will manage a 21”. My boat has the same ratio and 18ft with a stepped hull and pad. I’ve about 300hp at least and it will run a 19” well all round to the limiter. It will just manage a big diameter 21” only just touching limiter once in a blue moon at just over 5000 rpm. But that will be about mid to late 60’s and it’s a serious hull. No way your 260hp will ever turn a 21” anywhere near the range. It will struggle with a 19” too. Good luck.