Low WOT and MPH

[kbetts]

I have a 2000 21 foot bowrider with the 5.0 EFI Mercruiser and the Alpha outdrive. I currently have a 14.5 19 pitch stainless 4 blade prop on it now. With a light load and calm water, I am only able to get 4000 rpm and 41 MPH at WOT. I believe I should be around 4600 - 4800 for my WOT. If I go to a 3 blade aluminum, should this put me in my correct WOT, or should I look into going down in pitch also? Thanks,

 

[Crownie2]

Re: Low WOT and MPH

I have a 2000 21 foot bowrider with the 5.0 EFI Mercruiser and the Alpha outdrive. I currently have a 14.5 19 pitch stainless 4 blade prop on it now. With a light load and calm water, I am only able to get 4000 rpm and 41 MPH at WOT. I believe I should be around 4600 - 4800 for my WOT. If I go to a 3 blade aluminum, should this put me in my correct WOT, or should I look into going down in pitch also? Thanks,

It is really going to help to know the drive ratio. It can be found on the port side of the drive a few inches down from the top.

You are corrrect - your WOT RPM should be in the 4,600 to 4,800 range. To be positive, it should be stated on the top of the flame arrestor.

Your top speed should be "about" 50 MPH - that is a questimate.

Bob

 

[kbetts]

Re: Low WOT and MPH

Bob, I believe the drive ratio for my V-8 is 1.65:1. I thought I should be around the 50 mph mark. My spedo and the GPS are very close to the same reading on MPH, about 1mph difference. I am not overly concerned on the top mph, but would like to get in the proper WOT range.

 

[kbetts]

Re: Low WOT and MPH

I looked at the drive and it's 1.62:1 ratio.

 

[Crownie2]

Re: Low WOT and MPH

I looked at the drive and it's 1.62:1 ratio.

Hmm - according to Merc your ideal prop should be about 19-20 inches, which is what you have now.

Maybe there is something else slowing you down like weight higher than normal or a dirty hull/lowerunit or not trimming properly.

If none of these seem to be the problem, then you could try a slightly lower pitch or a good three blade.

 

[kbetts]

Re: Low WOT and MPH

Crownie, (I like your name)
The boat is a 2000 Crownline 205 BR and is spotless. The hull is clean and the drive is in perfect working condition with 135 hours on the boat and motor. I trim the drive up and the RPM's do go up, but only to 4000 RPM. The only time the RPM go up higher than that is when I trim it up to much and the prop starts to blow out. I have tried this on many occasions with only a light load in the boat ( two adults) and the motor sounds and runs strong. I was thinking of going to aluminum and going with a 17 pitch 3 blade. I am an seasoned boater, so I believe I am trimming it proper for max. RPM and speed.

 

[Crownie2]

Re: Low WOT and MPH

Nice to talk to another Crownie! I have a 202BR.

Well, the 4 blade SS is pulling the rpms down - probably by about 100 or a little more. So a 19" three blade aluminum is only going to raise it by that amount.

I guess my suggestion is to try to borrow a 17" 3 blade aluminum (or find a used one) and see what it does.

Depending on your use, a three blade SS 17" or 18" may be just the ticket, though. I just installed a 21" Merc Enertia and it is a fantastic prop.

Keep in mind the 205 may never see 50...

Bob