Prop for 1987 Boston whaler 20 outrage

[madman2021]

I have a 1997 Johnson ocean runner 225 hp with a 21p 3 blade prop on my outrage 20 and My wot rpms are a little high. 6400 rpms at wot 51 mph. Range is 5000-6000 rpm wot. Motor is also to far down on the transom. Moving it up a few holes this weekend. Should I step up to a 23 pitch prop? Stick with a 3 blade or move to 4 or 5 blade?

 

[steelespike]

Re: Prop for 1987 Boston whaler 20 outrage

The prop calculator says that at 6400 with a 1.85 ratio 21" prop 10% slip you should be doing 61.
Using the speed and the other specs the actual rpm may be 5300.
Before making a prop change I would try to verify the tach.
Borrow a shop tach.
There may be a switch on the back of the tach.It may be set wrong, also you could operate the swith a few times to clean the contacts.
See what the results are after raising the motor before a prop change.

 

[madman2021]

Re: Prop for 1987 Boston whaler 20 outrage

Raised the motor and took the 21x 13 1/4 off and put a 21 x 13 3/4 prop on. running 6500 rpms now wot and only achieving 51 mph. Verified tach with a shops portable tach and it's 6500 rpms.

 

[steelespike]

Re: Prop for 1987 Boston whaler 20 outrage

You seldom use diameter to change rpm.Diameter is a secondary consideration,just following the pitch. Prop size is always written Diameter X pitch.
Typically a 1" pitch change will result in about 100 to 200 rpm change. Increasing pitch will reduce rpm.
Going to a 23" should reduce rpm 300 to 400 rpm. Common prop sizes are usually in odd numbers.
Even numbers have fewer selections and cost more.
Very basically usually increasing pitch will slow hole shot but could improve speed. You may note that usually as pitch goes up diameter goes down. A further increase in pitch could further slow hole shot.
Try a 23" pitch and see how the rpm does.It is usually suggested to stay with a brand and model to make the change more predictable.

 

[madman2021]

Re: Prop for 1987 Boston whaler 20 outrage

Made another run today and got 56 mph with the top down. Still 6500 rpms. I'm hoping for a top speed of at least 60 mph in this boat. Motor is still too far down. Going to buy a jack plate to raise it more.

 

[madman2021]

Re: Prop for 1987 Boston whaler 20 outrage

Motor also needs carbs rebuilt cause its sluggish out of the hole and mid range but only for a second or 2 then it runs like a rapped ape.