3 to 4 blade, step down in prop diameter?

[henrye718]

I have a 17' power catamaran, the prop sometimes slips due to cavitation and ventilation. I need a 4 blade prop.

My current prop 3 blade 14x17 (size: Size 13 7/8 x 17) is good for the boat I have, WOT gives me 5250 RPM@29 MPH with only me onboard 200lbs. Engine: 2001 johnson, 60 HP, 2 stroke, 3 cyliders. RPM range in manual states 5000-6000. I use the boat for fishing crusing it is a fiberglass center console. Weights aprox 1200lbs with full gear NO passengers.

The prop company im interested in told me I should use their 13x18 4 blade prop. This doesnt seem right, Im no expert. The prop measures 13" exactly. Im dropping down 7/8" in prop size, gaining 1 pitch and gaining a extra blade looks like I will drop to 5000 rpms?. Does this seem right?

The other choice they gave me would be their 4 blade 14x18. this seems it would drop me right out of my good power range instantly.

Here is the same boat setup by another dealer with a similair sized engine.


http://www.budsoutboardmarine.com/uploads/60_performance_report.pdf


Thanks in advance for any input.

 

[steelespike]

Re: 3 to 4 blade, step down in prop diameter?

Are these boats sometimes refered to a shallow sports or something similar?
As far as I know it is suggested to drop an inch when going to a 4
I feel you need to get your rpms up more.5,500 or better.
While a 4 blade will surely improve your venting problem and low end performance I think dropping to a 16 will only maintain your rpm about the same.
You might check out the Turning Point Hustler in a 15" pitch usual results are an overall improvement including venting problems.They are said to deliver close to stainless performance at Al. prices.About $90.

 

[junior1113]

Re: 3 to 4 blade, step down in prop diameter?

agree 15' 4 blade. check for optimal engine hieght if no jack. should hit 5600+. that horse power is rated at 5500 so your running limp right now. you will be happy with the low end power. and should pick up some mphs. p.s your not looking to decrease diameter looking to decrease pitch(how far a prop travels in 1 revolution) if anything up in diameter 1/4-1/2".

 

[rs1lollie]

Re: 3 to 4 blade, step down in prop diameter?

problem might be that your motor sits to high or to low. (Transsom to keel)

 

[henrye718]

Re: 3 to 4 blade, step down in prop diameter?

problem might be that your motor sits to high or to low. (Transsom to keel)

I read about that the hole thing I think mine is set right. It doesnt happen that often so its not a huge deal, the cavitation that is. My current prop has seen better days so it will become a backup just want the "right" one this time around.