3 or 4 blade ros 24

dave54000

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I have a 24 ros with a flotation bracket with 2 little yamaha f50's I have 3 blade wheels and wondering if 4 blade would increase mid range and performance in rough seas.This boat is no speed machine basically running it at semi displacement it runs 14 kts at 4500 rpm and turns up to 5800 wide open so I have correct 3 blades
 

captainlarry84

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3 Vs 4

I am a big fan of four bladed wheels. At mid range they carry load well & in most but not all cases give a higher speed at cruising speeds. In you case with such small wheels and a semi displacement hull it would be a toss up.
If you do not need new wheels stick with what you have.
If you do then you may want to consider the four bladed wheels.

If you gave me the following we could figure out your efficiency which may help you make a decision.

Current wheels, both pitch & diameter.
Reduction gear
Exact speed at your cruising RPMs of 4500
 

dave54000

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and it is a planing hull tops out 19 kts most of there boats had 220 hp outdrives that went over 35mph 8.6 beam roughly 5000lbs the bracket is built into hull so water line is more like 27 ft thanks
 

captainlarry84

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I need the reduction gear of thje outboard motor so we know how fast the prop is spinning @ 4500 RPMs. with that we can go to work.
 

captainlarry84

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3 vs 4

OK.my math is the following;

You are running 12.5 X 9 propellers, @ 4500 RPMs and getting 14 knots with a reduction gear of 1.85.

Base on the above you are getting 78% propeller efficiency which is pretty good in a displacement hull.

100% efficiency which you would never get would be 18.03 knots @ 4500 RPMs

If you went to a four bladed propeller you would drop 1 of pitch to your new wheel in four blade would be 12.5 X 8. Let assume that with the extra blade at 4500 RPMs you pick up an additional 10% your speed would be:

If your boat made 88% efficiency with the four bladed propeller your speed at 4500 RPMs would be 14.10 knots.

If my magic you got 95% which is not happening it would be 15.22 knots.

In closing you will get nothing with the 4 bladed propellers

You may do better with SS wheels but that is also a crap shoot. You are where you should be.
 

dave54000

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Yes thats my boat i now its under powered i plan on putting a single 150 or 175 was just looking for a little tweaking i appreciate all your help i now every boat is a trade off its a amazing boat fishing boat can blackfish in six foot nasty chop and will do 14 kts in those conditions with more power i will only be able to use it in calm conditions veing that in my area there is a 3 to 4 chop most afternoons thanka again
 

dave54000

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what kind of performance to expect

I cruise 14 to 15 its at 4500 to 4700 rpm with 2 Yamaha 50's yes severely under powered what type of performance would you think with a single Yamaha 150 or Suzuki 175 running 4500 rpm guessing 8 gal hr just not sure on speed thanks
 

captainlarry84

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Hard call.... HP increase will help you out. Also get the weigh on the new power and comapare it to the old power. If you go that route go big and go to 175. I would also slick with a 3 bladed wheel based on those photos. Also S/S wheel a must. I would est you will pick up 3 knots.
 
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