Evinrude E-NATION, for those dedicated to water, power, fishing and fun
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Captain
Posts: 161
Registered: ‎09-12-2016

Re: 3 Blade Prop Vs 4 Blade


@arzwrote:
18” or better start with 20”?

I would start with an 18".

Skipper
Posts: 5
Registered: ‎03-22-2018

Re: 3 Blade Prop Vs 4 Blade

My dubt is that i have to buy the prop... here in italy is about 800€... 1000$.... 20” is a medium size.. I sow on many forum that lot of people use 20, 22 and also 24”. Goal is 5600rpm wot with normal load?
Captain
Posts: 161
Registered: ‎09-12-2016

Re: 3 Blade Prop Vs 4 Blade

I would shoot for 5600 to 6000, the higher the better. All I can say is if you are overpropped (rpm too low) you risk engine damage and voiding your warranty. If you are underpropped at least there is no damage risk.

 

I would like to see these internet posts where 23 ft 5000 pd boats, are propped properly with 24 or 22 inch rx4s. If these are the same guys recommending a Raker, I would disregard.

Skipper
Posts: 5
Registered: ‎03-22-2018

Re: 3 Blade Prop Vs 4 Blade

evinrude spec says the right range is 5400/6000 rpm... I just spoke with a guy who have an G2 250 on his saver 750 wa (like mine..). His prop is rx4 15" 20". Result is 43 knot at 5700 rpm with 2 people and 150 liters of fuel. How does it worth 50 hp more?  

Captain
Posts: 161
Registered: ‎09-12-2016

Re: 3 Blade Prop Vs 4 Blade


@arz wrote:

evinrude spec says the right range is 5400/6000 rpm... I just spoke with a guy who have an G2 250 on his saver 750 wa (like mine..). His prop is rx4 15" 20". Result is 43 knot at 5700 rpm with 2 people and 150 liters of fuel. How does it worth 50 hp more?  


 

 

Hi,

 

The best way to prop a boat is to gather information about similar boats as you have done.

 

The weight of the motor, fuel and people is about 1200 lbs, you stated 1800 lbs. If you added another 600 lbs to that boat you would drop the rpms considerable, probably 53-5400 rpms.

 

I recommended you try for 5600- 6000 rpm because targeting the low end is not a good idea, it's a cushion in case you take on extra load temporarily. My boat is propped to 5800, my boat before was 6000. 

 

50 HP might give you 3-4 knts, marine propulsion is tough because of the drag, you might get 300 rpms.

 

The last thing is I don't believe the 5700 rpm quoted on that boat. A lot of people hit full throttle and trim there engine to 100% and note the rpms. This is not correct. You have to note the rpms at the trim position when the prop begins to ventilate, this will be the point at which trimming any further produces no gains in speed.I believe the WOT rpms is much less than 5700.  Here's why. I have a 3300 lbs boat ( with engine, gear, people, fuel), it's a 225HO which is about 250HP motor, same as that boat. With my 20 inch RX4, at 5800 rpm my speed is 58knts, almost 40% faster than that boat. I'm at about 60% trim at that speed. This means that your guys boat is undergoing EXTREME slippage, way more than that hull would produce, the only explanation I can think if is the motor was trimmed up way too far and is ventilating. It likely means that boat is overpropped, or not setup properly engine mounted too high perhaps.

 

You might get away with a 20, but I don't see the value in trying to hit the low end of the rpm range personally.