3 blade prop vs 4

[charlie_kula]

I run a 4 blade SS prop today on a 300 hp Evinrude 88. It is a 21" Mercury Revolution 4. I hit 5500-5600 rpm at WOT making approx 60 mph. <br /><br />What pitch will I be able to pull if switching to an OMC Raker prop? I have heard 4 blade props lowers rpm compared to a same pitch 3 blade prop. True or false?<br />Will I get more vibrations?<br />Higher top end speed?<br /><br />Greatful for help!

 

[walleyehed]

Re: 3 blade prop vs 4

The particular set-up you have will determine what recommendation would be best.<br />High-horsepower, high-speed rigs typically benifit from 4-blade, larger diameter props due to blade surfacing.<br />Tell us more about your set-up...boat type, jackplate, etc.

 

[charlie_kula]

Re: 3 blade prop vs 4

It is a 24 foot lean race hull type of day cruiser. Weights about 2300 lbs.<br /><br />The engine is 20" sitting on a 20" transom. It is elevetad about 4-5".<br /><br />Its a real smooth rider especielly in tough sea and higher speeds.

 

[walleyehed]

Re: 3 blade prop vs 4

The revolution is going to be a tuff prop to beat for your application.<br />I don't feel you would gain anything noticable speed-wise with a Raker, and I think you would loose some hole-shot. You would also loose some holding power in rough water.<br />Calculations show top-speed at 5600 to be around 57MPH with 21" pitch and near perfect efficiency.<br />BUT, the revolution pulls the same as most other SS props of 1" MORE pitch.<br />If you have a prop-shop that will allow you to try other props, go ahead and try something else, but from the numbers you give, I don't think you will find a better combo than what you have now.<br />Can you raise the engine any farther??? are you on a jackplate??

 

[charlie_kula]

Re: 3 blade prop vs 4

Thanks Walleyhead,<br /><br />I think I stick with the revolution then.<br /><br />My engine is bolted and there is no jack-plate.<br /><br />Actually the prop have tendency to ventilate, using to great trim angles, so I think the height is near limit.

 

[walleyehed]

Re: 3 blade prop vs 4

the less you have to trim to get the required results, the better off you will be. If you can trim up quite a bit without cavitating, the eng. could stand to come up another hole.

 

[charlie_kula]

Re: 3 blade prop vs 4

Actually the boat does not benefit from trimming as it is set up now. <br /><br />It runs best having the engine pretty straight. Especially when trying WOT you have to be careful with the trim to avoid wobble.<br /><br />When the sea is choppy it is better to trim in a little to press the nose down.

 

[walleyehed]

Re: 3 blade prop vs 4

By "wobble" do you mean the side to side motion, known as "chine-walking"????

 

[charlie_kula]

Re: 3 blade prop vs 4

Something like that. <br /><br />Using to much trim on this one, the boat rides on a minimum of hull. With the full power engine, even moderate waves will kick it around. <br /><br />The feeling is somewhat like the grease in a fry pan.<br /><br />I guess it is possible to gain some knots, but it feels like the ride can get a very abrupt ending. Perhaps if you have split second reaction it could work. But thats nothing for me.

 

[walleyehed]

Re: 3 blade prop vs 4

I'm going to see if I can hook you up with a guy here that may be able to change that ride charactoristic into "smooth as silk". I'll see if I can line him up on here tomarrow.<br />Hold what ya got, we'll talk tomarrow!<br />K