3 vs 4 blade props

[scarab30]

I'm sure this question has been answered in the past, but I cannot find it so forgive me for asking something so basic. Is there a general rule of thumb (such as for changing pitch) for what the WOT rpm change will be when going from a 3 to 4 blade prop of the same pitch?

 

[ZmOz]

Re: 3 vs 4 blade props

http://www.iboats.com/cgi-bin/ubb/ultimatebb.cgi?ubb=get_topic;f=28;t=025695

 

[scarab30]

Re: 3 vs 4 blade props

Thanks for the link, but it did not answer my question. There is a rule of thumb that you usually change WOT rpm by 200-400 rpm for each inch in pitch you increase or decrease. All things being equal, if I go from a 3 to a 4 blade prop of equal pitch, in general, how much would you expect the WOT rpm to change?

 

[LubeDude]

Re: 3 vs 4 blade props

You will lose about the same, So if you are turning 6,000 RPM with a three blade, you would figure about 5,800 or so with a four blade of equal diameter., maybe less, maybe more.

 

[walleyehed]

Re: 3 vs 4 blade props

EDIT: I'm sorry, I posted a reply to a concern on another post, so I removed it.<br />LubeDude has given the general rule here, and the only place I see different is with specialty props-custom work, such as hydromotive, Mazco, and some of the OEM's props are flowing away from the typical "rules of thumb" we are used to.

 

[LubeDude]

Re: 3 vs 4 blade props

Originally posted by walleyehed:<br />Specialty props-custom work, such as hydromotive, Mazco, and some of the OEM's props are flowing away from the typical "rules of thumb" we are used to.

Very true!!

 

[markvan]

Re: 3 vs 4 blade props

I recently switched from a 3 blade to a 4 blade, my mechanic told me that if my WOT was good with the 3 blade I should decrease the pitch by 1 inch and the WOT would be the same on the 4 blade.........he was right, so with that said if you kept the same pitch you would lose 200-400 rpm.

 

[Hunky Dory]

Re: 3 vs 4 blade props

Warrior,<br /><br />What type of boat?<br /><br />What other effects did you observe, i.e. top speed, responsiveness, time to plane, steering???

 

[walleyehed]

Re: 3 vs 4 blade props

Walleye Warrior, that info is true in very few instances.....Blade design, material, even brand of prop adds many variables, so many infact, there is no rule of thumb with the exception of staying with the same pitch and testing.<br />When 4-blade props first started showing up as an alternative, the 1" rule "was" the guideline. With so many manufacturers building different versions of 4-blades now, they are at least attempting to make the prop perform in the same RPM range, as the same pitch 3-blade.<br />With the addition of jackplates, set-back, hull styles, etc, the pitch number is becoming less consistant over-all.<br />Testing is the only way to determine the correct and best performing prop for YOUR application.<br />If your boat has an outboard, I could quite possibly take the same prop you have and with a few minor adjustments to set-up, GAIN 300-400 RPM.<br />Simply changing props without regard to how a particular prop is intended to run, is like putting snow-tires on your car in the summer.<br />Lotta variables to consider here, and a lotta different props as well.....

 

[markvan]

Re: 3 vs 4 blade props

I switched from a 3 blade quicksilver to a 4 blade quicksiler, overall prop style is very similar bewteen these two props, so in this case the 1" rule was applied and worked. I never said it was black and white, I was simply stating what my mechanic said and the fact that in my case it worked. With the number of props out there today, it a least gives you somewhere to start.

 

[markvan]

Re: 3 vs 4 blade props

Hunky Dory - my boat is a 21' starcraft - deep v, very heavy, with 3 live wells, 2 bait coolers, and a 40 gal gas tank. The biggest changed I noticed was the time to plan with a full load was cut way back, handles and and steers marginally better....it handles great before so hard to notice a slight improvement. As far as speed goes I lost about 2 mpg not a big deal for me as it was 55mpg before now it 53mpg my biggest concern was getting the heavy beast to plan when I was fully loaded. The lake I normally go to it rather shallow until you get out a ways and when they built the marina they stopped dredging about a 1/4 mile too soon so if you cant plan before you get to the shallow part your only choice is to idle out into the deeper water which takes about 10 -15 min or so.

 

[walleyehed]

Re: 3 vs 4 blade props

W.W., is your engine mounted directly "on" the transom or is it above???<br />A few simple adjustments, and we may get that 2MPH back, equating to better fuel economy.<br />As for the 1" rule working, that's great! The Merc quicksilver is the only prop manufacturer I can think of that has the same blade geometry on the 4-blade in alum, as the 3-bld has.<br />Glad that worked. My point was most times, that isn't the case, and you don't "have" to give up speed for the added blade, if you make the proper adjustments. Just want to help you get the most from your rig.

 

[LubeDude]

Re: 3 vs 4 blade props

Walleyehed is "The Man" when It comes to props, The more info you can give him the faster he can help you. But the word is, "TEST, TEST, TEST.<br /><br />When we gonna change your name to "PropDude"?

 

[markvan]

Re: 3 vs 4 blade props

walleyehed I guess i got somewhat lucky with the quicksilver being the only one that keeps the same blade geometry. What are the odds that I lucked out and went with quicksilver...my buddy had one kicking around so i was able to get it cheap. My engine is not mounted on the transom there is about a 2-3 inch space between the bracket and the transom. I was considering a transom jack what do you think? I have always thought that a boat with a 225 should be faster than 55mph. It is a very heavy boat, my neighboor has a similar boat but 2 feet longer, same engine and he can only get 42mpg, so I guess in comparison i was happy with the 55mph.

 

[walleyehed]

Re: 3 vs 4 blade props

1-2" from the transom...I would say a jackplate would be an excellent addition I also have a Starcraft, although 3ft shorter, I have a 200+ on it, with 6 inches of setback and about 3 inches high, but these are 2 different boats-simular in design though.<br />Your 53-55 MPH is not bad at all with the loads you mention.<br />How far, apprx., can you trim from level before prop ventilates???<br />Does the boat want to porpoise at high trim angles????<br />W.W., there is one other prop of 4-blade design that kept it's blade geometry from the 3...the Renegade OMC "Bass edition" and the OMC Raker..In stainless of course.

 

[markvan]

Re: 3 vs 4 blade props

how much of a differance do you think I can expect form a jack plate, as far as the height goes its mounted about as high as it can go. I can trim maybe about 1/3 - 1/2 before it ventilates with my 4 blade and maybe 1/2 - 3/4 with my 3 blade (its 1/2 diameter larger) before it ventilates. Not sure what you mean by "does your boat want to porpoise at high trim angles"

 

[markvan]

Re: 3 vs 4 blade props

walleyehed - any insight as to my neighbor's boat...he does not have internet access at the lake so I told him I would ask a few questions on here for him. His boat is an 18 foot (i think before i said it was 2 feet longer than mine but my mistake) with a 1989 evinrude 225, the make of his boat is a 1981 Sunset---he told me that apparently they were bought out years ago by champion?? It is a heavy boat..if you are not familiar with that make i would say it is a cross between a bass boat style hull and a deep v....

 

[walleyehed]

Re: 3 vs 4 blade props

There are Soooo many variables to consider to make an educated recommendation.<br />The number one factor that seems to affect more boats, is the engine heigth, distance of set-back, and a prop designed to be used with the previous 2 variables in a particular situation. And, to top it off, all of this changes with hull styles.<br />The cross between hull style you list would most likely be considered "multi-species" type of boat.<br />I (we) would gladly give some advice if we know what the particulars are on his boat.<br />If it is an 18ft with a 225, and gets only 49MPH, he has some serious problems. My boat is very simular and 50 MPH is a nice mid-range cruise setting.<br />EDIT: I must add, all the above info is only realistic if the engine is running at optimum power (Good condition)

 

[markvan]

Re: 3 vs 4 blade props

he actually only gets 42mph and that was with two of us in it, about 30 gallons of gas, no water in the live wells, and neither one of us is over 200 lbs.......he said his machanic looked at the motor and said it is mechanically sound....i was looking on the boat ads on here today and unfortunately i was unable to find any boats made by the same manufacturer as his. i think i have a pic of his boat that i took for him when he was considering selling it. <br /><br />as far as mine goes what would you recommend for a jack plate, manual or hydraulic.........i have talked ot a few people at the lake regarding the height of my motor and the general opinion is that it probably cant go any heigher than it already is?

 

[markvan]

Re: 3 vs 4 blade props

walleyehed - i should add that he is currently using a 3 blade prop - 14.5" x 19 inch pitch he has never tried anything else.