4 blade prop selection...

[Boomyal]

I had been running a well matched 3 blade prop for my 302 OMC. It is a cupped 15 x 17. I've been wanting to go to a 4 blade prop for all of it's low end playing attributes. I usually go out and play, with a full boat load, using tubes, kneeboards etc. <br /><br />Are there any opinions on what pitch I should choose for a 15 inch 4 blader?<br /><br />This might be a little premature. When my boat goes back in the water, it will have Smart Tabs and a more powerful stroked motor, but there are some really well priced 4 blade aluminum Quicksilver props on Ebay and I'd hate to let them pass.

 

[walleyehed]

Re: 4 blade prop selection...

Your 4-blades will generally be smaller in diameter by a little-nature of the beast.<br />Not knowing what the new engine will do, I think I would try the same prop you have first, set your base-line and go from there.<br />There's some good props out there, many of them...I do have experience with the 391200 OMC (15X17), and I know it compares quite well to a Renagade SS 4-blade 14-1/4X17 Offshore version, but you may not be wanting an SS prop.<br />Propco makes a real good alum 4-blade called the Quad-R-Jet in a 14X17, and would perform very simular to the 15X17 3-blade.

 

[WillyBWright]

Re: 4 blade prop selection...

If the price is right, get a 16 and an 18 Quicksilver. Good props for that boat! If you have to choose only one, go with the 16. You'll lose speed but keep your holeshot. With the 18 you'd keep the speed but lose holeshot. In either case, the differences won't be much. Those props produce stern lift which is good for sterndrives.

 

[reference123]

Re: 4 blade prop selection...

On our OMC 225 with the OMC 800 outdrive we were running a 15 x 17 3 blade and found the 4x13.7x18 was the right fit in a 4 blade.<br /><br />Smart tabs had a much bigger impact on hole shot and minimum planing speed than the 4 blade prop did though. In fact, after testing the boat with both the 4 and 3 blade prop after putting on smart tabs I could not tell much difference.

 

[islandboat]

Re: 4 blade prop selection...

If you're going back with a stronger motor I would not reduce the pitch. The diameter is fairly standard for the outdrive that you are using but the pitch can really hurt your top speed. If any change at all, I would go with more pitch.

 

[crazy charlie]

Re: 4 blade prop selection...

Boomyal,I got 4 blade props this season from the guy on ebay.If you want info on them and want to deal with him direct like I did,his name is Mike and you can get him at diversifiedmarine.com Charlie

 

[Boomyal]

Re: 4 blade prop selection...

Originally posted by trophy175:<br /> If you're going back with a stronger motor I would not reduce the pitch. The diameter is fairly standard for the outdrive that you are using but the pitch can really hurt your top speed. If any change at all, I would go with more pitch.

It is an interesting conjecture. Every thing else being equal, when you increase power you should increase pitch (staying with a 3 blade) as the motor now has more power to drive a steeper bite. <br /><br />However, again everything being equal, when you go to a 4 blade (same diameter as the 3 blade) with the same power, you would decrease the pitch because now there is more blade area that needs to be driven.<br /><br /><br />As you say Trophy, it would seem that if you are both increasing power and going to a 4 blade of the same diameter, then maybe you should retain the same or increase the pitch as the previously well matched 3 blade had??<br /><br />Whew! Does that all make sense?<br /><br />Thanks for the info CC.

 

[reference123]

Re: 4 blade prop selection...

Boomyall,<br /><br />We actually tried a 16 pitch 4 blade and found it did not put us in the correct RPM WOT range where an 18 pitch did. So it may not be true that lower pitch versus the 3 blade is correct.

 

[Boomyal]

Re: 4 blade prop selection...

Originally posted by reference:<br /> Boomyall,<br /><br />We actually tried a 16 pitch 4 blade and found it did not put us in the correct RPM WOT range where an 18 pitch did. So it may not be true that lower pitch versus the 3 blade is correct.

Hey! I may be confused. The higher the pitch number, the lower the gear, right? Whatever it is, everything being equal, wouldn't you want a slightly lower gear when going to a 4 blade prop of the same diameter. If this is true and I did not say it before, I just mixed up pitch numbers vs effective gearing. <br /><br />Did you go with a 15 x 18? and do they cup 4 blade props?<br /><br />and BTB, reference, how did your outdrive repair go? What was the problem and remedy. Did Sullivan do ya right?

 

[walleyehed]

Re: 4 blade prop selection...

Remember, Not ALL props are manufactured the same...as in pitch, which can and does vary greatly, as well as blade area. You can get a 4-blade with large blade area, or smaller...just depends on the application...Larger blade area equates to better "push", but usually less RPM.<br />For the most part, 4-blades have "less" blade area (Per blade!) than the 3-blade they are intended to replace. This in turn, provides "some" of that RPM back.<br />The progression of pitch on a 4 blade is also different-you can't just "add" a fourth blade and expect simular performance. Of course this is taken into account in the manufacturing/testing process, and yet 2 props will never perform the same just because they are a certain pitch, and have the same # of blades.<br />In your application, I would expect increased hole-shot, better mid-range holding power, but loss of top-end, if we used the same pitch in 4-blade under the same conditions you started with...BUT, that isn't the case anymore...<br />Rate your boat (to yourself) as to whether you consider it heavy, but lots a power...Heavy with mid class power, etc...<br />The heavier the boat with less than max rec. power is a perfect example of a large diameter application. The "heavy" with max power would be a good example of dropping diameter. The faster "THIS" type of boat is, the less diameter you want to look at in sterndrive. There's alot of over-kill out there on prop diameters for general use boats of this type.<br />A 70MPH bassboat will break all the rules I just laid down....so will an 18MPH boat with that being the max designed speed.<br />I give you in my first post, the closest prop in numbers to your 3-blade 15X17, that being an SS Renagade in 14-1/4X17.<br />If your new engine is going to put out say 30-40 more HP, consider a 19 pitch, but as you already know, we can guess at this for months, or, test a couple of props, get a base-line for each, and I will assure you, we can better pin-point what you need.<br />Good luck and I hope I didn't confuse you even more!

 

[reference123]

Re: 4 blade prop selection...

Boomyall,<br /><br />So far not good news on the drive repair. In fact very little news. He broke his shoulder the week after I dropped it off so he has been somewhat out of commission. The season is over here anyway so I have not been in too much of a hurry. <br /><br />Once I know though I will post a follow up.<br /><br />The 4 blade we run is a 14 x 18. I dont know if it is cupped or not as I dont really understand that.

 

[islandboat]

Re: 4 blade prop selection...

I changed from a 3 blade prop to a 4 blade to decrease cavitation in heavy seas. The theory was that a 4 blade may have 2 blades in the water most of the time, versus the 3 blade that cannot get 2 blades to grip at the same time (if trimmed properly and at speed). I reduced pitch when changing from the 3 blade 19 pitch prop to a 18 pitch 4 blade. That reduces the amount of forward travel by 1 inch for each revolution. I had hoped to reduce prop slipedge and therfore retain the same forward speed. That did not work. I lost a lot of speed, both midrange and top speed, when turning the same cruising rpm. Being correctly proped with the 3 blade prop, I found little improvement in jumping out of the hole. I should have at least retained the same pitch when changing props. Cupping a prop and double cupping a prop effectively adds a little "pitch" and gives better stern lift.

 

[walleyehed]

Re: 4 blade prop selection...

I lost a lot of speed, both midrange and top speed, when turning the same cruising rpm.

This is usually a sign of engine heigth being too low...If you are talking about a sterndrive, this is the typical result.