4 blade vs 3 blade

[gst]

Hi Guys,<br /><br />I have read a bunch of the posts here, so let me see if I've got this right.<br /><br />3 blade props get higher rpms and mph because they are lighter and only have to push 3 blades thru the water. 4 blade props give a quicker hole shot, don't blow out as much when trimmed out, and have better response at cruising speed. <br /><br />In other words for if I want to go fast a 3 blade is preferable, but for normal operation and better handling a 4 blade is prefered. <br /><br />I have a SeaPro 17 CC, with a 1999 Yamaha 90hp, used mainly for fishing in Mobile Bay and the surrounding rivers. Max rpm rating for that engine is I beleive 5500 rpms, I usually run 38mph at 4500-4900 rpms with a well worn 13 1/4 X 17 aluminum prop, I very seldom run WOT. No trim tabs or foil, ventilation plate about 1" above keel.<br /><br />I don't care a lick about going fast, I just want to get there safely. Mobile Bay can churn up pretty good sometimes, and when running in that chop the motor will sometimes ventilate. <br /><br />If I understand correctly, a 4 blade prop will give me a better bite and possibly not ventilate as much, correct?? And since I don't care to take a beating running fast in a heavy chop, speed is of no importance. <br /> <br />Therefore a 4 blade SS 13 1/4 X 18 is better than a Aluminum 3 blade 13 1/4 X 17 for my circumstances, right??? <br /><br />Thanks for any help <br /> Gary

 

[Silvertip]

Re: 4 blade vs 3 blade

You don't know whether either one is proper for your boat and engine setup since you haven't made any WOT runs to determine what RPM the engine actually turns at WOT. This is a critical measurement since it determines if the engine is currently spot on, under propped or over propped. At WOT throttle you want the engine RPM to be 5500 (in your case) or even a tad higher for optimum performance. If its less than that you are lugging the engine. Significantly more than that and its over reving. The four blade would likely help with ventilation but it may be the engine is mounted too high.

 

[Dhadley]

Re: 4 blade vs 3 blade

And you really can't say that a 4 blade is always slower than a 3 blade. I can show you some 4 blades that beat the socks off of 3 blade props on certian boats. And weight has nothing to do with it. <br /><br />When testing props you have to compare apples to apples. Each one will probably run at a different height even if you are comparing the same brand and blde type.<br /><br />And Smoke is right -- you actually do care about what rpm and speed your rig runs at wide open. Thats how you know the set up is right and efficient.

 

[gst]

Re: 4 blade vs 3 blade

Thanks Smokee and D,<br /><br />If the weather cooperates, I might try to get the boat out this weekend and put it on the river.<br />If I can I will push it to WOT and make note of the rpms. I know I have had it WOT before, but the only thing I checked was that it was not turning too many rpms. I will pay more attention this time. Thanks for your input.<br />Another reason I was wondering, is I was trying to convince myself that I needed to go with the 4 blade prop, you see I stumbled accross a new Power Tech 4 blade SS 13 1/4 X 18 for only $270, and that's hard to pass up. <br /><br />Thanks,<br /> Gary

 

[Dhadley]

Re: 4 blade vs 3 blade

A general rule of thumb -- if you dont have a jackplate you probably cant take advantage of the technology in a four blade prop.

 

[Silvertip]

Re: 4 blade vs 3 blade

Make note of speed also. A GPS would be helpful since pitot speedos are generally reliable indicators of speed. Speed and RPM are the benchmarks you need for testing.