3-Blade VS 4 Blade?

[BBogey]

Guys, if any of you recall, I've been struggling with my boat coming out of the hole. It was suggested I was over-propped. I put my back up prop (4-blade) on which was a 13 pitch as opposed to a 15 pitch (3-blade) and WOW, problem fixed...IF I wasn't over revving. I was told to get a tach, which my son got me for X-Mas. I'm having some problems hooking it up, but then that's another story!<br /><br />I have a aluminum duck boat, 17', 60HP 4-stroke MERC, and I often times carry a lot of gear. Is pitch and RPM's more important, or does 3-blade VS 4-blade really come into play here???<br /><br />THANKS in advance!!!

 

[DJ]

Re: 3-Blade VS 4 Blade?

The pitch is the most important. It determines what rpm range your engine will operate in. You do not want to lug the engine. Not as important with four stroke as two-however still should stay within what the spec's. say.<br />The four blade will help you carry the load better. Generally, a four blade requires you to drop the pitch about two inches from a three blade.

 

[ccollins]

Re: 3-Blade VS 4 Blade?

What is the diameter of the 15 pitch prop? Out of curosity did you see my reply to your question on my post on Dec 18th "4 stroke 60 hp mercury"? I have tested both the 2 stroke and 4 stroke Mercury on this boat with the small gear ratio in the Mercury foot it will not turn a big prop and stay within the rpm range. We tried aluminum and stainless steel props and finally went with a 10 1/4 X 14 stelleto prop on the 4 stroke. It would turn a 10 1/4 x 15 but drop our rpms (5600) without increasing speed. I have also worked on another boat with the same problem you are having and the torque tab (not sure if correct terminology, the upside down fin below the cavitation plate) on the foot had been adjusted to try and relieve the torque on the stick steering. It had been turned so much it was creating a hole in the water so the prop couldn't get enough bite to push the boat up on plane. FYI: "Since my post on Dec 18th" There is an adjustment screw on the right side of the motor that we had to adjust to get more rpm's. Went to our dealer and out of the 5 four strokes he had on hand all were set different. We called Mercury and found that we needed to adjust this screw so that with WOT the linkage was allowed full travel. Mercury stated that motors are tested but some adjustments are left to dealers. Our dealer was not aware of this. Gave us another 300 rpm's on our boat. <br />Hope some of this helps.<br />

 

[BBogey]

Re: 3-Blade VS 4 Blade?

WOW! Sorry, I did not read some of this I guess? You actually got someone at Mercury to talk to you? I'm such a motor dork (in fact had to read your reply several times... ) but I will note what you said and talk to someone about it. I "think" I've found a dealer who will provide service when needed, unfortunately he's about 90 miles from me!<br /><br />You asked about diameter...other than the stamped # on the prop, how do I know what the diameter would be?<br /><br />Thanks for your help!

 

[ccollins]

Re: 3-Blade VS 4 Blade?

I'm sorry if my last post was confusing. I have a hard time putting my thoughts on paper. The first number is the diameter and the second is the pitch. I do know some props don't have both stamped on them. Diameter is basically the width of the circle the rotating blades make. <br /><br />We have called Mercury twice on seperate items and they have been very helpful.<br /><br />Here is a website that give some info to the different measurements for props. http://www.boatfix.com/how/props.html <br /><br />My 50 Johnson has bigger gears in the foot so I'm able to run a 12 1/2 X 15 prop while still staying in the proper rpm range. <br /><br />I'm not a motor guru by no means this is just from the experience we received from purchasing the two boats we bought coupled with a lot of reading on this forum and the internet. We are lucky that we have known our dealer on a personal level for several years. We were able to do a lot of testing with different props, transom hole settings, and also different motors to get the best performance for our boats. <br />Sorry for the long posts. Hope I have helped in some manner.