3blade or 4blade prop selection question

[chronicblazed]

I own a 18' bass boat w/150 hp outboard that I use manly for recreation (tubing, water-skiing....). I'm running a 23 pitch 3 blade and wanted to know if it would be worth switching to a 4 blade of the same pitch. I've read that a 4 blade will give you a better hole shot while maintaining the top end. Just how much better hole shot is what I'm looking for. Anybody with some XP on this topic, your input would be much appreciated. I would hate to drop $300 on a prop and there be hardly any difference.

 

[moderator1]

Re: 3blade or 4blade prop selection question

It will only maintain top-end if you raise the engine with the "proper" 4-blade.<br />Making a blanket statement that a 4-bld will increase hole-shot and solve other issues is a shot in the dark..now if set up properly, you may be able to keep the speed and you should see a slight improvement in hole-shot, but to suggest a pitch or type of prop is going to require some testing, and may require more than 1 prop.<br />What manufacture of 3-bld are you running now, and what is the diameter? What is your WOT RPM now???

 

[chronicblazed]

Re: 3blade or 4blade prop selection question

I'm taking the boat out sunday. I'll reply back with my RPM @ WOT and prop manufacture

 

[chronicblazed]

Re: 3blade or 4blade prop selection question

I don't know who the manufacture of the prop is, i looked all over it and didn't see any name. Its a 3 blade stainless steal prop with a 23 degree pitch. I took it out and was running 4500 rpm @ WOT. Is there a good rule of thumb for how many rpm's you will gain or loss when going up or down in pitch?

 

[LubeDude]

Re: 3blade or 4blade prop selection question

Originally posted by asher noddin:<br /> I took it out and was running 4500 rpm @ WOT.

That just aint right, with a 23P you should be over 5,000 RPM easily. Was that at full trim? You should be running close to 5,500 RPM with that engine at WOT.

 

[walleyehed]

Re: 3blade or 4blade prop selection question

what position (hole) is the engine mounted with.....top, 2nd, 3rd, or bottom hole???<br />Or, do you have a jackplate??<br />4500RPM will make that engine very short-lived. As LD said, we need to see 5500 minumum, but we need to know more about how it's set up.

 

[Navigator]

Re: 3blade or 4blade prop selection question

Asher,<br />Also keep in mind that going from a three blad to a 4 blade typically requires you to drop an inch or so in pitch to compensate for the additional drag caused by the extra blade. A 4 blade is not as efficient as a three blade and it takes more power to swing it. <br />As the rest of the guys are saying, you need to get that engining running in the correct WOT range first. It sounds like you are pitched too high. Maybe you want to switch to a 21" pitch. By doing so, you'll increase your Hole shot, plus you'll probably see a good increase in TOP end since you'll be unleashing another 20% of available RPM.<br />Typically, all other factors asside, a 1 inch drop in drop in pitch will give a 200-400 increase in RPM.<br /><br />You should be trimmed so that the cavitation place is parallel with the bottom on the hull. Any deeper and you're adding unnecessary drag.<br /><br />If all looks fine, another option you can consider is switching to a 3 blade prop that "vents" exhaust gas in front of the blade. Upon takeoff, the exhaust bubbles out in around the blade which allows large quantities of slippage. This lets the engine RPM climb faster. Once the RPM gets to a certain level, the exhaust no longer exits in front of the blade, the blade digs in and the boat takes off. This all happens in less than a second but the end result is a dramatic increase in hole shot. Kind of like smoking the tires and then they finally take hold and rocket you forward. <br /><br />Anyways, your first step should be to get your engine running in the correct RPM range. You're runnig too low and you're going to eventually damage the engine.<br /><br />Nav

 

[walleyehed]

Re: 3blade or 4blade prop selection question

Whatever the RPM is, we need to get the engine mounted as high as we can without loosing water pressure or ventilating badly. The AV plate may end-up 1-2" above the boats bottom, they all run differently. we still need some info regarding how the boat is set up before recommending a prop, because we may be able to gain 300-400 RPM with the prop you have. when we reach the max there, we can look at other options.

 

[ttp]

Re: 3blade or 4blade prop selection question

Another question? The prop you are using, is it progressive pitch or constant pitch? Also, is it cupped? I agree with Navigator,reduce pitch 1 or 2 inches (21 or 19)and you should increase your rpm to the correct range and very likely keep your speed but increase your hole shot. All considering that your motor is running correctly.

 

[Dhadley]

Re: 3blade or 4blade prop selection question

With todays 4 blade props we dont change pitch from a 3 blade. The set up is the difference. Thats why its not unusual for a 4 blade to be as, or more, efficient than a 3 blade. <br /><br />Just a couple of other things --<br /><br />You wont get a very good 4 blade ss prop for $300 unless its used. With good props, you get what you pay for.<br /><br />Pitch is measured in inches. There are things measured in degrees on a prop like rake. It can be confusing to refer to pitch in degrees.