4 Blade Prop?

[bkretchman]

Hello,

I'm looking to increase planning ability on my boat. The boat has a Merc 7.4 330HP with an Alpha One and weighs about 5000 lbs dry. Currently it has a steel 3 blade quicksilver mirage 17 pitch. My boat planes fine with one or two passengers at about 4200 - 4600 RPM trimmed with a top speed of about 30 mph. When 4 or more passengers are onboard (weight being in the stern) I have a lot more trouble planing and the engine tends to work a lot harder to make any considerable forward speed. I've spent a lot of time experimenting with the sterndrive trim and trim tabs but still doesn't make much of a difference.

I'm wondering if I should go with a 4 blade prop? Any feedback is appreciated.

-Brian

 

[steelespike]

Re: 4 Blade Prop?

4 blades do improve hole shot and the abilityto stay on plane.It is usually recommended to drop 1" in pitch when going to a 4 blade.The 4 blade will generally lose a little top end.Some props are designed with bow lift others with stern lift don't know which the quicksilver does.
Another way to go is to prop for the 4 passenger load whether 3 or 4 blade,and be careful of your rpm when running light.A 15" 4 blade might give
a big hole shot without driving the rpm too high.You should stick with the same brand and material to help make the improvment more predictable.
Someone may be along with more specific info.

 

[bkretchman]

Re: 4 Blade Prop?

Great... thanks for the info. I appreciate the feedback. So right now with a steel quicksilver 3 blade 17 p ... I should go with a steel quicksilver 4 blade 16 p prop. The current 3 blade has a 14 3/4 diameter..... how important is it to match the diameter of the 3 blade when I move to a 4 blade? For instance, if I could only find a 14 5/8 , 14 1/8, or a 14 ... does that make a difference?

 

[Tommyfmu]

Re: 4 Blade Prop?

I went through your exact scenario. I have a 5.0L Mercruiser with Alpha 1 Gen 2 , and had the factory spec'd 3 blade 15 x 17 aluminum Quicksilver prop. My boat is a Maxum cruiser 25" loa and is about 6000 lbs. with 4 adults and regular stuff on board.

I was looking to get more cruising speed at lower rpm's so bought and tried an aluminum 4 blade 15 x 16 Mercruiser; definitely got the desired effect of fast hole shot and increased cruising speed by about 5 mph at same rpm's.

Then my upper case gear teeth broke off and I had to get a new outdrive.

The outdrive specialist said that it could have been; 1. 10 year age of the outdrive, 2. too much torque due to the weight of the boat plus the extra strain of the 4 blade, but probably 3. both.

He recommended going back to the 3 blade 15 x 17 aluminum which I did. He also said the 4 blade is great for lighter boats doing water sports. Variance of 1/2 inch on diameter is normal and doesn't affect performance as long as it clears the cavitation plate.

If you want my relatively new Mercruiser 4 blade, 15 x 16 aluminum with hub kit, private message me. I'm looking to get about a hundred $ for it. Tom.

 

[bkretchman]

Re: 4 Blade Prop?

Thanks Tom. I will consider buying your prop if I decide that a 4 blade is right for my boat. I'm also wondering if aluminum is the correct material as my current 3 blade prop is stainless steel. How long did it take for the outdrive problems to occur after you installed the 4 blade prop? My outdrive is almost 10 years old so I certainly will keep your story in mind.

 

[Tommyfmu]

Re: 4 Blade Prop?

I would say a little over a month, so about 5 trips to the beach and back (6 miles round trip). Coming back from the beach all of a sudden we heard a bang, then a constant ticking from the outdrive (broken gear tooth going round and round), but we were able to make it back to the dock very slowly.

Regarding alum vs SS, here are some of the things I've heard ; SS much more rigid so faster hole shot but very unforgiving on your outdrive gears if you hit anything solid like rocks. Alum has some bend to it so slightly less ooomph out of the hole than SS, but if you hit rocks the blades will give before you do outdrive damage. A few people I've come across swear by their SS props.

 

[dan t.]

Re: 4 Blade Prop?

I also had a run in with merc 4 blade prop, it went like this, boat is Bayliner 2452, 5500 to 6000 lb loaded, 305 4bbl, alpha1 gen 2, 1.65 ratio, having tried several props, the best performance was from a merc#48-16440-16, 16x16. I was recomended a merc #48-834850A45, 15x16 4 blade. what a POS, I lost on hole shot, cruise rpm and top speed,so as the result I also have one for sale

 

[Tommyfmu]

Re: 4 Blade Prop?

Dan, if I'm reading your post right you tried going from a 3 blade to a 4 blade with the same pitch (16). It's no doubt that you'd be losing hole shot and putting much more of a strain on the engine, as the 4 blade is catching 25% more water than the 3 blade. Ordinarily you drop down a notch in pitch when going from 3 to 4. I had gotten the desired effect of fast hole shot and more efficient cruising speed when going from 3 to 4 blades, but the weight + age of the boat didn't work out.

 

[steelespike]

Re: 4 Blade Prop?

I'm no prop loading expert but how can a 4 blade put more strain on the drive system if its easier for the motor to push the boat.In low to mid range your are getting more speed with less power applied.

 

[Tommyfmu]

Re: 4 Blade Prop?

It becomes more of a load on the engine because 4 blades catches (theoretically) 25% more water than 3. The boat would cruise faster at less rpm's with the 4 blade, but adds more torque strain on the drive and engine.

 

[jay_merrill]

Re: 4 Blade Prop?

Four blade, stainless props also have a tendency to tear up gear boxes because they are heavy. As for extra stress due to additional bite, I would say that this is something that happens mostly during initial accelleration, when the load is the highest.

If you are currently running within correct WOT range when the boat is lightly loaded, but not when you put 4 people and gear in it, I would determine the difference in rpm and make a decision on what to do from there. Part of the decision that you make, might be to just stay with a three blade, but of a lower pitch.

As a general rule of thumb, you can expect about a 200 rpm change, with each inch of decrease in pitch. That is not an exact formula and differs with a number of factors, such as blade material, prop type (cupped,etc.), but its not a bad premise to start with. Once you determine what you need in terms of pitch, you can decide whether to buy a new prop, of just repitch the one you have.



???

 

[Dhadley]

Re: 4 Blade Prop?

The weight or blade area has very very little, if anything, to do with tearing up gears. There are tons of boats that run 4 blade props for years and still have the original gearcase.

In testing we've run props that have way too much pitch and nothing bad has ever happened to the gearcase. Even surfacing the prop doesn't instantly tear something up. Running too much prop will tear up pistons way before it tears up gears.

Props with 4 or more blades wouldn't be so popular if they would cause a gearcase to self destruct. If 4 blade props were indeed that evil the OE's would void the warranty on a new motor if you run one.

If a single 4 blade prop is so bad I guess the duo prop set up will put Volvo out of business soon. If 4 blades are bad imigaine how bad 7 blades are!

 

[jay_merrill]

Re: 4 Blade Prop?

OK, I'll take your word for it, since I know you do a lot of testing. That said, the information that I provided came from my local OMC dealer/owner, who has been a factory authorized tech for 30+ years.

Lots of info "out there" and sometimes its hard to know the useful from the not so useful.


???

 

[Dhadley]

Re: 4 Blade Prop?

If you talk to Evinrude or any engine manufacturer about prop weight and blade area they'll always go back to prop balance. While weight and blade area have an effect on performance and powerhead life, balance is a great concern.

 

[jay_merrill]

Re: 4 Blade Prop?

That makes sense. Balance in anything that rotates is a big deal, especially if it rotates at high speeds.



???

 

[steelespike]

Re: 4 Blade Prop?

It becomes more of a load on the engine because 4 blades catches (theoretically) 25% more water than 3. The boat would cruise faster at less rpm's with the 4 blade, but adds more torque strain on the drive and engine.

If you think about it increasing pitch should put more strain on the drive train than the number of blades. Ispecially if yor staying in the same pitch range.
I don't buy the blade number having any detrimental affect on the drive train.
If a 4 blade causes damage/wear then a 5 blade would be terminal.

 

[Tommyfmu]

Re: 4 Blade Prop?

If you think about it increasing pitch should put more strain on the drive train than the number of blades. Ispecially if yor staying in the same pitch range.
I don't buy the blade number having any detrimental affect on the drive train.
If a 4 blade causes damage/wear then a 5 blade would be terminal.

I'm only going by what the outdrive specialist said which was specifically for my boat, being a heavier cruiser, not a smaller boat doing water sports. Also the Alpha's are not nearly as hardy as the Bravo drives which also makes a big difference according to him. All he does is rebuild outdrives, and he's got a great reputation locally from a number of sources I've spoken to.

I loved what the 4 blade was doing on my boat for hole shot and cruising and wish I could continue using it, but would rather give up 3 knots than have to change the outdrive in another 2 years because the torque on the teeth was too much, so am staying with the 3 blade alum 15 x 17.

 

[walleyehed]

Re: 4 Blade Prop?

Understand your concern but let me take a shot at this.....
3-blades are typically faster, 4-blades are typically smoother.
The DYNAMIC balance of a 4-blade prop is "better" than a 3-blade when talking about "factory" props.
You have more "wear" from a 3-bld (long-term) than you do from a 4.
BUT, as Dhadley has mentioned, the important item here is that you stay at or near the top of your RPM range. The lower the rpm, the shorter the engine life, ie, powerhead-not gearcase.

 

[bkretchman]

Re: 4 Blade Prop?

How can one determine a prop's effect on RPMs without trying different props? Is it safe to assume that my RPMs from my current SS 3 blade with be the same on a SS 4 blade if the 4 blade has the same pitch and diameter as the 3 blade?

 

[Tommyfmu]

Re: 4 Blade Prop?

How can one determine a prop's effect on RPMs without trying different props? Is it safe to assume that my RPMs from my current SS 3 blade with be the same on a SS 4 blade if the 4 blade has the same pitch and diameter as the 3 blade?

From a 3 blade to 4 blade you would lose rpm's but gain a few knots I believe which is why you ratchet down the pitch by 1 ordinarily.