3-Blade vs. 4-Blade, and What Diameter/Pitch to use?

[greg82255]

Hello,

I currently have a 1986 Formula 206 LS with a Merc 260/Alpha One (1.5 ratio). The boat weighs 3250 lbs, and I have run 2 different props on it. First is a 14.5 x 19" Michigan Ballistic prop, and have gotten 52 mph at about 47-4800 rpm. Next is a Mercury Mirage 3-blade 14.75 x 21" and I get 50-52 mph at about 43-4400 rpm. Getting on plane is easier with the 19" pitch, but with the 21 and trim tabs it is not really hard. I want to know if switching to a 4 blade would help overall top speed or hole shot, or if using a higher pitch 3-blade would work (for top end, not hole shot). Basically, I'm looking for any suggestions that could give me a little more top speed, and any suggestions that could give me better hole shot without losing top end. I know those solutions are different, so any suggestions are appreciated. Thanks in advance

 

[Bifflefan]

Re: 3-Blade vs. 4-Blade, and What Diameter/Pitch to use?

From the numbers you posted, it looks like your about topped out. A 4 blade is not going to give you any more speed. You will need to lose some weight or gain some HP to get more speed.
Typically using a 4 blade you need to drop 1" p to get the same performance. A 4 blade will give you a little better hole shot. Your RPM's look good with the 19"p. If you can loose a couple hundred pounds or gain 50 HP you will see a speed increase with the 21"p. The 19"p may be over rev after the changes mentioned.

Diameter makes little difference and is mostly controlled by the manufacture.

 

[steelespike]

Re: 3-Blade vs. 4-Blade, and What Diameter/Pitch to use?

Your rpm are a little high with the 19.Rated is 4600.I think your better off with the 21.
the mirage is apparently a pretty good prop.You may have discovered that tabs need to be retracted all the way for max speed.While a 4 blade usually improves hole shot among other things. It allmost allways reduces top speed.Any boat your size that routinely runs over 50 is setup pretty good.Is your speed by gps?If my math is right;assuming a gps and tach is accurate you have about 20% slip thats high 10% is acceptable.Is the Mirage in excellent shape?If the tabs are not retracted speed might be lower than figures indicate.

 

[greg82255]

Re: 3-Blade vs. 4-Blade, and What Diameter/Pitch to use?

The 52 mph with both props was by GPS. I'm pretty sure the tach is accurate as well - it's a Livorsi tach, new this year. The Mirage had a small dent last year but I just had it reconditioned this year, so I assume it is in good shape. The 19 is in excellent shape also.

 

[steelespike]

Re: 3-Blade vs. 4-Blade, and What Diameter/Pitch to use?

Calculating speed etc is only as accurate as the input.
As usual with calculations I had the wrong ratio. at 1.50 your speed is right on @ 52.
Figured at a respectable 10% slip.It would appear getting more speed might be possible
with a sofisticated prop but I'm not the guy to help there.
You might consult with the guys at "screem and fly".Keep in mind a prop that squeezes out the most speed may struggle in some other areas.

 

[John_S]

Re: 3-Blade vs. 4-Blade, and What Diameter/Pitch to use?

In other threads you mentioned doing a reman on the engine this winter. Wait until after the reman and retest with the props you have. I recommend this even if you are not planning any hp increases, because your 800hr motor is probably already down some from original ratings. But, I do recommend looking at going to vortec heads with the reman. They will give you about 30hp over current.

While you have the engine out, check for water logged foam. That can add considerable weight and help explain the high slip numbers.

Now, once you retest, you might want to test some 4 blade props on that boat. I believe that boat is a bit stearn heavy and could use a prop that provides some additional stearn lift.

 

[Tail_Gunner]

Re: 3-Blade vs. 4-Blade, and What Diameter/Pitch to use?

Hello,

I currently have a 1986 Formula 206 LS with a Merc 260/Alpha One (1.5 ratio). The boat weighs 3250 lbs, and I have run 2 different props on it. First is a 14.5 x 19" Michigan Ballistic prop, and have gotten 52 mph at about 47-4800 rpm. Next is a Mercury Mirage 3-blade 14.75 x 21" and I get 50-52 mph at about 43-4400 rpm. Getting on plane is easier with the 19" pitch, but with the 21 and trim tabs it is not really hard. I want to know if switching to a 4 blade would help overall top speed or hole shot, or if using a higher pitch 3-blade would work (for top end, not hole shot). Basically, I'm looking for any suggestions that could give me a little more top speed, and any suggestions that could give me better hole shot without losing top end. I know those solutions are different, so any suggestions are appreciated. Thanks in advance

Mercury Mirage 3-blade 14.75 x 21" and I get 50-52 mph at about 43-4400 rpm. Getting on plane is easier with the 19" pitch, but with the 21 and trim tabs it is not

Im going to skip the Ballistic...but the Mirage is a good for heavy pushing and a known performer....21@4350@51mph 11% slip good numbers there.

You seem to be right in the middle of every prop i know of...a 20p 4 blade stilletto baypro 11 would give more lift and midrange but more top end ....no...a 20 p 3 blade solas.....maybe but more than likely not...Not even going to say much more as john pointed out heads and a cam is what you need....Your engine is running out of air at the high rpm notice the speed cap or drag @ at speed is constant 51 mph. You need more hp to push that boat....not gearing