Alpha one 3 blade to 4 blade prop change

[Benny67]

I hit a sandbar on Friday and damaged my 14" 19 pitch prop. I went to the local marina and they didn't have the same prop in stock. The guy behind the counter suggested that If I wanted to change to a 14.25" with a 21 pitch he had one available.

I decided to go for it because I didn't want to wait a few days for the replacement.

Let me tell you, what a difference in handling and speed the 1/4" and 2 degree pitch made. I always thought that my boat was a real pig as far as speed and handling goes and I just figured it was the nature of the game because it's a 26ft cruiser with a single 5.7 motor.

Anyway, my question to whoever can answer is, if I switched to a 4 blade 14.25 x 21 prop would I see an even more improved performance?

I like the idea that with this new 3 blade propeller the boat is faster and requires less rpm to do the same job as the old propeller. I would think that a 4 blade would increase my performance even more....anybody got input ?

 

[eavega]

As a general rule, going from a 3-blade to a 4-blade gives you better hole shot at the expense of top end speed. You need to define "performance". In the end your best prop is the one that allows your engine to run in its recommended WOT RPM range.

Rgds

-E

 

[Benny67]

I only took it out for 2 short runs(maybe 10-12 miles total)

I noticed this difference. (I know it's low RPM'S..I am not a big fan of running WOT unless it's necessary. At that RPM the motor sounds most comfortable to me)

old prop @ 2900 rpm = 13 mph and difficulty obtaining that sweet spot where I am steering more with the hull rather than the steering wheel.

new prop @ 2900 rpm = 19mph and easily obtaining the sweet spot where you are at a smooth comfortable and stable plane.

The boat just feels more responsive and smoother. Not working as hard as it was with the old prop.

I have no idea where I can obtain the information for the original specs, meaning what the prop size vs WOT rpm's

The setup is :

1988 Alpha one with 5.7 L (350 CI) I believe it's 260 HP

the Boat is a 1988 Regal Ambassador 255XL

 

[steelespike]

I think something was up with your 19" prop.
Prop size is given in inches. Written diameter X pitch in inches.
The pitch is the theoretical distance it moves forward in one revolution.It's actually a little less because its a liquid.
We call that slip.
For the health and performance of the motor we need to determine what wot (wide open throttle) rpm and speed the new prop gives.
Do a lightly loaded test run.We need the gps speed and max rpm at wot.Once we know the motor is able to operate within its wot rpm
rating.(4400-4800) You can operate at whatever throttle setting you prefer.
From your description It's possible the 21 is the perfect selection.

 

[H20Rat]

Your numbers are meaningless. Your new prop could actually be bad for the engine, or it might be perfect, but you need to get to either redline or WOT at least long enough for speed and rpm to stabilize.

The smaller pitch prop will usually be a better low speed manueuvability prop. The differences between how it feels at cruising speed isn't necessarily a product of the pitch but the design, stern lifting, bow lifting, or neutral.

 

[jcole59946]

Howdy, per the original question. We switched my families SeaRay 210 Merc Alpha One from a 3 blade 21 pitch prop to a 4 blade 20 pitch prop. Hole shot, speed control, and turning performance all improved. Also, the engine sounds smoother. I'm not sure why. We never go over 30 mph so I can't speak to the top end.

I switched my Glastron Gt205 Merc Alpha One from a 19 pitch 3 blade to a 17 pitch 4 blade. The purpose of the switch was to improve speed control for wakeboarding. The 4 blade 17 pitch is great for the18 to 22 mph range. I never drive over of 30 mph, so again, I can't speak to top speed.

Both boats have Merc Alpha One Gen II outdrive with 1.62 gear ration.

Generally most people drop one pitch when replacing a 3 blade prop with a 4 blade but there are other variables to consider like prop diameter, cup, stainless vs aluminum, etc.

The other posts are correct about calibrating the prop such that WOT is in the correct RPM range.

 

[Benny67]

Your numbers are meaningless. Your new prop could actually be bad for the engine, or it might be perfect, but you need to get to either redline or WOT at least long enough for speed and rpm to stabilize.

The smaller pitch prop will usually be a better low speed manueuvability prop. The differences between how it feels at cruising speed isn't necessarily a product of the pitch but the design, stern lifting, bow lifting, or neutral.

If that was the case the boat would have handled better with the old prop and how can you say my numbers are meaningless? Would you put the old prop back on just because you were told that it was not a good match by the manufacturer?

I get the fact that wot needs to be within safe rpms of the motors operating range and the prop size and pitch are the biggest factor in that number.

I did just get it back from the repair shop but it operated exactly like it did last season unless it was mis marked from the manufacturer... but its basically cast into the hub so I doubt that's the case.

The WOT with the old prop was somewhere in the ballpark of 3900 rpm.... at about 24mph(gps data) AND the boat never really felt comfortable when I pushed it to do so. I could never get it to plane smoothly regardless of how I trimmed the drive or my trim tabs. This new prop literally makes it feel like different boat. With the old prop I just accepted that this is the way the boat performs and thats that. That is not the case with the new setup

I'll run it this weekend if the conditions allow me to see what the WOT actually turns out to be.

 

[Benny67]

I think something was up with your 19" prop

So do I...

 

[Benny67]

The one difference is one prop is a michigan (14x19) and the other (14.25x21) is a mercuiser (quicksilver)

I am starting to wonder if the rake on the old prop is different than the new one. That would possibly explain the handling differences

 

[fishrdan]

Different between a Michigan prop and Mercruiser prop can be night and day, especially if it was an older Michigan prop compared to a newer Mercruiser prop. Prop technology has come a-ways.

Also, the boat should be propped close to it's WOT (AKA redline) RPM, or else you are lugging the engine, which is as bad or worse than running the engine at it's WOT RPM. Open it up and see what the boat runs for 15-20 seconds, speed by GPS and engine RPM at top end. WOT RPM should be something like 4200-4600 or 4400-4800, depending on engine and year.

Your new prop might "feel" better, but you won't know until you see what numbers the boat will turn.