3 blade prop slower then 4 blade ?

[89retta]

I usually run a Turning point 19" 4 blade . At 4800 rpm it hits 47mph on gps. Last weekend I purchased a 3 blade Turning point 19 and only hit 44mph at the same rpm. However I had to watch the tach as it easily climbed above 5000 rpm so I had to throttle back some so it wouldn't hit the limiter. But at the same rpm it was 3 mph slower.

 

[tpenfield]

Could be better stern lift from the 4-blade . . . or . . . less slip from the 4 blade . . . or combination of the 2.

funny thing about selecting props . . . it's like a box of chocolates

 

[alldodge]

Its prop slip and less power needed to turn the prop IMO
Try a 21-3 blade and see what happens

 

[dingbat]

Yep, in effect driving around in 3rd gear.

Going to a 21” will put you back in 4th gear at the determinant of your hole shot.

A change of prop design is in order to tweek things further

 

[211libwtfo]

Why would you want to go to a 3 blade anyway

 

[dingbat]

Why would you want to go to a 3 blade anyway

Why stay with an under performing 4 blade?

I went from a 4 blade to a 3 blade that improved my hole shot while increasing my top end speed.

 

[211libwtfo]

I guess I just figured with a 4 blade you’re getting more surface area cutting the water better bite which would perform better than any 3 blade. On my 454 mag 24’ sport boat it has a 22p prop with sooo much hole shot ALOT of throttle torque but I’m only getting 4200 wot rpms. So now that you mention it.....maybe there could be under performance?

 

[jimmbo]

Without seeing both props side by side to compare the blades(size, shape, rake, cup,) it is difficult to interpret the results you experienced. Since you weren't able to attain WOT without overrevving/limiter kicking in(I don't have one on my engine) you aren't able to determine what speed you would have attained. At what rpm does the rev limiter activate? Dam things make it hard to figure how much you can increase pitch as you don't know by how it is over revving.
I will guess your outdrive has a gear ratio of 1.79:1, most units on boats sized like yours came with 21 or 23 in. 3 blade AL

 

[dingbat]

I guess I just figured with a 4 blade you’re getting more surface area cutting the water better bite which would perform better than any 3 blade. On my 454 mag 24’ sport boat it has a 22p prop with sooo much hole shot ALOT of throttle torque but I’m only getting 4200 wot rpms. So now that you mention it.....maybe there could be under performance?

RPM is a function of Horsepower. You get a better bite with a 4 blade but if you don't have enough torque to push the motor to WOT your leaving HP on the drive shaft.

In my case, I dropped a pitch (17 to 15) going to the 3 blade to bring my RPM. The drop in pitch (and reduced slip) increased my WOT more than enough to make up for the loss in pitch. The smaller pitch combined with a vented prop allows the motor to spin up quicker out of the hole improving my hole shot as well.

Keep in mind I changed from one prop design to the other. I probably wouldn't have gotten the same results simply dropping a pitch in the same prop design.

 

[fishrdan]

Why stay with an under performing 4 blade?

I went from a 4 blade to a 3 blade that improved my hole shot while increasing my top end speed.

In my case, I dropped a pitch (17 to 15) going to the 3 blade to bring my RPM. The drop in pitch (and reduced slip) increased my WOT more than enough to make up for the loss in pitch. The smaller pitch combined with a vented prop allows the motor to spin up quicker out of the hole improving my hole shot as well.

17P 4-blade to 15P 3-blade, that was more a function of selecting the correct prop pitch to get the engine to it's WOT RPM, opposed to 3-blade being all around better than a 4-blade, since the 4-blade wouldn't allow the engine to reach it's WOT RPM. Dropping pitch while also going to a vented 3-blade prop, I could see that the boat would have much better holeshot than the non-vented over pitched 4-blade prop... You found a very good prop match for your rig, when before it wasn't a good match.

I tried several 3-blade props and was never satisfied with their overall performance. Then went from a 19P 3-blade to a 18P 4-blade and every performance aspect was vastly improved, except top speed as expected. Fits my needs perfectly, unless heavily loaded or at altitude, then a prop change is needed.

89retta chose a 3-blade prop that's not a good fit for the boat, needs more pitch since it's overshooting WOT RPM. Jumping up to a 21P 3-blade would most likely yield top speed improvements he's seeking, while sacrificing some holeshot.

 

[Leardriver]

I think of four blades as adding four wheel drive. More blade surface area, more initial bite and traction out of the hole, at the expense of more drag and less top speed.
Since most of us rarely go wide open for long, except for a blast up to the top to test a new prop, watersports may be more fun with four blades and less slippage.
Three bladed airplane props over two blades is the same tradeoff. Better climb, slower cruise speed.

 

[89retta]

Its prop slip and less power needed to turn the prop IMO
Try a 21-3 blade and see what happens

I have tried an aluminum 21 Volvo 3 blade. And was turning 4800 rpm at 47 mph. Same as my 4 blade 19 Turning point

 

[alldodge]

I have tried an aluminum 21 Volvo 3 blade. And was turning 4800 rpm at 47 mph. Same as my 4 blade 19 Turning point

Sorry, someone sold you a bill of goods or gave wrong information, because that would never happen

 

[89retta]

Sorry, someone sold you a bill of goods or gave wrong information, because that would never happen

It happened . I believe the Volvo prop was slipping a lot more and my tach is good . Speed is gps

 

[ahicks]

Failing to see a big deal here. There can easily be enough difference in blade shape and style between those 2 props to account for your results. If this were an exact science (eg all 19" props turned the same rpm), there would be a lot less guessing (and complete BS on the mfg's part) involved when it comes to prop choices.

Results here prove only what most of us have known for a long time. You can calculate all you want, but you aren't going to know the final results until you've tried it. Nothing new, nothing has changed.

 

[89retta]

Failing to see a big deal here. There can easily be enough difference in blade shape and style between those 2 props to account for your results. If this were an exact science (eg all 19" props turned the same rpm), there would be a lot less guessing (and complete BS on the mfg's part) involved when it comes to prop choices.

Results here prove only what most of us have known for a long time. You can calculate all you want, but you aren't going to know the final results until you've tried it. Nothing new, nothing has changed.

Why I posted this and tried the 3 blade is everyone always says 3 blades are faster. Both props are from the same manufacturer and same pitch. So it kinda puts doubt on the 3 blade props are faster theory.

 

[ahicks]

It does until you consider blade shape and style. You can't compare a prop with flat non cupped blades that look look Mickey Mouse's ears to a cupped late design prop designed using advanced engineering! The performance attributed to the Mickey Mouse ear shaped vs. late model props like these would be light years apart.

Mickey Mouse ear blade shape works awesome on a pontoon boat - but on something running much faster (over 50-60mph)? Not so much. ANY 4 blade would likely outperform it under those conditions. Just like ANY other 3 or 4 blade prop would likely outperform it under those conditions!

You can't say the 4 is faster than a 3 without further qualifying what you're talking about.

 

[89retta]

Both props are new this past year. Same amount of gear and fuel , lake conditions same. Speed was GPS , tach is accurate. Both props are Turning point and pitch. So yes I can say the 3 blade was slower then the 4 blade. When you do the same test with the same prop and pitch. Then feel free to comment. In other words I'm qualified to state what I said

 

[ahicks]

Maybe next time you could share the fact that both props were the same manf./series?

 

[89retta]

Maybe next time you could share the fact that both props were the same manf./series?

If you had read the very first post I made it says that.