19P 4 Blade vs. 21P 3 Blade???

[cjt757]

Guys, I need some good advise where someones isn't trying to sell me "the prop in the shop"!

I just purchased a 1998 24 foot Proline CC (5500lbs fully loaded). A bit old but in good shape. It has a Merc 250EFI. The current prop is a Vengeance (by Merc) 3 blade stainless. 13 3/4" x 21 P. I can get about 5000RPM at WOT. The book says the 250 redline should be around 5800.

According to the rules every inch of pitch is about 150-200 RPMs. So a 19P should spin at about 5400 which is perfect....

They say a 4 blade spins 150-200 slower than a 3 blade.

Thus a change from a 21P 3 blade to a 19P 4 blade should put me in the 5200-5300 range which is better than I have now.

They also say that 4 blades are better out of the hole and better mileage (I have a center console not a cigarette--so I don't care about top end).

I would appreciate any insight.

THANK YOU!!!

Chris
WPB, FL

 

[steelespike]

Re: 19P 4 Blade vs. 21P 3 Blade???

Very basically a 4 blade may seem smoother,usually gives better hole shot, will stay on plane better at reduced throttle,resists venting(cavitation), Handles better at speed and around the dock.Top end will be lower.
If you stay with a 3 blade a 19 will improve hole shot,It may stay on plane better at reduced throttle.
You probably won't lose as much top end.
If you give us your gps speed @ 5000 we may be able to tell how well your setup is working.

 

[Bifflefan]

Re: 19P 4 Blade vs. 21P 3 Blade???

Here are a couple assumptions on my part.
1) you just bought it.
2) you know the full loaded weight.
3) you took everyone that you know out on the boat when you got these numbers.

Do you know the weight with only a min amount of gear and people? And have you ran the boat like that? Your prop may be just right for less weight. Thus if you change and only have you in the boat, it may turn to fast.

To many people get hung up on going WOT, when for the most part you don't. Remember that you spend the most time at cruise speed, about 3000 rpm's.

I'd leave it alone. One day you will decide to take all the excess stuff out and "see what she can do". Then you may find that the prop you have is right at the red line.

 

[cjt757]

Re: 19P 4 Blade vs. 21P 3 Blade???

Very basically a 4 blade may seem smoother,usually gives better hole shot, will stay on plane better at reduced throttle,resists venting(cavitation), Handles better at speed and around the dock.Top end will be lower.
If you stay with a 3 blade a 19 will improve hole shot,It may stay on plane better at reduced throttle.
You probably won't lose as much top end.
If you give us your gps speed @ 5000 we may be able to tell how well your setup is working.

The boat does exactly 40mph (via GPS). I want to have a second prop on board and thus thinking the current 3 blade will be the "backup" and to make sure the new prop is the best possible. I'm looking for holeshot & cruise performance (low gas usage).

Thanks for your help!

Rgs,
Chris

 

[jestor68]

Re: 19P 4 Blade vs. 21P 3 Blade???

I put your numbers through the Mercury Marine prop selector. It came up with exactly the prop(s) I was going to recommend.

In aluminum: The Black Max 15.5 X 18 pitch.

In stainless steel: The Mirage Plus 15.38 X 18 pitch.

A larger diameter prop is required to efficiently move a big heavy boat.

 

[cjt757]

Re: 19P 4 Blade vs. 21P 3 Blade???

I put your numbers through the Mercury Marine prop selector. It came up with exactly the prop(s) I was going to recommend.

In aluminum: The Black Max 15.5 X 18 pitch.

In stainless steel: The Mirage Plus 15.38 X 18 pitch.

A larger diameter prop is required to efficiently move a big heavy boat.

Intersesting... As not many people talk about diameter, thanks.

 

[jestor68]

Re: 19P 4 Blade vs. 21P 3 Blade???

Intersesting... As not many people talk about diameter, thanks.

Your lower unit can handle prop diameters up to 16".

The general rule is that boats over 3000 lbs should use the larger diameter props in order to generate the static thrust needed to push that weight up on to plane quickly.

The main reason that diameter changes in relation to pitch is to maintain predictability ( that 150-200 rpm per inch of pitch thing).

A larger diameter 3 blade prop will generate more static thrust than the smaller diameter 4 blade prop.

The Mirage Plus props are specifically designed to power offshore fishing boats like yours.