Alan454

Does anybody know where I can find some efficiency curves of Mercury (or other) 14 5/8" dia 4 blade props vs advance ratio for some selected prop pitch's? Doesn't seem to be any info on this subject for any props that I can find on the net. I'm particularly interested in 15 through 19 pitch but any will do.

Griff

The prop's efficiency will depend on what boat its on. Certain props are very good on some boats and then the same prop is terrible on another boat.

bbladesprops

Nice response Griff. You've been getting all this for some time now! Keep up the great work!

There are many, many variables. Call me some time if you like and we can discuss some points in depth.

Brett

Alan454

You guys are right that the efficiency of the boat as a whole is a function of quite a few variables. However, one component of this complex problem is the efficiency of the prop all by itself. The prop efficiency is simply the output power of a the prop over the shaft power and is typically characterized as function of something called the advance ratio which is related to prop slip. Prop slip is well known on this forum so you can just think of a curve with efficiency that varies between 0 and 1 plotted against prop slip plotted from 0 to 100%. The idea is to operate at the peak of this curve, which typically occurs somewhere between 5% and 30% prop slip.

I know that Mercury engineers have loads of this data somewhere. Any Mercury engineers out there?

bbladesprops

12 years at Mercury. Most of it in the Racing propeller division.

Propellers as a single unrestricted unit have 0% slip and therefore move forward their rated travel in inches with one 360 degrees of revolution. This makes any propeller 100% efficiant on its own. (in a perfect situation)

Adding weight, hull design, "X" dimention, set back, diameter pitch and its progression, cup height and location, rake angle, number of blades air temp, H2O temp, humidity............to name a few, determine the actual efficiency of the prop in that senario.

However, to answer your question as I believe you are trying to ask, More blade area can create the best efficient numbers. In effect, 5 blades can be more efficient that 3 blades in the majority of cases. However, and very important, 3 blades can be faster and as efficient on the correct set up. The lack of material in the water reduces drag and again, if the prop is hooked up well, it will out perform the 5 blade. On the flip side, add 3 more people to the boat and the 5 blade may become the efficient performer.

As is stated before however, you can't have a certain efficiency number without using a craft of sorts to obtain and compare the different geometrys. Therefore, the props efficientcy by itself is irrevevent till you know what set up and the goals desired are. At that point you will determine the peak rpm to obtain the best fuel/slip/speed for the individual boat.

10-15% slip would be a good number for your Formula 26 footer. I'd be real happy with 11-12% @ WOT and 13-15% at cruise with a Bravo 1 4 blade Lab Finished specific to a obtained base line.

Brett

Alan454

Brett - my current low cruise slip is about 28% and 17% at WOT. I am adding over 125 HP though as I mentioned, and putting my boat on a diet so I won't have any baseline to start from. Can't use the Bravo I's unless I get some different gears as it looks like they don't come in low pitch versions (have 1.36:1).

bbladesprops

We can go to the Revolution 4 series or others.

Let us know when you are ready to test and we can help you from there.

Brett

Griff

What boat and what is the new HP going to be????

What HP now and what prop are you using???? What RPM and speed are you seeing at WOT???

Strip Poker 388

iTrader: (6)

I would think the gear ratio would be hurting it.

bbladesprops

1.5:1 gears would be better.

Brett