I have a 2000 Velocity 280 with a Whipplecharged 502 mag with bravo 1 drive. I found it way too easy to break the Hydromotive lose. I'm not talking full throttle starts because I don't do them ... I'm talking coming out at say 3000 RPM. Once you lost the bite with the hydromotive you usually had to almost stop the boat and start over. The 30" Bravo 1 has worked great and is much more forgiving and useful as an everyday prop.

I think what you're experiencing is the efficiency of the Bravos that are 32" pitch or less. It seems, both in my testing and others, that once above 32" the Bravo, at something more than a conservative X dimension, and indeed the 4 blade Hydromotive will tend to blow out. This happens either getting on plane or in rough water with a bow high attitude. Possibly the fault is not entirely in the Bravo prop design but also in the lower unit shape of the standard Bravo drive, be it XR or otherwise. For example the new Merc Sportcase is showing a dramatic increase (3 to 4%)in speed (in specific applications) when bolted at the exact same X dimension with the same propeller.

When I was running 500 EFI's with a 1.50 ratio, top end was app. 86 MPH with labbed 31 Bravos. No cavitation, pretty reasonable slip numbers <15%. With the addition of the Whipples and WOT now approaching 100 MPH, and going to 36" bravos, that's when the inefficiency became noticeable. BTW, it was just as noticeable with Hydromotive 4 blades as well. It was not until I ran the 5 blades that slip dropped back to <10% and the other ride attributes were enhanced.

In 2 other cases the large pitch inefficiency V smaller pitch efficiency theory has held true. Jeff Brauer and Steve Northington from the OSO Board have used different pitch labbed Bravos of mine and have each found different attributes. It wouldn't be proper for me to dislcose those speeds, etc. However if they'd like to respond to you, please contact them directly.

Their tests, mine and others continue to prove that empirical testing on your set up is the only way to know for certain what will work best.

Thankyou for your continuing dialogue on propeller design. The more each of us can learn from one another, the closer we can be to making the right decision on various parts, in this case, props.

Take care,

Steve

No. thank YOU Steve. Your tests are required reading on this board. We are all trying to piece together the information from various sources, theories and test data. I've been searching the web on this subject and can share my synopsis so far.

You said that many have noticed an increase in slip at higher pitch, when combined with a higher X dimension. But the reason you needed the higher pitch was the extra power you had. So could it be that the Bravo simply doesn't have enough blade area to handle the power? Since you're limited in diameter by the drive, adding a fifth blade is the only way to get the slip down.

It makes sense that a better gearcase would also reduce slip if it improves the flow of water to the prop, but doesn't it also reduce a major component of hull drag? This would allow higher speed with whatever thrust the prop is delivering. This is not the same as reducing prop slip, but the way we calculate prop slip, you can't separate the two factors.

Slip and efficiency are not the same thing but they are related.

Prop efficency = Thrust HP/propshaft HP (power out/power in)

Thrust HP = Speed(ft/sec) X Thrust(lb)/550(ft-lb/sec/HP)

Mercury and others say that every prop has an efficiency curve. Maximum efficiency occurs at a certain slip. All other things being equal, including pitch, the major thing that you can change on the prop that affects slip is blade area. On airplane props this is usually done by changing diameter. Too little diameter/blade area and slip goes up/efficiency goes down. Too much diameter/blade area and slip goes down, but so does efficiency as you use more power just pushing the blades through the fluid. This is why four blade props at low X dimensions are usually slower than three blades. So maximum efficiency occurs at a certain slip in between.

Mercury also says that, in a given propeller series, maximum efficiency increases as pitch increases. I don't know why they say this or under what conditions it was true, but I have seen on two other websites, graphs showing a family of efficiency curves for a series of propellers, and as pitch went up, so did maximum efficiency. I don't know why this is.

It seems to me that we need to do two things when we are trying to match props to a given hull/engine combination. Yes, we want to match the pitch to the RPM, but we also need to match blade area to the HP, such that we hit that certain slip value at which the most propshaft HP is converted to thrust. It's easy to try props of different pitch and blade #. Adjustments to blade diameter and drive height are more difficult.

Here is a link for one of the websites I found. I have attached a graph from that site with a family of efficiency curves. I think it would help us to have this kind of information from the prop manufacturers.

This graph plots efficiency against Advance ratio, not slip, but Advance ratio is closely related to slip, as you can see from the equation below.

Advance ratio (J) = Speed/RPM x Diameter

Speed is the actual boat speed, RPM is proportional to theoretical boat speed and Diameter is a way of controlling blade area. When Diameter increases, Advance ratio decreases, so the curves start at low efficiency with too much blade area.

http://aerodyn.org/Propulsion/propeller.html#other

used with permission www.aerodyn.org

Here's another graph from www.aerodyn.org. This one plots the power coefficients for different pitches against the advance ratio. I'm not sure what this one means because I haven't found the equation that defines the power coefficient yet.

But I think of it this way. The power coefficient just tells you when you are getting the most bang for the buck. Spin any prop at different RPM and there will be a certain RPM at which you get more thrust per RPM than any other RPM. And power is just thrust X speed. Since it takes more power to spin a higher pitch prop at a given RPM, we see much different curves on this graph.

So increasing pitch does increase the amount of HP that the prop will absorb, without increasing blade area. We all know this from experience, more power = more pitch = more speed. But all the curves drop off as advance ratio increases (not enough blade area), and without plotting your own test data on graphs like these for the props you are using, you are left to find out the hard way through trial and error.

I'm not saying that testing is avoidable, it isn't, but I for one would like to have graphs like these for the props I'm testing. It might help narrow down the choices and leave more time for boating.

Tomcat,

Thank you for your insight and statistical approach to the discussion. I had a wonderful statistical analysis professor during B school that passed me out of his class only out of his kindness and Mercy. The simple formula for a straight line leaves me bewildered........

Thus, to overcome my lack of such skillsets, I'm stuck with empirical testing. When we combine the results of the in water tests with engineering skills you and others on OSO posess, we'll come up with some pretty rock solid evidence.

If you will just let me know what kind of tests you need from in water testing, I'll be happy to gather that real time data and share it with you.

Thanks again!

Steve

SteveDavid - You don't need to change your testing. The data you normally collect to calculate slip should be enough. The missing piece is the prop efficiency graphs for the props you are testing. I know you have top notch contacts in the industry. Maybe you could ask for this information. I picture it being a simple matter to calculate your slip or advance ratio for the prop, and see if the peak efficiency occurs at that point. If it doesn't, you need more or less blade area or diameter (I think). I wonder about two things:

1) Is this what good labbing shops are doing?
2) Is this information considered proprietary due to the F1/F2 wars?

Thanks

Tom

P. S. I'm no expert either, I barely passed statistics and calculus. I'm sure there are real mechanical engineers lurking on this board, laughing there asses off at my clumsy attempts to understand this subject! Hey you guys, if you're out there, post!

Steve,

I don't know if I missed it or you guys haven't said anything about them, have you tried the 5 blade Herrings? I've seen them on the 38' TG's and heard that the mid range response was great (3 to 5 MPH) with no loss on the upper end. I agree with TooOld, this thread is REQUIRED READING!! Thanks for the info, Robbie

Hi Robbie,

Yes, we've run the Herrings on the F-2 35' Fountain, my Black Thunder, and the twin engine BT we had at the Miami Show.
Great props, very precise in their measurements and as a result, quite pricey.

We found the top end virtually equal on our F-2 boat between the 5 blade Hydromotives and the 5 blade Herrings.

On my personal Black Thunder, the 5 blade Hydromotives had no cavitation at any phase while the Herrings had some getting on plane. Mid range was identical as was top end +/- 1 MPH.

On the twin BT (575's) we had at the show, we ran 27 Herrings and the mid rage was 4 MPH faster than the 28 stock Bravos at 3800 RPM. Top RPM with the Herring 27 inch was 5000 and top end was app. 75 MPH. Top end with stock 28 Bravos was 79.9. MPH at 5300 RPM. Jim Herring, Bob's son was with us for these evaluation runs. By the way, both Jim and Bob Herring are first class folks.

I think if we had 26 inch Herrings we might have achieved similar top end numbers to the Bravo 28s. We did not have any 5 blade Hydromotives to try on the twin engine BT for the Miami show but will be testing some in the near future.

Take care,

Steve

Steve,
You mentioned earlier in this message string that when you changed to the 5 blade Hydromotive prop your slip went down to ~9% from the Bravo. Did your speed increase over the bravo and if so how much. Or did it just load the engine better, lower your rpm's and speed remained about the same.

I am sorry if I am asking a this and you have already indicated but is your X higher than normal for your boat.

Thanks for the info,
Rick