Twin O/B Sonic

Have heard the same and would agree based on limited experience w/them.

Jadento’s experience above is a great example of how the calculator/slip works.

The #’s the shop gave you would be the ones to use.

Similar to advertised HP’s.
Its a range.

A while back, I was w/a friend in Fla picking up a set of 30” Ron Hill cleavers from the friends local prop shop and they told him (in front of me), there was 3” (pitch) variable between the blades!

The basics is simple math (I’ve done it on a bar napkin) but the final 5 - 10% is black art!

Part of the variable is the people involved too.

I used to work for a dealer who swore there was no difference between stainless props!

Then there’s the expert that preaches what he read on the internet as gospel.

If you want to have fun sometime, take a grinder and a hammer to a beater prop!

Ive done it many times and have improved its performance 95% of the time.

On the variables in the calculator, if your only using it on your rig, you can normally track them down and you should still see the results follow the changes.
The real issue is two identical rigs/set ups that run way different.

The slow guy is always scrambling.

PARADOX

All good stuff. THX.
When I put the Avanti together, 2008 (long story, new empty hull, then rigged with all new stuff, 525's XR shorties, ITS) out of the box with 28 standard Bravo I, spinning in, I got a top speed, (don't even remember what it was, but it's on OSO somewhere, Cash Bar guessed the closest) Then I tested a bunch of props, spinning in, out. Some labbed some standard. While I used the slip chart as a guide, the best way to do and dial in a set up: is test, test,, test. and test some more. SO,. After all the tests, The best I got at 5350 -5400 RPM is 89.3 MPH, (GPS) with quarter tank and 7 people. I was happy with the data, might get to 91 with a light load.
TEST and TEST some more.
P

Powerquest230

Rookie question: Wouldn’t slip on same boat using same props vary by rpm and speed? Who says maximum efficiency is at max speed? Interested to see experience and views…TIA

bajaman

A lot of the boating mags will note this, too. They will show speed-at-rpm and one can do the math from there. So I just did this on a recent test of a boat running a 20" prop with 1.75:1 ratio on the drive. Here's the numbers:

• 58.6 mph @ 5900 = 9.932 mph/1000 rpm = 8.2% slip
• 54.8 mph @ 5500 = 9.963 mph/1000 rpm = 8.4%
• 48.9 mph @ 5000 = 9.780 mph/1000 rpm = 9.6%
• 44.1 mph @4500 = 9.800 mph/1000 rpm = 9.4%
• 38.7 mph @4000 = 9.670 mph/1000 rpm = 10.6% slip

So in this example, the highest efficiency in mph per rpm does not equate to lowest percentage of slip. Therefor one can infer that maximum efficiency insofar as PROP SLIP goes is achieve at max speed.

BoatKeep

id think around 10%