Ideal Prop Slip %
 

Even with my prop shaft at 4.5" below the keel of the boat (just like everyone else) my prop slip % is super low (7-8%).

Is there a magic # with respect to slip. Do I want/need to be up in the 12% range to free up the prop. I can't seem to get close to the 6100 I need to spin and it seems to make sense if I come up 1/2" or 1" it should free it up a bit.

Please throw some numbers out there. Most cats dream about 7-8%. I can't imaging a Heat with those kind of numbers.
RPM, speed, Gear ratio, Prop (and type) and work done to the prop if any.
Thanks,
Dave

0%

1.5 ratio, 6100, 89 GPS, 26 (25) Stock B1 was base # the boat came with.

1.5 ratio, 5800, 86 GPS 25 P5X in Havasu.

Standard height drive, prop is 4.5 inches below the keel of the boat. IMCO SC Xtreme (I thought I might have to much drive in the water but most everyone is about the same in terms of prop depth).

I was working down the path of the wrong gears , I.E. 1.34's because the slip actually pencils out at 12-13ish with a 1.34but I actually pulled the lower and confirmed 1.5's.

I need to spin 6100.... The boat usually runs in the 56 to 5700range with the 26.

From talking to you.... you might want to point out that you have yet to personally see the boat go over 86mph. The 89 mph figure was stated by others, but never seen by you.

5800rpm, 25 pitch prop, 86 mph = 6% slip

This seems really low.

You should also post this question over in the Prop section, and hopefully get some expert opinions.

How fast have you personally seen a 26 Bravo go? RPM? Speed?

Chris

I have a labed 28 from mercury racing that would work great on your boat. If you are at 8% slip you are doing good but some boats like slip to get the best top end numbers. Most fountains run in the 20's with slip to get the best top end speed.

I've actually run the boat 87 on Elsinor w/two people and a little over 1/4 tank.

87 @ 5850 with a box stock B1.

Once again, 6%.

Maybe I need to take a little cup out to free up the prop..
or come up a 1/2.

I don't know.

Have you checked to find out what prop was on the boat to get it to run 89 mph?

I seriously doubt it was a stock prop.

You can have the stock B1 thinned and labbed to pick up aprox 200 rpm. Or try a Mercury Racing labbed 26. It also will turn more rpm's than a stock 26 B1 while also probably being faster.

Chris

SouthOCG

Julie at Throttle-Up told me that the ideal slip for our type of hull is between 10% - 14% .
Steve Z's and BobL's slip is right around 13.5% using a P5x prop.

You might indeed need to raise your outdrive in order to increase slip and pick up speed.

Before going to that expense, I would have your current 26 Bravo thinned and blueprinted in order to increase rpm and speed. Or buy a Mercury labbed 26. Both these options are sure to increase your speed by a couple of mph.

Both these options will also increase your slip.

Chris.

On a side note, If you take the speed per horsepower on my boat, Steve Z's boat, and Bob L's boat, and plug this constant (227-229) into BAM's boat speed calculator, along with your aproximate weight of 4850#'s - 5000#'s, in addition to your 700hp. It perdicts your boat speed at between 86 and 88 MPH. Your boat speed to horsepower is right in line with everyone else. Both BobL and Steve Z have done a lot of calculations on our boats, and I find their math to be correct on my boat as well.

and
 

SouthOCG,
Altitude, weather, water conditions, and loads (people, gear, fuel) have a big effect on speeds. My boat varies up to 5 MPH from ideal conditions to bad conditions.
Do you boat at a higher altitude than the Havasu test that resulted in 89 MPH?
I assume your boat was lightly loaded during that test.
What was the weather like when they ran the test? I assume they tested in the winter to make deadlines for spring publication.
If you factor in ideal conditions, ie cool weather, light load, lower altitude than your locale, that could easily make a 3-5 MPH differential from the Hotboat test.
Have you asked Nordic to borrow that "magic" :rolleyes: prop they used for the HB test? I would.
A friend of mine owns a new 27' Lavey step vee with a 710 Ilmore, and he usually runs 87-89 here in summer heat. That boat has a much more radical double step hull than ours, and being a former drag boat racer, he's an excellent driver. I know he's squeezing everything he can out of it.
With a TU blueprinted 28" P-5X, my boat runs 90 @ 5850, 91 @ 5900, and 93 @ 6000 on the limiter. This propeller has about 13% slip, and equates to 30" Bravo 1 4 blade in performance.
I agree with Chris that your #s @ 700 HP are correct and in line with our (Bob, Dave, me) testing.
Best Regards,
Steve