Prop slip numbers
 

Is there a general range of "normal" prop slip percentages, or does it absolutely depend on the setup?

Re: Prop slip numbers
 

Yes, there are some guidelines established for acceptable ranges of slip or the % of slip.

4B = 10% - 14%
5B = 8% - 10%
6B = 3% - 8%

And this does depend on hull type and set-up as well.

Julie

Re: Prop slip numbers
 

Does that same rule apply to a twin application. What is acceptable for a 3 blade ? Who else besides herring is making 6 blades nowadays ?

Re: Prop slip numbers
 

3B propellers typically have 12%-15% slip, they are not quite as efficient as 4B propellers. Engine count is not relavent to slip.

Hydromotive and Hering make 6B sterndrive props.

Julie

Re: Prop slip numbers
 

Although Julie has set forth a group of numbers to use as guidelines, don't use them religiously. We have numerous customers running below 10% with 3 blades and some customers running above 10% with 5 blade props.

I just wanted to reiterate what Julie said, that is......Alot of things contribute to your efficiency. If you don't fit into those ranges, it doesn't necessarily mean your not dialed to your boats best potential.

For those of you that remember dialing boats 15 years ago, anytime we got below 15%, it was great. Below 20% may be acceptable. Boats running under 10% were typically only race set ups. The old mod VP boats could really run some good numbers!

Today the technology has improved the efficiency so far as that people feel like they should all run under 10%.

The point is, if you still run what you consider too high of a slip number. Consider all the possibilities and talk to knowledgeable set up people before making a possibly expensive mistake.

Re: Prop slip numbers
 

While we're on the subject of slip, I'll throw out my question and experience. I've got a 28' Nordic currently running about 750 HP with an ITS/XR 1.5:1 drive package. I've tested quite a few props on it through several engines. I've bought several P5X's from Julie over the last couple of years. Currently I run a stock 29 P5X at 5500 RPM and 84 MPH. If you calculate the slip, it's very high. At cruise it's really high, like 30% or so. In fact I turn about the same RPM at 40 MPH as I did with a 25 P5X. I also run a 30 Bravo 1 prop and it slips a lot worse. I'll be sending the 29 back to you in the next week or so, Julie. I haven't really had time to experiment this summer because we've been so busy. I thought I'd post this here and get some opinions. I'm a Nordic dealer and have quite a bit of experience with the 28, and they all run high slip numbers at cruise. They seem to really need a lot of bow lift. My boat has an open bow so invariably I have a lot of weight up front while cruising. This really drives the slip way up. I assume adding some cup to the tips will help.

Bob Lloyd

Re: Prop slip numbers
 

Bob,

We should take a look at the slip numbers you are experiencing. You may be getting to much stern lift and if your drive is realatively high this may be causing some of the slip you are seeing. Post you numbers so we can take a look.

Regards,
Julie

Re: Prop slip numbers
 

OK Julie, here are some numbers from this weekend. This is with the boat carrying 60 gal. fuel, 5 adult passengers and the other necessary accessories. At 3000 RPM it was running 36 MPH. 3500 RPM was 45 MPH, and 5500 RPM at 84 MPH. The speed is GPS and rpm is from the laptop. The cruise numbers are better with no weight in the boat...but who uses it that way? This is with the 29 P5X prop. The Bravo 1 prop is much worse. I don't think there are any magic answers but it'll give you something to think about. Like I said in the other post, these numbers are fairly close to what it did with the 25 P5X until it reaches over 4000 RPM. If adding cup will help the midrange then I'm all for it. I'm not worried about WOT RPM. In fact I'll be adding another 75-100 HP once I crank the boost and timing on up to where it's supposed to be. I know the hull needs more bow lift.

Re: Prop slip numbers
 

How deep is the prop shaft?



Sometimes using a smaller diameter with less blades will actually increase the bow lift. If the prop is deep enough this will work.

It is normally tough to beat the low end and mid range of 5 blade props. But, 4 blades can work better on certain applications. More blades more stern lift.

Your slip #'s are high. The C.G. must be quite forward in the boat. As you also stated, open bows can contribute to this.

Re: Prop slip numbers
 

Bob,

We have definately needed to increase bow lift with the Nordics. As you said you have a lot of weight up front so indeed it is probably throwing the CG's off. let us take a look at the propeller and we will see what can be done to improve your situation. Talk to you soon!

Julie