311 Prop slip

[Formula88]

I was wondering what % prop slip are you getting on a 311 sr1. I have 30P Bravo1 4 blades and at 2800 rpm I'm going 40mph. That came out to be 25%. Is this too much or does that sound right for a heavy classic boat? My WOT rpm is 5400 and I have 22% slip then, but also the drives need to be fully trimmed to get the boat to run flat.

Reese

[4mulafastech]

It is normal to see your slip numbers increase some a slower speeds. Normally you will see slip decrease with increase in speed. You are doing good if you can get between 10-15% at full throttle. Ideally the props carry the hull nicely when running close to neutral (not trimmed really high).

Here are slip numbers on my 292 which is a twin stepped deep V that weighs 7500-8000 lbs running 28P Bravo1 4 blades.

[Formula88]

Thanks for the info 4mula. I was not sure if some thing was way off or what. The WOT slip should improve if I get some bow lifting props that are the same pitch, that way I can keep the drives straight out. If I run WOT with them straight the bow is way down in the water and feels like the tabs are down.

Any more info will be greatly appreciated.

Reese

[ThirdBird]

You're running 30" four blades on a 311?!?!?!?

What the heck is in that thing for power? That's a ton of prop for a 311 with standard power.

[jeff32]

what is the speed calculation formula again to get the theorical max speed?

max rpm x drive ratio x pitch?

[4mulafastech]

Theoretical MPH = (RPM/Gear Ratio) X (Pitch/1,056)

Dividing by the 1,056 gets the inches/minute to miles/hour.

[got twins ?]

Originally Posted by ThirdBird

You're running 30" four blades on a 311?!?!?!?

What the heck is in that thing for power? That's a ton of prop for a 311 with standard power.

I am running 24" four blades w/330s and I am getting 40 mph on gps @ 3000rpm ....

Something doesn't make sence ??? ....

Jeff

[Formula88]

Originally Posted by got twins ?

I am running 24" four blades w/330s and I am getting 40 mph on gps @ 3000rpm ....

Something doesn't make sence ??? ....

Jeff

What drive gear ratio do you have? I think I have 1.5, I have not confirmed it yet.

My next question is, could I have to much prop and I'm just cavitating? I boat does need to be brought up on a plane gently or it will blow out the props.

[got twins ?]

Originally Posted by Formula88

What drive gear ratio do you have? I think I have 1.5, I have not confirmed it yet.

My next question is, could I have to much prop and I'm just cavitating? I boat does need to be brought up on a plane gently or it will blow out the props.

I have 1.5 .....

I used to have major cavitation when I used my 23" labbed Mirages (3 blade).

Call Brett @ BBlades he will give you the best solution or answers you need .... he helped me out a lot .....

Jeff

[LAKE EFFECT]

Originally Posted by Formula88

I was wondering what % prop slip are you getting on a 311 sr1. I have 30P Bravo1 4 blades and at 2800 rpm I'm going 40mph. That came out to be 25%. Is this too much or does that sound right for a heavy classic boat? My WOT rpm is 5400 and I have 22% slip then, but also the drives need to be fully trimmed to get the boat to run flat.

Reese

If your spinning 30p Bravos @ 5400rpm on a 311, that calculates to 102mph with out any slip, and 79mph with 22% slip. If thats right I would have to guess you have some pretty big power, and too high of an X dimension.

I had 550/575hp in a 88 311 with deep TRS's, it ran 77/78mph on GPS with labbed 25p Mirages @ 5400/5500rpm with 10/11% slippage.

If all your data is accurate, you need to get your slippage in check. What drives are you running? Bravos? If so you can buy drive spacers to get the drives a little deeper without buying new props. The only other choice would be 5 blades, which shouldnt be necessary on a 311.

The 311 was designed for a 3 blade prop and a deep drive. Get the X dimension right and keep your 4blades and you should get some pretty good performance from that hull.

LE