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Propeller slip numbers- 3 blade vs. 4 blade

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Propeller slip numbers- 3 blade vs. 4 blade

Old 10-09-2020, 07:50 AM
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Default Propeller slip numbers- 3 blade vs. 4 blade

I have read about propeller slip over the years and know it's not the end-all regarding correct prop, but did not see info on comparing 3 to 4 blades in the calculated slip percentage. I have a new to me boat with Mercury 4 blade offshore and slip looks to be high teens.
Thanks or any info!
Old 10-09-2020, 08:03 AM
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My experience has been that a 4 blade yields better results with higher prop shaft heights. I'd be shooting for 10-12% slip.
Old 10-09-2020, 08:09 AM
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You can’t talk about propellers in simple terms like number of blades. It depends entirely on what model of propeller it is.

The Offshore / VenSura is a four blade that typically yields high calculated slip, so your numbers do not surprise me.

Remember, low slip is never a goal, it’s just a data point to see if a given model of propeller is performing within known expectations.
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Old 10-09-2020, 08:24 AM
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Is your objective low slip or best efficiency?

All things being equal (which they almost never are) a four bladed propeller will be less efficient than a three bladed propeller. When it comes to converting engine power to thrust.

But if and when one changes from a like kind three bladed propeller to a like kind four bladed propeller, the slip numbers may look better, given that the four bladed propeller blades will have less pitch. Assuming the same WOT engine RPM is attained with both propellers. Even though the thrust, and the boat speed, may be less.
Old 10-09-2020, 08:26 AM
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Originally Posted by Tom W Clark View Post
You can’t talk about propellers in simple terms like number of blades. It depends entirely on what model of propeller it is.

The Offshore / VenSura is a four blade that typically yields high calculated slip, so your numbers do not surprise me.

Remember, low slip is never a goal, it’s just a data point to see if a given model of propeller is performing within known expectations.
For sure, not all propellers with the same number of blades have the same characteristics. Stern lift, bow lift, etc.

I'm not following you last statement. Why would someone want to run a prop with excessively high slip numbers? Low slip numbers means efficiency to me. More MPH per RPM.
Old 10-09-2020, 08:28 AM
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Originally Posted by Tom W Clark View Post
You can’t talk about propellers in simple terms like number of blades. It depends entirely on what model of propeller it is.

The Offshore / VenSura is a four blade that typically yields high calculated slip, so your numbers do not surprise me.

Remember, low slip is never a goal, it’s just a data point to see if a given model of propeller is performing within known expectations.
In aviation, propeller pitch is calculated from a measurement of the propeller blade angle. With the blade angle being measured at some point outward along the propeller blade.

Do you have any references to how outboard propeller pitch is calculated amongst the various propeller makers? I have been unable to find anything.
Old 10-09-2020, 08:31 AM
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Originally Posted by 30Concept View Post
For sure, not all propellers with the same number of blades have the same characteristics. Stern lift, bow lift, etc.

I'm not following you last statement. Why would someone want to run a prop with excessively high slip numbers? Low slip numbers means efficiency to me. More MPH per RPM.
What if the boat was faster with a propeller having a high slip number (and better fuel efficiency) vis a vis a boat being slower with a propeller having a lower slip number?

A propeller maker seems to be able to label his propeller with whatever slip number he wants to use. They can game the system.
Old 10-09-2020, 08:36 AM
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I like to mess with props.

On most of my boats I see 9-12% slippage with 3 bladed props.

With Merc Racing 4 blades I've been running 4-6%, which i thought was basicly impossible.

Under 15% is generally considered pretty good. Over 15 you likely have some setup issues.
Old 10-09-2020, 08:37 AM
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I changed from a 3 blade to a 4 blade mid season this year.

Very happy with the feel, but have not had enough time on the water after the swap to determine the new prop slip calculation. The feel is simply stronger, speed and out of the hole is comparable.

If my slip was in the teens I probably would not have changed it. My slip was close to 40%...it was the prop on it when I bought the boat end of last year.
Old 10-09-2020, 08:37 AM
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Originally Posted by alloyboy View Post
What if the boat was faster with a propeller having a high slip number (and better fuel efficiency) vis a vis a boat being slower with a propeller having a lower slip number?

A propeller maker seems to be able to label his propeller with whatever slip number he wants to use. They can game the system.
Take the calculators for what they are worth, but this is a simple math equation with gear ratio, pitch, and RPM to find the theoretical speed. Then, factor in slip, and the speed can only go down. Assuming the RPMs, gear ratio and pitch are the same, no way is higher slip going to have a higher speed.
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Old 10-09-2020, 08:43 AM
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4b props usually have lower slip, that's a plus. But also have more drag, that's a downside.

Which effect is dominant, no hard rules, depends on boat and motor and prop and all sorts of other stuff.

Minimizing slip is good up to a point, but you can lose the advantage if drag goes up.

More blade area (as in bigger or more blades) increases skin friction of the water over the blades, thus more drag. More drag means lower top speed and more fuel at cruise.

Props will always have some slip. Water is a fluid (duh, I know) and if prop is making thrust, some water will move out of the way. Too much slip is bad, though.

Best to try a few wheels and see what works best for your ride.
Old 10-09-2020, 08:48 AM
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Originally Posted by RiptideCharters View Post
I like to mess with props.

On most of my boats I see 9-12% slippage with 3 bladed props.

With Merc Racing 4 blades I've been running 4-6%, which i thought was basicly impossible.

Under 15% is generally considered pretty good. Over 15 you likely have some setup issues.
That sounds good. I'm running bravo 4 blades right out of the box and I'm seeing 8-10% slip.
Old 10-09-2020, 09:00 AM
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"4b props usually have lower slip, that's a plus. But also have more drag, that's a downside." This tends to be true.
That's why folks report better acceleration, backing, vibration, and slow speed performance with the additional blade. What's far more difficult to predict is any mileage improvement at cruise speed. That's a metric many worry with. This is approximately where the 4blade performance curve (trending downward) intersects the 3blade curve. As an aside, I was disappointed in that my favorite prop puller didn't fit my 4bl props. Some designs have the blades overlapping at the hub.
Old 10-09-2020, 09:28 AM
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Originally Posted by 30Concept View Post
Take the calculators for what they are worth, but this is a simple math equation with gear ratio, pitch, and RPM to find the theoretical speed. Then, factor in slip, and the speed can only go down. Assuming the RPMs, gear ratio and pitch are the same, no way is higher slip going to have a higher speed.
Take two propellers. Different models from different makers. But otherwise virtually identical.

Both propellers propel an identical boat with an identical motor to the same speed at X RPM.

One maker labels his a 17" pitch. The other labels his a 19" pitch.

The 19" pitch propeller has a higher slip number than the 17" pitch propeller. Which will lead some/many to believe that the 19" propeller is inferior to the propeller with the 17" pitch. Is it?

Old 10-09-2020, 09:40 AM
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Thrust pattern plays an important role in how a prop performs. It goes beyond number of blades. The pattern affects how the total thrust is shared laterally and rearward. A propeller will move X amount of water (100%) at a given RPM and velocity through water. Some of it goes back, some of it slips off the tips, but you can't have 100% of both. Also, there is static thrust, when there is little or no forward movement, and dynamic, when the prop is moving through the water at various speeds.
A prop that is designed for stern lift will typically have a wider thrust pattern and direct some of the thrust laterally. These tend to have purposeful foil designs, less rake angle and less cupping to allow some of the water to be directed laterally, thus losing some rearward thrust.
A prop with higher rake angles and considerable cupping will hold the water better and direct thrust rearward with a tighter pattern, which tends to have bow-lifting tendencies when thrust height is low.
It would be helpful if some of these design numbers were more readily available, but it would generally cause more confusion.
This is all theory and there is no substitute for testing, more testing and lots of testing. Testing can get expensive.

Old 10-09-2020, 09:41 AM
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Originally Posted by 30Concept View Post
Why would someone want to run a prop with excessively high slip numbers? Low slip numbers means efficiency to me. More MPH per RPM.
Very common mistake. Low slip does NOT necessarily mean efficiency.

An example: One of the more common rigging errors is mounting a motor too low on the transom. Raising it will often gain a boat both speed and fuel economy, yet it will invariably increase the calculated propeller slip, regardless of what model of propeller is being used.
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Old 10-09-2020, 09:54 AM
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Originally Posted by alloyboy View Post
In aviation, propeller pitch is calculated from a measurement of the propeller blade angle. With the blade angle being measured at some point outward along the propeller blade.

Do you have any references to how outboard propeller pitch is calculated amongst the various propeller makers? I have been unable to find anything.

link below explains, progressive pitch props are an average.

https://www.ptprop.com/prop-basics.html
Old 10-09-2020, 10:10 AM
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Originally Posted by Tom W Clark View Post
Very common mistake. Low slip does NOT necessarily mean efficiency.

An example: One of the more common rigging errors is mounting a motor too low on the transom. Raising it will often gain a boat both speed and fuel economy, yet it will invariably increase the calculated propeller slip, regardless of what model of propeller is being used.
I fully understand what you are saying here. But, now there is another variable being added, prop shaft height. Testing starts again. The OP may very well benefit by making a change there, but that is not what he asked. Depending on the volume of a particular hull, most of the guess work of motor height and best propeller should already be eliminated.
Old 10-09-2020, 10:16 AM
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Originally Posted by Tom W Clark View Post
An example: One of the more common rigging errors is mounting a motor too low on the transom. Raising it will often gain a boat both speed and fuel economy, yet it will invariably increase the calculated propeller slip, regardless of what model of propeller is being used.
How does that work? Calculated slip is a simple formula, given the same motor and the same prop it's just a function of speed at the given rpm. More speed for the same rpm implies less calculated slip, does it not?
Old 10-09-2020, 10:25 AM
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Originally Posted by Drako View Post
How does that work? Calculated slip is a simple formula, given the same motor and the same prop it's just a function of speed at the given rpm. More speed for the same rpm implies less calculated slip, does it not?
Drag from lower unit is decreased when motor is raised. Slip numbers can increase, but speed can increase also when motor is raised.

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