hwsiii
Commander
- Joined
- Jan 25, 2009
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Prop Slip and Blade Surface Area
Prop Slip is conceivably one of the most misunderstood aspects of prop theory there is. Prop slip has nothing to do with the actual efficiency rating of a particular prop in a model or series of props, it is in fact the speed difference between what the manufacturer states as the theoretical pitch of a prop and what the prop actually produces in speed and effective pitch on any particular boat and motor combination. You can have either too low or too high of a prop slip, too low of a prop slip can be caused by the prop having too much blade area or geometry, either because of blade diameter, an extra blade or the actual size of the blades of the prop, and too high of a prop slip is caused by just the opposite of this, meaning that the blade surface area or geometry is too little for the load and associated drag that the prop is pushing. BUT it is also caused by the design elements implemented into a particular prop, although in this discussion the only thing we are talking about is blade surface area. I will be posting another white paper on blade design and the elements in a prop and their effects at a later time, after the next white paper on Effective Prop Pitch.
The only times that there is no prop slip for a propeller is when there is no power being applied to the propeller, such as on a sail boat under sail, or a boat that is being towed by another vessel or when the boat is coasting to a stop. In order for a prop to function as designed it will slip, as water is not a solid but a liquid, but the faster a boat travels in the water the more water becomes more like a solid than a liquid, and that is why very high speed boats use propellers that are much closer to twice the pitch than the diameter and have a Pitch/Diameter ratio very close to 2:1. Because the smaller the diameter is, at much higher speeds, the less blade friction there is to steal power and energy from the motor, and thus the more RPM your motor can turn, because of this increased efficiency. If this is hard for you to believe then just stick your hand out of a car window at 25 MPH and then raise your speed up to 65 MPH and tell me if there is not a major difference in the resistance you feel on your hand, water is no different, other than it has much more mass and creates much more drag because of this extra mass.
Let?s start with high prop slip, as this is the most common problem with boats and motors. When your prop slip is too high you are wasting a lot of energy from your prop, because the prop is not advancing in the water as efficiently as it should, and by proxy it is wasting energy from the motor as well. When you add extra weight to the boat like extra people, gear or trying to pull skiers out of the water then the prop slip just gets considerably worse and thus the prop becomes even less efficient. What we are trying to find in a better prop for our boats is a balance between too little blade surface area and too much blade area. I discuss blade surface area rather than diameter on a regular basis, for the simple reason that in props built today the prop designer decides the actual diameter of props, as the diameter follows the pitch of a prop when they design a particular series or model of prop, and as the pitch increases in any particular series normally the smaller the diameter is and the higher the efficiency of that prop becomes. When the prop has too little blade area it can also cause the prop to cavitate easier, and then you have the possibility of actually having full blow out of the prop because the cavitation bubble makes the prop have even less blade area. I am inserting a picture of a past proposal I made to someone about what prop I thought would be a good candidate for a better prop than the two he had already, one was a stainless steel and the other was an aluminum prop but both of them were showing a prop slip of 20%, which is too high for recreational boats, except heavy cruisers, in my opinion. I recommended a 4 blade aluminum prop to lower his prop slip, as it was too much. He just bought the prop I recommended and on his first trial with the prop he was able to attain 46.5 MPH and an RPM of 4,600, and this was faster than the SS prop he had before and it is in the right RPM range and with a prop slip of only 8%, versus the 20% he had before. These were my initial estimates on the performance of the new 4 blade aluminum prop.
jrampey Prop Slip
Propeller slip can also be too low, and most people don?t understand how it can be too low. But when a prop has a slip ratio that is too low, let?s just say below 7% arbitrarily, this usually means that a prop has more blade surface area than is required to equalize the drag from the boat, or the stated pitch is less than the prop actually has. For a particular boat and motor combination, when the prop slip is too low because of blade area, there is a lot of wasted energy used in blade friction instead of the forward propulsion of the boat. And in fact, if it is much more blade area than is needed the prop will not be able to attain the manufacturers recommended RPM for that motor, the same way that too high a pitch stops a prop from reaching the recommended RPM. I am inserting pictures and a chart here to show exactly what I mean, all three of these props are from Yamaha and the stated pitches of these props comes directly from the manufacturer.
Larry Prop Slip
The Performance series props have much more blade surface area, have a diameter of 13.25? and are designed for heavier boats than the small semi cleaver props, so when they are used on lighter boats they just waste potential energy that can be used for RPM and speed in blade friction and drag.
The 17? black painted steel prop is a semi cleaver with a small blade surface area, have a diameter of 13? and is known as a stern lifting prop as well, as it likes to run very high in the water column, which means you can raise your motor higher on the transom and thus gain more RPM and speed because of less drag from the lower unit in the water.
You can see from the pictures and charts above that all of these props have a prop slip at or lower than 7% and in fact the Performance 16? has a prop slip of only 4% at WOT, and that is exceedingly low, even though neither one of the Performance series props can attain the manufacturers upper recommended RPM range. This just goes to show that prop slip is one of the factors to be considered when deciding the correct prop for a boat, because if the prop slip is too low then you are wasting energy in blade friction and more drag. Blade design and pitch also control RPM and speed as well as blade size, which is part of the blade geometry of a prop. If you will notice in the charts that even though the Performance SS 16? pitch is less pitch than the 17? Painted SS it produces almost exactly the same speeds as the Painted SS prop at most RPM until it is overcome by blade friction, because of its much larger blade surface area, which creates less prop slip and thus more speed, and it can?t reach but 5,050 RPM before prop drag and friction take over and it can?t turn anymore RPM. This shows that both of the Performance series props have much more blade surface area than is required by the boat and motor setup on this particular boat, and that when your prop slip is too low that can cause the prop Not to be able to reach the recommended manufacturers top RPM rating, which is bad in my opinion.
Effective Prop Pitch
This tells me that the effective prop pitch of the 16? Performance prop is very close to the Painted 17? prop and creates increasingly more effective pitch and drag the more RPM it turns, because of its larger blade surface area, which lowers the prop slip. There is a lot more I can say about blade surface area and it might make it easier to understand, but I have found that if I make these papers too long that people will not bother reading them. I used these props and this particular boat as my example, as it is the only one I have in my database with three props and full speed numbers and RPM as well as for the differences in blade sizes.
The next white paper in this series will discuss Effective Prop Pitch and how this effects the performance of a prop on your boat, and how it can help you determine whether a particular series or model of prop will work satisfactorily on your boat.
You might also try reading this to learn more about Changing Props for Performance and SS versus Aluminum
http://forums.iboats.com/showthread.php?t=369057
H