Fixed vs. Free Wheeling Prop - Updated !
- Thread starter Maine Sail
- Start date
Isn't anyone going to poke some fun at MainSail?
Like too much time spent in the barn? I don't have the guts, someone else?
Like too much time spent in the barn? I don't have the guts, someone else?
Scott, I'm not afraid..... he owes me a HUGE favour
Maine, Dude, you spend WAY too much time in the barn coming up with some weird stuff.... you need to get that boat in the water and have some fun, you're obviously getting a severe case of cabin fever..... maybe deep down inside you're Canadian
Re: Fixed vs. Free Wheeling Prop - Coming Soon !
Maine Sail—
Can you do the test with a two-bladed prop as well.... and how about a feathering prop??
Maine Sail—
Can you do the test with a two-bladed prop as well.... and how about a feathering prop??
Interesting set up...
that you have designed Maine Sail. Have you thought about how accurate it will be? I am not by no means being critical but just putting this out there for discussion and I look forward to reading the results of your study (even though I have a Kiwi).
I question a couple of things that may affect the readings. Are your knots secure? After the line/knots get wet/cold will things stretch/contract? Is your fish scale an accurate one?
You have designed and built a gage. In industry we can (and many times are required to) check the accuracy of the gage by performing what is called a measurement system analysis, MSA, or what is more commonly know as a Gage R & R (repeatability & reproducibility) study. It's quite an exact test. I used to do them. You can bet the guys at MIT did this for their test. Not saying you need to do this but you might do a simple abreviated test to satisfy your own curiosity.
A suggestion would to be to run a few test runs at different boat speeds with one prop. Say one, two, three, and four knots. It's important to get the full range (lowest scale and highest scale readings you expect to find) on this test. Record your findings from the scale. Repeat the same test a few more times. Did you get the same results? If not there is something wrong with your gage or the user. Did you forget your glasses? Did you give the GPS enough time to stabilize? Was there current present at one test and not the other? Did the line contract from one test to another?
If it turns out that there is 60% more drag on a fixed prop than a free wheeling prop the gage R & R is less critical. But when you tell me there is a 10% difference and you have no Gage R & R I would be skeptical.
Thanks for taking the time to share with us.
that you have designed Maine Sail. Have you thought about how accurate it will be? I am not by no means being critical but just putting this out there for discussion and I look forward to reading the results of your study (even though I have a Kiwi).
I question a couple of things that may affect the readings. Are your knots secure? After the line/knots get wet/cold will things stretch/contract? Is your fish scale an accurate one?
You have designed and built a gage. In industry we can (and many times are required to) check the accuracy of the gage by performing what is called a measurement system analysis, MSA, or what is more commonly know as a Gage R & R (repeatability & reproducibility) study. It's quite an exact test. I used to do them. You can bet the guys at MIT did this for their test. Not saying you need to do this but you might do a simple abreviated test to satisfy your own curiosity.
A suggestion would to be to run a few test runs at different boat speeds with one prop. Say one, two, three, and four knots. It's important to get the full range (lowest scale and highest scale readings you expect to find) on this test. Record your findings from the scale. Repeat the same test a few more times. Did you get the same results? If not there is something wrong with your gage or the user. Did you forget your glasses? Did you give the GPS enough time to stabilize? Was there current present at one test and not the other? Did the line contract from one test to another?
If it turns out that there is 60% more drag on a fixed prop than a free wheeling prop the gage R & R is less critical. But when you tell me there is a 10% difference and you have no Gage R & R I would be skeptical.
Thanks for taking the time to share with us.
Re: Fixed vs. Free Wheeling Prop - Coming Soon !
Pat—
I don't think Maine Sail is going for absolute accuracy, but more to get a basic idea of what the relative results are. There are far too many variables in a given boat/prop/keel/transmission setup for his fairly simple test rig to give any specific data... but it should at least give a ball park answer as to whether the prop should freewheel or be locked.
Pat—
I don't think Maine Sail is going for absolute accuracy, but more to get a basic idea of what the relative results are. There are far too many variables in a given boat/prop/keel/transmission setup for his fairly simple test rig to give any specific data... but it should at least give a ball park answer as to whether the prop should freewheel or be locked.
Welcome to SailboatOwners Pat, hehe. Mainesail is pretty much a stickler for accuracy. Although, as he stated, he doesn't care about exact measurements. I'm sure the knots aren't going anywhere. As far as stretch of the line goes, the tension will still be the same on the scale if the line stretches or not. As for the scale, he's comparing it against itself, so if the scale isn't 100% accurate at least it will give him something to compare. And if his collection of voltimeters are any indication, I'm guessing he probably has another scale (or 7) around. This isn't his first barbecue. Check out his site and you'll get an idea of what a perfectionist he is.
Check out his site and you'll get an idea of what a perfectionist he is.
Re: Fixed vs. Free Wheeling Prop - Coming Soon !
Maine, can you explain exactly how this will show differences in drag...I am no engineer so please excuse my ignorance, however I don't see how this will measure anything as the entire unit appears to be one peice and does not move with the exception of the hinge. I imagine as you start, the hinge will conform to the angle in the wood and stay there in both cases, thus the measurement will be the same.
Wouldn't the drag be shown if the shaft portion was on a heavy spring that would be pulled based on the drag??
I am sure I am missing something. Like I said, I am no engineer just a beer drinking sailor...
Thanks!
Maine, can you explain exactly how this will show differences in drag...I am no engineer so please excuse my ignorance, however I don't see how this will measure anything as the entire unit appears to be one peice and does not move with the exception of the hinge. I imagine as you start, the hinge will conform to the angle in the wood and stay there in both cases, thus the measurement will be the same.
Wouldn't the drag be shown if the shaft portion was on a heavy spring that would be pulled based on the drag??
I am sure I am missing something. Like I said, I am no engineer just a beer drinking sailor...
Thanks!
True, but it would be interesting to see what kind of reduction in forces they generate...
Ah, you meant compare them to the fixed blade numbers....True, but it would be interesting to see what kind of reduction in forces they generate...
Re: Interesting set up...
Pat, Welcome! As you have probably noticed I have supported Maine sail on this effort. Without regard to the actual drag values ten per cent difference will be insignificant. Snagging a piece of seaweed on the rudder may induce enough drag to make up the difference. The result that I expect to see from this effort will be relative. A very sophisticated test apparatus would be required to get absolute numbers.
If you want to determine if a three blade prop will have enough added drag to adversely affect your boat speed you may tow a food can with the same cross sectional area as one propellor blade and observe your speed display. Will it be absolute? No! Will it represent an approximation ? Absolutely! This test is not for two decimal place accuracy but for real world relative effects of fixed or free props.
Pat, Welcome! As you have probably noticed I have supported Maine sail on this effort. Without regard to the actual drag values ten per cent difference will be insignificant. Snagging a piece of seaweed on the rudder may induce enough drag to make up the difference. The result that I expect to see from this effort will be relative. A very sophisticated test apparatus would be required to get absolute numbers.
If you want to determine if a three blade prop will have enough added drag to adversely affect your boat speed you may tow a food can with the same cross sectional area as one propellor blade and observe your speed display. Will it be absolute? No! Will it represent an approximation ? Absolutely! This test is not for two decimal place accuracy but for real world relative effects of fixed or free props.
Re: Fixed vs. Free Wheeling Prop - Coming Soon !
Once again!! OUTSTANDING, Main.. after considering this carefully over a nice Merlot, the only thing I see is that the force on the scale will be less than the true drag force on the prop. It will be reduced by approximately the cosine of the angle between the string and the “strut”.. assuming that the strut is kept nearly perpendicular to the horizontal part of the test rig and the hinge has minimal bend. From the pictures, I measure about 40 degrees so the scale reading should be multiplied by 1/cosine. If the angle is 40, and the scale reading is 10 pounds, then the (approximately) real drag on the prop is :
10 X 1/cos40 = 13.05# Since this is true, the difference between spinning and locked will be bigger than measured too.. (Injecting my bias now) when you measure 20 # with the prop locked and 10# with it spinning, the actual drag difference will be 13and some change and not 10.. So when it looks better to freewheel, it will be LOTS better. (except maybe in Canada)
Once again!! OUTSTANDING, Main.. after considering this carefully over a nice Merlot, the only thing I see is that the force on the scale will be less than the true drag force on the prop. It will be reduced by approximately the cosine of the angle between the string and the “strut”.. assuming that the strut is kept nearly perpendicular to the horizontal part of the test rig and the hinge has minimal bend. From the pictures, I measure about 40 degrees so the scale reading should be multiplied by 1/cosine. If the angle is 40, and the scale reading is 10 pounds, then the (approximately) real drag on the prop is :
10 X 1/cos40 = 13.05# Since this is true, the difference between spinning and locked will be bigger than measured too.. (Injecting my bias now) when you measure 20 # with the prop locked and 10# with it spinning, the actual drag difference will be 13and some change and not 10.. So when it looks better to freewheel, it will be LOTS better. (except maybe in Canada)
Re: Fixed vs. Free Wheeling Prop - Coming Soon !
Doing any kind of "test" without some very sophisticated equipment and computing will not provide any difinative answers. The measured drag of fixed and free wheeling propellers will be greater or lesser as compared to the opposite, depending on hull shape, strut configurations prop design and position of prop relative to hull. In other words everybody is right, depending on what their personal experience was. Therefore you cannot predict or calculate what the drag is going to be with any certainty. Only emperical data from a tow tank will tell accurately.
Doing any kind of "test" without some very sophisticated equipment and computing will not provide any difinative answers. The measured drag of fixed and free wheeling propellers will be greater or lesser as compared to the opposite, depending on hull shape, strut configurations prop design and position of prop relative to hull. In other words everybody is right, depending on what their personal experience was. Therefore you cannot predict or calculate what the drag is going to be with any certainty. Only emperical data from a tow tank will tell accurately.
Mainesail is using a towing tank! It is call the Gulf of Maine. His test set-up is an attempt to put the prop in non-trubulent water. There are as many varibles that can be applied to this as there are sailboat models. I don't think that we will find that there is a substantial difference unless you're are racing like boats with matched crews over a long course. But for cruisers it is a good subject for winter nights in front of a fire with a bottle of port.
Re: Fixed vs. Free Wheeling Prop - Coming Soon !
Some of you engineering types are really funny!!!
My money is on the locked screw having more drag.
Paul
Some of you engineering types are really funny!!!
My money is on the locked screw having more drag.
Paul
Re: Fixed vs. Free Wheeling Prop - Coming Soon !
I have a difficult time thinking that there can much of an issue of drag when comparing a fixed to a freewheeling prop. Seems like all one need do is go sailing and look at your knotmeter or GPS under either circumstance. I know the result on my boat. The issue to me would be whether there is "significant" (however defined) wear on the running gear, i.e., transmission, packing gland, and cutlass bearing when freewheeling vs a fixed prop.
I have a difficult time thinking that there can much of an issue of drag when comparing a fixed to a freewheeling prop. Seems like all one need do is go sailing and look at your knotmeter or GPS under either circumstance. I know the result on my boat. The issue to me would be whether there is "significant" (however defined) wear on the running gear, i.e., transmission, packing gland, and cutlass bearing when freewheeling vs a fixed prop.