I konw there must be a formula for determining the correct prop size/pitch for an inboard engine on a displacement hull. What is it? Bob
Correct Prop Size
- Thread starter bobtigar
- Start date
- Status
Prop sizing is more of an art than a science, as it also depends on the HP, RPM range, hull shape, aperture size, etc. If you gave more information, the chances you'll get a good answer go way up.
There are a few different formulas, all of which share one important variable. Different symbols for it are used in different formulas and programs but, here, we can call it "F". This stands for "Fudge Factor" and it the primary determinant of the final diameter/pitch combination.
The larger the diameter, the more effecient the prop will be and the less fuel you will burn. However, there is a maximum speed the tips can go through the water without wearing away the metal so you have to keep raising the reduction gear ratio of the transmission. Since you probably aren't going to change transmissions, that is one size limit although it's a bit fuzzy. Tip speed that a yacht with less than 100 hours a year running time could get away with would be a big problem on a commercial vessel operating nearly full time. That formula needs a big "F" in it.
You need to have 10% diameter tip clearance if you are willing to tolerate a fair amount of vibration, 15% if you want it reasonably quiet, and 20% if you don't want to hear any significant propeller noise. Few sailboats can be arranged with the tip clearance for a really quiet prop. Put and "F" in that formula for owner noise and vibration tolerance.
Once you have a diameter, probably determined by the above, you then want to select a pitch such that the combination of the blades going around and the boat's forward speed will cause them to encounter the water at the optimum angle of attack. This means you have to know how fast the boat is going to go. That's a whole other question. Part of getting it right is knowing how effecient the prop is going to be and you don't know that yet.
The big "F" in the formula is the fact that the hull drags some water with it and this slows down the flow past the prop. The water speed at the prop is not the same as the speed of the boat through the water. There are rules of thumb for this based on the fact that fatter and fuller ended boats slow the flow down more than long skinny ones. Still, to get the prop exactly right, you have to know this factor. Without a super computer, the only way to know it exactly is to have data from a very similar vessel. This needs to be accurate trial data and not bar talk.
Most places that can sell you a propeller will have a computer program with enough "F" values established for common boat types that they can sell you a prop you can live with. If you want an exact match and are willing to pay for a day of careful sea trials with exhaust temperature and fuel flow measurements as well as a haul out, you'll probably have to switch props once or twice. This is usually only done on commercial power craft where a 1% fuel savings a year can pay for all that engineering.
The larger the diameter, the more effecient the prop will be and the less fuel you will burn. However, there is a maximum speed the tips can go through the water without wearing away the metal so you have to keep raising the reduction gear ratio of the transmission. Since you probably aren't going to change transmissions, that is one size limit although it's a bit fuzzy. Tip speed that a yacht with less than 100 hours a year running time could get away with would be a big problem on a commercial vessel operating nearly full time. That formula needs a big "F" in it.
You need to have 10% diameter tip clearance if you are willing to tolerate a fair amount of vibration, 15% if you want it reasonably quiet, and 20% if you don't want to hear any significant propeller noise. Few sailboats can be arranged with the tip clearance for a really quiet prop. Put and "F" in that formula for owner noise and vibration tolerance.
Once you have a diameter, probably determined by the above, you then want to select a pitch such that the combination of the blades going around and the boat's forward speed will cause them to encounter the water at the optimum angle of attack. This means you have to know how fast the boat is going to go. That's a whole other question. Part of getting it right is knowing how effecient the prop is going to be and you don't know that yet.
The big "F" in the formula is the fact that the hull drags some water with it and this slows down the flow past the prop. The water speed at the prop is not the same as the speed of the boat through the water. There are rules of thumb for this based on the fact that fatter and fuller ended boats slow the flow down more than long skinny ones. Still, to get the prop exactly right, you have to know this factor. Without a super computer, the only way to know it exactly is to have data from a very similar vessel. This needs to be accurate trial data and not bar talk.
Most places that can sell you a propeller will have a computer program with enough "F" values established for common boat types that they can sell you a prop you can live with. If you want an exact match and are willing to pay for a day of careful sea trials with exhaust temperature and fuel flow measurements as well as a haul out, you'll probably have to switch props once or twice. This is usually only done on commercial power craft where a 1% fuel savings a year can pay for all that engineering.
The real rule is that ANY motor MUST be able to reach its rated RPM under its normal load or it will be dammaged
Other boats in your boats family are the best place for info
Other boats in your boats family are the best place for info
Roger: " Without a super computer, the only way to know it exactly is to have data from a very similar vessel. This needs to be accurate trial data and not bar talk. Most places that can sell you a propeller will have a computer program with enough "F" values established for common boat types that they can sell you a prop you can live with."
On the B323 forum I offered that Gori Props gave me a size X prop. The other 323 owner called Gori and they gave him a different prop just on the basis of 2 or 4 less horsepower- both Yanmars. The horsepower and transmission ratio are big factors in sizing a prop, so leave it to the pros, and not the guesswork on this forum unless there's a prop-shop online here.
On the B323 forum I offered that Gori Props gave me a size X prop. The other 323 owner called Gori and they gave him a different prop just on the basis of 2 or 4 less horsepower- both Yanmars. The horsepower and transmission ratio are big factors in sizing a prop, so leave it to the pros, and not the guesswork on this forum unless there's a prop-shop online here.
The factors you need to consider are:
Engine Horsepower
Transmission gear ratio
Engine WOT RPM
Boat displacement
Prop aperture opening, if any
Number of blades
Yes, a good prop shop will be able to put you in the ball park at least, but even then, you usually have to fine tune it a bit. For most sailboats, as pointed out previously, a larger prop makes more sense, but will generate more drag as well. Folding and feathering props are often a good, if expensive, option as they will reduce drag under sail and often have better performance under power than a fixed prop.
Engine Horsepower
Transmission gear ratio
Engine WOT RPM
Boat displacement
Prop aperture opening, if any
Number of blades
Yes, a good prop shop will be able to put you in the ball park at least, but even then, you usually have to fine tune it a bit. For most sailboats, as pointed out previously, a larger prop makes more sense, but will generate more drag as well. Folding and feathering props are often a good, if expensive, option as they will reduce drag under sail and often have better performance under power than a fixed prop.
I've heard/read same as sailingdogs' sentence:
"Folding and feathering props are often a good, if expensive, option as they will reduce drag under sail and often have better performance under power than a fixed prop"
Also I've encountered posts from owners that prop walk in reverse was lessened after installing a folding prop.
Any opinions why folding props are reported to have the edge on motoring performance? If so, why aren't there fixed props that have the same type of profile as folding props blades when extended?
Just a question.
"Folding and feathering props are often a good, if expensive, option as they will reduce drag under sail and often have better performance under power than a fixed prop"
Also I've encountered posts from owners that prop walk in reverse was lessened after installing a folding prop.
Any opinions why folding props are reported to have the edge on motoring performance? If so, why aren't there fixed props that have the same type of profile as folding props blades when extended?
Just a question.
Dave Geer's Propeller Handbook is a good place to start if you want to learn the details of prop selection. Besides Diameter, pitch, number of blades, horse power and reduction gear ratio there is blade profile, as mentioned, blade area ratio, underwater hull profile, skeg location and rudder size and location.
Generally, getting pretty close is enough.
I know a man who builds and races hydroplanes with several hundred horsepower inboard engines. His propellers are much smaller in diameter and much higher pitch than mine and I run ten horsepower. But I am content with 5 knots and he want 125 knots.
The difference is race cars and farm tractors. A couple of hundred horsepower can be applied in more than one way.
Generally, getting pretty close is enough.
I know a man who builds and races hydroplanes with several hundred horsepower inboard engines. His propellers are much smaller in diameter and much higher pitch than mine and I run ten horsepower. But I am content with 5 knots and he want 125 knots.
The difference is race cars and farm tractors. A couple of hundred horsepower can be applied in more than one way.
Oh My ! How can I ever face him again?Ross—
It is Dave Gerr.. not Geer.
It is still a good reference book.
Castal Marine has a free program to calculate pitch. I have used it to verify prop info from Michigan Wheel, Max-Prop and others and they will all come out within one to two inches of this program. The nice thing is that you can run variations on horsepower, hull shape, displacement and see the diffrances in pitch, if any. As noted by others, pitch calcs is also an art. There are several prop designs from different manufacturers and they have there own calculation formulas that they use to determine pitch ( broad blade, narrow blade "sail" prop, etc.) so you need to know your existing prop manufacturer to make comparisons.
For the free program (with lots of disclaimers), see:
http://www.castlemarine.co.uk/pitch.htm
For the free program (with lots of disclaimers), see:
http://www.castlemarine.co.uk/pitch.htm
My folding two-blade has most of the blade out towards the ends, away from the hub, whereas the OEM had the most area in towards the hub.
It is called Trial & Error.
Each hull shape and engine combinantion will present variables. Cost is a factor that manufacturers will look at. A given engine develops its rated horsepower at a given RPM range. An adequately sized and pitched prop will allow that engine to reach that RPM range without exceeding it. Acceleration and reversing ability are factors of engine power and hull shape respectively but can be affected by prop choice. Three blade and high end props may not appeal to manufacturers due to cost but may yield better overall performance. Nowdays it would be rather costly for an individual to conduct the Trial and Errors test so we have to rely somewhat on other's tests and experience. Usually a manufacturer will exalt the virtues of their props while downplaying the faults but a fellow sailor with a same model boat and similar loading habits may provide a valuable recommendation unless he is selling you the prop. Those that claim that they can give you the best prop for your boat if you tell them the horsepower, type of hull and displacement of the boat can only offer a starting point. A bigger budget can provide you a better prop but it still has to sized appropittely.
Each hull shape and engine combinantion will present variables. Cost is a factor that manufacturers will look at. A given engine develops its rated horsepower at a given RPM range. An adequately sized and pitched prop will allow that engine to reach that RPM range without exceeding it. Acceleration and reversing ability are factors of engine power and hull shape respectively but can be affected by prop choice. Three blade and high end props may not appeal to manufacturers due to cost but may yield better overall performance. Nowdays it would be rather costly for an individual to conduct the Trial and Errors test so we have to rely somewhat on other's tests and experience. Usually a manufacturer will exalt the virtues of their props while downplaying the faults but a fellow sailor with a same model boat and similar loading habits may provide a valuable recommendation unless he is selling you the prop. Those that claim that they can give you the best prop for your boat if you tell them the horsepower, type of hull and displacement of the boat can only offer a starting point. A bigger budget can provide you a better prop but it still has to sized appropittely.
A really good
A really good prop guy who has been doing it for a long time, can get closer than probably any formula can, right off the top of his head. But even then, if you want it exactly right, might take a tweak or two. The correct way to do it is have the engine reach full rated RPM with normal load in the boat. I prefer to have mine turn a couple hundred more, if the boat is empty, cause I been known to throw a bunch of stuff aboard.
A really good prop guy who has been doing it for a long time, can get closer than probably any formula can, right off the top of his head. But even then, if you want it exactly right, might take a tweak or two. The correct way to do it is have the engine reach full rated RPM with normal load in the boat. I prefer to have mine turn a couple hundred more, if the boat is empty, cause I been known to throw a bunch of stuff aboard.
- Status