Reply With QuoteI don't think cupping is designed to create lift.
Hello all
I run a sailfish 5.5 alloy cat, wondering if anyone is running a cat with cupped props?
Do they create more lift ?
I don't think cupping is designed to create lift.
What are they designed for ?
Cupped props help with cavitation
so I theory they LIFT your nose / bow.
which in turn drives your transom in to decrease cavitation,
so if transom is down and nose is up
that is lift.
now that’s the theory.
but not my question
Another design concept, called cupping, can also come into play here. Cupping means curling the trailing edge of the blade slightly to better grab water as it comes off of the blade face. This facilitates hook-up, but it can also load the engine more, much as adding pitch does. Different series of props have different amounts of cup in them. It is not uncommon today to have props with cupping added to the tip area of the blade to minimize tip losses and maximize efficiency.
This is a good old cut and paste job from the net: so lets think this through.
Yes cupped props help with cavitation due to them grabbing the water better. If you dont suffer from cavitation then this would only benefit your motor if it is underloaded at a certain RPM that you cruise at. In theory by grabbing the water better, your motor would undergo greater load but you would also travel further as the blade is having less slipage,
Does it lift the nose/bow? I'm no naval architect but i cant see cupping having that much of that effect. Yes you have better bite into the water, but engines arnt fixed and obviously with trim we can adjust the angle of thrust.
I think all you can take away from cupping is better bite into the water, more load on engine, less cavitation. Dont understand why it helps efficiency at higher speeds apart from less slipage in the water which would be accentuated at higher speeds (higher slip and more cavitation)
I dont know if cupping can extend to creating more lift. What are you trying to achieve with more lift ? are you transom heavy?
When people talk about lift they talk about stern lift not the bow
Bow lift is just a function of engine trim
^ agree: stern lift would be a function of the planing nature of the hull (planing characteristics) along with angle of thrust and overall weight/displacement .
Cavitation is not the prop sucking air and it is not affected by height in the water, so no, you theory isn't right and you need to work out the difference between ventilating and cavitating. Prop cupping increases water resistance and therefore, like a 4 blade prop, increases stern lift but not to the same degree. Done correctly you'll lose top end revs because of the extra resistance.
If your props are pitting and have burn marks on them from water boil, then yes - you have a cavitation issue which may well be fixed with some cupping.
If your props are ventilating now (blowing out, over revving and not biting the water), adding cupping will most probably make it worse due to the added stern lift. Ventilation issues are rarely fixed by changing just props, it is a motor/running line setup issue and ventilation is the result that disappears once the setup is fixed.
For the record, props can also create bow lift.
nil carborundum illegitimi
Just to kind of explain in very simple terms what cup is, and how it affects water flow, a prop spinning in the water kind of simply screws itself through the water, because the prop blade is a basic flat surface twisted into an angle, some of the water just slips off the outside edge, instead of pushing back, now if you bend the back edge of the prop up, this "cup" then catches some of the slipping water, and channels it backwards, creating added efficiency, but this cup, causes resistance, requiring more HP to turn the prop.
Once upon a time someone coined the phrase "stern lifting prop", I say we should track him down & neck him.
FWIW I once saw an ad for a prop that was both stern lifting and bow lifting at the same time. ROTFLMAO.
They must be stern lifting as when I'm underway I can see more of my transom than when at rest, LOL.Originally Posted by Fed
I was under the assumption of cupped trailing edge is to provide the next blade with clean undisturbed water ie no cavitation or air bubbles from the previous blades wash
The leading edge of all props would create some form of airation as it slices thru yhe water it would split the atoms and cause aeration
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And race teams would no longer have to carry 2 gearboxes for each engine to change the rotation and tune the amount of stern lift they want for the boat for each race given the sea conditions............. You've saved them a fortune.
nil carborundum illegitimi
No, not exactly, because the prop is "screwing" through the water, each blade is kind of in clear water, because the boat has moved forward.
But if u look at a planes airfoil in a wind tunnel it has a big bulge of air over the leading edge now we are talking props it doesnt have a airfoil profile but it does have a thickness that would create the same i would imagine
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on a slightly different tack,my suzuki was running a 17 pitch prop but was making 5500 on the tacho (should be around 6000) and as there are no 16 pitch available i got a cupped 15 which in theory takes it to a 16 (1 size up in pitch),so as stated above , cupping might not benefit the motors in the first place.
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