Ytmsn wrote: Wed Feb 13, 2019 8:09 am
I already know what I'm going to do, but I thought for the sake of discussion and for the benefit of some of our newer members we could talk about satin versus polishing. Labbing/Lapping (depending on what you type into the search bar) What clocking means, what Cupping means, what it does, etc..
I learned most of what I know from reading the Enquirer and on the innerweb....
Matt?
No worries just Coffee.
Duo prop out-drives are purpose built for efficiency. (Merc or Volvo) they are designed for reducing slipping in propulsion at steady speeds. This is Not pertaining to Volvos DPX Duo prop performance drives, or Mercury Black Hawk duo semi surface piercing drives) as those are a shallow depth set ups with high X - dimensions (out-drive location in relation to bottom of the boat) to leave as little of the out-drive in the water as possible. Less Drag equals speed!!
Prop Pitch is calculated a ZERO slippage for example a 26 Pitch prop is calculated to move 26 Inches in one rotation. So now lets chat!
Your limited on the low hanging large 6 blade set ups as they are great for taking off and holding cruise speed with limited slip, but they are awful at overcoming higher speed drag. So lets not overthink duo props trickery in obtain performance as they fight themselves just in the design as what they are intended to do.
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2019-02-13 08_00_54props.jpg
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Race boats use 4 blades.... 5 blade and in a Cleaver style to keep them slipping and accelerating at the same time. So when prop shops talk about tricking out Volvo F series props and or Mercs Bravo 3 props.. Take a breathe.....You must keep it simple and go for Balance and Blade rake equality. Thats about all you can do unless you try companies different theory on Bow or Stern lift as they design in the angel of rake built into each blade.
Labbing
Reducing the exiting resistance of a Prop blade to help it free up and obtain a higher pitch number along with a higher rpm. It usually includes thinning the edge blade (razor sharp) and creating a low resistance prop.
A boat that struggles to hit the rev limiter with a prop can benefit from this but now the prop is fragile for nicks and general abuse.
Satin Vs Polishing
Satin is the most common after a prop shop will change rake or repair bent blades. They also trim edges and tips that have chips and gouges so this method hides a lot of the repair work, and gives the prop and nice look over all. Some props thin the last bit of the edges with a satin 1/4 inch tail as this thin tips and inside no burrs. Its looks cool as well !!
Polishing is thought to allow the resistance of friction to be reduced but is time and labor extension on a repaired prop.
Cupping
"I put some cup in it " the prop guy said...
Take that same edge of the Blade we just discussed in Labbing, and give it a slight forward lean to create increased blade traction and reduce slipping.
More common on a 3 and 4 four blade single out-drive props, as they tend to hold the high slip percentage, and as a benefit of achieving a better cruise speed. Not designed to gain higher rpm or to increase MPH.
Clocking
The 6 blades ( 2 X 3 = 6 ) you have on these wonderful in effective duo props have been theorized to create such a turbulence in full throttle operation that cavitation and resistance to forward propulsion is felt to happen .
Since the props are counter rotating, the props may cross paths at the same time under the Cavitation plate of the lower unit. By clocking the props to avoid this occurrence is can reduce the turbulence and cavitation... so you can achieve a higher rpm along with forward propulsion when accelerating. Top speed improvement.
So in closing .. Keep it simple .. Keep them balanced and clean and don't over think whats Volvo and Merc have already designed for your Hull design, and all-around best handling and performance.
Prop shops are there to repair props more than they are to improve them. Companies like "B Blades" and other study rake angle and design as they can Taylor a need for your specific goal and most likely achieve it.
