Prepping my prop for fresh powdercoatiing - any performance tips or tricks I can do?

[Homerr]

Prop: Mercruiser 4-blade (834852A45)
Engine: 5.0L
Dia: 14.500"
Pitch: 18


I stripped off the old finish and smoothed out the battle dings and scratches. This prop is your basic cast OEM unit. Once I got it stripped to bare aluminum, I was amazed to see just how ROUGH the surface areas are. I would have thought the factory would have polished it better? Or perhaps they left it this way so the finish can attach better? Not just rough blades, but spots of flashing sticking up, etc. If they took 5 minutes with a Dremel or tumble it in plastic media, it would have done a world of improvement.

Question: Would it improve performance if I were to polish the prop to a very smooth finish before I have it powder coated? Or should I leave that rougher surface for the powder coat adhesion? I could probably get away with the standard stern-drive paint, but I can get the powder coating done for next to nothing, so I'd rather do that. Powder coating will last much longer and look nicer.

Also, any "high performance" prop I've ever seen is glass-smooth. I figured since I'm down to bare metal, now would be the time to make any improvements.
Or to heck with it and slap some Krylon on it!

Thank you!

 

[alldodge]

Prop: Mercruiser 4-blade (834852A45)
Engine: 5.0L
Dia: 14.500"
Pitch: 18


I stripped off the old finish and smoothed out the battle dings and scratches. This prop is your basic cast OEM unit. Once I got it stripped to bare aluminum, I was amazed to see just how ROUGH the surface areas are. I would have thought the factory would have polished it better? Or perhaps they left it this way so the finish can attach better? Not just rough blades, but spots of flashing sticking up, etc. If they took 5 minutes with a Dremel or tumble it in plastic media, it would have done a world of improvement.

Question: Would it improve performance if I were to polish the prop to a very smooth finish before I have it powder coated? Or should I leave that rougher surface for the powder coat adhesion? I could probably get away with the standard stern-drive paint, but I can get the powder coating done for next to nothing, so I'd rather do that. Powder coating will last much longer and look nicer.

Also, any "high performance" prop I've ever seen is glass-smooth. I figured since I'm down to bare metal, now would be the time to make any improvements.
Or to heck with it and slap some Krylon on it!

Thank you!

There is one thing you never do to the hull of a speed boat, and that is never wax the bottom. Props like boat hulls are best for speed when they a slightly rough. A shark swims very fast because its skin is rough.

A highly polished prop looks good and the one advantage is you can see a hair line crack easy. So an offshore racer wants to know if they have a crack starting. The satin finish prop is the best surface for a prop because it reduces drag.

To prove the point, take a piece of glass and lay it flat in the water, then try to pick it up. As for your prop it came out of a cast mold and they spry painted it, cheap and easy. I would just prime and Krylon it.

Just my .25 worth (inflation)

 

[gm280]

I don't know if I follow AllDodge's thinking on this one. He usually has spot on ideas that I normal agree with 100%. I know for a fact that when I waxed the bottom of my bass boat, it would easily pick up about 3mph for an outing or two before the wax was gone again. And I know a smooth surface cuts through the water easier then a rough surface. So if you want to polish your prop, have at it. To use his same example, lay a rusty old metal panel in the water and try to lift it out. Same things happens with that too. It is surface tension that causes that effect, not the smooth surface of the glass. Anytime you can reduce friction, you gain speed. Proof of that is how a boat will go faster if the motor is adjusted to the correct height and not dragging in the water... Hence jack plates assist all types of boats go faster simply by lifting the lower foot section and reducing drag! And that same effect is seen when the water has a slight chop instead of glass smooth. And while that seems to go against my premise, if you think about a glass smooth water, it flows over the hull at 100% instead of only hit and miss of the chop. So the slight choppy water has less drag on the hull... JMHO

 

[bruceb58]

Waxed hulls are always slower. That is why sailboat racers sand the gel coat on their hulls to make them faster. Having a slightly rough surface creates a boundary layer that actually reduces overall friction. The water is sliding across the boundary layer of water instead of the surface of the hull.

It's the same reason golf balls have dimples. Boundary layer of air around the ball reduces friction and makes the ball go farther.

 

[Homerr]

Thanks for the replies everyone! Very useful information. That makes sense about a rougher hull. I never thought of it like that, but I can see where it would create a buoyancy layer. Far as props... I think I'll leave mine alone and not polish it. I don't know a lot about powder coating, but it seems to me that a rougher surface might be better for the powder to cling to. Beside - it worked for years when the factor did it... why should I change it now? I just wanted to have it re-finished after I dug out all the dings and scratches. The powder coating probably won't cost me anything but a favor or two, so what the heck.

H.

 

[steelespike]

There was a thread here some time ago. A fellow did a test before and after polishing prop performance.
He polished the heck out of his al prop no wax just polished.If I recall correctly besides looking nice
it was very marginally faster. I think performance faded as the prop went back to its natural state.
Might be interesting to see the polished performance compared to painted performance.
I'm sure it will take a gps carefully watched to tell the difference if any.
I wonder about spray on Teflon coating.

 

[bruceb58]

If anything poweder coating your prop might make you slower just because you are making the blades thicker. For example, the main advantage of stainless props over aluminum is the fact that you can make the blades thinner for the same strength and thereby having less drag..

 

[gm280]

bruceb58, I do understand what you stated. But I can guarantee you my bass boat actually achieved more top speed (by the speedometer in the dash gauge ~3mph) after waxing the hull. I know it did because that boat got waxed about once a month when I had it. I actually would jack up the hull so I could wax in the runner areas as well. It was garage kept and looked like brand new when I sold it near ten years later as well. I was a fanatic about detailing it after every outing. No we didn't have GPS at the time, but using the in dash gauge was the same before waxing and higher speeds after waxing. So even if the actual speed was incorrect, it was incorrect for both speeds.

 

[bruceb58]

I don't know what to say. Waxing slows you down!

I used to race sailboats. No body ever waxed their boats. Trust me, the guys I raced against were fanatical about getting everything out of their boats.

 

[alldodge]

GM, The more the hull that is in the water the more wax can slow you down. A bass boat has very little in the water when at WOT. With so little I would like to see some actual numbers done with a GPS and I would bet there is next to no difference due to very little surface. That said in your first post

Hence jack plates assist all types of boats go faster simply by lifting the lower foot section and reducing drag! And that same effect is seen when the water has a slight chop instead of glass smooth. And while that seems to go against my premise, if you think about a glass smooth water, it flows over the hull at 100% instead of only hit and miss of the chop. So the slight choppy water has less drag on the hull

Jack plates put the blades in smoother water and also allows "on a bass type boat" allows the boat to be lifted out of the water more. Less hull in the water, less friction.

I slight chop in the water causes air to get caught under the hull, that air brakes the surface tension allowing the boat to go faster. This is way they started making stepped hulls on speed boats.

Here is a trick to beat a 75mph bass boat with a 60 mph speed boat. The speed boat gets to pick the day, and the day a speed boat will want is a 2 foot chop

 

[Watermann]

So when something is powder coated don't they put it in an oven? What abut the rubber hub inside the prop or is your prop one of the hub kit types with the prop shell?

You guys talking about drag should see this video...