prop finish

[Roflhat]

I've been trying to read up about this online but there are clearly mixed views.
What kind of finish gives the best results on a prop? some people say polished but others say its best when you used 120-grit sandpaper

[zul8tr]

Here is an interesting link on the subject, site owner Ron Hill is in it

http://www.screamandfly.com/showthread.php?24877-Polish-prop-or-not

[Roflhat]

interesting read, not a very definitive answer though

[Ron Hill]

I'll add my thoughts here. Over the years, in kneeldown boats I've never seen a satin finished prop in the inspection area of the finals at an APBA Nationals. Usually, the props are thin as hell and sharp as a razor.

BUT, I used to make OPC props like that and they usually broke after not much use and that was in the 150 HP days.

I my onw testing, I believe a satin finished bottom is in fact faster than a polished bottom. Jay Smith said 400, but I like almost 80 grit...sanded long ways, which brings us to the question of satin finished propellers. I would not want the satin to go across the blade. I try to make the satin lines follow the rotation of the blade, not exactly but in that motion. I am convinced that Speed coat is slower.

The trailing edge is very important in how a props really runs. We, family relgion, always took a very sharp knife and ran it across the trailing edge of a blade to where there was a slight burr, just before a Kilo run or a Nationals final heat.

The slightest "EDGE" is what you are looking for in racing. Polishing, under a microscope, tends to roll off the back edge. But this can be sharpened back..

Bottomline, if the prop works, keep it...Polishing a satin finished prop could ruin it. Same as satin finishing of a polished props could ruin it. I take a machine polished Mercury Bravo One, thin it, balance it, sharpen the edges, and satin finish it and it picks up three mph and gets on plane faster. Nothing I did SHOULD make it plane faster, I wouldn't think anyway, except the satin finish.

Sand blasting or glass beading the trailing edges, to me, was done to cause the cup to "Slip". See, I believe cups slow props down......Anyone want to go there??

DeWald seems to own Formula One, his props are polished. Most Formual One dudes don't use Merc Props, but I've heard it is a customer service (He loans you several to buy one). I've heard Mercury sells you one or several. When I've reworked DeWalds, the winningest one I did was satin finished.

Jim Boo has built some damn fast props over the years. His polishing is close to perfect.

All the above are my opinions. I guess that is what makes "HORSE RACE".

Bottom line: I don't know that there is an answer...maybe, I should run for politics!!!

[Skoontz]

Ron let me axe sumpin. Is it not easier to see stress cracks and would be trouble areas with a polished prop?
Don Hendrich would polish my props and then sharpen the edges so you would get a mirror finished prop and then about 1/4" of the edges ground sharp. I even smeared a little shaving cream on my left arm and sliced the hair off one night when I had a few too many Heilemans Old Style beers to show my buddies.... You could see the machine marks running the same direction as the edges parallel along the curves if that's the correct term.

[zul8tr]

All my props from Pinner were satin, I never polished them out and they ran well in 25ssh. If they were polished I doubt that any cost effective measuring device could detect the difference under controlled real world test conditions because of the many variables that would be hard to keep constant. No doubt controlled lab conditions could detect a difference in drag (interesting test for a ventilated surface piercing prop) but not the same as real world running.

As for bottom finish tried gloss varnish and sanded flat black with 400 in long direction and sanded was better feel with my SOTP Buttmaster (patent pending). BTW Bob Wartinger, Bob 110 records as of 11/04 and read he uses sanded bottoms.

[Ron Hill]

My dad had a small cabin at Lake Elsinore. He made his own water skis, he had heard of them, but never seen any, He made them out of 1 X 8's and actually made 20 or so sets. One thing he did while fooling with his skis, was to try different things on the bottoms to reduce drag. He always said sanded varnish with square trailing edges had the least drag. When fiber glass bottoms came into being, he always sanded the fiberglass and made sure the small grooves ran long ways.

As far as Race Props go, I have yet to see a SATIN FINISHED propeller win a Nationals. I'm not saying it hasn't happened, I just have never seen it. The last time I was in the H1 pits at San Diego, I was actually quite surprised to see how highly polished all the props were on these turbine powered boats.

Thin and sharp have always been fast.......but they don't always last!

[Roflhat]

great replies, I was thinking in terms of the air on the prop, which when sanded is obviously more. In boat racing all the top offshore boats have stepped hull to reduce contact area with the water, does the same principle apply to propellers?

[Ron Hill]

You want a prop blade to push water, not slip through it. I'll try to post some pictures tomorrow of some prop surfaces!

[zul8tr]

No, stepped surface does not apply to props. On a hull the steps reduce bottom surface area and introduce air downstream of the step thus less drag with the air entrained mix. The ocean racers I have seen have rough surface where the water contacts the bottom. It is a laminar vs turbulent boundary layer issue with the drag component on the bottom. As Ron Hill correctly notes on props you want the water to stay in contact with the push face (aft facing for thrust. It of course is complicated by the surface piercing fully ventilated props that are used on the race hulls which operate with turbulent boundary layer air mix on the push face.

[phillnjack]

so would it be better to have one side of a prop polished and the other side satin ?
What i mean is the side closest the boat mirror polished to slip into the water, and the side pushing the water away to be satin ?
that probably sounds a bit daft to those who know,
But would (or could) it make sense if it slips in easy then grips to push out !!

obviously the look of the props are much nicer in mirror finnish, but thats fine for the trailer viewers.
I would imagine the racers have already tried all the combinations possible ha ha

phill

[zul8tr]

That is worth an experiment but doubt if the difference could be accuratly measured in testing with so many variables that change so frequently.

[phillnjack]

Yeah your probably right
cant even go out 2 days on the run and get the exact same speeds.
so it must be a case of personal choice.
myself i do like the look of a nice polished prop

phill

[Roflhat]

It's an interesting idea. I reckon full polished is the most efficient, because the metal doesn't have to grip the water, it's the shape of the prop that forces water over it, so I think the smoother the better (i.e. the faster water can flow over the blades)