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-   -   Propeller position? (http://www.sailnet.com/forums/racing/44154-propeller-position.html)

ereuter 06-15-2008 04:24 PM

Propeller position?
 
I race a P30 PHRF cruising, and have the adjustment for a fixed 2-blade prop.

I've often wondered if the position of the prop when I shut down the engine makes a difference in drag - i.e. whether the blades are pointing vertical or horizontal or in between.

Does anyone know if it matters?

I always sail with it in gear so that the prop doesn't spin freely.

Thanks-
Eric

sailingdog 06-15-2008 04:27 PM

Yes, especially if the keel and prop are close together, since a horizontal prop won't be shielded by the keel, and will generate more drag as a result. If the prop is stopped in the vertical position, most of the area of the prop will be "shadowed" by the keel—resulting in far less drag.

ereuter 06-15-2008 04:36 PM

Well, it's a fin keel, so the prop is pretty far away. Looks like this:
http://southroad.net/temp/IMG_0094.JPG

The rudder is turned around backwards in that photo.

paulk 06-15-2008 10:03 PM

Even with such distance between the prop and keel, vertical is probably better than horizontal. Things like entrained flow and turbulence along the hull surface may mean that having a blade closer to the hull doesn't slow you down as much as having both blades sticking out horizontally into relatively less turbulent water. What with heeling and wave motion, though, it might be worth testing with the blades in both positions to be sure. You'd need a fairly finely calibrated speedo to tell the difference, though.

maestro 06-16-2008 01:52 PM

what we do
 
on our boat, we have marked the shaft near the transmission (while the boat is out of the water) where the prop is in the vertical position. Then all we do is set the prop so that the 2 marks line up and put it in reverse.

DavidWhitney 06-16-2008 03:12 PM

I always set mine straight up and down, before I put a folding prop on.
I also removed that raw water strainer and raised my rudder up until it just cleared the hull. That and wet sanding seemed to make a major difference in my finishes. I now win more that I lose..Dave Whitney 1975 P30

ne57301 06-26-2008 09:04 PM

Nice Bottom
 
Wow Eric, that bottom looks great, nice job.

Mark the coupler between the shaft and the transmission with white paint. A line fore/aft on top of the coupler when the prop is vertical, two lines for either blade up.

Here's the big tip. You can see the coupler if you take the bottom out of the top step (use a flashlight). Make a little tool to turn the engine over using the pin in the middle of the flywheel (the thing you would use to manually start the A4). The tool I have is a couple of pieces of (3/4" ?)pipe connected with a 90 degree elbow. Use a hacksaw to cut a couple of notches in the end of one of the pipes. Now you can leave the engine in gear, look over the top of the engine at the coupler, turn the engine over by hand until the white line is on top. Prop centered. (really helps with the folding prop)

ereuter 06-28-2008 02:45 PM

Quote:

Originally Posted by ne57301 (Post 334059)
Wow Eric, that bottom looks great, nice job.

Thanks. I had it soda blasted, but it was still a lot of work.

Quote:

Originally Posted by ne57301 (Post 334059)
Mark the coupler between the shaft and the transmission with white paint. A line fore/aft on top of the coupler when the prop is vertical, two lines for either blade up.

Here's the big tip. You can see the coupler if you take the bottom out of the top step (use a flashlight). Make a little tool to turn the engine over using the pin in the middle of the flywheel (the thing you would use to manually start the A4). The tool I have is a couple of pieces of (3/4" ?)pipe connected with a 90 degree elbow. Use a hacksaw to cut a couple of notches in the end of one of the pipes. Now you can leave the engine in gear, look over the top of the engine at the coupler, turn the engine over by hand until the white line is on top. Prop centered. (really helps with the folding prop)

That's good to know. I currently have all the paneling out of the quarter berth because I've been doing work on the engine. I'd been wondering how I'd know where the shaft was. So you've eliminated the floor to the little storage compartment?

I actually found the original hand crank for the A4 under the port settee. Justin didn't even know it was there. It's rusty, but I've been able to start the motor with it, even with that short throw.

Which folding prop do you have? Have you found that it's worth the hit you take on your PHRF rating?

Thanks-
Eric

sailingfool 06-28-2008 07:01 PM

I don't think it matters a bit the prop orientation if it isn't in an arpeture...but do yourself a big favor and get a folding prop. They are far and way well worth the 6 second per mile rating reduction, if they were not, you'd see the hotshots cheating the rule with fixed props. Never happen, a knowledgeable racer will have a folding or feathering prop.

maxmunger 06-29-2008 09:15 AM

Yes vertical behind the strut is better.
But all this really matters is in light air. Once a boat can get near hull speed the prop doesn.t matter much.
Those old P30s can still beat most boats1


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