H37C rudder "flaps".

[Ed Schenck]

I decided to start a new topic and keep the rudder topic strictly that.

I could post a picture of the "flaps" but Ike came through here and the condo is without power. So I am using my fiances's laptop. I cannot remember what was there when I bought the boat but I think something made of sheet metal. I used that as a pattern and made new ones out of rubber stair tread. Those always lay flat against the rudder to what purpose I am uncertain. I suppose if we knew anything about laminar flow of water it might make sense.

I also glassed in pieces of wood on the inside because I worried that those screws went directly into a void. A couple of loose screws could let in a lot of water. Still I worried that maybe my glass work was not filling the void. So I decided to eliminate all concern that bilge water is coming from those ten screw holes.

So far as prop walk I do not know if the flaps would contribute or not. The folder was on the boat for one season(2006) and part of 2007 until it came off in the Erie Cansl. I can tell you that it had very little prop walk. And I really needed it in the canal because to stop the boat the stern would always walk away from the canal wall where I was trying to grab a line. The prop fell off in the very first lock so I went all the way through with the replacement, a 14 x 14 three blade Michigan.

 

[sandpiper10471]

In a Flap

When I bought my 37 C the flaps had been long gone, just screws and caulking keeping out the water. I made new flaps up from two layers of finishing cloth and epoxy. They seem to be doing the job. Pics to follow.

I have huge prop walk in reverse. Were you saying you had little with the folding prop? If so, how come?

 

[Ed A]

rudder flaps

I seemed to loose the last post i put up on this.

The flaps are designed to reduce turbulance on the rudder.

The flaps smooth the water flow going back over the rudder. that translates into less drag and better attached flow across the rudder. my flaps are thin sheets of fiberglass.

The flaps will have nothing to do with prop walk in reverse.
The water flow from the prop in forward blows water on the surface of the rudder increasing velosity. The water moving in a circular pattern as it leaves the prop. So when it hits the rudder the water creates more pressure on one side than the other. So you get resistance at the helm.

In reverse the water from the prop goes forward away form the rudder. so the flow on the rudder is limited to hull speed thru the water, and has no rotational motion. The flap if properly installed is tight against the rudder. so you get little interference. the area forward of the rudder does not move while steering anyway.

So the flaps dont effect prop walk in reverse. they do help clean up water flow in forward. but dont effect the pressure that develops at higher throttle settings. That is more likely the result of the angles of the shaft relative to the rudder.

The walk in reverse is the rusult of inefficient thrust in reverse ( prop design), and the fact that that flow is not stabilised by the rudder because the flow from the prop is going the other way. The rotation of the prop is not minimised by a rudder.

It is a misconception to think that prop rotation is not a factor in forward too. If you pay attention when you drop the boat in forward the stern will also push to one side. I have seen power boats that just dont track straight. On outboards they use a trim tab on the foot of the motor to compensate.

 

[Jim Boernge]

fiberglass flaps

Hi Ed,
My 36 also has fiberglass flaps. I agree, the area the screws penetrate is a void.

Jim
ps. Short Circuit is finally back in the water after Katrina.