Rudder flaps

[monocerosin]

1979 Hunter 37c.
There were broken remnants of rudder flaps when we hauled out that we threw away. Since then I have filled the holes and i am reconsidering if I want to try to fabricate flaps or just leave the holes filled and not mess with it.
I have read about cavitation and I tried to find a thread here talking about it with no luck.
Am I fine to leave them off?
Thanks!

 

[Johnb]

The sky will not fall if you don't replace the flaps, but if you want to preserve the original design this is what they look like. I still have them.

 

[shemandr]

Looks like a nice smooth bottom.
For sailboat speeds I would be surprised if cavitation in front or on top of the rudder is a significant increase in drag.

 

[monocerosin]

Looks like a nice smooth bottom.
For sailboat speeds I would be surprised if cavitation in front or on top of the rudder is a significant increase in drag.

I just don't like the idea of more holes in awkward places that will eventually leak. Thanks for the 'props'!

 

[Jim Legere]

On my old H37C, the glass was thick where the flaps attached - at least 1/2". I redid the screw holes, using a 10-24 bottoming tap and just used short SS machine screws (with a dab of silicone as 'Loctite') to hold the flaps on. I never had any issues and never worried about leaks because the holes didn't penetrate through the glass. I suspect the flaps are worthwhile, otherwise John Cherubini wouldn't have specified them in the design. It would have cost Hunter some money to incorporate the recess for the flaps in the molds and to make and install the flaps, so they must have thought it worthwhile too

 

[gockleyd]

What are the flaps made from? Mine never had them and I always wondered what that was for

 

[Marmalade]

Different boat, but same application.
On a Chaser 33, I used 0.010" mylar sheet, with a 1" wide stainless strip at the fasteners. Worked great, was thin, and the water kept it tight against the rudder, even when turning.
Would get 2-3years on a set, but after that, the yard would hit them hard with the power-washer and 1 would eventually crack.

I made them longer than the rubber version probably would have been, but I felt this allowed the water flow to hold them on a more fair-shape.

YMMV

first pic, rudder straight

Second pic, rudder turned full-stop

 

[sandpiper10471]

I made my flaps from a couple of layers of fiberglass wetted out with polyester. Thin and flexible. Did that thirteen years ago and still going.

 

[chichiflys]

Different boat, but same application.
On a Chaser 33, I used 0.010" mylar sheet, with a 1" wide stainless strip at the fasteners. Worked great, was thin, and the water kept it tight against the rudder, even when turning.
Would get 2-3years on a set, but after that, the yard would hit them hard with the power-washer and 1 would eventually crack.

I made them longer than the rubber version probably would have been, but I felt this allowed the water flow to hold them on a more fair-shape.

YMMV

first pic, rudder straight
View attachment 227802
Second pic, rudder turned full-stop
View attachment 227803

I realize this a hi-jack - but is that a copper coat bottom paint job? If so - you happy with it so far?

 

[Marmalade]

I realize this a hi-jack - but is that a copper coat bottom paint job? If so - you happy with it so far?

VC17 for fresh water... not for saltwater or brackish... but yes, on the Great Lakes, it works well... but our boats only swim 1/2 the year!

 

[DianaOfBurlington]

... I suspect the flaps are worthwhile, otherwise John Cherubini wouldn't have specified them in the design. It would have cost Hunter some money to incorporate the recess for the flaps in the molds and to make and install the flaps, so they must have thought it worthwhile too

I don’t know that they were specified. It appears to me that the rudder was meant to flare more at the top (like the bottom of a Cherubini 48’s rudder does) and for some reason it was not done and so the flaps were added as a band-aid.

If you have two mushroom-head through-hulls forward of the keel they’re probably more drag than that gap is. And I agree with the idea of avoiding blind holes into fiberglass under water. What if you used too long a screw in one place?

The best reason for the flaps has to be in keeping junk from jamming into the gap. You can’t imagine more drag than strings from six or seven Chesapeake crab pots dragging occupied traps behind you; and you need only to sail out of the Sassafras once to enable that.

 

[chichiflys]

I've sailed on our '82 H36 for about a decade without the flaps. Always noticed a little vibration or turbulence in the rudder when sailing under light winds (well trimmed), around 4 kts or so. I've since added them back (used some thin plastic cutting board material from Amazon). That vibration is now mostly gone. Now, I also had the rudder dropped and repaired this past winter, the yard ensured the rudder stuffing box was packed properly (at least they were supposed to) - so it could be the rudder was a bit "loose" before and now it's tightened up a bit. Or it could be the flaps. Not 100% sure.