swallowtail sugar scoop

[kettay]

hi, i just bought a boat that has a swallowtail scoop stern extension. after some thought and observations, i m thinking that the swallowtail design is actually creating drag on my boat rather than increasing the speed due to a longer water line . does that make sense and is it possible? any guess at what sort of hit i may be taking in as far as speed? more importantly, how can i fix it - do i have to take of the extension or do i just fill in the V? also its a transom hung rudder, i guess that needs to be moved back also? maybe too big of a hassle?
thanks and cheers

[kettay]

fyi, further info, the boat is a 39foot sailboat/australian made, the scoop is about 2 feet, and looks like the picture about (thats not the actual scoop but the design is similar). the rudder sits in the gap

[messabout]

Use the search function to find a thread that was here a few months back. Another Aussie had a similar notion. There were numerous responses to that thread. In the end the boat owner added extensions and believed that they were helpful. I doubt they were helpful but he was pleased with them and thats all that mattered.

Appendages like that can be very good or very bad depending on a lot of other factors, as well as the design of the appendages themselves. The boat in the picture might be more apt to plane with those additions. Maybe, maybe not. It all depends on the shape of the run, the position of the running waterline, the speed of the boat, and so on.

[RHP]

I had a Beneteau First Class 8 that looked similar but without the cut out. She left a very smooth wake in nearly all conditions on all points of sail. My technical knolwedge is limited to if it looks right, it probably is right.

[kettay]

thanks for that, i guess i m curious re the problem the cut out presents and how to resolve it

[kenJ]

Your choices to get rid of the "v" are to either remove the two sugar scoop sides which is alot of work or fill in the "v" and add a sturdy mount to hold the rudder. I think any speed advantage you think you might gain by filling the "v" will be lost by the extra weight you are now carrying to support the rudder. Moving the rudder aft may also require a longer tiller. Enjoy the boat. If you need something with a bit more performance boat shop until you find it.

[kettay]

hmmm, can you explain further? how can your rudder not have to move if you are filling in the V? thanks for the help

[tspeer]

How about adding some horizontal pieces to the rudder that would slide along the bottom to form blade seals to fill in the gap. They would have to be flexible enough to conform to the bottom, but stiff enough to still form a smooth contour.

Installing and removing the rudder would be more difficult, because the rudder would have to move up from below to position the seals, but probably wouldn't be able to drop down onto the pintles.

[kettay]

interesting thought. i wonder how it you could keep it from being a drag and how you could keep it from becoming a trash/weed catcher. anyone try this before?

[Tactic]

I was going to try this,I wasn't able to seal the hole effectively and ended up dragging water around.The idea was also to keep the rudder in the original place and hence the extension need not be so strong thus lighter.
I ended up making it stronger and hanging the rudder on the back.

[LP]

My first question since you haven't mentioned it is, are you dragging around a bunch of water in the notch? Since I'm not intimately familiar with the design, I'm hesitant to say too much, but I would think that when your boat was sitting on it's lines, the "tails" would not be submerged or minimally so. If that is the case, then filling the gap would result in little if any gain.

If the gap is submerged, are the ends of the swallowtail above waterline when the boat is upright and/or when heeled? Are you getting a clean flow out of the "V" or is it turbulent, backwashing into the stern? If filling the notch doesn't clear the waterline, you're still going to get drag from the backwash, but possibly at a reduce amount depending on how much you've improved water flow off of the stern.

If you do proceed with the modificaton, is the rudder to be easily removed?