Centercase sealing - SailNet Community

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  #1  
Old 01-10-2008
Classic30's Avatar
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Centercase sealing

In the middle of doing some minor patch-ups to the hull following our recent holiday on the lakes, I noticed that the centercase is completely open at the base (around 2" wide by 4' long).

I remember from my dinghy days that the bottom of centercases are usually covered by rubber flaps to decrease drag - but then they were planing dinghies and the Hartley doesn't!

How significant is this likely to be? Would it be worthwhile finding and fitting something to close up the space for extra boat-speed??
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Old 01-11-2008
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If you are racing the boat pictured, putting centerboard trunk flaps to cover the opening (and guide the flow of water around the centerboard when it's down) might add .000004 knots to your boatspeed. That is definitely not what you tell your competitors, however. To them, your very expensive teflon-coated speed-flaps increase your hull speed by introducing added unmeasured waterline length. (4' of centerboard trunk length with a flap on each side should equal an additional 8' of water-surface contact length. If speed varies as to the square root of the waterline length, this means your boat , with flaps, can now go almost three knots faster than it used to. THAT is the number to tell your competitor(s). Of course your helpful rating handicap official needs to be advised about this change to your hull as well. The increased drag caused by the large surface area of these heavy flaps will slow your boat so much that you may have to carry extra food and supplies (slowing the boat still further)in order to have them last until the end of your trips. He will have to let you know how much your rating will need to be changed to compensate for this. Perhaps 6 or 8 seconds per mile would be enough. All in all, installing flaps might make for a possibly quieter ride. If it's worth it to you, you should know from your dinghy experience that it's relatively simple to install flaps. There are probably a good many other things you could do to improve boatspeed that would be more productive.
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Old 01-12-2008
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4 foot by 2 inches would be 96 square inches. This would be less then 2/3 of a square foot. The drag from the add on flaps would slow you down more then the added wetted surface could ever gain you.
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Old 01-16-2008
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Thanks guys for the detailed replies.

You are correct that it's an easy thing to do, since I could simply tack something either side of the centercase, but I had not given any thought to the possibility of it introducing more drag with no real corresponding change in boatspeed.

The centerplate doesn't make much (if any) noise now, so there's no real driver for it other than a reduction in turbulence from the aft section of casing, but at 5-6 knots hull speed??? - I'll cross that one off the list.
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Old 01-16-2008
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A San Juan 21, made for many yrs near me, used a rubber flap on the bottom of the boat along the CB. I have to admit, I do not know how much speed etc it gained, But I am sure it is more than .000004 knots in any given wind! Then again, .000004 knots, over a certain distance, does add up to maybe 10-100 ft or yds/meters if the race is a few nautical miles in length..........So it could be worth it, if done correctly! Clark boat company was the manufacture, Good Old Boat has a write up this month on that company. i am sure if you googled San juan 21, You would find a assoc/class to figure out how that boat does the rubber flap, maybe find, buy one of them and shorten if need be........


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Old 01-17-2008
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Thanks, Marty - I'll have a look.
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