Just wondering, how much increase in drag will you see if you dont have a perfectly fair bottom on your hulls? Are we talking percentages or less?
For the serious racer I assume every second counts, but for us hobbyists, is it worth it refaring the hulls under the waterline or is the time better spent on the water? I tend towards the latter, but my crew isn't that sure.

Rolf,

I'd say it is worth spending some time up front to get stuff right. Hulls are a big job to get right and so I'd spend time up front getting them fair, and I would always fix stuff below the water line PDQ if possible. Ditto Foils.

In my experience:

Hulls - fair (no bumps or scratches) and then use a good teflon polish. I don't like wet sanding my hulls if they are already fair as that just opens the pores of the gel coat and invites staining. Keep it sealed and clean with a good teflon polish. Wax may be OK but many feel it slows you down a slight amount.

Foils - fair (no bumps or scratches) and then wet sand with 400 grit. A world class sailor/designer told me a 400 grit finish on the foils keeps the water attached better and prevents cavitation so I follow that advice. That same sailor felt you should not put polish or wax on your foils.

Hope this helps.

Bob Hodges

The surface on foils are critical, as they need to generate lift for as little drag as possible. But just how much will bumps, dinks and scatches at the bottom slow you down (increase hull-drag)? Is it measured in percents of your race time or less? Will one bad tack or jibe make up for it, or is it more if the underwater hulls are in rather poor shape?

My understanding is that the dings and scratches on foils don't necessarily increase drag too much but instead reduce the lift of you foils significantly, which increases your leeway angle, which decreases your VMG. Especially any dings on the leading edge, these should be fixed at once for performance. Some 15min curing car bog and a bit of light sand paper comes in handy for this.

Sun Cure works well for rapid repairs on boards and rudders - 5 minutes to cure. On a really sunny day you have to work under an umbrella to get the repair shaped before it starts kicking.

But how about dinks, bumps, scratches etc. on the bottom of the hull? How much point is there actually in re-fairing the hulls?

Rolf, at Hobie 16 races you can see pretty scratched hulls that go at the top of the fleet. I personally don't think it is so important. At high wind speed, sail plan resistance plays a much more important role I'd say. Of course, your daggerboards and rudder blades should be in a good state.

Rolf,

If you want a definitive answer to this question contact Lars Guck at Guck Inc who does one of the best fairing and finishing jobs that I have seen. Great work on Tornados and A's.

For me, it's all about the "head game". If I know my hull bottoms are some of the sleekest in the fleet, I have one less excuse to crowd my brain as to my performance. Likewise with the rest of the boat, if I know it's tip-top, I don't have to think about it.

rolf, I agree that the foils are those parts to worry about. those are small in size and deal with very specific function. the hulls in contrast are quite large and much less sensitive to the loss of flow. significant damages you probably will notice but scratches...?
just imagine the loss when pulling once too hard on the tiller, taking a wave wrong, having your sail slightly not trimmed perfectly... there is so much more to loose than with the little scratches on the hulls...

I heard once the story that the A Mitch Booth became worldchampion with was build with a 2cm difference between the two hulls as they took the first hull as mould for the second... In contrast Boyer did not like the moulds of the German Flyer when they started to build the MK 5 and AUS Flyer as they were not plain and smooth enough...

But remember all those events you have to put sticker on your hulls and they start to fall apart with some boat in the middle of the race? Now, one would assume that these causes noticeable drag, still you find the top boats encountering this problem and still finishing first...

Saying this, there is so much to win and loose in good versus bad sailing, but the smoothness of the hulls is really a minor. If you are an olympic sailor and on top of your game, you surely also don't want this <0.1 % possible lack of performance but for the rest of us I think we still deal with 10% losses by simply sailing badly from time to time... ;-)

Ahhhh...just finished tonight.

Yes, those camper windows sure are shiny! Good work!

<img src="http://www.catsailor.com/forums/images/graemlins/smirk.gif" alt="" />

It's almost strange that not even Bethwaite have done tests on this.. It sure would be good to have some quantifiable information on the subject.

Jake, that is really slippery! Must be hard tough to know that it's all in your head and not the boat when you dont do as well as you want to <img src="http://www.catsailor.com/forums/images/graemlins/grin.gif" alt="" />
Is it your guard dog we see in the background?

Bethwaite says that only the bows see laminar flow, the rest sees turbulent. With turbulent flow, surface smoothness is not so important, so just polish your bows. <img src="http://www.catsailor.com/forums/images/graemlins/wink.gif" alt="" />

I'd say this effect only has some importance when all other drag is very small, i.e. no waves, very little wind, low speed. In order to find out you have to do some towing experiments...

Yes - that's Gunther. 70lbs of Bull Terrier and Boxer mutt. Good guard dog.



Yeah, well...I can fix the boat...my head, however....

Years ago a Factory rep for one of the largest windsurfer custom fin designers/manufactures gave a talk on the importance of foil shape and repairing any nicks, scrapes or gouge in a fin. He said they found a definitive test by placing the fin in a fast flowing stream of a Artesian well (solid water...no aeration)...He said when a properly foiled fin with no surface imperfections was introduced in the flow, the fin would offer very little resistance to holding it in the flow...however if a fin was introduced into the flow with surface imperfections the fin would be ripped right out of their hands. It became a standard practice for them to check their personal race fins for resistance using this method...I would think this same method would work for cat rudders and dagger boards as well.

Mathematical Calculations and scientific research are the accepted ways to define and quantify resistance, but this simple method can show whether the theory lines up with real world results.

Best Regards,
Bob