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Old 10-05-2008, 17:08   #1
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Reworking a mono for reduced underwater drag?

Hi guys,

I've been doing a lot of reading about laminar flows and drag reduction, but have reached the wall where book learning stops and the need for getting hands dirty starts.

Anyone spent time on reducing the under water drag? Care to share your experiences, what you did and the results?

Flush through hulls? Hard paint? Blue printed keel? Foil rudders? Could you tell if anything changed from daily runs?

Thanks!

Zach
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Old 10-05-2008, 17:32   #2
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Anyone who wants to soup up a Triton has too much time on his hands--start by getting a boat where all that work will make a difference.
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Old 10-05-2008, 17:51   #3
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A Pearson Tritan is a well built but heavy sailing boet. That actually was the first (US anyway) production sailboat, and it was made when they really didn't know how strong fiberglass was. A couple of things you can do is make any underwater hull protrusions flush, Where they can't be made flush like Keel coolers, Radio ground plates, hull zinks, and intake strainers, move them as far aft as possible to allow the water flow to stay laminar for as long as possible before it reaches the protrusions.

Probably the biggest gain you can make is just by going to a folding prop, or the more practical feathering propeller. I believe your prop was originally a solid 2 or 3 blade in the aperture of the rudder, so you have to be careful to make sure the new prop will fit.

You have a good older boat that is never going to be fast mostly because of its weight and wetted surface. These tips may help a little so good luck.

Joe S
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Old 10-05-2008, 22:26   #4
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I know that a few years ago "they" experimented with a rough finish on the paint in the areas that you'd think should normally be the smoothest like where the keel meets the hull. "They" were looking at the skin of a shark (an extremely fast moving creature as we all know) and their skin is rough like sand paper. Tests concluded that this roughness actuallt led to reduced friction by creating a lubricated surface (for lack of a better description) against the sharks skin. Also there is/was a company out of Washington state that has developed a type of rudder cap that doesn't reduce drag so much as increase the effectiveness of the rudder for better/faster manueverability.
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Old 10-05-2008, 22:32   #5
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Its not just surface friction and laminar flows. It is also wave making resistance and the best way to test that is with tank testing. Ship hulls and prototypes are tested this way. Tank testing also tests how a vessels hull reacts to waves. So really, there are four factors that cause resistance.

http://en.wikipedia.org/wiki/Ship_model_basin
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Old 10-05-2008, 22:44   #6
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What on earth are you trying to achieve? From the current condition to the best you could achieve, you may gain fractions in excelleration. But the boat will still only go the same top speed.
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Old 10-05-2008, 22:58   #7
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Alan is correct, near hull speed wave making resistance is far greater than resistance than friction from wetted surface or eddy resistance. It only at very slow speeds that the first two factors matter the most.
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Old 10-05-2008, 23:11   #8
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If you are not sailing some new rocket ship type sport boat, you are never going to be able to keep up with the new boats out there regardless of what you do.

If you are interested in being able to pass the rest of the boats in your class on Wednesday night races then try fairing the hull. Fill every depression, nick, chip, scratch, etc. that you can find. Wet sand the bottom and then apply something like VC-17.

Make sure that your sails are in optimum condition. Without really great sail shape, you can't go fast. It will have a far greater effect than anything else you can do.

Good Luck
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Old 10-05-2008, 23:59   #9
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My interest has more to do with the price of fuel, and the fact I'm going to be sailing in lower wind speeds as a budgetary concern... Anything that improves efficiency on all points of sail, all the time is worthy of consideration. Top speed isn't my goal, rather improved averages.

I am trying for better light air, particularly down wind efficiency. My boat is a fractional rig with jumper stays that complicate a mast head spinnaker. Sail area is at a premium!

I've switched from an inboard to a fuel sipping outboard, under sail I lift the prop clear of the water. The prop aperture is going away completely on the next haulout, along with the raw water intake, and exhaust outlet which is submerged at high angles of heel.

I'm a hotrodder at heart, and am tantalized by the possibilities of improved efficiency!

Thanks guys!
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Old 11-05-2008, 00:53   #10
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You would have a few options as far as I can see. Replace the rig and/or spend some money in some top end sails. You have the option of selling and buying something lighter and smaller. The option of going from deep hull to flat shallow hull with spade keel and rudder. What ever of those you choose, none would warrant the cost to fuel saving ratio in my opinion. I wouldn't worry about it.
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Old 11-05-2008, 02:10   #11
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Quote:
Originally Posted by Zach View Post
I've been doing a lot of reading about laminar flows and drag reduction, but have reached the wall where book learning stops and the need for getting hands dirty starts.
Sometimes the simplest answers are the best.

Reduce parasitic drag by keeping your hull clean.

I know a guy who had a $2500 stereo in a 10,000 Chevette. I leave the rest of the though in ellipsis.
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Old 11-05-2008, 09:53   #12
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Quote:
Originally Posted by Zach View Post
been doing a lot of reading about laminar flows and drag reduction, but have reached the wall where book learning stops and the need for getting hands dirty starts... Flush through hulls? Hard paint? Blue printed keel? Foil rudders? Could you tell if anything changed from daily runs?

A fascinating topic… As a decided amateur some years ago I spent a bit of time on this topic in marine, auto and aero environments… laminar flow, and the chasing of pressure gradients goes quite a bit beyond the usual, keep the hull clean and keep the protrusions to a minimum… or at least that is my theory… but in the displacement marine category, wave-making raises a scenario not seen directly in the other (except sorta in the lift/downforce realm) and I never seemed to crack that – in part because I don’t have very sophisticated math/engineering background…

However, one of the things I saw that fascinated me from the aviation world was a little aircraft that set a speed record of something over 200mph with what was essential a ultra-light Rotax engine… the amateur designer had done all the usual laminar stuff – usually, like most, concentrating on skin friction and protrusions in the forward 50% and came up against a brick wall at about 180-190mph as I recall… then he got to thinking about the wing/fuselage intersection (keel/hull…??), specifically the rear portion… and found after doing some dye studies that the usually fairing aft of mid-chord was no where near full enough and was zapping many of the gains – in short the sizable pressure gradients aft of mid-chord were negating many of the gains usually seen in the forward area… in the end, his studies seemed to me a bit like marine area distribution analysis…

I’ve long been curious whether exploration of the bustle area on the trailing edge of the keel and/or rudder wasn’t a potentially fruitful area, assuming the “rule” didn’t penalize one, and paying attention to the usual prismics, etc…

Good luck finding that extra tenth of a knot…
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Old 11-05-2008, 09:56   #13
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Quote:
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Sometimes the simplest answers are the best.

Reduce parasitic drag by keeping your hull clean.

I know a guy who had a $2500 stereo in a 10,000 Chevette. I leave the rest of the though in ellipsis.
I think this is true. Going swimming to clean off the hull is probably the best thing you can do to achieve light air performance, given that you aren't going to fly a spinnaker.
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Old 11-05-2008, 11:39   #14
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Closing the prop aperture will really help. I would also fair the trailing edge of the rudder to quite a sharp edge.

The long overhangs mean that your hull speed is really low when the boat is level. Sail with some heel to lengthen the waterline (e.g. tack downwind or even have everyone sit on one side when dead before the wind).

In the more expensive area, have a sailmaker add roach to your mainsail. This will add a lot of power. The sail will hit the backstay when tacking but it will slide by. There won't be much chafe since it's only for a moment. Steve Dashew experimented with how big he could make his roach - it's in his cruising encyclopedia but as I remember it was something like 20% overlap of the sail and backstay.

Finally, I would think (but don't take my word for it) that you could fly a masthead cruising spinnaker in light air from a Triton if you rigged a halyard up there. Used spinnakers are really cheap online and the size would only have to be approximate. You'd have to be careful and it would have some risk - but you seem to like to live on the wild side. Has anyone seen a Triton do this?

Good luck

Carl
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Old 11-05-2008, 12:35   #15
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sell the Triton and get a Melges or Elliott
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