Fastest Bottom?

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Fastest Bottom?

Postby GreenLake » Wed Mar 13, 2013 12:48 pm

There was a discussion of graphite bottom coats in the thread on coamings (DS1 forum). Perhaps we can move this here and generalize it a bit.

What things have you tried to make your bottoms faster and do you have any evidence it worked?

When I joined the local all-comers beer-can race (not sure why it's called that way, I've never seen cans, but they do give away beer once a year), I was taken aback when one of the competitors reached under my hull to check it for smoothness. What he found was that the state of the (painted) hull had deteriorated to the consistency of sandpaper. The coating had failed (undetected by me) and created several really rough patches, all of them where they were nicely out of sight.

So, thinking that this might explain my last-place finish, I careened the boat, sanded off the paint, fixed the gouges in the gelcoat, and brought all the patches to the same condition as the surviving paint around them. The boat felt a lot faster and I was still coming in last.

Next, I went for a complete redo. Sanded off all the paint, fixed more defects, added fairing compound to smooth out medium scale waves and bumps. Painted with a PU topcoat. The result is that the boat looks great (from below) and feels a lot faster. I'm still coming in last.

At the same time as the first bottom upgrade I had unearthed a really banged up CB which I refaired and re-gelcoated. That stopped a noticeable flutter at high speeds. Since I had been coming in last, I decided to improve the rudder. Since that didn't change anything other than make the rudder much lighter and nicer looking, I used the second pass over the bottom to further improve the CB. Alas, no change in results.

Now I bought a different, faster boat and found some expert dinghy sailor as crew, and that fixed things (fourth place).

Of course, the real problem with the DS was that it was outclassed by the other boats, and in a no-handicap race that's the end of that. Except when luck, traffic or what have you mixed up the tail end and I got second to last.

Coming back to the question of a faster bottom. Unless you sail against another DS, I don't see how you can actually observe any improvement. It might be there, but conditions are too variable (wind, waves, trim etc) to allow one to tease out the contribution by any one factor in a single boat test. That's why hulls are compared with towing experiments using towing balances where differences in drag between two hulls can be observed in real time in the same conditions.

It might be possible to notice some effects when racing another DS, but my suspicion is that you'd have to two pretty expert sailors to be able to attribute any difference to such a single factor. That's why I included the question of evidence above.

So, what have you tried and what are your results?
~ green ~ lake ~ ~
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Re: Fastest Bottom?

Postby Mike Gillum » Fri Mar 15, 2013 12:06 am

My current Thistle and a recent Thistle both have graphite bottoms by the previous owners that I have wet sanded starting at 600 grit and ending up at 1000 grit wet/dry. Some people will use even higher grits like 1200, 1500 & 2000 but I can't justify the expense and time.
I then polished using a 9" wool buffing pad on Makita Polisher with adjustable rpm and 3M Finesse It II Finishing Material followed by a coat of the best Carnuba Paste Wax.
Same method can also be used on the factory gelcoat of your Day Sailer with Dave Keran restoring #11026/Rebel, #2365/O'Day & #13150/McLaughlin to nearly new over the past couple of years.
You might also want to see if the rudder, centerboard and mast are aligned by tying a long string to the main halyard from the top of the mast, lift the boat up using a hoist and then pull the string taunt while looking from astern trying to line up the aft edges of the mast, rudder and centerboard with the string. If anyone of them aren't aligned then you need to work on aligning them.
Finding another Day Sailer to tune against is your best bet in making both boats faster over time by using one boat as the base and making incremental adjustments to the second boat.
If the adjustments make a noticeable improvement then swap boats and see if the other crew can reproduce the improvement. If the test boat is still the faster boat thenchange the base boat to match and then repeat. If the adjustments make the test boat slower then try the next best idea but don't change the base boat until the test boat is faster.
I'm fortunate in that I get to actively race against some of the best Day Sailer racers currently in the class and they give me immediate feedback on any changes I make to #2772 and the same goes for them!
It doesn't hurt
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Re: Fastest Bottom?

Postby GreenLake » Fri Mar 15, 2013 2:26 am

When I painted the bottom, I used a PU topcoat (that's something that can't be left in the water for longer than about a day or so, but I don't moor my boat). I put a few extra coats on, and then I sanded and polished the paint - that seems crazy, but I was able to take out many of the small imperfections from painting that way. I finished with Finesse II and a wax as well. Unfortunately I have no other DS to test anything against... I must admit my main motivation for restoring the finish was the rather noticeable amount of damage to the existing finish, from spider cracks in the gel coat to failing paint.

Interesting idea about checking on alignment. Many of the boats are known to be a bit off in some dimensions, so it seems reasonable to make this check - I do have a hoist of sorts, but that one requires me to clear my garage enough to fit the DS. Hasn't happened in 10 years, but this might be a reason :)
~ green ~ lake ~ ~
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Re: Fastest Bottom?

Postby Mike Gillum » Fri Mar 15, 2013 3:04 pm

GreenLake,
It's been said that "sailboat racing is a game of inches" and I have certainly found it to be very true over the years!
Time spent sailing the boat is usually more effective than time spent working on the boat!
Improvements to the bottom and foils of your DS will certainly help to make it go faster through the water but if you can't be on the line at the start, effectively roll tack every tack, be on the right side of most every shift, and/or quickly and cleanly set and strip the spinnaker then the fastest bottom is for not.
As an example if you and your crew can gain 2-3' every time you tack against another boat then in twenty tacks over a long leg you'll have gained 2-3 boat lengths overall which is the difference between rounding ahead or rounding astern of the other boat which is probably more than the fastest bottom will net you.
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Re: Fastest Bottom?

Postby jdoorly » Fri Mar 15, 2013 5:00 pm

KC, I think Mike has given us all of the secrets to racing glory, so I'll just cover some background.

Graphite (and Teflon) both have the lowest friction rating (of 0.04) and are are considered 'dry' lubricants. The graphite you buy from West or others to mix with epoxy comes as a powder and there is a suggested maximum amount per unit of epoxy. Using more than this amount results in a better distribution but greater risk of flaking or chunks tearing out.

Regular epoxy, when impacted, tends to 'grab' the impacting item (for instance a rock). Epoxy mixed with graphite will act the same unless prepared properly. This is, I think, where the controversy lies. Many people have 'tried' epoxy-graphite and reported no improved performance, but they failed to prepare the surface with ascending grits of sandpaper and compounds (BTW I like spray teflon wax in lieu of carnuba but agree with the Mike on the rest. When the graphite powder is mixed with the epoxy and spread on the surface the graphite particles migrate away from the surface because of surface adhesion, so the actual surface of the cured epoxy contains no graphite. Sanding removes the empty surface level and exposes the graphite particles. Further sanding smooths surface aberations and the lubricative property prevents 'grabby' impacts. The surface now feels smoother than anything you ever felt before (for instance a pane of glass is smooth but when you pull your finger along it your skin vibrates and your hear a squeak. Since graphite is a dry lube your finger won't vibrate, there is simply NO FRICTION!

When I tested, using a GPS and multiple passes and 2 directions, my 14 ft kayak with an epoxy only surface but smoothed to 400 grit I could easily cruise at 3.5 kn and could hold 5.1 kn for 100 yards. After adding graphite and smoothing to 2000+ grit, cruise speed was increased to 4.2 kn and top speed to 5.4 kn. The best effort vs. speed increase was at low and moderate speeds (where the friction vs. form resistance has a higher percentage of frictious drag). But the biggest difference was the FUN quotient; The boat was more lively and accelerated faster!

The maintenance became more than I wanted to bear and I soon sanded only a couple times a year and only to 400-800 grit.

West Systems suggests graphite on CB's and rudders. Wonder how they prep A45's?
DS2 #6408 "Desperado"
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Re: Fastest Bottom?

Postby Mike Gillum » Fri Mar 15, 2013 6:09 pm

Guess what I should have said regarding the graphite bottoms on my Thistles is that they were painted with "Speedkote" that used to be readily available and doesn't seem to exist anymore.
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Re: Fastest Bottom?

Postby jdoorly » Sat Mar 16, 2013 10:18 pm

DS2 #6408 "Desperado"
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Re: Fastest Bottom?

Postby Salty Dog » Tue Dec 10, 2013 9:31 am

I was told once by a guy that taught me to sail, That a smooth waxed btm. was not the best. for speed. He said it created a suction on the hull in the water that slowed the boat down. He told me that the fastest btm was one that had small ( almost scratches in size) lines that ran stim to stern length ways down the hull. He said these lines could be made by just dragging sand paper along the length of the hull. He attributed this to the condition of tiny bubbles being traped in the lines and acting as small ball bearings for the hull to ride on and keeping the water surface tension from sucking on the hull when moving thrue the water. I have not perfected my sailing abilities to the point of worrying about about small bubbles so I haven't thought about it much since then. This post made me think of it but I did not see it addressed here so. I ask you..........Was he smoking crack back when crack wasn't cool.

SD
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Re: Fastest Bottom?

Postby jdoorly » Tue Dec 10, 2013 9:01 pm

Sandpaper, I'm thinking 220 and not 1000, along the whole 'wetside' would probably cause (negative) cavitation type effects but I'm unfamiliar with the technique.

A hull is detered from forward motion by surface friction and form friction, which both increase with speed. When a hull begins to accelerate the water in direct contact with the hull tends to stick to the hull ( I'm thinking surface adhesion). A layer of water next to that tends to move a bit slower, and so on until you get to water undisturbed by the hull. This set of layers is called the Boundary layer around the hull. The boundary flow begins smooth and uniform, but at a certain point some feet aft from the stem the surface adhesion breaks and flow becomes turbulent. There is a correlation between the length of laminar flow and boat speed.

There are not only studies but real life applications where a "rough" surface used in a particular way improved performance. Like the 1977 Ranger 23 which had turbulence stimulators (small bumps) on the front of the mast above the 7/8 fore stay that improved laminar airflow at the mast top and increased speed in light air and overall beat angle. The idea here is to extend the location where the laminar flow breaks away into turbulent flow. I'm guessing that the stimulators create vortexes whose interlayer path and spiralling energy result in keeping flow attached momentarily near the critical point. I think if you wanted to use stimulators on a hull you would probably want to put them, not everywhere, but just before the break in flow occurs, in an athwartships semi circle. Current thinking would suggest 'denticle' shapes (i.e. shark skin). The rest of the hull surface should be smooth, especially the leading 1/3 of the hull and appendages. I remember a boat that leached out a long chain polymer substance near the laminar break point and won a race only to be disqualified by its use.
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Re: Fastest Bottom?

Postby Salty Dog » Wed Dec 11, 2013 9:24 am

jdoorly

Well this was his theory and he passed it on to me. (Your right he did say rough grade sand paper). I'm not sure where he got it. I am sure he was not an engineer. I was curious if any one else had heard of this or if he was alone in his thinking. I say "was" because he is no longer above ground and no way of asking.
He is the one that talked me into buying this "Neat little boat" he knew was for sale. It turned out to be the DSII I have now.

Thanks SD
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Re: Fastest Bottom?

Postby Mike Gillum » Wed Dec 11, 2013 2:17 pm

Regarding my previous posting of March 14, 2013 regarding finishing the bottoms of my Thistles and Day Sailer it reflects the proven practices of my younger brother who has previously maintained several top end sailing programs including the 1999 Farr 40 World Champions and built a couple of TP52's.
If you go online and look at recent photographs of the bottoms of the boats of the top sailing programs from dinghies on up you'll see that the topsides, bottoms and foils are all so shiny that they rival glass or mirrors in their reflection of objects or people in close proximity and its not because they just washed them!
The bottom and foils on my DS #2772 BUBBA are so slick that when my 11 year old skipper and I capsized in a wicked gust at the 2011 Junior DS NACR's in Clear Lake, Seabrook, Texas I slipped off of the end of the polished and waxed Waterat centerboard three times wearing Gill Hiking Boots with rubber soles while trying to right BUBBA. I finally figured out that I could only go halfway out on the centerboard while holding onto the rubrail and timing my attempt to right BUBBA with a gust of wind to kick the main and mast up.
My previous experience with wet sanded bottoms is that they'll collect and retain any and all dirt, debris and contaminents requiring constant wet sanding to be competitve while the polished and waxed bottoms only require a damp rag to look good and perform well again.
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Re: Fastest Bottom?

Postby Salty Dog » Thu Dec 12, 2013 11:57 am

I know I have a lot to work on Rigging, Starting, and Tacking before I have to concern myself with the bottom. But one thing I can say with all confidence. is that so far my bottom has not gone any faster than my top. I may want to keep it that way. :D

SD
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