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  #16  
Old 03-18-2017, 07:22 PM
Ilan Voyager Ilan Voyager is online now
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@No I didn't any calculation as it's not worth. The explanation I got by some aerodynamicians was to control the boundary layer on very low Re boat. The coat has to be kept as thin as possible, or it will feed a separation layer with eddies. It's simply more than a simple friction. That would be too easy just friction.
At the difference with the ideal situation of the calculations, a sail boat moves up en down, side to side and "snakes". Bref the flow never will pass in straight line like in the computer animations, the conclusions are only indicative. The flow changes all time of direction. That becomes worst with small models spending their lives in a terrible chop with 2 inches waves. So a statistical experimental approach gives a fast answer; navigating and seing if the rudder stalls or not, the profile of the keels drags or not, and if the hull stays scotched or not in the water.

During the craze of the M class among the Multihull French Flying Circus, and in France and Germany, systematic trials have been around 1990 done by a Parisian group with the same boat and rig, with 3 skippers on the same lake and same weather conditions, and same regatta triangle. Hull bare from mold, sanded 1000, sanded 3000, polished around 5000, mirror state, waxes, teflon, turbulators.

The conclusions were simple. Good polishing of the surface but not too too (yes twice) much. You get a good boat, able to tack fast. Hydrophobic additives (waxes, teflon etc) result in all around problems, specially with the rudder. Coarse, depolishing, grooves, spoilers, golf, tennis, baseball ball surface, are totally useless. On simple objects simple solutions...

In fact the obtained answers are only valuable for this model of class M. But it appeared that the general empiric recipe was usable with 95 % of the very diverse class M of the period. The roughness of the surface of the skin is important on a small boat, not so much on a 1000 feet tanker.

In the thread the question was simple and the answer evident.

So I joined some pics of Class M so people can have an idea of the object. The One meter are even simpler but learn a lot from the Class M.

A typical class M is 1.27 (50") long, 3 meters high including the 60 cm draft, 0.5 m2 of sail, weights 4.7 kg including the 3.2-3.5 kg bulb ballast. That leaves you 1.2 to 1.5 kg for the hull, mast and rig, sails, appendages, radio, winch, servo and batteries.
Most of the pics are from the Class M Margo designed by Paul Lucas, NA at Brest France. Margo has a very big success story in competition. The second pic shows Margo full speed upwind on flat water. You can admire the lack of wave.
On the third pic Margo is on the right. The first pic is an unknown boat the number 75, bigger than a class M. It can be seen again in the third pic on the left.

The good recipe is known since ages. Good surface and respect of the dimensions and profiles. No complications.
Attached Thumbnails
Hull surface smoothness-m-09.jpg  Hull surface smoothness-vpg_margo_3.jpg  Hull surface smoothness-m-07.jpg  

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  #17  
Old 03-18-2017, 09:02 PM
Ilan Voyager Ilan Voyager is online now
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Out of subject. Very anecdotal but simply to show Mr Paul Lucas other fields of interest, and mines also.

Dieselis was the first small plane 2 seats initiated by Serge Pennec, another guy from Brittany, on 1996. Lucas helped a lot in the concept and design thus co-signed.
The result was an ultra light 2 seater weighting 265 kg which won a economy rally using 4.7 liters (1.24 gallon) hour at 153 km/h (95 mph) mean speed.
The two first pics Dieselis. Made in Plywood.
After this initial success, Pennec worked on the Gaz'aile 2 with an AX Ciroen diesel turbo car engine of 53 HP. Lucas and many others participated in the project.
Post 68 ex hippies left wing social democrat European engineers are incurably stupid. Instead of making money and patents, they work part time for free for the good causes, from the saving fuel small plane to fishing boats, very cheap diesel engines working with straight vegetable oil, solar pot dutch ovens, or water pumps and many other things in poor countries. These "islamized" communist Europeans are frightening. LOL.
Result of the Pennec work a 2 seater cruising speed 220 km/h (137 mph) with a consumption of 7.8 liters (2.1 gallons) hour of diesel fuel fully loaded. The plane flies 800 km (almost 500 miles) with 28 liters (about 7 and some gallons) in 4 hours.
That's brain juice at his best. The plane is built in wood, uses an ordinary small turbo diesel car engine and costs around 10 to 12000 USD. That is not the price of an engine Rotax 80 HP... Add two barrels of elbow grease (3500 hours). An ultra light (bigger wings with low speed landing) version can be used in short dirt tracks in Third world countries for a bush doctor.
Last pic Gaz'aile2.
Attached Thumbnails
Hull surface smoothness-dieselis.jpg  Hull surface smoothness-dieselis-fuselage.jpg  Hull surface smoothness-gazaile2.jpg  


Last edited by Ilan Voyager : 03-20-2017 at 10:20 AM.
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  #18  
Old 03-20-2017, 04:07 PM
CrunchyFrog CrunchyFrog is offline
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Thanks for all the input.

Clarification:
The boat is indeed an IOM - Goth XP design by Frank Russell, built out of cedar strip. Dipslacement, as per IOM rules, is 4 kg.

I recently faired out a bunch of voids in the hull which occurred during construction (I didn't build it). There are, at places, up to 1mm of thickened epoxy to make the bottom smooth. I then used primer followed by automotive paint, and then had it professionally clear coated.

After clear coating, I wet sanded it with 1000 and then 2000 grit. I can say after all this that the boat is noticeably faster than it was before. The difference is small, but it's the difference between being slightly slower than the boat next to me and being the same speed. Anybody who's raced sailboats at any size will know how important this is.

To finish my hull prep, it sounds like I should hit it with some 3000 grit and then follow up with a finishing polish.

As a side note about golf ball dimples, this seems to come up a lot in relation to boat hulls. It was explained to me by a hydrodynamic engineer that the divots in the golf ball do less to decrease air friction and do more to create lift. This is what makes the golf ball go further.
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  #19  
Old 03-20-2017, 07:13 PM
Ilan Voyager Ilan Voyager is online now
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The Goth XP is a lovely design. You're almost done, use a light hand for the final steps 3000 and a polish and have fun with your boat. RC sailing is a great affordable pleasure. (Yes the dimples of the golf ball are mainly for flying further)

https://www.youtube.com/watch?v=YSOJxQ_y7Ek
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  #20  
Old 03-21-2017, 10:36 AM
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gonzo gonzo is offline
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The high polish is only good for cosmetic purposes. For example, race boats that need sponsorship can't look dull. However, sanding to more than #360 has been shown by tests that it makes no difference. Check "Principles of Yacht Design" by Eliasson and Larson.
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  #21  
Old 03-21-2017, 11:08 AM
Joakim Joakim is offline
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Quote:
Originally Posted by gonzo View Post
The high polish is only good for cosmetic purposes. For example, race boats that need sponsorship can't look dull. However, sanding to more than #360 has been shown by tests that it makes no difference. Check "Principles of Yacht Design" by Eliasson and Larson.
As that book points out the smoothness requirement depends on speed. So 360 is a total overkill for a RC boat. But sanding with 2000 seems to be part of the hobby and easy to do with a RC boat. It could make sense for a fast dinghy or a foiler doing 20 knots, but not for one doing only a few knots.

Fairing the surface is another story and it can help, especially for the appendages. But not because the surface gets smoother.
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  #22  
Old 03-21-2017, 01:30 PM
CrunchyFrog CrunchyFrog is offline
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Does the Principals of Yacht Design book account for the much reduced waterline length of an RC sailboat?
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  #23  
Old 03-21-2017, 01:33 PM
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gonzo gonzo is offline
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It is related to speed only, not waterline length. There is also turbulence related to shapes, where a rough surface can be beneficial. Basically, it creates a thinner turbulent layer.
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  #24  
Old 03-21-2017, 03:58 PM
CrunchyFrog CrunchyFrog is offline
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I'm no expert (hence my asking questions here) but to my understanding, the boundary layer is effected by the waterline length (among many other things). Is this incorrect?
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  #25  
Old 03-21-2017, 04:56 PM
Joakim Joakim is offline
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The boundary layer for a 10 m sailboat or a 300 m tanker is just the same for the first 1 m as it is for 1 m RC boat, if all are going at the same speed in the same fluid. Since a sailboat and a tanker are typically going much faster, they have thinner boundary layer and thus need smoother surface for the least possible drag.

The link I gave earlier (http://www.mne.psu.edu/cimbala/me320...rbulent_BL.pdf) gives all the formulas for the boundary layer thickness.

E.g. at 1 m/s speed (2 knots) the laminar boundary layer is 1.6 mm thick 10 cm after the leading edge (bow or leading edge of a keel). At 20 knots it is 0.5 mm at the same spot, if the boundary layer is still laminar. If it is fully turbulent, it is 2.2 mm thick. The thickness of the boundary layer is the distance where the velocity is 99% of the far field, thus wall has slowed down only 1% at that height.

Laminar boundary layer has linear velocity profile. So at 10% of the boundary layer the velocity is 10%. Imagine the hull is made of 200 sanding paper, which is made of 75 um particles. Now there will be peaks going maybe 40 um above the average surface into the boundary layer. That is 2.5% of the height of the BL at 10 cm so the velocity there would be 0.025 m/s, if the surface was totally flat. Not far from 0.0 m/s at the average surface, thus not any measurable additional drag.

Now imagine sanding that 200 hull with 200 paper. The surface will definitely get smoother. So you don't need 200 paper to make surface that smooth.

When the BL is turbulent the velocity profile is totally different. There is always a laminar sublayer, which is thin and has linear profile. Above that the velosity profile is y^1/7. Thus 10% BL from the surface velocity is still over 70% of the far field. Thus surface roughness vs. BL height is much more critical for a turbulent BL. If the roughness is higher than the laminar sublayer, the peaks see high velocities and drag is created.
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