That link is the best data I have seen on surface roughness and impact on hull friction. The 18m proa when freshly anti-fouled has a measured drag 1.4 times higher than the calculated drag at a speed of 5.5kts. The chart in the link indicates that would occur with surface roughness of 250 microns. We have never measured the surface finish with freshly rolled antifoul but I think 250 micron is close - its is rough to touch. The antifouled used is the ablating type so it may get smoother with use but it gets slimy fouling within a couple of weeks. Typically any change on the boat is less significant than fouling so if testing anything new the hull gets a scrub.
By cleanliness I mean no slime; just the finish coat. Pointing ability is a function of the overall sailing efficiency. Any boat that has barnacles is, at best, a motor-sailor because its windward ability under sail alone will be horrible.
The lower wetted surface on the tri hulls only occurs when neither ama is loaded. That is almost impossible with a long slender central hull unless the amas have lifting foils. The individual hulls of a HP have near zero stability so a tri with a comparable central hull that has low wave making through its speed range will have near zero stability making it almost impossible to keep the leeward ama off the water.
Any boat that sits in a marina without use for a couple of weeks will have slime that will take some hours of sailing or high speed motoring to remove. Boats that are raced twice weekly or even weekly are usually clear of fouling if they have a good ablating antifouling. If the boat can be taken out of the water then they will stay clean. The rate of fouling depends on the location of course. I have seen boats with more life attached and adjacent to the hull than a respectable public aquarium.
Remember that fouling data only applies to viscous drag. The drag on a well designed slender hull is around 90% viscous drag and 10% wave drag. The viscous drag is less significant on a barge where wave drag is more significant than viscous drag. Hence fouling has a proportionally lower impact on a barge or barge like motor/sailing boats.
Rick
On 2 Mar 2019, at 5:05 am, Björn bjornmail@gmail.com [harryproa] <harryproa@yahoogroups.com.au> wrote:Rick, regarding "Neither hull makes substantial wave drag at low speed making hull cleanliness the key factor in light wind performance."Cleanliness, do you mean from algae/barnacles?A trimaran of the same length/displacement as a catamaran or proa, should have a smaller wetted surface area by a factor of 1.4. So if the growth has a larger effect than that, it should be true.I remember finding and posting this:Google translation:It seems like the permissible roughness is 100microns per m/s. Or about 200 microns per knot. Is this the boundary layer thickness of that speed?There is also a diagram showing how much extra friction arough surface adds, depending on the roughness. At two knots, a surface roughness of 1mm in size will increase the friction by a factor of 1.5. So then the trimaran would be slower if the proa is completely clean. Then, the faster the boat is, the more important the surface finish is. At 4 knots, 400 microns gives a factor of 1.5. And at 7 knots 200 microns, But to me it looks likeHow dirty is a typical multihull in a marina, and how much will it effect the performance?
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Posted by: Rick Willoughby <rickwill@bigpond.net.au>
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