Rick, "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.
"
But will the ama really add 40% when just touching the surface?
There is a diagram (which I can't find now) showing the wetted surface for a cat and a tri as a function of heeling moment.
The cats surface will steadily decrease with heeling until its flying one hull. The tri starts at rest with almost the same low surface as the cat when flying a hull. Which is when low wetted surface is really needed, in light winds. Then the tri will increase the area with more heeling moment, having higher wetted surface in mid range, until it also starts to fly a hull. I think that's fine, because in the midrange there is enough power and speed anyway.
There is also another thing to it. When sailing in uneven winds in the midrange, a drop in pressure will lead to the WW hull dropping deeper on the cat, causing an increase in drag, slowing the boat, which means a lot of sheeting to compensate. In the tri, a drop in pressure unloads the ama, decreasing the drag, and the boat glides through with less resistance, loosing less speed, and less sheeting is required. In theory.
On Fri, Mar 1, 2019, 23:25 Rick Willoughby rickwill@bigpond.net.au [harryproa] <harryproa@yahoogroups.com.au> wrote:
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.RickOn 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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