Spray drag - how much of a difference does it make, and how can you reduce it?
If you add external rails, then those wouldn’t markedly increase the buoyancy in the bow, but if the rails are horizontal, then if you are in a near-pitchpole situation, they increase drag at the bow, and have a negative AoA, which increases the risk of pitchpoling. If on the other hand you make the rails sloped, so they are higher up at the bows and closer to the WL a little further back, then you have solved the problem in a near-pitchpole situation, but you have enormous amounts of extra drag (and a positive AoA creating upwards lift on the bow) every time you drive the bow and the rails right through a wave.
So option (1) a chamfered step, increases bupyancy and thereby pitching, and (2) the external rails may increase pitchpoling risk, increase pitching, and increase wetted surafce significantly when going through a wave.
Marco
So option (1) a chamfered step, increases bupyancy and thereby pitching, and (2) the external rails may increase pitchpoling risk, increase pitching, and increase wetted surafce significantly when going through a wave.
In waves, I would have thought that momentary increases (and decreases) in wetted surface are insignificant in the scheme of things…....and the bigger the wave, the greater the insignificance.
The more you dig a nose in, the greater the risk.so at the end of the day increased pitchpole resistance can only come from increased forward buoyancy (static or otherwise) or rig / sail area reduction.
Nothing more to say…As Bill S once said somewhere on Proafile: “Don’t speculate, build!” or words to that effect. I will let you know in due course how “Sidecar” performs.
Rob
—And I will be listening eagerly. 😉 The offshore trimaran designers no doubt spent a long time getting that balance between excessive pitching and pitchpoling risk just right for those boats, but that may or may not be the best solution for our proas.
The tricky question for me is: do I want the extra volume at the bow that a chamfered spray step brings with it, or not? And do I want that as close to the waterline as what those authors suggested for their spray rails? If I knew my boat had a tendency to pitchpole, I’d definitely add them (the extra space in the cabin is a really nice plus too), if on the other hand I knew that it pitches too much in a seaway, then I’d leave them out. I haven’t built and sailed the boat though, so I have absolutely no idea how it’ll behave. :/ Your solution of using the bottom pod panel to do that is very elegant, and it’s high enough that it comes into play quite late where immersion and the extra buoyancy is concerned…
I spent some time today looking at the drawings of Sidecar’s cabin and iakos which you posted earlier by the way, and I have to say I really like your cabin layout! It’s amazing that you’ve managed to fit what looks and will probably feel like such a roomy and comfortable cabin into such a small boat. It’s nothing like the cramped cabin that I will have to settle for in my boat. 😊 I really like the sheltered cockpit and all the stowage space in the iakos too. My design has none of that at the moment, the extra windage of a shelter / canopy is just really excessive if you use small iakos. With a larger wing-shaped iako like yours the whole thing looks really elegant, and you get all that extra stowage space too. How are you going to build the iako / wing? Tortured ply? And the little shelter, is that permanent, or a soft canopy which you can stow, or what are you going to do there?
Cheers,
Marco
It’s amazing that you’ve managed to fit what looks and will probably feel like such a roomy and comfortable cabin into such a small boat….............. With a larger wing-shaped iako like yours the whole thing looks really elegant, and you get all that extra stowage space too. How are you going to build the iako / wing? Tortured ply? And the little shelter, is that permanent, or a soft canopy which you can stow, or what are you going to do there?
Most proa designers go for the single central hatch to below, which in my opinion wastes the best space in a boat and creates all kinds of conflicts with the mast and all the stuff hanging off of it…...2 hatches even though it seems wasteful avoids all of that and gives you drier access to below (you only use the aft one when it is wet and windy).....and better ventilation in warm weather. And by putting the cockpit in the wing, you get a lot more space below, more weight out to windward and it means that I am to windward of the vaka spray blast zone. You only have to look at the flapping spray dodgers on Jzerro to see how much of a problem it is.
The inverted aka wing, is less (wind) draggy than 2 aka beams, trampoline netting, bench / slat seats and an outboard sled. It should be a lot drier, provides storage and I hope that it will in itself provide some down lift righting moment. The RM gain is peanuts at low wind speeds, but over 30 knots apparent, the potential gets quite interesting. I will build it using strip planking.
The frameless canvas canopy (supported with ties off the lower weather stays and mast) folds in and rolls up and being fixed on the weather bulkhead end of the cockpit can be used as a backrest, ready to be erected at a moments notice. Cold, and wind / spray blast on proas that are doing double figure boat speeds and much higher apparent wind speeds in cold places should not be under estimated.
BTW, all my strip planking is 190mmx12mm ( 8x1/2 inch in old money) paulownia (260kg/m3) and I narrow it down and / or route 6mm wide half depth U shaped grooves into the back of it when the curves get difficult, and then fill the grooves with lightweight bog. All the bulkheads and frames are 14mm paulownia panels and all the stringers and other framing is also paulownia.
http://www.storerboatplans.com/Faq/paulownia.html
Rob
There is something I’m wondering: if spray rails / steps are as obviously good for performance as we have concluded here, how come you don’t see them on C-Cats and the big offshore trimarans? The designers at VPLP and elsewhere know more about high performance boat design than most of us ever will, so why didn’t they include rails?
Telefonica had spray steps on their VO70s in the 2008-09 race, and at least the Juan K. boats in the 2011-12 race had a sharp edge above the waterline as well, which would have had the same effect as a rail. But what about the offshore trimarans (MOD70s, Banque Populaire V, IDEC, etc) and prefoiling c-cats?
Marco, to partially answer your question:
1: A lot of these large trimarans are quite old (for racing boats) and have been modified and rebuilt, often many times and at minimum cost, to keep them reasonably competitive and to comply with changing rules and design fads. They were then renamed, making it hard to trace out the original boats. A close look at some of the photos on the page Rob linked will show a number of tris with archaic (so to speak) design features.
2: A lot of these big tris were not originally designed as offshore boats, but were actually inshore harbour racers. As such, the ability to punch through short chop without slowing much was a lot more important.
3: Have a look at Nigel Irens’ website, under all three categories. There you will see- shall we call them “spray edges” or “spray flares”- on many boats. Nigel rarely gets it wrong.
http://www.nigelirens.com/yacht-categories/racing-boats
http://www.nigelirens.com/yacht-categories/power-boats
http://www.nigelirens.com/yacht-categories/sail-boats
4: C-cats are inshore harbour racers and have the same priority of punching through short chop as the inshore racing tris. I also think wing-sail C-cats operate at such a high wind-speed/boat-velocity ratio that little about them will apply to a small wooden proa.
I would go with spray rails. From too many years of beach cat sailing, I was tired of getting a face-full of water every time the windward hull touched down. So I put them on my first proa and they made sailing it a much more pleasant proposition. They were particularly helpful for non-boat-crazy guests. If you are worried about extra buoyancy forwards, you can always take it back out by angling or bending the hull above the spray rail a bit narrower, so the deck is also narrower and will shuffle off green water faster.