Spray drag - how much of a difference does it make, and how can you reduce it?
I don’t follow, especially the first part. If you find the time later today/tomorrow, could you make a sketch?
Marco
Marco,
Detail attached…...
Comparative sections: A is the “norm”, B is what we are talking about, C is “Sidecar”, my proa.
“Cross hairs” are the centres of gravity of each section.
Rob
The clinker build of Viking longships have the same function. A clinker built hull will have many “spray rails”, and the edges below the surface will give some dynamic lift, keeping the bow up and damp pitching. I don’t think they create that much drag, if the lines are right and the hull very slender.
I have been thinking a lot about a steel lap-strake/clinker build. I think there are a lot of potential benefits with these natural spray rails.
We visited Sigrid Storråda Sigrid Storråda in her harbour in lake Vänern this summer, and i am very impressed by the ingenious design of the viking longships.
Cheers,
Johannes
Very nice diagrams there Rob! I definitely understand what you mean now. One question though: is the dashed blue line the waterline? If so, those spray rails are way too high up relative to what Faltinsen suggests in his book: 0.03 * LWL right at the bow, dropping to 0.01*LWL pretty quickly. On an 8m boat we’d thus be talking 24cm height right at the bow, dropping down to 8cm clearance and lower in fairly short order, those rails look like they are quite a lot further up than that, especially since that’s the section 0.75m from the bow. While Faltinson may be wrong, it does seem like there’s a lot of potential wetted surface (from spray) that you can save by moving the rails down low…
For me there’d be yet another reason: in Firstborne the floor is raised at the expense of headroom for (1) increased stowage space under the floor since that’s just about all there is in that regard, (2) more width for the person sleeping on the floor, and (3) to make the cabin self-draining, should it ever be flooded because someone left the hatch open and rolled the boat over for some reason. 😉 Now a 4cm wide (chamfered) spray step on each side of the hull at maybe 10cm above the waterline at midships would be pretty convinient for me, since it just so happens that my cabin floor is at precisely that height too, making my secondary ‘berth’ on the floor 8cm wider—what a coincidence! 😊
Here’s another question: wouldn’t a full-length fillet along the length of the entire boat, along with the stringer weigh quite a lot, even with low density filler? Faltinson’s suggestion for the rail width is quite large after all. I shudder to think of double digits here…
Cheers,
Marco
P.S. - By the way Rob, does this up and coming mystery beauty of yours have a thread of its own on the forum here? 😊
One question though: is the dashed blue line the waterline? If so, those spray rails are way too high up relative to what Faltinsen suggests in his book: 0.03 * LWL right at the bow, dropping to 0.01*LWL pretty quickly. On an 8m boat we’d thus be talking 24cm height right at the bow, dropping down to 8cm clearance and lower in fairly short order, those rails look like they are quite a lot further up than that, especially since that’s the section 0.75m from the bow. While Faltinson may be wrong, it does seem like there’s a lot of potential wetted surface (from spray) that you can save by moving the rails down low…
The blue line is the water plane…...What you see as spray rail and step on my cross section are the upper (for extra dryness and reserve buoyancy) spray rails. The flat bottom is the lower (for speed) sprayrail(s). If I were to have a semi circular bottom of equivalent underwater cross sectional area, its underwater girth would be ~ 31cm, plus 2 x 4cm spray steps = ~ 39cm. Rails would be more again. The flat bottom underwater girth is ~ 34cm and much less draggy and lighter than either steps or rails. My boat put through Michlet software says that at 10 knots the bows will lift ~13cm and the stern sink ~ 13cm, so my 0.75m section is (just) airborne in dead flat water.
Here’s another question: wouldn’t a full-length fillet along the length of the entire boat, along with the stringer weigh quite a lot, even with low density filler? Faltinson’s suggestion for the rail width is quite large after all. I shudder to think of double digits here…
What supports your floorboards? Use the spray “stringer” ......My upper ones are horizontal and support outer bunk top edges….If you want to save a little bit of weight, don’t have spray rails. If you want the possibility of extra dryness (less weight?) and speed, accept the weight penalty…...There are no free lunches.
“Sidecar” is in build….I am glassing the Vaka now, the ama is already glassed so I am not far now from turn over. They look exactly like the 3D images below,.The carbon spar blanks are already in the shed and I hope to be sailing by the end of January, which is late summer down here.
Regarding storage, don’t forget to maximise usage in the ama and akas…...
Rob
PS: if you do go down the spray rail route, don’t forget to put them on the ama as well…...arguably, they should be more effective than vaka ones.
The clinker build of Viking longships have the same function. A clinker built hull will have many “spray rails”, and the edges below the surface will give some dynamic lift, keeping the bow up and damp pitching. I don’t think they create that much drag, if the lines are right and the hull very slender.
I have been thinking a lot about a steel lap-strake/clinker build. I think there are a lot of potential benefits with these natural spray rails.
I just love viking long ships, they are one of the all time classics….....Isn’t it a pity, they didn’t make them narrower and attach an ama??!!!!!
And I agree about clinker / lapstrakes acting just like lots of (little) spray rails. My only comment on doing steel lapstrake is that you wont get much of a rail out of 2mm plate, maybe use the stringers supporting the plating as step rails???
Rob
Wow, she’s gorgeous!—What LWL, beam, displacement and sail area are we talking about here?
I honestly haven’t put much though into using the ama or akas for stowage since I want to avoid non-moveable weight in the ama, and with its large ballast tank, airtight compartments for self-bailing when inverted, and low freeboard, there is not a whole lot of space to use there, especially if you want all-weather-access. That said, I will think it over if there is anything on my list that I could stuff in there.—How do you use the akas for stowage? I really wouldn’t want to cut holes into a major structural member of my boat… “Sidecar” has a somewhat unusual aka-configuration, do you use it for stowage, if so, where?
Where the spray rails are concerned, I definitely wouldn’t put a rail under the water for exactly the reason you mentioned: horrible things happen to wetted surface then, and the 5-10% overall reduction which I am trying to get myself with the round bottom, will disappear easily. That the bows will lift 13cm at moderate speed surprises me a bit, I had no idea the effect could be so pronounced with a boat like this.—I have absolutely no clue about bow-up moments at speed, but does the calcualtion factor in the bow-down moment from the rig? If so, at what course?
Overall the tradeoff here becomes really interesting… 😊 If we take the 31 vs 34cm girth as a fair measure of comparison there’s a 10% difference in wetted surface (spray effects aside), and an additional 10% difference in the mass of the underwater part of the hull skin, which means something like a 1-2% increase in total displacement. So in terms of displacement the flat bottom costs approximately the same as external wooden spray rails. In terms of wetted surface when at speed (so including spray now), I’d venture to guess that adding spray rails/steps to a flat-bottom boat, will result in lower effective wetted surface than a boat with a semicircular section without the rails.
That point really gets me thinking again about whether those semicircular sections are worth it or not—they’re really a headache compared to a flat bottom, strip planking instead of sheet ply for the bottom, installing trunks and everything becomes more difficult, and the vaka won’t stand upright in the shop on its own anymore either, and there’s the slight increase in draft as well. How I’d love to have a definitive answer on how much a narrow flat bottom like yours slams in a seaway… If I knew with certainty that slamming was absolutely no concern, then I’d probably drop the semi-circular sections in my design; the performance benefit alone doesn’t outweigh the other factors.—The construction of my vaka is slated to start only in March/April of next year, as soon as it’s warm enough (my workshop is unheated), so I’ll definitely be following news of your first sea-trials eagerly! 😉
Anyway, I think we both agree on that the rails definitely need to stay above the waterline, both at rest and when underway. 😊
I personally like the hull step approach best, for the cabin room improvements, and I think if the weight penalty will be almost nothing if I just give the hull a 4cm wide chamferred step and where stringers are concerned I just place them on the inside of the hull skin normally, not sandwiched in (a 4cm thick stringer with a corresponding fillet would be really heavy).
Cheers,
Marco
Marco,
SIDECAR:
LWL: 9.5m. Displ: 1180kg. Sail Area: 63m2 max, 47m2 working. Cp: 0.53. B(c/c): 4.5m
Michlet calcs re boat trim does not include for rig loads (too complicated for me)
Ama has a reserve buoyancy leepod / spraystep as well and contains 80 kg batteries and will stow mooring lines and anchor(s).
Although I have the facility for water ballast, I am hoping not to use it much and rely on the uplift / downlift differential of the pivoting foil. (+/- 100kg @10 knots) It is a lot easier to “dump and refill” quickly than waterballast. Also will be using DaveCulp’s Auto flight link between foil and mainsheet. I don’t expect autoflight, autodump in itself is a real result.
IMHO, do yourself a favour and forget half round bottoms.The small theoretical advantage isn’t worth it and in your case, I would rather have shallower draft anyhow.
Rob
Wow, what a boat… I’m going to have to take all that in tomorrow, it’s way too late to spend hours drooling over the renderings and the numbers now, but I will say that I really do hear you where the round bottom is concerned… You wouldn’t believe how much trboule has already gone into the ‘simple’ task of trying to find suitable wood for strip planking, and a place to get the planks made in the required dimensions, at a price that a poor student like me can afford. Slap a sheet of plywood under there and I’d have a couple of things less to worry about.
I absolutely do not want to compromise the seakindliness of the boat though, since I really do want to do some serious cruising with it, so I’m really worried about the impact of the flat bottom on slamming, and the increase in experienced accelerations due to waves, which comes with decreased draft. If you want a smooth ride, you want as low a CG and CB as possible (Marchaj). If I had proof that a flat bottomed non-heeled boat like that could handle a real slop without problems, then I’d go for it, but as yet, I don’t. Russell Brown’s boats and Cheers work really well and at least the former are very seakindly from what I’ve read, but they all have deep-V sections.
Cheers,
Marco
I absolutely do not want to compromise the seakindliness of the boat though, since I really do want to do some serious cruising with it, so I’m really worried about the impact of the flat bottom on slamming, and the increase in experienced accelerations due to waves, which comes with decreased draft.
Marco,
....Nor do I, but I want upwind planing (at least on the ama) and I reckon the bows are narrow enough to be OK . At 0.75m back from the bow, “Sidecar” is only ~ 16.5cm across the bottom, there are any number of IOR / IRC boats that had flats bigger than that (rating gain), even heeled and they have much bigger bow angles and lead keels.
You know my fall back solution, but if you are still worried, then your boat could still be flat bottomed in the middle and develop as little (tortured if necessary) Vee towards the bow as possible to make you comfortable with it and still keep it easier to build and get some inherent spray benefit. You could also build in some oversized chine logs up front and chamfer them off?
“Sidecar” has a double bottom, ie twice as thick as the rest of the hull plating. You could do 2 or even 3 thin bottoms laminated on top of each other to make sure you can get your (tortured) Vee. We used to double bottom Cherubs back in the late 60’s / early 70’s and it was just as light, stiffer and more robust than single ply and stringers, especially if you could get a bit of bend or twist. Two way bend is even better.
BTW, FWIW, I had 3 Cherubs back then, the first fine bowed, the other two fuller bowed. The first was wetter and never planed upwind. There was no noticeable pounding with the other two, and they both planed upwind, SO much faster.
PM me if you would like…........
Rob
Just for comparison, here’s the section 750mm from the bow for my 8m boat (without the spray step). Seeing as your design is a fair bit longer than mine that’s proportionally a bit further back than the 750mm section of your boat, but it still has a very fine entry. With a section like that I’m not the least bit worried about working my way through a good seaway. 😊
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?
I get the feeling we’re missing something here…
Cheers,
Marco
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
Good question…...
http://trimaranproject.blogspot.com.au/p/john-cadwalladers-photos-from-his-trip.html
IDEC: 7/63 and 46/63….could be structural…..
I could be wrong , but I have put my money where my mouth is and whenever I have any doubts, I recall these two pieces of text:
“Skene’s Elements of Yacht Design” P 108:
“Any rounding of the forward sections will cut down pounding and produce an easy-riding hull. If lines are too fine forward, there is a tendency for such a boat to dive into the waves; thus the fine, round-bottom boat is soft riding but impossibly wet. On the other hand, V-bottom, planing hulls with a wide chine kept low forward are dry but impossibly hard riding.
Spray guards are the solution to the spray problem and really make a tremendous difference. On some round-bottom boats without spray guards, a thin sheet of water will climb up the side and sometimes go as high as the top of the deckhouse. If there’s a stiff breeze, this spray will soak boat and crew like water from a fire hose. If the cockpit is not designed to be self-bailing, the boat could even swamp and sink from her own spray! This is bad design. But spray guards force water, as it starts climbing up, to separate from the hull. They throw spray out and down. They also provide some lift, as well as making a planing boat more stable dynamically.”
Excerpt from the Commodore’s Story:
http://www.thecheappages.com/proa/commodore.html#1900
“With the growing wind on the quarter, outrigger in air and spray flying, they were traveling at a very high speed for a small boat. Then suddenly, with a little extra puff, the fore part of the boat lifted under them and, skimming the surface like a skipping stone, leaped into a surprising burst of speed. The sea beneath swept by like a cataract, vague and foam-streaked, the tugging steering oar cut a clear open cleft in the water and threw up on either side a long rainbow of spray, while the boat itself rested so lightly on the water that she made little, if any, disturbance in her flight. They stole hurried, ecstatic glances at each other. How fast were they going? What sort of creature was this proa? And what on earth was she doing?”.....Planing.
If Munroe could do it with a pine planked cotton sailed boat over a hundred years ago, I want to do it…...Ironically, I didn’t realise till recently, that my boat is not only flat bottomed but also wide planked, down to the (outer) bottom being cross planked…..Just like his.
And the Frank Bethwaite Sail Carrying Power Formula (>30%) says “Sidecar” (35-45%) should be able to plane upwind as well. Only one way to find out…..
Rob
Those are some really great pictures, with some angles on the boats that I hadn’t seen yet. 😊
I’ve always thought that the multi-chine construction on the vaka of IDEC was probably a cost and/or weight-saving measure. What the upper portions of the vaka look like doesn’t really affect the performance, and if you can cut costs there, then you can build a slightly larger boat for the same amount of money. It could help with the spray too though. At first I was thinking that the shape right at the bow doesn’t support that hypothesis, since you don’t see an extra chine there, but usually the very tip of the bow is out of the water, so it’s the deep V underneath which counts when you’re looking at the very front of the bow, and that should be mighty good at defelecting spray. A few meters from the bow, you can see the multi-chine construction take over, so for me it looks like spray deflection could actually be an integral part of the vaka design, I just never noticed it before because it’s quite a subtle feature. The amas on IDEC also have an extra chine above the WL, starting from the bow, which may help with spray. You can see them on the inboard side (images 19/63 and 31/63), but the outboard sides have them too (though it doesn’t look like it in 8/63). Here’s a nice video of IDEC sailing in a little bit of a seaway, and there you can see the chine on the outboard side of the amas very clearly: http://youtu.be/KtY_B1j5A4M?t=3m48s
Looking at the other pictures, I don’t see that on many of the other tris, notably the MOD70 doesn’t seem to have anything in that regard… Here’s a nice video of them sailing in the 2012 Krys Ocean Race by the way: https://www.youtube.com/watch?v=kO_4gEsSqvM—While wet and bumpy, it sure looks a lot drier and gentler than a VO70 bashing its way through the seas! :D
Where the planing is concerend, I don’t know. You have a very high L/B ratio, which makes for a really low aspect ratio planing surface, and thus a very low lift coefficient. Also, with rocker in the hull, it’ll be difficult to give the surface a proper trim angle so that planing lift can develop (the proa you cited didn’t have rocker as far as I know), so I’m skeptical, but yeah, there’s only one way to find out, so I’ll be following you keenly. 😉 Interesting would be to hear from someone who’s done some seriously fast sailing with a Harris-designed Mbuli. It’s got a flat bottom, if we found someone who has one, then we could ask them about planing…
Cheers,
Marco
I think I’ve got it… I think it’s about pitching! If you add a chamferred step to the front of the boat, then you’ve got more volume immersed when you hit a wave, which will result in greater vertical accelerations on the bow. It will help prevent pitchpoling, but it also makes the ride less smooth, slows the boat down a lot when it comes crashing down again, and it ensures the trim of the entire vessel is constantly changing.
The offshore tris almost universally have inverse bows these days, not nearly as extreme as the AC45 or AC72 bows though, because they can create a lot of dynamic downward lift when they go into a wave, exaberating the pitchpoling risk, but the offshore tris do have low volume inverted bows to keep them from bobbing up and down. It’s probably much easier to maintain a high average speed that way. Racers don’t care if it’s wet anyway, and probably the performance gain from having a steadier trim and less pitching, significantly outweigh the performance benefits of wide spray rails / steps.
Cheers,
Marco