Spray drag - how much of a difference does it make, and how can you reduce it?
Hey guys,
I’m not at all well informed on the topic of spray drag, but I’ve seen it mentioned here and there as being a significant factor for high speed boats if you want to squeeze maximum performance out of it. Does anyone have some forumlas or empirical values for how much of a difference it actually makes? And how do you go about avoiding / minimizing it? Is the bow-radius the deciding factor for that, e.g. the sharper the bow the less spray drag, or is there some optimum radius, or are other factors at play there too, if so what are they?
I can’t help but compare the absolutely beautiful footage of Invictus’ C-Class cat at times slicing through the flat water with no spray at all, around 3:20 in the video, with the firehose spraying up from Jzerro’s bows in this video. Jzerro is obviously a fair bit larger than a C-Class cat, but as far as I know the two boat types (non-foiling C-Cats) are quite similar in performance, at least in terms of top speed, both maxing out somewhere in the 20-25 knot range. To be fair to Jzerro, there’s quite a bit more of a chop out there, and probably a fair bit more wind too and thus higher speeds than the C-cat in the video, but in the end that firehose is just unnecessary drag, so the question remains: can you do something about it, and if so at what cost?
Edit: And in terms of physical phenomena, what causes spray in the first place, other than a hull slamming down on a wave? (there was plenty of that with the C-Cat)
Anyone? 😊
Cheers,
Marco
The best way to avoid creating spray is making vaka and ama long, slender and lightweight.
Every kg mass in the proa is a liter water that has to move out of the way. The faster you try to move the water aside the more spray and waves. A high PC will create a faster and more abrupt parting of the water, creating more wake. A longer low PC hull (0,5 - 0,55) will create less spray and wake, compared to a shorter but high PC hull, for a given displacement.
Johannes
I think it is even a little more involved than that (but not by much). Base thing to look at is Displacement/length ratio, if the boats in question don’t have similar-equal D/L ratios it’s a matter of apples and oranges.
Pc analysis is tricky. Take two hulls equal length and displacement. Both will have the same D/L ratio but the lower Pc hull will shine in lower speed range and the higher Pc hull will blast by lower Pc hull when the wind picks up. This assumes both hulls are equally well crafted.
As to the original question, I’m not sure of much but here’s my take.
1. As long as the spray isn’t hitting any thing on the boat it’s pretty much a low penalty foul.
2. As soon as the spray starts hitting something (lee pod or the usual side mounted rudder brackets come to mind) drag cost skyrocket.
3. Jzerro looks to be a little dryer ride than the C cat in any case.
Cheers,
Skip
Marco,
A number of points to consider:
1) Multihull design
Do sailing monohull generalities where the norm is speed length ratios of around 1 and length beam ratios of 3 apply absolutely equally to proas where we are talking speed length ratios of 2 to 3 and length beam ratios of 12 to 20?
2) Power boat design
Speed length ratios of 2 to 3 are getting well into powerboat territory. So maybe some of the things they do in powerboats (like spray rails) are relevant? For dryness and improved comfort and strength if not speed. If they are steps (a la Pizzey), as opposed to rails, you get more reserve buoyancy and internal volume as well. A lot of the most recent cats and tris are sporting steps in various configurations now for similar reasons.
3) Forward hull shape
Any number of textbooks on yacht design will tell you that the finer the forward sections are, the wetter the boat will be and the fuller and flatter the forward sections are, the drier the boat will be. Down side is that the fuller, flatter sections forward will pound and slam more .......But how bad will that be on a proa with an l/b ratio of 15 which is inherently fine-bowed anyhow?
FWIW:
1) My proa is flat bottomed all the way and will have a sprayrail and has a spray step. The weather rail will extend and blend into the bottom sidehung rudder gantry. The leeward step is in the form of a full length leeward pod. Flat bottoms save weight (no floorboards) are easier to build, enable shallower draught and should provide some dynamic lift. If the flat bottoms forward really do end up being a serious issue, then I have enough rocker in the boat to foam up and round them off as necessary.
2) A link to one of Michael’s friends / collaborators: http://www.mcgowanmarinedesign.com/Zip_Tri.html Check out the steps underneath.
3) http://www.sponbergyachtdesign.com/Bluebill.htm
Good Luck.
Rob
An interesting study on the 49er skiff has been published by Simon Watin. They straightened the bottom slightly and sharpened the chine to get a drag reduction of up to 25%.
AArgh can’t link to it. Search for 49er performance enhancement report.
A hull throwing spray from the bow at speed indicates that there is an intense high pressure zone pushing water up above the surface. It is a very obvious sign of drag.
I have pulled my deep V hulls through the water at about 3 times hull speed, with no spray and very little wake. They are low PC, but very low D/L with a very smooth gradual constant radius - change of flow around the hull. Any sudden change to the flow of water around a hull will create these high pressure shock waves and spray.
Thanks for sharing that paper on the 49er performance enhancement tdem! 😊 Just as Rob pointed out, some of the newer high performance multihulls do feature spray rails and sharp chines (just like the updated 49er) to help ensure the spray stays down / is deflected to the sides, preventing a massive increase inw etted surface area, and thus reducing drag. An example would be the Hobie Wildcat:
http://www.catsailingnews.com/2009/01/new-hobie-f18wild-cat-bow-rails-in.html
http://www.hobiecat.com/sail/wild-cat/
I completely forgot about it, but there is a small section on spray in “Principles of Yacht Design”, which has a quite good diagram (see below) explaining the basic mechanism behind spray formation: stagnation points. When flow has to either go to one side or another of a body, say to the left or right of a hull, there’s a “stagnation point” in the flow, where the flow stands still (perfectly in the middle, it can’t decide which way to go), where it has a velocity of zero and increased pressure. Its near that stagnation point where spray is formed.
For a planing boat, or any boat with enough bow-up trim to get the bow out of the water, the stagnation point will be on the underside of the hull, otherwise it will be at the bow. There’s another at the back of the boat where the flows converge.
Now assuming your bow is in the water, then I could imagine that the radius at the bow plays a big role there. When I say the radius at the bow I mean the foremost couple of centimeters of the boat: is the boat a flat plate at the front, or is it ground to a knife’s edge? I would think that plays a decisive role in how much spray is formed, but that’s purely intuition; does it actually make a difference? And if so, what is the optimum radius at the bow, how ‘sharp’ should it be? The same would apply to spray reduction on stern or side-hung rudders for instance…
Cheers,
Marco
Marco,
Pages 18 and 19 in: http://www.9eronline.com/library/49er Performance Enhancement Report by Simon Watin.pdf
gives a good explanation…...And we are not just talking about spray, but displaced water.
Re the Hobie Wild cat…..Nice to see, but it feels more of a marketing gimmick rather than being truly effective…...I reckon the spray ledge would work better if it was longer and the same height above the water as it is below the sheer. The more you allow the water to rise, the more energy you lose.
Rob
PS: As an aside, the rig section in the above document is interesting as well…...
Good point about the marketing. 😊—What I also find interesting is that I don’t recall ever seeing a C-Class cat with spray rails. I wasn’t looking for them, and I’ll have a look around, but if the rails really did bring a marked advantage, then I’d think the C-Cats of the non-foiling era would have them. Unlike in the F18 or A-Class boats the C-Class rules don’t specifiy a minimum weight though, so it may be that they weren’t worth the extra displacement. Non-foiling C-cats also had round hull sections, so the very opposite of sharp chines. ‘Alpha’ did have a very streamlined iako-hull interface to avoid a bit of the drag from the ‘firehose’ spraying up from the bows at speed though.
What price do you pay for spray rails at the bow near the WL? Just the extra wetted surface of the rails (when they are not doing anything) and the slamming on the rails? I’d imagine the latter would be rather insignificant, and if anything it’s only dissipating the energy from hobby-horsing (thereby damping it) and not your forward momentum. So other than a bit of extra build-time and complexity I’m not really seeing a major drawback here. It’d be great to have some hard data though…
Cheers,
Marco
PS: The rig section got me thinking a bit too. I’d always wondered about how the high performance skiff and catamaran classes seem to have way too much sail area for their RM, I wasn’t aware of the fact that actively depowering the rig is a must, to sail the boats at higher windspeeds. That makes me wonder a bit what strategy I should adopt for the sailplan of my proa; the same as the skiff? Put ‘too much’ sail area on the design so it’s good in light airs as well, depower the rig when needed, and then reef when that isn’t enough? Or be more conservative with the sailplan so that even if you do happen to suddenly get caught by an 8Bft gust and aren’t there to dump the sheets, the boat won’t go over further than gently leaning on its pod? It’s kind of a foolproofness vs. performance tradeoff.
What price do you pay for spray rails at the bow near the WL? ......... It’d be great to have some hard data though.
If you find any let me know. At the moment it is more likely to be found in powerboat land, where there is a whole side industry based on retrofitting sprayrails. They are too new in sailboat land and there is a lot more development and refinement yet to come.
BTW Its not spray rails, and it is a foiler, but if you look at Oracle’s cross sections, they are flatter up front than they are towards the stern. So even in lighter weather / pre-foiling, they are still looking for dynamic lift / spray deflection up front.
Re bendy rigs and gust response, it would be good if you can get some. 18 ft skiffs have 3 rigs, all set up to flex and dump excess power….the big dilemma often being: which rig to use on the day. I would do it on my proa but mast bend and flex doesn’t work with my rig.
Rob
Hey,
A google search pointed me towards this where the author states that at Froude numbers above 0.5 (our much beloved proas come in around Fn = 0.8-1.0), spray rails can reduce overall drag of the naked hull (no appendages) by as much as 12%, but increase overall drag by as much as 20% at Fn < 0.3, and the author also claims the system has been proven to work well for both round bilge and hard-chine catamaran and trimaran hulls as well, so it sounds to me like them there rails will make our boats go faster, if done right. Plus it makes the deck drier.
The boards are going to account for a significant fraction of the overall drag, so that dampens the 12% figure when looking at the total, but even a ~5% overall drag reduction would be pretty neat to have: it could be enough to add about half a knot to the top speed of a boat like ‘Firstborne’, which would be pretty awesome for just adding some spray rails. 😊
The best thing of course would be to do some testing on a racer, with some hard-core data-logging equipment on board to really investigate the spray rails and how to best do them. Too bad I don’t have a small proa to try all this stuff out on. Maybe I should build one, though that would push the start of construction of the big boat even further back…
I’ll ponder over those AC72 sections some other time, but to say I’m surprised would be an understatement… It’s nearly the opposite of what you see in A-Cats and high performance offshore tri hulls these days. Thanks for sharing, I’m definitely going to be pondering of that for a long time… Do you have any theories what on earth is going on with the flat bow sections? Increasing longitudinal stability to help avoid pitchpoling perhaps? Or maybe they are trying to produce a bit of extra bow-up moment there as well due to spray / the bow wave?
Cheers,
Marco
PS - I think optimizing mast bend is out of my reach, considering I’m essentially working alone (with plenty of help from you guys though!). I think I’d need to be developing a boat in a team, to be able to go as far as mast bend and the effect it has on the characteristics of the rig. :D
Marco,
Thanks for the link…...
...... so it sounds to me like them there rails will make our boats go faster, if done right. Plus it makes the deck drier.
Don’t forget, they can be used structurally as well, and if you use steps instead of rails, you get increased reserve buoyancy and internal space. and when a step is fully immersed, you get around half the increase in surface area and drag when compared to a rail.
I’ll ponder over those AC72 sections some other time, but to say I’m surprised would be an understatement… I… Do you have any theories what on earth is going on with the flat bow sections?
I am totally guessing, but if you look at the rocker, it is pretty long and straight forward and all the cross sections along it are flat bottomed. The aft rocker is slightly shorter and more rounded and has the more semi - circular cross sections. So I reckon it is dynamic lift, maxmium lift forward to get the bow(s) to rise and minimum lift aft to get the stern(s) to drop / stay low which suggests apart from maybe slightly increased (dry) speed, it is to do with pitchpoling. With flat sections forward, the dynamic centre of buoyancy must be further forward than the static one. If you had flat sections aft as is / was the norm , it must move aft.
So there could be an worthwhile inherent gain in pitchpole resistance which also magnifies the effect of moving weight aft.
Rob
I would guess the Oracle answer is in the attached picture. If spray rails are above WL they shouldn’t affect drag at low speed, right? The 49er report with sharp chines also did not show greater drag at low speeds.
Good point about the structural aspect Rob! Here I was thinking: ok, just build the boat without spray rails, log the performance, then add spray rails, and see how it goes. If they don’t turn out well then you can always sand them off and muck around with them until they’re good. So much for that idea! If you do put them in right from the start, and get it right, then you can save yourself at least one pair of internal stringers, since the spray rails can make up for them. A single full length pair of 20x40mm stringers makes something like a 5-6kg difference in displacement, or about 1% of the target displacement for Firstborne, and then you can add a little more for potential savings in stringers near the bow, due to the small additional spray rail which is present there too (the “Advanced Spray Rail System” calls for two rails). Overall you are probably going to make the boat slightly heavier with the rails either way, at least if the rails are full timber and not hollow plywood (hehe! 😉 ), but if the 1% displacement from the stringers is avoidable, then you might as well take it! That would also have the advantage that ‘proper’ wood and fiberglass rails would be a lot more sturdy than some flimsy lightweight foam/glass thing you add to the boat at some later point in time… So it’s a tradeoff of ease of changeability of the rails vs. ~1% displacement and their durability.
I like the durability and displacement savings of ‘proper’ rails more, but then I would have to commit myself to the rails, and get their design right the first time. Fortunately that figure provides the required details, so as long as what they are saying is on the money, then it’s only a bit of extra design and construction time.
@tdem: The author proposed that continuing the spray rails to down below stationary WL to increase dynamic lift when at speed, dampen rolling (irrelevant for a multihull), and increase dynamic transverse stability (page 38 in the link). If you do continue them to below the WL (and even without, those rails proposed by the author are seriously low so they are bound to partially be inside the hull’s wave system), so you are bound to pay a penalty at low speeds because the rails are partially in the water on way or another, increasing wetted surface. I’d just opt to not continue the rails below the water to help avoid that, and our rails need to go up on the other side to allow shunting anyway, so my guess is we’re not going to pay anywhere near the 20% increase in pure hull drag at low speed, simply because we’re avoiding the extreme version of the design. Drag due to waves, at low speed, would also increase a bit with the presence of the rails, so that may help explain the 20% as well. Once the boat is moving though (for a 8m hull Fn < 0.3 is at v < 5 knots), the decrease in wetted area from the rails will really be helping out though. 😉
Cheers,
Marco
P.S. - I wonder what sort of other goodies might be hidden in the book…
—Or you pick some height you like, and give the hull a step there, sloped or not. Didn’t mean to cut you short there Rob. 😉 I wonder which is better in terms of construction time…
At the recommended ~0.005 * LWL the step would make an 8m long hull 4cm wider on each side; having 8cm more width to play with somewhere 8-16cm above the waterline makes for a really significant increase in cabin space! You could also put a piece of plywood over that sharp step on the outside, so it’s not a 90° bend, and thereby hit the values which the author there recommends for the angle beta. That would also reduce wetted surface in case of immersion, and reduce possible slamming on the 4cm wide steps.
Cheers,
Marco
—Or you pick some height you like, and give the hull a step there, sloped or not…...I wonder which is better in terms of construction time…
.... You could also put a piece of plywood over that sharp step on the outside, so it’s not a 90° bend, and thereby hit the values which the author there recommends for the angle beta. That would also reduce wetted surface in case of immersion, and reduce possible slamming on the 4cm
I would have thought to keep the stringer rectangular, tapering off at the bow as you want and fill the resultant inverted 90 degree V on the underside with lightweight epoxy bog shaped to suit would be the easiest and fastest way to go.
Don’t forget you have also gained some reserve buoyancy and extra deck space as well if you want it. If you don’t want the extra deck space, or even would like to reduce it ( weight and particularly windage on the windward side), you could slope back the topside above the step to suit as well, in which case you will have to chamfer the outer face of the stringer.
I haven’t time now, but I will post a forward section of “Sidecar” rails, step and all FYGI later.
Rob