Chine and strake design

Discussion in 'Powerboats' started by meren, Dec 14, 2006.

  1. surfieadam
    Joined: Apr 2010
    Posts: 1
    Likes: 1, Points: 0, Legacy Rep: 27
    Location: Western Australia

    surfieadam New Member

    Chines and Strakes on Planning Catameran

    Gday All,

    What a great forum with some obviously very knowlgeable members, thanks for taking the time imparting your wisdom and so to the forum moderators. Reading this thread alone has already taught me some things I did not know and I grasp thats there's much for me to learn....eh I was almost sweating at that trick question at the registration! :p

    Before I touch on the topic in this I thought I'd briefly touch on my experience, limited as it is. I have zero boatbuiling experience...though I've been reading bit of late. I'm getting near 40, been around smaller boats most of my life, mostly small monohulls under 6m, more recently I had the pleasure of a 7.5m sailsish catameran + 150 hondas in the Cocos Island for a year, where I got my Coxswains ticket and did a bit of Island hopping, fishing and surfing tours to a remote atoll about 25ks north. I say remote because the seas are often a bit rough for comfort even with a decent cat thanks the prevailing tradewinds.

    I must say I'm hooked on Catamerans after that experience for the purposes I required at the time, but there's some other "creative" design concepts I would like to see in a catameran that I do not seen any of the current model - to the extent I would like see :idea: . I found of particular interest the comments made by member u4ea3 on the v versus concave bottom (softer ride) and on the definitive to this day tests on life size model leading to what we have today (v bottoms).

    I am in the process putting on paper a scale model of 5-6m Cat with some creative new hull concepts involing a significant chine and strake combo..which may have already been tried or there's some existing knowledge in this area. I'd like your comments on a design concept for a planning cat hull with a difference involving a I guess a fairly radical chine and strake combination with the view to create additional lift via the concave affect (faster moving water through the shorter radius creating lift). Ultimately the view is to tool/mould up, go into production and establish a brand.

    A drawing would do far better, as would a mini scale model (which I might have compete this week) and I'll look to post one but how does this sound for a basis:

    I'm looking to have the slim fine semi planning/displacement ultra soft riding hulls like the Glacier bay cats - but the downside I see to that is the very high tunnel clearance required (less internal room, top heavy) versus say a lesure cat with it's fatter, higher speed but rougher riding planning hulls. My idea was to change the bottom in the Glacier bay style from a v to flat to a v concave along the bottom of the hull to point where the dead rise begins. The idea here is that I can achieve the softest ride with the finest hulls but also getting the hulls to rise out of the water creating less drag/more effieciency requiring less hp for same speed and quicker planning via the concave affect, in turn also requiring a lower tunnel.

    Obviusly I need to find the right combination of width of the concave at the bottom of the hulls to achieve the displacement required and improved (I hope) hydrodynamics, but with the concaves not so wide that it rides harshly defeating the objective of a softer than lesure cat ride - similar to a Glacier bay but with improved efficency and top end.

    Add to this concept that the outer edges of the of the bottom of each hull (the outers edge of the concave) would be slightly higher than the inside edge - giving rise to an asymetrical profile at the BWL on each hull - so as to affect a lean into turns as with asymetrical cats, as opposed to the lean out on symetrical hulls (like the sailfish).This design concept allows surfboard to more easily transition rail to rail. This design makes the chine offset towards the the tunnel centre and this line is continue to the bow - versus the usual centreline chine arrangement found in all the cats I see (in what orientation an outboard might be placed in its wake is yet to be determined) This design also gives rise to more forward volume and planning surface and less of a tendency to bury the nose/broach in a following sea....a sometimes scary downside to cats.

    Oki, thats it...all the current design perfromance flaws in a catmeran solved :D and I hope I'm on topic with post and that I havent confused the heck out of everyne reading this. Yes, I'm looking for something different and maybe I'm being over arrogant in straying from the more universally accepted. I'm a surfer with practical experience in the affects of v bottom versus concaves and outer edges/rails, too, and the endless refinement in bottom shapes I guess biases me towards the belief that there's always room for improvement. V bottoms in surfboards are good for big powerful waves where power unlimited and on tap (think super high speed boats, 50 knots pus), concaves are good to get going when the power is not so plentiful because they provide lift and reduce drag, which makes the board travel faster across the same section of wave than a v bottom would (think, the average boaty wanting under 50 knots). To my eye and rationale....I think we can do better than a basic v, with the right combo of concave, hull width and displacement.

    Looking forward to your valued comments and advice - thanks!

    Cheers, Adam :)
     
    1 person likes this.
  2. sottorf
    Joined: Sep 2007
    Posts: 192
    Likes: 20, Points: 0, Legacy Rep: 202
    Location: South Africa

    sottorf member

    There are a few good reference papers on chine and strake design, that can be used as guidelines One of the best works on the subject i know is by Muller-Graf. Does anybody else have any good references of scientific works on the subject??
     

    Attached Files:

    1 person likes this.
  3. Easy Rider
    Joined: Oct 2009
    Posts: 920
    Likes: 46, Points: 0, Legacy Rep: 732
    Location: NW Washington State USA

    Easy Rider Senior Member

    Surf,
    Sounds like you don't think the Glacier Bay boats got it right. If sales are a good indication they did but I think a boat should have the ability to slide sideways in big seas. GB has high CG and almost no sideways ability. So I agree that they didn't. I'm not fond of most cats .. lots of extra weight for structures to keep the 2 hulls together, flat tunnels to pound on or shapes in the tunnel to soften same that even adds more weight, extreme twisting forces when quartering head seas, when quartering seas the bow can't rise up smartly (as it should) when plunging into a wave because it's only got half lift as it's only half a bow and when the bow gets in deep the deck keeps it from rising .. as it should. Not my favorite boats but I digress as this is not on topic at all.
    What I wanted to add to this strake/chine party is:
    1 I don't think the strake should sweep upwards in the bow as Willallison suggests. Just causes extra drag and spray. I think it's done for appearance.
    2 I've often thought a strake design that would be very effective would be to have one or two strakes that would be like a half round or less than a half round just inboard of the chine for the purpose of seriously slowing the water down before it hits the spray rail. Theoretically the water would gently roll out from the chine and plop right down where we want it.

    Easy
     
    1 person likes this.
  4. u4ea32
    Joined: Nov 2005
    Posts: 416
    Likes: 14, Points: 0, Legacy Rep: 192
    Location: Los Angeles

    u4ea32 Senior Member

    The Muller study yet again mentions that strakes that extend under water only add drag. The positive effect of strakes is to get the spray to release from the hull. They are NOT effective to increase lift.

    Think about it for just a moment: L/D from an airfoil is dominated by aspect ratio: the higher the aspect ratio, the higher the lift for a given amount of drag. The lower the aspect ratio, the more drag for a given amount of lift.

    A strake or chine flat is a very, very, VERY low aspect ratio lifting surface. Lots of drag, very little lift.
     
  5. Easy Rider
    Joined: Oct 2009
    Posts: 920
    Likes: 46, Points: 0, Legacy Rep: 732
    Location: NW Washington State USA

    Easy Rider Senior Member

    As I see it when the water around the "lift" strake is moving parallel to the strake there is no lift generated ..so you are right, BUT that only happens at 4 knots or less. Any speed that promotes an increase in the angle of attack produces what Aeronatical Engineers call spanwise flow. Anytime in a boat where there is water flowing crossways over the lift strake there will be lift created in the bow or at the stern .. spray or no spray. The designer must decide if the lift from a strake well aft is worth the added drag. Ther'e a lot of variables involved in that call. My opinion.

    Easy Rider
     
  6. u4ea32
    Joined: Nov 2005
    Posts: 416
    Likes: 14, Points: 0, Legacy Rep: 192
    Location: Los Angeles

    u4ea32 Senior Member

    I agree, Easy.

    I think the lift forward added by the strakes in the Muller study is due to this spanwise flow: its a displacement hull form that normally makes a huge bow wave (really, spray), and so the flow is going up alongside the hull, and therefore the strakes -- above the waterline -- do provide good L/D. The bow lifts, and the total drag decreases when stern flaps are also added to avoid the drag increase due to simple bow rise.

    Note that the Muller study and many others find that the spray rails or strakes should end more or less where they go underwater at speed. Underwater, where the strakes are acting on flow that is almost perfectly aligned with motion and therefore acting as very low aspect ratio lifting surfaces, the strakes add drag.

    On the Muller study they found an improvement even if the strakes went underwater about 10% of LWL. Perhaps there is still substantial spanwise flow in this area, its not clear to me.

    Sound reasonable?
     
  7. sottorf
    Joined: Sep 2007
    Posts: 192
    Likes: 20, Points: 0, Legacy Rep: 202
    Location: South Africa

    sottorf member

    I fully agree, increasing the length of the spray rails beyond the waterline do not add lift. THey are however there for other reasons: they improve the directional stability of a boat significantly and also affects how it banks into a turn.

    Another useful reference on spray rails and spray is from the Master himself: Daniel Savitsky:
    http://www.stevens.edu/ses/cms/file...f_Whisker_Spray_in_Performance_Prediction.pdf
     
  8. fg1inc

    fg1inc Guest

    Please keep in mind that some are "spray rails" and some are "lift strakes"
    and some....well, it reminds me of a Hatteras MY from years ago that had vertical strakes at the forwrd sections. Looked a little like multiple gills on a fish, 90 degrees to the keel! Turned out they built 3 of 'em that way because the suction caused by clean laminar flow wouldn't let the bow rise enough to get planing. They tried conventional "lift" strakes but that didn't do anything.
    So they glued on these crazy vertical strakes and she planed! (sort of)
    Trial and error , i guess. It was pretty cool having Jack Hargrave living in the same town and being able to ask stupid questions!
     
  9. Easy Rider
    Joined: Oct 2009
    Posts: 920
    Likes: 46, Points: 0, Legacy Rep: 732
    Location: NW Washington State USA

    Easy Rider Senior Member

    So, You guys think the water flow is going to straighten out and run aft parallel to the keel from midships aft? What force could straighten the water out??? And why wouldn't the crosswise flow create lift aft of the water line while under way. As I see it .. as long as there is angle of attack there will be crossflow and lift .. all the way to the transom. Of course they create drag but the whole hull does that. It's really a question of whether or not lift strakes create enough lift to make the price of their drag worth leaving them on. With aircraft they put "tuffs" on the wing and go fly it and watch. Anybody up for some scary scuba diving?

    Easy Rider
     
  10. u4ea32
    Joined: Nov 2005
    Posts: 416
    Likes: 14, Points: 0, Legacy Rep: 192
    Location: Los Angeles

    u4ea32 Senior Member

    Sottorf, great paper!

    Here is a gem in there near the end that I'd read before in another much older paper. Edited for brevity: "The sharpness of the outer edge of the strake cannot be overemphasized. Even a quarter inch roundness eliminates the effectiveness of the strake."

    So if its not a sharp edge, its better to not have the strake at all!
     
  11. Easy Rider
    Joined: Oct 2009
    Posts: 920
    Likes: 46, Points: 0, Legacy Rep: 732
    Location: NW Washington State USA

    Easy Rider Senior Member

    I agree David .. and the sharpness of the trailing edge of the bottom (as in transom meets bottom) needs to be even more so. Seems to be an advantage of metal boats.
    Also I may have been wrong about the performance of lifting strakes aft of the WL on plane. As the crossways flow over the strake produces lift the water must return to the hull and that could cause the boat to some degree to be sucked downwards (or anti-lift) eliminating the effects of the lift that may be canceled or reduced to the point that the remaining lift (if there is any) would not be worth the drag of the strake. That's the only thing I can come up w that could explain why (if it is so) lifting strakes don't work on the after plane of a boat. What think?
     
  12. u4ea32
    Joined: Nov 2005
    Posts: 416
    Likes: 14, Points: 0, Legacy Rep: 192
    Location: Los Angeles

    u4ea32 Senior Member

    The above paper by Savitsky includes photos of the flow of water under a V hull using both dye and tufts. Both show the flow is essentially fore-and-aft aft of the "stagnation point" or the leading edge of the hull, as the planing hull surface goes underwater.

    There is some athwarthship angle to the flow near the chines, but not much.

    This lack of sideways flow means the strakes are in that "very low aspect ratio" configuration, so lots of drag for a given amount of lift.

    Since surface area contributes to drag at a more or less linear function of speed (double the speed, double the drag due to surface area), and the strakes add surface area, I think that's probably sufficient to explain the slight increase of drag due to immersed strakes.

    Its still worth noting that even optimum strakes make perhaps a 1-3% reduction in drag. They really make so little difference that they may not be worth the effort and weight.
     
  13. Easy Rider
    Joined: Oct 2009
    Posts: 920
    Likes: 46, Points: 0, Legacy Rep: 732
    Location: NW Washington State USA

    Easy Rider Senior Member

    OK David .. Hard to imagine there ins't much cross flow but tests sometimes show what seems to be obvious isn't so. There's lots of people here learning lots of stuff ..and that's great. Something good that I just thought of is that the absence of strakes on the afterplane allows the stern to swing laterally giving better turn performance. I have an aluminum skiff that sticks to the water so tenaciously that in a real tight turn it feels like it's about ready to flip over like a car. I can't take the little micro-keels off as they are structural. I would think the best skiffs would have the structural stuff inside the boat.

    Easy
     
  14. FlyingTime
    Joined: May 2010
    Posts: 10
    Likes: 2, Points: 0, Legacy Rep: 51
    Location: Stuart FL.

    FlyingTime Junior Member

    Wayne,

    A good place to start is 1" for every 5' of chine length... You can either keep it flat at the transom and have in converge towards centerline tapering out at the forefoot OR start with a 3/4" vertical on the outboard sides and run them parallel to centerline and blend them in around the forefoot. Either way works well but seems like most use the vertical outer edge. Some say it offers more protection while up on blocks in the yard.
     
    1 person likes this.

  15. Wayne Grabow
    Joined: Aug 2003
    Posts: 251
    Likes: 17, Points: 18, Legacy Rep: 297
    Location: Colorado

    Wayne Grabow Senior Member

    FlyingTime,
    Thanks for the information. If I understand correctly, a 20' hard chine boat should have about a 4" wide keel pad? Seems narrow from what I have seen on the mentioned examples. The trade off in pad width seems obvious: more keel flat surface will bring more of the characteristics of a flat bottom; less emphasizes the deep V aspect. The ideal is that happy medium.

    Thanks
     
Loading...
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.