Best L/B ratio for power catamaran hull efficiency

Discussion in 'Boat Design' started by kengrome, Dec 27, 2009.

  1. kengrome
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    kengrome Senior Member

    Is there an accepted waterline L/B ratio that I might focus on as I begin to design a power catamaran for maximum efficiency at 20-25 knots? If so, what is it?

    Here's why I ask:

    I know that a L/B ratio of 10/1 is more efficient than 5/1 for example, but is 20/1 more efficient than 10/1? Or does the added surface friction of a 20/1 hull result in so much more drag that a 10/1 ratio is actually more efficient?

    It seems like there must be a physical limit of efficiency for a very slim hull, such that it no longer makes sense to "keep going longer" because the drag eventually overcomes the advantages of being skinny. If this is correct I'm hoping that someone with experience in this area might give me a L/B ratio to start with ... so I can begin by design in the right ballpark.

    My goal is a super-efficient and very smooth riding 20-25 mph power catamaran for fishing in the Gulf Coast USA. The boat should be capable of taking 3-4 fishermen out in moderate or rough conditions when shorter monohulls will stay home.

    I'm thinking about a hull length of maybe 40' max because of trailering length limits ... and yes this will be a trailered boat. Don't worry about the boat's width, I have a plan to push the hulls away from each other after launch and retract them before trailering, so I'm mostly concerned about the hull length right now ...
  2. sabahcat
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    sabahcat Senior Member

    Easy enough but a doubt that 20-25 knots will be comfortable in rough conditions.
  3. kengrome
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    kengrome Senior Member

    Thanks for the reply, unfortunately you seem to have missed the question which is basically this:

    What L/B ratio should I be starting with in terms of maximizing the efficiency of a power catamaran for speeds of 20-25 mph?
  4. tom28571
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    tom28571 Senior Member

    Ken, I haven't seen data that gives you the answer your seek. However, I suspect that you will be limited by practical considerations that dictate the max L/B you can use. Start with displacement and length and the max L/B will be clear. The only other variable is draft and you have practical limits on that also. I recently went through this and found a practical limit of the mid to upper teens as the max I could expect. Would be even less if more stuff is added in the form of weight.

    Other designers experience seems to point to 20 knots as a practical max for displacement power cats of less that 30' LOA or so.
  5. kengrome
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    kengrome Senior Member

    Thanks for the info Tom.

    The bottoms of the hulls will actually be delta or triangular in shape, most likely flat or perhaps with very shallow rocker, in the manner of Peter Payne's Seaknife boats:


    The advantages of such a design are knife-sharp wave slicing at low speeds, very narrow bottoms that dramatically reduce vertical acceleration at planing speeds, easy trailering on a simple flatbed trailer, etc.

    If I go with a 40' length and a 20/1 L/B ratio each hull will be only 2' wide at the transom. With such wide aft hull ends the boat will have a lot more buoyancy than I need ... and if I bring the aft end width down to 16 inches the hulls then have a L/B ratio of 30/1. This puts me far beyond that L/B ratio in the teens that you mentioned. I don't mind being in this range if it is more efficient, but I just don't know.

    One thing I do know is that the longer the hulls the more comfortable the ride in most conditions, which is why I'm not afraid to go 40' long if I have to. But I need to have an idea of the optimal L/B ratio first, so I can pick a length that works with my target displacement.

    My guess is that Rick Willoughby might be the man to ask about this since he seems to have done more with extremely long and narrow hulls and efficiency optimization than other folks in this forum. If he doesn't see this thread maybe I will PM him and ask privately.

    I don't think draft will be much of an issue in this case. The boat will be trailered and launched at decent ramps and a 2' draft is plenty, but the hulls themselves will only draw 6-12 inches (with the engines tilted up). To get into shallower waters I can take along a rowboat or trolling motor powered dinghy and launch it from the catamaran that's anchored in deeper water nearby.

    One reason for the seaknife-type hull bottoms is to attempt to bypass this limitation and actually be able to plane at speeds exceeding 20 knots. Payne's Seaknife boats do 50+ knots in rough water. I'm not trying to go that fast, but it's nice to know that if I want to I might be able to simply by adding more horsepower.

    There's nothing wrong with 20 knots in displacement mode, but if a simple hull design change can add the ability to plane with only minimal negative effects on displacement performance, why not?

    The drilling rigs about 60 miles offshore in this area attract a lot of fish and they are popular fishing destinations, so one of the things the local boaters look for around here is the ability to "get out to the rigs fast". Unfortunately it take a lot of fuel to get out there as fast as the local guys like to run, and when the conditions are less than optimal the ride pounds you half to death in the v-bottom monohulls they typically use around here ... so the owners of these boats really do not go out as often as they want.

    My goal is to design and build a boat that solves both the fuel efficiency problem and the issue with the boat beating their humans up in rough water at moderate speeds. With a boat that can get out to the rigs and back FAST (if I don't mind using more fuel) and that rides nicely in marginal conditions that discourage the monohull boaters I should have a great all purpose boat for this area.
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  6. tom28571
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    tom28571 Senior Member

    L/B I refer to is based on the max WL beam, so the ratio does not change with reducing the transom beam. I would think the displacement drag would be higher for planing hulls than for optimum minimum drag hulls. I would also expect surface friction drag of very high L/B ratios when planing to be higher than optimum. You can't optimize everything at the same time.

    Rick or some others can probably answer this question straight off.
  7. kengrome
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    kengrome Senior Member

    Not exactly, here's why: The hull bottom is basically just a long, skinny triangle and the sides will have minimal flare, so the max WL beam is always going to be at the transom in this design.

    To keep the same L/B ratio I have to shorten the boat if I narrow the transom.

    My goal is to start off with the most efficient L/B ratio for the stated speed of 20-25 knots, then adjust the hull length (and width) to accommodate the required displacement while maintaining the efficiency optimized L/B ratio. Everything else stems from this primary goal.

    I'm pretty sure it will, but I want this boat to be able to plane fast with bigger engines than I personally plan to put on it -- so I can resell it to some speed freak some day, or so I can put big engines on it myself and speed across the water if I get the notion -- and this means I cannot use simple displacement hulls ...

    ... UNLESS I install "planing wings" above the static waterline along the aft sides of the hulls so that the boat will ski on those wings instead of squatting at planing speeds. This may be a better approach to the problem, but I'm not sure just yet because I don't have enough info at this time -- so for now I'm focusing on the seaknife type hull design to see where it leads me.

    I'm guessing that the transom drag, which will probably create the biggest detriment to efficient low speed propulsion, is not going to be that much anyways given the long skinny nature of the hulls.

    Naturally the hulls must be designed to plane if I want to push them faster than displacement speeds, and the seaknife hulls are designed specifically for high speed planing, so it seems this hull form may work well in this application. The bows of these hulls will slice through the water efficiently at low speeds since they are very sharp. Yet when enough power is applied the leading edges rise out of the water (see the picture above) and the boat planes on the bottoms of those two very narrow but perfectly flat hulls.

    I agree, but I expect that some of this drag will be negated by the fact that only the narrow aft end of each hull is in the water at higher speeds ... and hopefully the overall efficiency will still be very good.

    Right, that's why my goal is to optimize for the lowest power consumption at 20-25 knots since this is the speed I expect to be running at most of the time.

    At higher speeds than 25 knots the boat will be less efficient but it will still plane comfortably without the bone jarring pounding of a typical monohull. And at lower speeds it will be less efficient that a pure displacement hull mostly because of the transom dragging in the water. But I don't expect to be running in either of these two realms that often.

    The typical weekend fisherman here has a boat that goes as fast as possible so he can get out to his favorite fishing holes quickly. Then he anchors and fishes for a few hours, or possibly moves at high speeds from one fishing hole to another to find where the fish are biting and anchors again. Then after the day is nearly over he high tails it back to port as fast as the conditions allow.

    I'm just trying to come up with a design that gets the fisherman out to his fishing grounds fast, makes the ride smoother, consumes far less fuel, and offers a lot more space to move around on the boat (on a more stable fishing platform) when the boat is actually anchored at a fishing hole.

    And a boat that rides better at any speed.
  8. TollyWally
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    TollyWally Senior Member

    Back in the states eh?
  9. erik818
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    erik818 Senior Member

    I've done some reading and number-crunching on problems similar to yours, but claim minimal hands-on experience. I think I can safely assume that any errors in what I write will be corrected by others on the forum, so here's my input.

    Leo Lazauskas has done theoretical research on long and thin displacement hulls, with both a fine entry and exit. The optimal L/B seems to be around 20 for each hull. Achieving 20 knots with a 40' displacement CAT is fully realistic. Google Leo Lazauskas + Multihulls and you should get a link.

    Leo's case study was for 300 kg and 6 m length, which would scale to 2.4 tons at 12 m. I don't know what displacement you assume.

    You want the possibility to operate the hulls in planing mode, which means you are not going to have a fine exit. When in displacement mode, the turbulence will add drag. I expect that less beam should mean less drag, so making the hulls as narrow as possible still seems right.

    Planing or not planing, the sum of the lifting forces will have to act at the center of gravity. (Rules of physics also apply to boats.) The cat is not going to plane on the rear parts of the hulls unless you have put the center of gravity there, which is not practical when in displacement mode. You must expect the hulls to plane on the full length bottom surface. Consequently the planing surfaces should be as small as possible. Narrow hulls still seems the right way to go.

    The little hands-on experience I have leads me to believe that there will be no planing hump of consequence. The long thin hulls will not create a large bow wave to climb over. The transition from displacement to planing will be gradual. I expect that the transoms will be dry at quite low speeds (5-10 knots?) but the majority of the lift will still come from displacement. At higher speeds the forces will shift to dynamic lift.

    It will be interesting to read how you solve your problem, so please continue writing about it on the forum.

    It would also be interesting to know how much drag the turbulence at the transoms can be expected to add compared to a fine exit, if anyone knows.

  10. kengrome
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    kengrome Senior Member

    Yes, for about 7 months now. Initially I returned to help my father with some medical issues in Florida not knowing if I would stay or go back to the Philippines. But after a few months those issues resolved themselves, so I moved to my current location of Long Beach MS -- more or less in the middle of the "Katrina Zone".

    After seeing some excellent real estate investment opportunities here my wife and I decided that this is a good time for us to remain in the USA, and we've decided to "fix and flip" properties as our primary business now. Fortunately I still find myself designing boats in my free time, and this one will be for my personal use.

  11. kengrome
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    kengrome Senior Member

    I thought I remember such a number myself, and it probably came from Leo Lazauskas or Rick Willoughby or maybe both of them, so thanks for refreshing my memory.

    I will do this, thanks for the suggestion.

    I'm hoping for a lighter boat, but I can scale down to make it lighter if necessary. I do not want to lose the advantages of the long hulls though, so for the purpose of weight reduction I'm considering the possibility of carving the forward 1/3 or 1/2 of the hulls out of solid blocks of foam then glassing over it like a surfboard, or strip plank this part of the hull with foam rather than wood.

    The part about wanting to operate in planing mode is absolutely true. However, I think that a fine exit is possible if I add a little bit of rocker to the hull. I believe that on a 40' hull it may be possible to add enough rocker to get the transom out of the water at displacement speeds -- yet retain acceptable steering performance and handling characteristics at planing speeds.

    In the literature it is reported that planing hulls should have little or no rocker in the aft sections because too much rocker in this area will cause the boat to wander or become unstable (unsteerable?) at speed. But I have yet to see a report that this "rule of thumb" applies to long slender multihulls -- or any multihulls for that matter.

    In fact, I think that the triangular shape of the bottoms of these hulls will counteract any tendency the boat might have to wander because of a little bit of aft rocker. I also think the boat's rather extreme L/B ratio improves straight line tracking enough to counteract any potential for wandering, especially when there are 4 chines in the water instead of just 2.

    So unless someone can tell me why my theories are wrong here, I may be willing to incorporate enough rocker in the bottoms of these hulls to bring the transoms out of the water while the boat is at rest and/or running in displacement mode.

    Agreed ... and if it is possible to use enough rocker to get the transoms out of the water then this turbulence might not even exist. But even if it does, I don't think it makes a whole lot of difference when the boat is normally going to be run fast enough to plane, and it will be used at low speeds for a much smaller percentage of its time on the water.

    I cannot optimize for all conditions so I want to optimize for the most important conditions. As I see it now this means running at 20-25 knots, and for all I know I may still be in displacement mode at this speed.

    I will learn more as I go along, but no matter what, I still intend to design for the possibility of planing at 50+ knots. Then I can easily sell the boat some day to someone who actually wants to go that fast on the water ... and most of the guys around here would go this fast or more if they could -- especially if it costs them half as much as it currently costs them in fuel consumption.

    I expect the CG to end up about 1/3 forward from the transom. My design concept is to have a boat that's more or less designed as a 26-27 footer, but with 13-14 foot hull extensions forward to cut through the chop or gradually rise over the wave crests and give the boat a smoother ride.

    I've seen pictures of similar concepts on boats being used commercially for rough water transport in (I think) Australia or New Zealand. They call them "wave piercers":


    This picture shows how the forward 1/3 of the hulls have no structure attached to them. They are basically there to smooth out the boat's wave entry and to reduce vertical accelerations for a nicer ride.

    But in my design there will be no weight forward. The wing deck will cover the aft 2/3 of the boat and not hang over any part of the forward 1/3 like this boat does, so most of my boat's weight will be located in its aft end. Coincidentally that's where the center of buoyancy will be too since I'm using triangular planform hulls.

    I still agree that narrow hulls are the best solution, but when I look at all the pictures of those Seaknife hulls they all have about 1/3 of their hulls out of the water at planing speeds, so I disagree with your belief that this boat will plane on its entire bottom length.

    I think this is typical of nearly all long slender hulls.

    Especially if I add enough rocker to get them out of the water! :)

    Right, but I think this shift will come gradually. I also think that such narrow hulls will never develop as much dynamic lift as wider hulls of the same displacement ... but this is irrelevant as long as the boat can still be driven more efficiently at these higher speeds.

    Remember, one of the advantages I'm looking for in this design is a smoother ride and/or a faster ride when the waves come up. Monohulls will have to slow down in rough conditions if they don't want to pound the life out of their occupants, but these long skinny hulls will slice through the waves and/or ride on top of them with far less vertical accelerations at speed.

    I don't know about this myself, not yet anyways. I'm also not sure if the term "fine exit" is the one I/we should be using here ...

    While I would like to get the transoms out of the water for more efficient low speed use, I still want flat transoms for mounting outboards. This means the transom bottoms will still be 1-2 feet wide, not sharp like the bows, and they will still be perfectly flat from side to side. Can (or should) this be referred to as a "fine exit"? Or is there a better term?
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