Daggerboards vs keeled hulls

Discussion in 'Multihulls' started by nickvonw, Apr 11, 2011.

  1. nickvonw
    Joined: Oct 2009
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    nickvonw Junior Member

    hello everybody

    what are the differences/advantages between a hulls with small fixed keels and hulls with retractable daggerboards??? lets say for arguments sake we are talking about a 40 foot cruising catarmaran buit form plywood epoxy construction

    cheers

    Nick
     
  2. Doug Lord
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    Doug Lord Flight Ready

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    It can come down to the reason you need one or the other: lateral resistance. If that was the only consideration the daggerboard wins hands down because it can generate the required lateral resistance with lots less area and more effectively since it behaves more like a wing than does a long shallow keel.
    But there are always other considerations-a daggerboard can take up interior space, but have a shallower draft when retracted. A long keel may draw more but you don't have to fiddle with it. It all depends on what your usage of the boat will be.
     
  3. Petros
    Joined: Oct 2007
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    Petros Senior Member

    dagger boards allow you to beach a catamaran, even a large one. It also allows you into very shallow draft waters. They are usually more efficient (slightly less drag), and allow you to adjust the amount of dagger board you have in the water, giving you one more dimension of fine tuning to the conditions while underway, not so with a fixed keel.

    fixed keel is usually lighter, lower maintenance, and as already stated they do not take up interior space as a dagger board does.

    Either can be vulnerable to damage if not careful in shallow waters, usually the dagger board can be repaired without lifting the hull out of the water, except in a bad collision (in which case both design can suffer hull damage).
     
  4. michael pierzga
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    michael pierzga Senior Member

    ...when a fixed keeler comes out of the water for an antifoul of whatever it rests on its keel. A dagger board boat rests on its hull skin , so it must be cradled...big hassle over the life cycle of a boat. A daggerboard needs a trunk..this trunk takes up valuable interior living and machinery space. A keel is a battering ram that protects the boat, the prop and rudder, A daggerboard is a foil that catches junk.

    Never...I repeat never, go daggerboard on a cruising boat.
     
  5. Doug Lord
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    Doug Lord Flight Ready

  6. CatBuilder

    CatBuilder Previous Member

    Talking about drying out a dagger vs. a mini keel for a minute...

    I understand your post, Michael and what you're saying about it being a hassle to cradle a boat with no keel. Let me ask about something...

    What if you have a boat with a dagger board, but have reinforcements on the hull to take up the load of standing up on the hard? Would this not be the same thing as mini-keels?

    Mini keels are subject to the same stresses and put them at a spot on the boat (where the keels join the hull), so is there really any difference in just running a strip of extra glass down the entire length of a boat with boards and drying out, just the same way the boat with the keels does?

    Regarding valuable space taken up by board trunks: I'm way ahead of you on that one! ha ha ha Struggling with that now.
     
  7. PAR
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    PAR Yacht Designer/Builder

    Boats are generally stored on their keels, deep fin, long fin, built down, or retractable appendages don't have much difference between them. They all rest on their centerline, with bilge supports to prevent them from flopping over. The same is true of trailer borne vessels.

    Anyone attempting to support a boat by it's hull shell doesn't know what they're doing and risks localized damage, be it laminate, skin or structure.

    The only true advantage of retractable appendages is shoal draft. A fin keeler has as much area as necessary, as does a retractable appendage. The dagger, or centerboard or leeboard offers no less area then a similar area fin keel, though in it's favor is the ability yo partly or fully retract the appendage as the sheets are well eased, reducing wetted surface.

    So, that's it, retraction for shallow waters and off wind ability are the advantages of a deployable appendage. Now you can get into areas where special, retractable appendages serve other roles, but this is a different can of worms.

    As a general rule, fixed appendages have more advantage, because they can carry their ballast lower.
     
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  8. Richard Woods
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    Richard Woods Woods Designs

    I think Michael is generalising and oversimplifying.

    I quote from my FAQs page

    "No wonder that - as this photo of a well known design shows - many catamarans need props under the bows and often also under the sterns (a bit hard to fit them while drying out on a beach I would have thought!)"

    My 35ft Banshee had a round bilge hull and daggerboards so, although we kept it on a drying mooring (it was aground 12 hours a day) I was always nervous about beaching on a rocky shore.

    So all my newer round bilge cruising designs that have daggerboards, like the 30ft Sagitta, 32ft Eclipse, 38ft Transit etc also have a deep (150mm) beaching keel. This keeps the hull clear of any rocks and only slightly reduces performance. A worthwhile compromise I feel. Chined hulls don't have quite the same "hull bottom sitting on rocks" problem

    The daggerboard cases can easily be hidden if the interior is carefully designed. On the Banshee one case formed the side of the chart table, for example. On the Eclipse it formed the side of the dry hanging locker for shore clothes.

    I have built sailed and owned both daggerboarded and LAR keeled catamarans and indeed had the same design fitted with keels and with daggerboards (the 24ft Strider). The daggerboarded boat is faster, points higher and pitches less. The LAR boat is easier to sail, the keels protect fixed props and rudders, is cheaper to build and usually offers better load carrying (extra buoyancy in the hull and less weight - no boxes)

    If you read the other recent posts I think you'll see the original poster is interested in a 40ftish live aboard cruising sailing catamaran, not a powerboat on sailing monohull

    Richard Woods of Woods Designs

    www.sailingcatamarans.com
     
  9. Steve W
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    Steve W Senior Member

    Many,many daggerboard multihulls are kept on drying out moorings without issue year after year, you would certainly want to walk the swing radius when you first aquire the mooring and remove anything that may puncture the hull which is no big deal as you are usually on mud or sand. Lar keels would sure make it easier to scrub and paint the bottom at low tide on your mooring as well as being safer on unfamilliar beaches. Daggerboards cats ,as Richard notes, perform better all around.
    Steve.
     
  10. CatBuilder

    CatBuilder Previous Member

    This is a good thread. Of course, I have a vested interest and asked a loaded question. My boat is a 45' cat with daggers and reinforced keel join that the boat is happy to rest on when dried out.

    I'll personally take the hit on not being able to wallow around in the mud trying to apply bottom paint between tides for the extra performance of the boards.

    I'm pretty confident I can dry out safely though. Worked fine on my last catamaran which used a crazy amount of rocker instead of keels or boards to reduce leeway. (the rocker didn't reduce the leeway very well)

    Other advantages to retractable appendages are just like described above: Drying out is still pretty easy. If you have no props down there and just daggers and kick up rudders, drying out is a piece of cake.

    The trick to a successful dry out (coming from on water experience, rather than design) is not going into it blindly. You must first and foremost pick a place that will be like a calm lake. You sure don't want the boat to float, then hit bottom, then float, then hit bottom, driving a rock through the hull or cracking a bulkhead loose. If you've done it right, you won't know the boat has touched bottom until she stops moving in the breeze/current.

    Next, you want to make sure the bottom is firm, yet not rocky or full of hard things jutting up (coral, very sharp shells, etc...) Sand or firm mud are best since you still want to be able to walk around the boat.

    Here's a picture of me drying out on the other boat with the rocker. Saildrives were protected by skegs aft. I was drying out here to change the zinc anodes on the saildrives.

    Notice... no keels required.



    [​IMG]
     

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  11. michael pierzga
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    michael pierzga Senior Member

    To each his own. Retractable appendages create complexity , cost and consume space .... a high price to pay for an increase in windward performance. For a cruising boat, the abilty to enter an unknown brown water harbour and strike the bottom without damaging the hull skin or running gear is very desirable. Its easier to handle a keel boat out of the water for yard work when no attention must be paid to daggerboard trunks, crush zones and foils. To many times I see daggerboard boats hauled for expensive, time consuming, foil and trunk repairs resulting from minor impact with offshore long lines , harbour mooring gear and floating junk...

    I strike the bottom with a lead bulb dozens of times per year with only the loss of antifouling. The leading edge of the keel has many deep grooves cut by offshore longlines. .
     
  12. Alan.M
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    Alan.M Junior Member

    Lead bulb on a multihull???
     
  13. Alan.M
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    Alan.M Junior Member

    We are cruising on a daggerboard equipped catamaran, the main advantages are better performance on all points of sail, and extremely shallow draught. We point much higher than minikeel boats, and off the wind board up, we have less drag.

    We can float in less than 1/2 a metre of water, we can dry out with no problem - spent 5 weeks in Hill Inlet, drying out twice a day. (We also have kick - up rudders, and outboards.)

    Entering unknown shallow water is no problem - raise the dagger, let the rudders float, and the only problem would be if the water was REALLY shallow - less than 1/2 metre. If it's that shallow you'd be going pretty slow.

    Slipping the boat - simply stand the boat on blocks under it's keel where bulkheads are, and antifoul everywhere you can access, then before launching you antifoul where the blocks were. Pretty much standard practise with all boats.
     
  14. michael pierzga
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    michael pierzga Senior Member

    Well Alan...Ive been working busy boats worldwide for thirty years. LEAD BULB..is an analogy for a collision , grounding shoe...not ballast. My rowing boat has a keel with a brass grounding band.
    I have no idea what your underwater profile looks like but since Ive been aground many times I can testify to the fact that as the tide goes out...the transition between floating and aground...is punishing on a boat as it dances to a cycle of lifting then grounding out for the next half hour. I wouldnt want to expose the hull skin to this aggressive treatment.

    If you would like to build a sport boat...be my guest and use foils. If you need a durable cruising boat that can operate year on year without incident..., beware of daggerboard appendages.
     

  15. CatBuilder

    CatBuilder Previous Member

    This can go in circles for days, but I'd like to point out that a dagger board that hits something and is broken is far better than a keel that is broken the same way. It can be repaired while under way. No haul out even required.

    Michael, it sounds like you are coming at this from a monohull mindset. Things work differently on cats. Dagger trunks have impact absorbing, replaceable, foam crash blocks in the trunk, if built properly. There is no trunk damage.

    The boards are the deepest thing when fully down. Rudders kick up on impact.

    Like the poster above, ill have outboards with this setup. A perfectly workable and better performance setup than keels with no damage to motoring gear possible, as it retracts.

    Only downside is the loss of interior space as we see in that galley layout thread.
     
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