Catamaran Daggerboard Position

[cbrown]

Hi,

I am currently in the process of designing my own 14' catamaran based loosely on what I've seen that I like and what seems to work well. I have decided to use daggerboards as to allow the boat to tack better and to reduce the extra surface area and drag that skiffs inevitably add to the bottom of the hulls.

My problem is that I can't seem to find very many resources at all that want to discuss catamaran design.

My question is this: How far behind (or in front?) of the center of effort of the sails should the dagger boards be positioned?

A link to a render of my current design (without a jib which definately will be added) can be found here: http://www.freewebs.com/treeformula/isurus.htm

You might also notice that the shown design looks like it might be built with cedar strips. This is the plan until someone tells me why I shouldnt. I believe that cedar strips will produce the beautiful curves, colour and strength that im looking for.

Any info, comments, critiques or pointers to info about my question or catamaran design in general would be MUCH appreciated.

Colin

[P.I. Stazzer-Newt]

Take a long hard look at the Unicorn and Tornado designs and history.

Long soft curves but not strip built.

http://www.catamaran.co.uk/unicorn/unicorn.htm

[Wedge9]

Hi Colin,

You can take a long look at the side view plan illustrations at the Performance Cats site http://www.performancecat.com/

These are the guys who build Prindles and Nacras and some of their recent boat developments are truly state of the art for beach cats.

I think you'll find that most good performing beach style cats have their Center of Lateral Resistance (CLR) positioned very closely and only slightly forward of their Center of Effort (COE) Many of the smaller designs shown in the Nacra range utilize integrated hull based keel forms to eliminate the daggerboard. These boats are aimed at the entry level market where the extra hassle of daggerboards and when to have them down, is a hindrance to beginners and their thinking patterns.

The existing drawings show the boards just aft of the forward beam and you'll need to move them aft some for good boat balance.

Nice, translucent illustration, by the way.

Chris

[bainbridgeisland]

Boats use a combination of foils and hull to resist leeway. Have you noticed that fast multihulls have smaller foils than most monohulls with the same sail area? The reason for this is that the sharp entry provides considerable resistance to leeway. In fact many high-speed cats can manage to go upwind in a breeze with the board housed though they do have more leeway. In this case the leeway resistance is provided mostly by the rudder and the sharp entry.

So, where to put the board? It depends on how sharp your entry is and how big your rudder is. All together must provide the correct center of leeway resistance. For example, you could have a big rudder and a small board placed well forward or a smaller rudder with a larger board placed further aft.

The point is that you must take into account sharp entry and relative rudder size when placing the daggerboard. So choose existing boats with similar parameters to model your dagger board configuration.

[Woxbox]

Also -- the faster you go, the less blade you need. Same reason supersonic jets can get away with such stubby wings.
Eyeballing your plan, I'd say the boards are way too far forward. The rudders look pretty big, too. (One of those might do the job) Compare your plan with known successful designs, as suggested above. Nothing wrong with borrowing from others' experience.

[BigCat]

Catamarans aren't all that sensitive to the position of keels or daggerboards. Centering the board under the center of the sail area will be fine.

[pjwalsh]

Take a look at other cats with similar dimensions and you'll find that most jib and mainsail boats have their dagger board leading edge just a bit behind the aft edge of the mast. I went through this exercise once with a photocopier and a stack of sailing world magazines looking at Hobies, Nacra, Prindle etc. You can also get a good idea of the depth and area required and rudder area.

If you want to use the method of calculating "lead" fron the CE and CLP lots of cats have more or less neutral lead. On such a small boat the trim of the hull due to crew weight also has a huge effect on balance. To my eye your board looks a bit too far forward unless you plan on a headsail too.

[paladin]

I would move the daggerboards back skip the frame next to where it is now and use the second one back, placing the board just about centered on the foot of the sail. Your hulls are symmetrical. Bend and reinforce the support arm tubing ends up at an 7-8 degree angle on each side, making the amas cant outward when at rest...it's easier than building assymetrical hulls.

[barrington]

Also, don't forget to curve the tiller arms inboard by about 6 inches each. They cannot be parallel to the centreline or one rudder will drag as you come about and slow you down. Kinda like 'toe-in' on front wheels, the rudders have to turn at different rates.

Chris

[Chris Ostlind]

You can't determine board position until you determine the final sail plan. You indicate the addition of a jib as part of the sailing setup, so calc an area and location, determine its COE and locate the overall sail plan COE based on the COE of the main and jib togther. Drop the board just aft of the combined COE. A multihull needs hardly any, or no, lead for proper balance.

Fill out the forward sections of the hull form as shown on the drawings. There is much too little volume forward and this boat will always want to drop its nose and then keep it dropped, leading to regular pitchpole exercises.

Relocate the hull volumes to a more forward position on the hull. This is a sport cat and does not need to be designed as one might for a cruising style hull. Almost all the sail force and certainly all the vector angles are moving forward on boats of this type. The buoyancy prioities should match those vectors to resist pitcholing. Keep in mind that on virtually all points of sail, save for extreme light winds, the boat will be seeing apparent wind angles swinging well around until the boat is almost always in a near upwind sailing effect. The faster you go, the more pronounced this effect. Draw the hulls and location of sail plan to reflect that reality.

Move the beams to the tops of the hull forms. You have them too close to the water where they will really slow the boat with each and every wave encountered. The low position will also make for an exceedingly wet ride which may, or may not, suit your sailing needs.

Move the entire rig and forward beam aft. It is set too far forward and will give similar behavior responses as those outlined above regarding the hull form. Pitchpoling frequency

Strip built is OK for a boat like this one, but special care will need to be taken to keep the resin amounts as low as possible without hampering the strength of the glass wetout. Heavy cats are slow cats and then why bother.

Consider a dolphin striker, depending on the beam section and material you choose for your final design, as mast compression on these little boats can be surprisingly potent.

[Chris Ostlind]

... Bend and reinforce the support arm tubing ends up at an 7-8 degree angle on each side, making the amas cant outward when at rest...it's easier than building assymetrical hulls.

Paladin has two different things going on at the same time in this sentence and it is potentially confusing for the first time designer/builder.

The purpose behind the canted hull approach to a cat is to allow for an optimal hull form as presented to the flow of the water while underway. When the boat is heeled under sail, the hulls remain as vertical as possible, presenting a more efficient form and are thus, faster.

Truth is... as a recreational sailor, you'll probably never see the difference that this change might make. What you will see is a boat that is harder to fixture and/or fabricate parts for. Bending a tube, the size needed for your forward and aft beams, will need to be done at a shop that specializes in such business. They will need to be equipped with a mandrel style bending machine suitable for the tube sizes and wall thickness you will be using. Typically, this is rght around 4" OD and there are not that many benders around who can handle that size tubeing with a correct mandrels.

OR

You can cut and weld the angle.

Both of these options are a pain in the butt and will either make the part less strong, or it will be ugly to look at. It is far easier, if you choose to go this route, to create beveled deck mounting positons on the hull surfaces that reflect the cant you wish to achieve. It's also much more pleasing cosmetically and will be recognzed as standard build style by other cat sailors. It's important to keep in mind that if you do cant the hulls, you will need to allow for the change in the overall beam of the boat so that it will remain trailer legal, as well as still fit on a chosen trailer should you already have one in mind.

Presenting the hull in a canted angle style of build has nothing at all to do with the hull being asymmetrical or symmetrical in form. On cats, an assy. hull is used to do away with the need for a daggerboard or centerboard in the hull, thus making the boat more beach sailing friendly. This is not the same thing as canting the hulls for advantageous positioning relative to the waterflow. It does afford a simpler boat, but it also gives away some measure of pointing ability in the tradeoff.

[ahp]

Just to muddy the waters, why have a daggerboard at all? The great thing about a small cat is that you can run it up on the beach. Wait too long to pull the daggerboard and you can have a major repair job. Hobie 14's and 16's had no boards, just assymetrical hull and they went to windward very well.

[Chris Ostlind]

...Hobie 14's and 16's had no boards, just assymetrical hull and they went to windward very well.

They do sail to windward, but not very well. You don't see "very well" in upwind beach cat performance until you start sailing daggerboard and centerboard equipped designs such as Nacras, Prindles (beyond the P16, which was assy. hulled) Hobies with boards and the like.

A quick look at every performance oriented beach cat will show you that the really good ones all have boards of some sort. The H16, while very popular worldwide with something ilke 300,000 of them built, is not one of those on the list.

No assy. hulled cat is going to sail to windward as effeciently as one that is equipped with a board in the hull. Daggerboards are, typically, more efficient than centerboards. Centerboard boats are easier to beach, as the board can be made to just fold up and into the hull upon contact with a hard surface, such as the beach. Two main exceptions to that rule are the Prindle19-6 and the time tested and very fast, Tornado, at 20' LOA.

[ahp]

I beg to differ. Hobie 14's and 16's go to windward very well. I raced one for six years. Yes. those with boards do better, but not by much. The swinging center boards I have seen are dingy little things that don't do much good. How important is that last five degrees of pointing to you? Enough to build a CB trunk? Not to me.

[Chris Ostlind]

Aside from the emotional connection to the boat type, ahp, are you really saying that you'd be happy giving away a five degree pointing difference on every upwind racing leg just because you have an aversion to building a pair of boxes?

That's interesting and noted. To each his own.

[ahp]

Exactly! Is he going to race? Against what? Tornadoes? I would gladly avoid building a CB box, and rebuilding it other things every time I misjudged the depth of water. In a cat you can come on shallow water Very fast.

[Chris Ostlind]

ahp... (just what is your real name, anyway? I just hate it when someone uses an arbitrary title for discussions. One would think you have something of which you are not ready to openly discuss)

Anyway, we are talking about a catamaran hull(s) here, my man. These are two really, really simple hulls to build, no matter the build techniques used. Other styles of boat are far more complex in their hull structure than are the ones of which we are talking... So, what is the reservation, if it isn't simply arbitrary and emotional?

Here's my take: I can not argue your position if your point is derived from pure emotion. Hey, dude... you win that one, just have the temerity to honestly agree that your argument is emotional in nature and not based in fact and we can all move on from this silly haggle.

No sailor would gladly give away pointing ability when all they have to do is simply construct a very elemental structure to handle a much better ability to point higher, if and when, they choose to do so. Think of the times when you wished you could point that three or four degrees higher (you and I both know that those times exist if you have done the amount of sailing you suggest) You could have eliminated one extra, time consuming tack. You could have driven the boat with serious, straight-line speed, instead of setting it up for the eventual tack needed to make the layline obligation... and on and on it goes.

Recreational sailors have the same issues, even though they are not racing. Ever tried to work your way through a tight mooring field in light airs, knowing that you will have to throw one tack after another because you can't possibly point the boat as efficiently as can the other cats with boards? Yeah, I thought so.

Point made.

The built structure point is further enforced by the fact that cat hulls are some of the most basic built forms that are out there and the addition of a trunk, whatever the purpose, is next to nothing in the build regime needed to complete the boat. Next to nothing.

You are arguing for your limitations on this topic, php and it's going nowhere.

Take a long look at the current types of cats being built commercially and you'll see that all the designers and builders have long ago abandoned the use of assy. hulls in favor of daggerboard, centerboard and integrated keel form structures. The latter being the modern answer to the H16 form that has no underwater appendages. Even the integrated keel form boats sail to windward better than do the older, H16, variety of hull shapes, so why would the poster want to adopt old technology when he has access to better, more efficient forms and solutions?

If you wish to persist in arguing the point from an emotional perspective, then have at it, my friend. I can only show you so many points in the discussion. If you don't want to, or are not capable of recognizing the points, then there is really no further purpose in the discussion.

Perhaps the original poster will be able to take all this into his sphere of knowledge, do some additional research and produce a very nice example of a boat that suits his needs. I'd like to see that happen.

[P.I. Stazzer-Newt]

This long since stopped being anything other than hypothetical - check the dates on the posts.

[Chris Ostlind]

Check references in subsequent, up to date, posts.

Your observation is understood and integrated within the current observations.... and what is wrong with hypothetical, anyway?