Question: I have seen several flat bottom boats out there and they are great for ease of construction and shallow draft use but I have not seen any flat bottomed multihulls. If daggerboards were used to improve stability, would a flat bottomed cat or tri be viable?
Reply: Before discussing flat bottomed multihulls, let me first correct the misconception that dagger boards are added for stability. The only reason for adding a daggerboard, centerboard or leeboard to a multihull, is to reduce the side drift while sailing with the wind from ahead or from the side. Such a board will not totally negate the side force on the sails but it goes a long way towards that goal—depending on the efficiency of its shape, sectional form, location and surface fairness.
Monohulls also need a similar surface to provide this so-called 'lateral resistance', but in this case, they also need a significant weight low down to provide stability so such keels are typically tipped with lead or steel to achieve this. One of the main reasons that multihulls can be faster than monohulls, is that they do not require weight for ballast—their spread of buoyancy being generally quite sufficient. Without this ballast to support, the typical multihull can then have finer hulls with less buoyancy/displacment and therefore offer less resistance to forward motion.
Now back to flat bottoms. There are just a few multihull designs with flat bottoms and the K24T by Bill Kristofferson is one. It was actually reviewed in my recent Small Tri Technical Report available through my website. Yes, they do offer shallow draft but keep in mind that virtually all multihulls are basically shallow draft once their boards are raised. Richard Woods (originally from the UK but now in BC, Canada), has also had much success using simple ply catamaran hulls with a small flat of bottom and his easy-to-build designs have much to recommend them.
One other advantage is that they provide a flat keel surface to rest on and this distributes the load over a larger surface than when a sharp central keel is employed. And yes, it would generally be true that this form of hull is very easy to build. For a small boat that you want to build at low cost and start sailing just as soon as possible, this may well be a viable solution and I am in fact reviewing and trial-sailing a small boat with this format sometime in August and will report back. (See photo below)
Added note—Oct 2009:
The tests with the little tri went very well and confirmed what I've said elsewhere on this website, that a narrow flat bottom on a long slim hull can be surprisingly efficient. The shallow chines helped to prevent leeway and there was very little resistance shown by the slim box form—definitely an interesting and viable option for the new 17-footer I'm working on.
But for a boat say over 6 m (~19 feet), one needs to keep in mind that unless you already have a suitable mast, sails and fittings etc, the hulls of multihulls are still only a small percentage part of the total cost of a completed boat and it makes little sense to compromise the expensive equipment one needs to buy and install, by fitting them on low cost hulls that in the larger, heavier size range will likely have more drag and other undesirable traits than well rounded ones.
As flat bottomed hulls can pound in any sort of waves, it's important to not make the flat part too wide and of course, this applies to the amas too. One may also wish to consider other flat skin options possible with plywood as noted at: Comparing different chine hulls.
A very light, flat bottom multihull boat could be built that was fairly wide, and if it also had minimal keel rocker would possibly plane as well, but such a form would also have very negative attributes for sailing in most general conditions and therefore would only be recommended as an experiment for speed sailing in ideal conditions on a reach off the wind.
See article Do Trimarans Plane? for more thoughts on this.
Thanks for your interest and happy sailing!
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