Daggerboards or Mini Keels for Catamarans?
Comparing fixed keels and daggerboards.
Fin keels, fixed keels, minikeels, skegs. Call them what you like, we know what you’re talking about. Choosing between fixed keels and daggerboards is probably one of the most debated options for a cruising boat and a difficult decision for a first time boat builder or purchaser. There’s no doubt high aspect ratio daggers give the optimum lift to windward and quicker tacking, but how significant is the downside of going to fixed keels, and are there significant disadvantages to living with daggerboards?
Fixed keels often get a bad rap. But it’s largely because there’s a lot of fixed keels out there that aren’t well designed. Poor section shapes and too shallow are the main problems.
Not all fixed keels are created equal, and because fixed keels by nature have a lower aspect ratio than a daggerboard, the foil section and planform (side view) selection are critical to minimise the tip loss - or the flow that runs under the keel and reduces the lift which provides windward ability. Obviously a relatively deep keel of moderate length will develop more lift than a long shallow keel, and this can be further improved by fitting an end plate to the base of the keel.
Construction Time and Cost
The dagger and dagger case combination probably take a little longer to manufacture and install and you will need to use carbon in the spar caps, but otherwise there is no significant difference in materials costs.
However for daggerboards you do need to pay particular attention to the hull to dagger case bonding areas which need to be de-cored and strongly reinforced. Also there is the additional hardware involved in the controls required for daggerboards, and a little extra deck clutter.
Asymmetric boards? These are totally inappropriate on a cruising cat in my view. With asymmetric boards the windward boards should ideally be lifted each time you tack or it will be causing drag. This means each board needs to be larger to get the same lift. An unnecessary complication for a minimal difference in performance on a cruising boat.
Building Implications
The keels are at a disadvantage here in that the building space will require 500mm or more of roof height if they are attached to the hulls at the start of the project. This also means more climbing and more scaffolding to work on the boat. One option here is to fit the keels and glass tape them to the hulls after the boat is extracted from the building space and lifted to the required height.
Performance Comparison
The difference in windward performance between fixed keels and daggerboards on cruising cats varies depending on the conditions. In very light airs the daggerboards have a clear advantage, providing reduced wetted area and greater lift at angles of attack where a large percentage of the fixed keel’s area is likely to be suffering from flow separation. The boat with a fixed keel is likely to point nearly as high as one with dagger boards, but will lose ground though increased leeway.
As the wind strength increases the fixed keels will typically operate much more efficiently and from personal experience the difference in performance between two similar cats -one with efficient fixed keels, the other with daggers - is minimal in 10 knots or more of breeze, and negligible in 15 knots and above.
The boat with the fixed keel might be expected to suffer more friction drag downwind due to increased wetted area, but once again from personal experience this disadvantage is only clearly evident in light airs.
Other Factors
Most of the cruising boats I have designed have been fitted with fixed keels - even though an option for dagger boards is provided in the plans. This is mainly because of the advantage of being able to dry the boat out on the beach with the hull bottom easily accessible for cleaning, but also for the protection the keel provides to the rudder and sail drive leg.
These factors don’t rule out dagger boards - most boats will sit on the hull bottom and the rudder without suffering damage to the rudder stock - however I do recommend that the dagger board, if fitted has a restraining strap or line which can be released to prevent the board being raised above the height of the rudder tip or sail drive leg when sailing.
Keep in mind that with fixed keels you will have less draft than a boat sailing with the boards fully down so the chance of a grounding is increased with daggers, and because the fixed keel has an angled entry profile it is less subject to damage from a sudden collision. Dagger boards take very high loadings and cedar and glass construction is generally more forgiving than amateur composite construction.
Daggerboards can be noisy inside the case, both when raised or lowered. This problem can be minimised with soft bearings at the top and bottom inside the case (synthetic grass or carpet) but is not always easy to completely avoid.
On a boat any bigger than about ten or eleven metres the daggerboards are a big chunk of stuff and can be difficult to raise and remove from the boat for maintenance.
Safety
We sometimes hear the argument that fixed keels could cause the boat to trip and capsize sideways in a sea. In steep seas you're most likely to be running (possibly with a drogue) or laying to a sea anchor.
I've never heard of a cat tripping sideways over its' keels but please enlighten me if this has been the case.
Making the Decision
I generally recommend that daggerboards are a worthwhile investment if the owner has a preference for sailing to windward rather than motoring or waiting for a favourable breeze, and if the boat is going to be kept light enough to have a power to weight ratio that will do justice to the dagger board configuration.
As a rough guide I would suggest that a 40’ cat which had a sailing weight of six tonnes or less would benefit from dagger boards, while a relatively heavy cruising cat would gain very little benefit from the more efficient foils. On a 50’ cat the cut off point is probably about 10.5 to 11 tonnes.
Dagger boards should be at least partially raised when reaching at high speed, and the need to raise and lower the board at various times is an added complication that a lot of cruising people would prefer not to have.
Daggerboards do break from time to time. Until now I have not heard of a cat having a structural problem with fixed keels, except for one that was washed sideways across a coral reef and then mercilessly pounded by waves. In that case the keels may have provided some additional safety.
Generally speaking cruising cats with fixed keels are probably easier to resell than cats with daggerboards. If you’re finding it difficult to make the decision and you're not interested in racing then it’s likely that fixed keels are right for you.
