Most sailboats have one of three drafts, based on their keel type. Inland water boats using centerboards have the least draft, while bluewater boat keels run much deeper.
The depth of sailboat keels depends on their design. Boats designed for inland waters usually have a draft of around 0.6m (or 2ft). Bluewater boats can either use a full keel or fin keel. Full keels run on average around 1m (or 3ft) deep. Fin keels, wing keels, and bulb keels run on average 1.8m (or 6ft) deep.
Below, I'll go over each keel type's depth and also show the range of depth based on different boat lengths, both in metric and imperial.
Draft of 9 Most Common Keel Designs
The average depth of different keel types is not readily available on the internet. To find these numbers, I've measured dozens of technical scale drawings of yachts in an old Dutch sailing book published in 1977. The numbers are quite consistent.
In order to keep things simple, I'll go over the rough averages first, before diving into a bit more detail based on boat length. After that, I'll discuss why different keel designs use different depths.
Average depths based on keel type
- Full keel: 0.8m or 3ft
- Modified full keel: 1.2m or 4ft
- Fin keel: 1.8m or 6ft
- Bulb keel: 1.8m or 6ft
- Wing keel: 1.8m or 6ft
- Bilge keel: 0.6m or 2ft
- Daggerboard: 0.6m or 2ft
- Centerboard: 1m or 3ft
- Leeboards: 0m or 0ft
- Canting keel: 5m or 16ft
Keel depths for different boat lengths
|Keel Type||Boat Length||Keel Depth (m)||Keel Depth (ft)|
|Full keel||6m (20ft)||0.6m||2ft|
|over 8m (26ft)||1m||3'3"|
|Modified full keel||6m (20ft)||1m||3'3"|
|Regular fin keel||8m (26ft)||1.8m||6ft|
|Shallow fin keel||8m (26ft)||1.5m||5ft|
|Bulb keel||8m (26ft)||1.8m||6ft|
|Wing keel||8m (26ft)||1.8m||6ft|
|Bilge keel||6m (20ft)||0.3m||1ft|
Keel Depth Explained
Different keel designs use different depths due to several factors. Fin keels are by far the longest keels, as they use their length to increase the leverage of the ballast. Full keels, on the other hand, require less depth since they carry more ballast and provide more wetted surface, which improves directional stability in itself.
Centerboards and daggerboards are often used in sailboat designs meant for use in inland waters, which is why they run on average less deep, with the daggerboard running the least deep of both at around 60cm or 2ft.
Bilge keels also run less deep, as this design was intended for use in tidal waters, and allows for safe beaching of the boat. The use of two blades instead of one doubles the wetted surface and ballast, allowing for a shoal draft.
Shoal draft simply means the keel doesn't run deep.
How deep should a keel be?
A keel should be deep enough to provide enough wetted surface and enough leverage for the ballast to provide adequate righting moment for the used sail area, while simultaneously allowing access to desired waters.
For bluewater conditions, the keel tends to be at least 1 meter deep, in order to cut through the surface drift layer of the water.
The water's surface drift layer is the layer of water directly affected by the wind.
For most inland-water sailboats, this translated to a keel depth of between 0.5-1m. For offshore boats, keels tend to be deeper, at 1-1.8m.
Factors that affect keel depth:
- Keel design
- Ballast weight
- Desired sail area
- Usage conditions (inland, offshore, cruising, racing)
- Hull width
How keel design affects keel depth requirement
Keel design plays a major role in the amount of ballast required and the total weight of a sailboat keel.
Full keels span over a longer area fore to aft, which is why they can be less deep while achieving the same amount of wetted surface. They also carry more ballast, which provides similar or more righting moment at lower depths than fin keels.
Since fin keels are generally quite narrow, they need to run deeper to achieve the same amount of wetted surface and to provide a similar righting moment. However, since the ballast is further away from the hull, they can carry less ballast, making the boat lighter overall (and thus faster).
How hull width affects keel depth requirement
How is keel depth measured?
Keel depth is measured vertically from the waterline, which is where the hull touches the water, down to the lowest point of the keel. Keel depth is also called draft or draught.
What is the difference between draft and depth?
Depth is measured from the top of the keel to the top of the deck beam, at the center of its length. It refers to hull depth, as opposed to draft. Draft is measured from the center point at the waterline, down to the lowest point of the keel. It refers to the minimum water depth a boat requires.
When calculating minimum water depth, you want to take your boat's draft, not your boat's depth.
What is depth below keel?
Depth below keel, sometimes called keel offset, or depth below transducer, is the distance between the lowest point of the boat and the ocean floor. It is measured by the depth meter, which often shows it as a negative number.
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