I'm working on a seaplane and find myself doubting about the specific nautical term for a keel placed between the bilge keel and the main keel. This might be a bit academic in nature since I have a working way of naming it, but I'd rather take the chance to find if it's correct or just a fossilized mistake.

My working term is "sister keelson", which is commonplace in seaplane design, but something tells me it's not really right, as in naval engineering a keelson is the inner structural element that holds ribs against a keel.

The second nearest term would be "docking keel", because of its location, but that implies it can be used to beach the craft, which it may not be necessarily designed for.

I can't add images right now, but a google image search for "sister keelson seaplane" will give you pictures of what this is, while a search for "sister keelson" will point you to the inner elements that would be used in a wooden hull boat.

  • $\begingroup$ If you don't get answers here, you may try aviation.stackexchange.com as well. $\endgroup$
    – SF.
    Commented Mar 10, 2017 at 9:45
  • $\begingroup$ Try runners for a generic term. $\endgroup$
    – Phil Sweet
    Commented Mar 29, 2018 at 2:25

1 Answer 1


Sister keelson definitely does not translate to conventional marine usage. There are a couple of terms that apply to similar structural elements.

  • Strake. A longitudinal hull element that may be backed by an internal longitidinal structural element. In planing hulls the strake at the turn of the hard chine may be referred to as a lifting strake. Lifting strakes may also be fitted on the flat of bottom to improve hydrodynamic efficiency.

  • Which leads to the other possible term. Chine. In a multi chine vessel the hard angle between strakes is referred to as a chine. There will typically be a chine log or composite chine log backing the chine longitidinally.

There is a reasonable comparative illustration here.

This usage is primarily restricted to planing hulls (there are no chines on faired hulls) and on a faired hull a strake is a longitudinal run of planking / plating extending from the garboard strake adjacent to the keel to the sheer strake at the hull/deck join.

However I guess your seaplane float is a planing hull & so the terminology applies!

  • $\begingroup$ Strake does seem to be a suitable term for this; I should have probably mentioned that Chine is already taken as our hull already has a well defined chine on it where the spray rails happen to be. I tried too hard to keep it to nautical terms. I'm a stickler for technical correctness so I shall now incorporate that into the documentation, thank you. $\endgroup$ Commented Mar 10, 2017 at 20:18
  • $\begingroup$ Went back to my Nav Arch text books and had a further think. A chine is the hard angle where two strakes of flat hull material meet. Strake is more flexibly used to refer to external longitudinal hull elements that are not keels. In the sutuation you describe, without knowing a little more about the structural details I think you have a (lifting?) strake fitted at the chine. These usually designated a letter from keel to deck. A->?. $\endgroup$
    – pHred
    Commented Mar 10, 2017 at 23:21
  • $\begingroup$ I'm pretty certain what I'm calling a chine does fit the usual definition. Here's a clumsily edited pic that shows the elements I'm referring to: https://dl.dropboxusercontent.com/u/14561665/avc_00175934.jpg We have a spray rail at the chine, while the strake I'm trying to name is not as much a lifting element as it is a roll dampening one. $\endgroup$ Commented Mar 10, 2017 at 23:46
  • $\begingroup$ Is this a picture of the actual craft? $\endgroup$
    – pHred
    Commented Mar 11, 2017 at 1:33
  • $\begingroup$ Similar enough, this overall design is quite common, see: Icon A5. Although on this model the strake is smaller and alsmot cosmetic in nature $\endgroup$ Commented Mar 11, 2017 at 17:16

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