FPB 97 Stabilizer Position Logic

971 BOW ON 1

One of the more difficult design issues is positioning the stabilizer fins. The considerations are as follows:

  • Bottom of fins are above the bottom of the canoe body or keel when the fins are centered
  • As close to the point of maximum beam as possible to maximize leverage
  • Near the center of buoyancy to minimize steering impact
  • As close to parallel with the surface as possible for better force vectors
  • Sufficiently below the waterline to minimize loss of end plate effect in rough sea states
  • Good access for inspection and maintenance
  • Structural integrity to minimize hull damage in the event of a grounding

If you work through this list it will quickly become apparent that there are a few conflicts.

971 stabilizer fin 1

In the case of the FPB 97, the fin is a touch further forward than is optimal. However, this gets us access in the landing area between the forward owner’s suite and the guest accommodations, a location where you can quickly inspect the actuator mechanisms. The tip of the fins is above the canoe body bottom, and they are positioned deep enough so that ventilation in big seas should rarely be an issue.

FPB 971 stab framing

The stabilizer shaft comes through a transverse frame, and there is a longitudinal girder reinforcing the hull to help dissipate impact loads when the boat runs aground, although with sub-five foot/ 1.5m draft, running aground should be a rare occurrence.

971 stab mechnism

Posted by Steve Dashew  (May 2, 2012)

9 Responses to “FPB 97 Stabilizer Position Logic”

  1. Martin Says:

    I didn’t know that closer to parallel to the water surface was better. I’ve always assumed the shaft should be as near a normal to the centre of roll as possible, so that the force vectors are a right angles to the roll motion, and also the stabiliser ends are as far from the centre of roll as possible for the most leverage.

  2. Dario Says:

    Hi Steve

    Why not GIRO stabilizers?

    Are them not beter than fins with a mechanism through hull.

    I have been on a yacht with this type of stabilizer, it was dificoult to notice the sea condition at all.

    Keep with your hard work to envision the dream boat we all want have some day.

    Best Regards


  3. Steve Dashew Says:

    We have discussed this before, but basically, the units now available do not generate enough force in all directions for the heavy weather in which we want to be comfortable (compared to external fins).

  4. Alex F Says:

    Hi Steve
    The openning picture of this post brings to life the fine engineering that you, Linda and team have put on these FPB designs …. Look at how little draft you have been able to achieve vis a vis the amount of superstructure above the waterline, and yet with fantastic stability levels !!!
    Congrats to you & team, and the lucky ones who will have the privilege of owning and sailing one of these beauties !

  5. Steve Dashew Says:

    Thanks Alex:
    The formula is pretty simple. Waterplane inertia, mass, its center of gravity, and how buoyancy develops with heel. Ignore the dogma that passes for wisdom, and get back to first principles.

  6. JLF Says:

    Hello SetSail,
    I’m thinking my first trip in that boat will be the North Atlantic, the land of the big tides. It looks like the under-body got sharper in the evolution. Can I still ground it out on the mud and sand with no worry? And I’m curious, from you expert point of view, even if she was designed for it, would you feel safe intentionally grounding her?

  7. Steve Dashew Says:

    The FPB 97 is designed to use tidal grids and to dry out on tidal rivers, assuming a reasonably smooth bottom.

  8. Geir Ove Says:

    What is the angel of no return ?
    she does have a lot above waterline.

  9. Steve Dashew Says:

    Above 140 degrees (i.e. 50 degrees beyond horizontal).