  The Modboat is a low-cost, underactuated, modular robot capable of surface
swimming. It is able to swim individually, dock to other Modboats, and undock
from them using only a single motor and two passive flippers. Undocking without
additional actuation is achieved by causing intentional self-collision between
the tails of neighboring modules; this becomes a challenge when group swimming
as one connected component is desirable. In this work, we develop a control
strategy to allow parallel lattices of Modboats to swim as a single unit, which
conventionally requires holonomic modules. We show that the control strategy is
guaranteed to avoid unintentional undocking and minimizes internal forces
within the lattice. Experimental verification shows that the controller
performs well and is consistent for lattices of various sizes. Controllability
is maintained while swimming, but pure yaw control causes lateral movement that
cannot be counteracted by the presented framework.
