return type of sog is utils::optional<units::velocity> not utils::optional<units::knots> in the rmc sentence about marnav HOT 2 CLOSED

auto v = rmc.get_sog(); // returns utils::optional<units::velocity>

// v is an optional,
// -> accesses its data assuming there is,
// get<...>() converts it into the desired unit
auto v_kn = v->get<units::knots>(); // v_kn is now units::knots

// for example also possible conversions from the same quantity:
auto v_mps = v->get<units::meters_per_second>(); 
auto v_kmh = v->get<units::kilometers_per_hour>();

auto v_kn = v->get<units::knots>();
std::cout << "SOG = " << v_kn.value() << '\n';

// or, of course, directly:
std::cout << "SOG = " << v->get<units::knots>().value() << '\n';