Below are the by-laws of US Sailing about voting and non-voting members. Especially note 1.(a)(v) below relating specifically to multihulls.

You ask: "OK... WHO ELSE in sailing has an organization like NAMSA or for that matter NAHCA (a collection of classes)?" You added: "Yacht Clubs and individuals join US Sailing. That is the membership.... Not NAHCA or NAMSA."

As you can see from the following, ONLY regional and national organizations (like NAHCA and NAMSA)and class associations and offshore and cruising associations have a vote within US Sailing. And there are many such associations.

Yacht clubs and individuals are NOT voting members.

NAMSA is doing precisely what is provided for in US Sailing's rules so that we CAN start working within the US Sailing system and provide another vote on behalf of things that will benefit multihull sailors.

What fleets do to survive and thrive at the local level is entirely up to them -- different things work in different areas. But I do notice in 1.(a)(i) below that RSA's are to be made up of yacht clubs, and it does not make any provision for fleets to join.
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(From the US Sailing By-laws)
ARTICLE THREE—MEMBERSHIP
1. Membership in this Association shall consist of:
(a) Voting members represented by delegates hereinafter described and composed of:
(i) Sailing associations (sometimes titled yacht racing unions, yachting associations, yacht racing associations or sailing associations) made up of yacht clubs, which by their application for and election to membership in
the Association accept the responsibility to carry out, in their respective areas, the purposes for which the Association was organized;
(ii) Sailor-Athlete Advisory Council delegates who are Sailor-Athletes (as defined in Article 15 of these Bylaws).
(iii) Class associations composed of one-design classes organized through fleet and/or districts as one-design, restricted, open or rated classes;
(iv) Offshore cruising/racing associations organized through clubs, fleets or classes for the conduct of racing locally, regionally or nationally on a level, handicap or rated basis.
(v) Multihull sailing associations organized through associations, clubs, fleets or classes for the conduct of racing locally, regionally or nationally.
(vi) Inter-Collegiate Sailing Association (ICSA).
(vii) United States Windsurfing Association.
(viii) Community Sailing organizations.

(b) Nonvoting members composed of:
(i) Affiliated associations which conduct college, school, Sea Explorer, Sea Scouts, armed forces and similar sailing programs;
(ii) Yacht clubs which are members of a member sailing association;
(iii) Fleets or stations which belong to a member class association or offshore cruising/racing association;
(iv) Individuals or families who are interested in sail racing;
(v) Other sailing organizations, including yacht clubs, which are not members of a member sailing association and fleets or stations which are not members of a member class association or member offshore cruising/racing association;
(vi) Corporations wishing to support the work of the ssociation.

(c) Such other categories as may be established by the Board of Directors.