Delphi UTF-8 strings inside records

I use a record like this...

type
  TNameRec = record
    Nam: string[maxNameLength];
    Var : real;
    Siz : integer;
  end;

to store and access data to files of class TRecordStream = class(TFileStream)

using functions like...

var
    fileStream: TRecordStream;
begin
    fileStream.ReadNameRec(NameRec);
    fileStream.writeNameRec(NameRec);
end;

declared as...

function TRecordStream.WriteNameRec (Rec: TNameRec  ): Longint;
begin
  // This function writes the record Rec to the stream
  Result := Write(Rec, sizeOf(Rec));
end;

function TRecordStream.ReadNameRec (var Rec: TNameRec  ): Longint;
begin
  // This function reads the record Rec from the stream
  Result := Read(Rec, sizeOf(Rec));
end;

This works fine for me. The problem rises when i want to save utf-8 strings.
In this case the strings cannot be stored properly (uknown characters), unless I remove the declaration of constant length for the strings as below:

type
  TNameRec = record
    Nam: string;    //[maxNameLength];
    Var : real;
    Siz : integer;
  end;

if i do that, the utf-8 strings can be saved and then be accessed keeping their actual characters. Unfortunately, when i try to read these records from the filestream, sometimes the process works fine, and sometimes there is access failure, obviously due to miscalculation of the record length.

Is there a way to save and read these utf-8 strings properly, but by keeping this record structure? Of course, if it's not possible, any solution close to this is welcomed.