The peculiar velocity of the Local Group - I. H I observations of Sb and Sbc galaxies

H I observations of a sample of 128 Sb and Sbc galaxies, limited by angular diameter, are presented. The physical and star-formation properties of these galaxies are discussed. In a later paper the data will be used for a new study of Local Group motion. The galaxies range in linear diameter from 9 kpc for UGC 10554 to 128kpc for UGC 2885 and in total mass from $$6.3\times{10}^{9}\,{M}_{\odot} \,\text{to}\,1.3\times{10}^{12}\,{M}_{\odot}$$. Corrections for the inclination dependence of observable galaxy parameters are re-examined. No evidence is found for the variation of H I self-absorption with inclination in Sb and Sbc galaxies, although there is a substantial dependence of observed diameters and luminosities on inclination. A correlation between total surface mass density and colour index is found which implies that star-formation in these galaxies is governed by disc density rather than total mass or gas richness. H I surface density is relatively constant in Sb and Sbc galaxies although there is a dependence on morphological type and luminosity. Local Galactic H I observations at 12-arcmin resolution are used to derive new correction factors for diameters and luminosities to allow for the effects of galactic obscuration. These observations show that there is substantially less absorption at b>45° than that given by a 0. 2 mag cosec |b| law.