Harry and Dave ate not related within a genealogical time frame. Normally, they would have to match 41/43 or better. However, 39/43 or 40/43 would be a gray area and would require testing different branches.
They are probably in different haplotype subgroups. See:
The easiest way to get a haplogroup prediction is to enter the results at www.ysearch.org. You need to change the results on 4 markers tested at Ancestry before entering, because of differences in nomenclature. They are:
Both test autosomal, Y and Mt DNA. They use slightly different nomenclature, but that is noted on the ISOGG site. The second test is actually done by FTDNA.
You can get the DNA results of anyone at Ancestry by doing a comparison with the person, unless the person has asked that his results be hidden. For example, if you are logged in as Harry, do an individual surname search (on the "Groups" page) with "Rudd" or any other surname. Click "View" next to a person of interest. Then, click on "Compare."