It might be helpful to put the model/design & year or decade built, ex; k39. or kss 38 etc.. and if you have them, plans/drawings.
You're not going to find polars, it's vintage, it's custom/homebuild not production.
Polars from a "similar boat" would be useless -even a visually (or specs wise) "similar" tri wouldn't be close enough to make anything more than a wild guess..
Multihull dynamics (nowadays paid site) might be a good source of getting the type of data you seem to be after- (if you have the full specs/design/drawings & you're willing to spend $ on getting the design added or a custom report etc, that is. No
kelsall tris in the general pre-existing database, only Kesall cats) - But even if you're not getting a full custom evaluation, for a small
membership fee - you might still consider signing up- likely would still be able to find a "somewhat similar" ~40 ft tri on their site, look over the #s- and do some comparisons to other
boats.
"Similar" cruiser/racer 40ft tris might (perhaps a little optimistically) hope to *average* ballpark 7 knots on passages, likely a bit less, maybe a tad more pushing hard with a
race crew/mindset or on downwind runs.
Keep in mind the
multihull "average" speeds you see thrown out there are often accompanied by some bias and could, often, be more accurately described as "average in ideal conditions".. On similar
boats pushed hard with good
sails - 200 mile days (8.3
knot average over 24hr) for example are probably do-able but not exactly easy or common.