How much speed is actually lost between the two hull designs with a longtail or SD?

Forget speed - think performance....

Which means getting thru stuff with your stuff and not getting out and walking....

Mod V bow is OK but full length Mod V just adds 2" - 3" of drag and discourages
hull lift over shallow bottoms that you get with true flat hulls even at 3 to 5 mph...

MMs have limited power so you need max floatation - flatter equals more floatation
however true flat square chine doesnt turn at speed - so the real question should
be what chine design is best for where you want to run !

Flat - modified - round are the three major types... And 18 foot is the shortest hull
you should even consider or you will regret not going long...

I like round chine because it carves turns like its on rails and rolls off stumps and
cypress knees the easiest.... I give up some valuable floatation - so on sandy
bottoms I loose a bit of performance from "dead stop".... But that was my choice...

Every boat design choice is a trade off....

not likely to get a meaningful answer on this. flat bottoms on here tend to be heavier gauge aluminum while most true MV hulls on here are lighter weight (lots of times riveted).

I freind was running a mod v rivet 1648 was running 34 with a gdsd. Now has a unc j 1748 3/16. Same motor heavier boat runs 38 now

Surface drives are designed for flat bottom boats, so common sense would prevail that they would perform better on what they were designed to run on. Will C confirms this

What exactly are you calling a mod V? Most mud boat builders refer to a mod V as a flat bottom hull, with a V bow that ends at the rake.

If that's what you are referring to, there should be zero difference at top speed. The only part of the hull contacting the water should be the aft 1/3.

Mod-V ---- is badass with dual 200s ----

.

As with anything, speed is secondary to performance in your application. If you're looking for speed, a MM is probably last on your list of motors you should be looking at. Now, if you need a boat to get into areas where depth, vegetation, etc is an issue, then a MM is best for that application. A purely flat bottom will serve that application best but if you have to run any open water with chop or the potential to get rough, you're gonna get wetter in a flat bottom. My next boat will be a Hybrid or Mod-V bow as they cut through rough water better and slice through tules/willows etc better with a pointed bow. The difference in speed is negligible IMO in that hull design.