Rough surface on sponson insides on Dunlap tunnell ?

Hi all last winter i refinshed my 10 year Dunlap W.O.F. 30 by sanding back to bare wood then applying 5 coats of Epifanes marine varnish as per instructions. This past summer i noticed that at about after 15 runs after wiping the bottom of the boat that the  inside port and starboard sponson inside vertical areas from the transom  11'' towards the bow looked like some one took # 80 grit sand paper to them i ran several scenarios through my head that maybe twigs or other objects where coming in contact with this area but the varnish on the sponson bottoms  is in perfect condition . But what i personally found odd is that both sides are a mirror image of one another where the varnish looked scuffed up.  So this past fall again i sanded the area back to bare wood applied 3 coats of Pacer Finishing resin followed by 3 coats Minwax Helmsman spar urethane . Took the boat to last nights boat club meeting and explained the situation to club members one member with a engineering background believes this situation is caused by something called cavitation not prop cavitation ! Where the rapid formation and collapse of vapour pockets in flowing liquid in regions of very low pressure e.g like sand blasting and mentioned varnished wood will not stand up to this . The  Dunlap 30 is the best performing tunnel i have built in the last 25 yrs. Has anyone had a similar experience like the one described ? Regards Andris Golts.