If they are what I think you mean I had them on my 25' Almand. Basically they are tabs held in the down position by a strong spring. As you go faster the water pressure on the tab forces it up so it's less effective the faster you go. The trick is to match your speed to the wave action so you get enough tab downward force to cause your bow to knife thru the water but not so much as to slow you down. They definitely work, but not as good as electric or hydraulic tabs where you can fine tune the amount of tab needed. Additionally, unlike adjustable tabs, they are only good for fore and aft boat trim. They cannot trim a boat from side to side like what you need when your 200 lb buddies all sit on one side of the boat.
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1987 V20 w/1987 150HP Yamaha on a Shoreland'r Trailer
1978 16.5 Airslot w/1996 120HP Force on a Four Winns trailer
1996 V21 w/1993 200HP Mercury on a Shoreline Trailer
All towed by a 5.7L Hemi Durango.
If God didn't have a purpose for us we wouldn't be here, so
Live simply, Love generously, Care deeply, Speak kindly.
(Leave the rest to God) 
Silence, in the face of evil, is itself evil. Not to speak is to speak, not to act is to act. God will not hold us guiltless.
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