Good to count with an electric air and foot pump, large Sibs must ride fully inflated to their working pressures to get the max out of them.
Don't buy any doel fin or any other fin type, save the money for some cold ones. Useless installing one on top AV plate when already having prop aeration issues which a fin won't cancel, a fin must ride parallel to the water level and fully submerged to work as was designed to work unless wanting to use it a a step latter to climb on board from behind the motor....
The issue with your current combo is that has an awful motor/transom match, the pic seen on post 6 corresponds to the maiden voyage of the 430 and 30HP motor out of their respective boxes. This is where the water flow was passing at plane right over the exposed AV plate where all the excessive prop aeration was being produced...
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The only possible way to make that 430 powered with a Honda 20 HP to perform as expected was to cut the transom down at least 1 full inch for the prop to go deeper, have a taller water bed, bite much better and achieve a much better prop thrust than when at the maiden voyage...
Cutting the Transom down....
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Check the new lower leg height with water flow passing right under the small upper plate. No need to center the motor on transom, just drop it down and secure both clamps...
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The owner is so content with the water performance of his reloaded Seamax 430 that now uses it a lost more than his yacht. Check in your area if there's someone sufficiently skilled to do that kind of work, will be a day night boating difference guaranteed...
Happy Boating