Chance,
Anyone that says you don't need tabs on a 20 hasn't really spent much time on the boat or tried to run very far in a serious chop! The one exception might the I/O Seafari 20. I asked Carl Moesly about the CG difference between the I/O and OB models, and he confirmed that the CG is further forward on the I/O model, even with a lightweight 60's vintage 260-300 lb OB on the transom! A recent ride in an I/O Seafari w/o tabs revealed that it did indeed plane at about 12 mph, and rode very well in a short steep 2' chop.
On my first Bahamas trip in 1976, we made a 20 mile run from Green Turtle Cay to Man O'War Cay headed SE into a 20-25 kt SE trade wind breeze in Abaco Sound, which is only about 20' deep. That long fetch in shallow water produces "square" waves, 2' high and about 2' apart. I didn't have trim tabs then but I discovered that the boat actually rode better at 25 mph than it did at 20! The reason is that once you get on plane, the running angle continues to decrease up to about 25 mph; it's flat above that. (Check out the plot of running angle vs. MPH for the 20' Seafari in the Boating Magazine test in the Literature section.) At the time I was running a 300 lb 1975 115 Evinrude motor on a 20" transom. I concluded that what would really improve the ride was to have that flat running angle, but at a lower speed so I didn't hit the waves so hard! I installed 12x9 Bennett tabs for the trip the following year and what a difference . . . I swear the boat thought it was about 3' longer with the trim tabs! With the tabs, I could plane at about 10-12 mph and easily ride through 3' square waves, SITTING DOWN, at 14-15 mph! That was undoubtedly the best $300 (in 1976 size $!) that I ever spent.
Another benefit of the tabs is that in conditions where the waves are approaching at some angle off the bow instead of head on, you can use them to roll the boat AWAY from the waves, which essentially increases the hull deadrise relative to the waves. Just a few degrees of roll can soften the ride substantially. (Any deep V will tend to lean into the wind . . . I believe the reason is that the wind blows you off course, so when you crank the helm over to get back on course, the hull leans over, just like it does any time you make a turn.) Now that I'm running a 25" motor on a bracket, that different geometry seems to provide more leverage, such that engine trim has a much more powerful effect on running angle than it did with the previous configuration, so I use engine trim to control running angle, and use the tabs to set the optimum roll angle relative to the waves. You can accomplish the same thing by moving crew and/or ballast around, but it's a lot easier to just tap one of the tab switches a couple of times! Denny
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