Originally Posted by fryedaze
I dont know anything about hull design and wonder if trim tabs would make my boat plane. I have twin 210 HP Cummins and the top spped for the boat is 11.5 Knts. She is 42' with a 14'6"beam and only drafts 39". I am not sure what her displacement is but I think its around 30,000#. She does dig the stern in when running above 9 knts. Does anyone know if trim tabs would give me more speed and allow the boat to plane?
I'm not a credentialed marine engineer, by any means. However, I do have decades aboard boats, owning boats, piloting boats, and working (in my early years) with shipwrights in boat yards of NY and Maine... building, restoring, refinishing and adding items as well as sections to boats - back then the vast majority were made of wood, what fun!
It clearly appears by your picts that your boat has planing hull. Some may call it semi displacement or semi planing. The way I see it: If it can plane it's a planing hull. If it can't its displacement - Period!
Therefore, regarding your hull design, size boat, weight, power availability and whether trim tabs could make your boat plane:
It sounds by your description of digging in her stern above 9 knots (your length boat's approximate top end of its "displacement hull speed") and her top speed being 11.5 knots that you simply are just shy of enough power to pull her over the bow wave for leveling out and planing atop the water... rather than continuing to "dig in" her rear, and, push lots of water with her “somewhat upward pointing” bow. I bet she drinks fuel at the 11.5 knot speed, should be very inefficient fuel usage then as deep dig rear and bow wave push takes loads more energy per knot compared to just about any other speed (attitude in water) a boat's hull may attain.
In answer to your question... large trim tabs might (notice I say “might”) be able to help your boat barely get onto plane at WOT. But, from what you say circumstances are with current power, props and without tabs, then even with tabs I doubt you could hold plane unless staying at or real close to WOT continuously.
Couple suggestions that may help you have your boat plane. Also, I highly recommend that you discuss this with hull designers/engineers, mechanics/engineers familiar with your motors, and propeller designers/engineers.
1. Increase horse power of your engines if it is doable – somewhat of an $Ouch$ (install more powerful engines - BIG $$$-OUCH-$$$)
2. Have props designed/altered to better make use of current motor power or to suit potential for increased power
3. Have real good trim tabs that the hull engineer recommends professionally installed.
I believe your boat could probably be engineered to plane – but, it may (would) be at a cost... how tall the cost is depends on how much of a potential exists via different parameters mentioned in 1 – 3... and... how much you are willing to spend to potentially plane.
Best luck my fellow boater, and of course - - >
Happy Boating Daze!