View Full Version : Trim Tabs

06-28-2010, 10:48 AM
Are Trim tabs neccessary and what is the advantage? If I buy a used Sweet 16 I/O isn't the boat balanced enough without trim tabs or are trim tabs something to be seriously considered?


Dr. David Fleming
06-28-2010, 11:17 AM
Well this question you have asked is bound to get a lot of discussion. To begin with lets give Donzi some credit for designing and producing your boat without the tabs. They knew what they were doing.

Secondly your hull design dates back to the 1960's and was the first really hot item Donzi did under the direction of Don A. the founder of the company. The 16 was one wild ride in its day.

When a boat is manufactured without the trim tabs the "x dimension," which is the distance the propeller is from the bottom of the boat is changed. This gives the trim with out the tabs. Tilting the drive in and out gives basically the same effect as tabs.

Trim tabs are a more modern invention and function similar to the flaps and elevator on aircraft. In a hull designed for them, the "x dimension" is reduced and the propeller then is operating close to the surface of the water - the reason for this is that there is less drag from the drive - the boat is faster. Trim tabs allow a measure of control in this low "x dimension" situation.

When trip tabs are installed they are usually about one inch above the bottom of the boat to eliminate drag when the hull is at speed. When they are down they lift the back of the boat and lower the front but this causes drag. If the boat has a heavy load, such as passengers it will get it up out of the water easily. If the water is very rough and choppy you can get the bow down into the chop for a smoother ride - very nice for running in the rough. They can also be dangerous because they can imbalance the boat by making its nose dig in the water at speed causing an unstable condition in which the boat will spin out.

Another not too talked about condition that the Donzi chief engineer told me, Chris C., the tabs in neutral position effectively make the boat longer and handle as a longer boat - 16 becomes effectively a 17 foot.

All of this is just the basics -

06-28-2010, 05:17 PM
Having owned a 16 for three years no way would I be without tabs!!!!!!!!!! The little buggers porpoise like crazy at cruise speeds, they will wander down a canal like a drunken sailor. With tabs you can do away with the porpoising and do away with the wandering just put the tabs full down at idle speeds. Note ALL single I/O deep V boats wander at low speeds without tabs! You want to be uncomfortable in several ways do not install tabs, Oh by the way they improve handling in a bad sea and allow you to correct for passenger loading. Of all the Donzi's the 16 really benefits more than any other with tabs.

06-28-2010, 08:02 PM
So the $64,000 is how much are trim tabs and why do folks seem to mention they have the Bennett model?

06-28-2010, 09:38 PM
Bennet's are the most popular with great customer support!

Dr. David Fleming
06-28-2010, 10:37 PM
Have a set of used Bennetts from 1999 that came off my ZX - the entire system as I switched to Mercury K planes - for sale if you want them.

Just Say N20
06-28-2010, 10:56 PM
Used Bennetts are good.

New, appropriately sized for the 16 are around $400 if you do some shopping. I bought a set for the 16 I am currently redoing. I wouldn't own one without them.

06-28-2010, 11:43 PM
Does anyone have an "out of water" transom picture of a 16 with them installed?

06-29-2010, 02:33 AM
I have owned and rigged numerous deep V boats over the years since the early 70s. The first set of tabs I ever installed were on my 1971 Donzi 18', and I have never had a deep V since then without installing tabs. They will correct the problems already mentioned, but I also use them to set my angle of attack for the oncoming waves by slightly listing the boat away from the the waves coming in at an angle so I can avoid the heavy bow slapping. Bennetts are simple and easy to install and I have never personally had a set go bad except for a leaking actuator. I think of them as bullet proof. Bill