On The Level: Hydrofoil, Trim Tabs, & Power Trim Devices

 On the Level: Hydrofoil, Trim Tabs, & Power Trim Devices 

 

 

           

 


 
 
 

Hydrofoils, trim tabs, and power trim devices all can improve your boats planing and driving performance.  
Adding a hydrofoil device to your outdrive will get your boat onto a quicker plane.


When it comes to driving a boat, there is nothing more aggravating than trying to steer one that has a serious list or labors too long before coming onto a plane.

What might aggravate the situation even more is if everyone aboard decides to move to the rear of the boat before you are able to accelerate onto a plane.

Regardless of the size of the vessel, excessive weight concentrated in one area will generally impede how long it will take a vessel to come to a plane if it can come to one at all.

Trying to see over the high rising bow of a boat lugging along, desperately trying to "break over," and not knowing what is in the pathway as the minutes seem like hours before sight is regained, can be more than just a hairy dilemma. Of course, this might not be a problem, as far as being able to see, if you happen to have a tuna tower with which to navigate your vessel.

But you say you do not have that problem because of having a plenty of horsepower. Your boat slingshots you out of the hole like a dragster seeing green lights.

That could be true for some, but that still might only resolve the "planing" problem. What about lateral control?

Even boats with the most powerfully equipped engine(s), too much weight to either side can cause a listing problem while underway. Such problems can only cause a poor ride, hard steering, exorbitant fuel consumption, and unnecessary wear and tear on both engine and lower unit parts.

Some have even thought that such problems with planing and lateral control are just something that comes normally with boating and are non-rectifiable. Others have even been under the false impression that power trim is the whole answer to planing faster, thus many either order their boat equipped with such or have a unit added after the fact. In either case, no boat is complete without it.

Sometimes the answer for faster planing may additionally lie in having power trim tabs and a hydrofoil stabilizer. You know, that ugly looking whalelike fin that bolts on to thehydro.jpg (33110 bytes) outdrive unit. That piece of plastic (metal in some cases) that many have reasoned to themselves couldn't work because if it did manufacturers would already have them on outdrives to begin with.

To further help you appreciate that such reasoning can be misleading, ask yourself why it took so long after the invention of the automobile to find out that aerodynamics play an important factor in fuel efficiency and handling?

The simple truth is that it often takes awhile before inventions are accepted by others as really workable solutions.

Let's consider three components that will affect the planing and lateral control of your boat: Outdrive trim (power or manual), trim tabs (power or manual) and hydrofoil stabilizer. These three devices all must have moving water for them to function. Understandably, the faster the water moves around them, the better they will perform.

It is probably safe to say that most boatmen are familiar with the function and importance of a properly trimmed outdrive by either manual or power activation.

For those who don't know, the trim device for the outdrive will control how the boat moves through the water by controlling the tilt of the outdrive and thus will either move the bow of the boat down or up correspondingly.

For example, if you move the trim switch on a power trim unit to "In" position, the outdrive will move toward the transom and will cause the bow to move down simultaneously, placing more of the boat in the water. The opposite will occur if the boat's outdrive is trimmed to the "out" position. Generally, the trim switch will only allow a 15 degree movement out from the transom when the boat is underway or over a given RPM so that the prop is always submerged along with the water intake ports.

The outdrive trim on any boat, whether it be manual or hydraulically controlled, will ONLY move the bow up or down and will have no control on the lateral position list to either side.

The principle function of this device is to allow the driver to control and adjust the planing of the vessel, fore and aft, according to weight aboard and prevailing sea conditions.

In rough or choppy water an outdrive trim unit should be adjusted to move the bow more down than up, so that the hull is more in the water and able to nose through the waves. This will also keep the boat from leaping out of the water at each wave encounter.

Tuning the outdrive trim and speed of the boat as it travels through the water should be done gradually as you attain the best possible dry ride while maintaining reasonable comfort for those aboard. Under ideal conditions, efficiency is best when the outdrive unit is positioned to maintain near levelness. Under the same conditions, over trimming the outdrive to cause the bow to come up too high will result in poor fuel efficiency, cavitation, and cause the boat to porpoise.

Also, consideration to loading the boat in a level manner will also be best, rather than resorting to extreme adjustments of tilt angle. It must be noted that steering controllability will be hindered if the outdrive is over trimmed to either the "in" or "out" position, whether moving at low or high speeds; the latter being more evident.

The benefits of having a power-driven trim on an outdrive as compared to manual type should be obvious.

But there is even more that can be done to your boat that will affect the way the hull moves through the water even if you already have a power trim unit.

You've read the claims about hydrofoils: ďAdvanced Hi-Tech design developed by computerized aerospace engineering .... Gets on plane in half the time without bow pop-up .... 40% fuel savings .... Gives smoother ride, eliminates chine walk and
porposeing, and capitation." (Note: Hydrofoils have been known to cut fuel economy.)

Upon reading such, you've also thought, how can a piece of molded plastic, bolted to my lower unit, achieve so much? Seems like a lot of hype!
 

As earlier alluded to, those who have added this amazing winglike contraption to their outdrive have found it to do exactly what had been claimed. Most of us realize that what sounds too good to be true, probably is. That's because we, sad to say, live in a world full of flimflams and money-making schemes; thus, it is often hard to see through some claims until you have been victimized.

However, this is not the case with this invention and, those to whom I spoke, were totally astounded by the drastic improvement of their boat's ride and performance.

When we think about the theory behind the function of this winged device, coupled with the movement of the outdrive power trim, you can see how more area contact with the water is made by expanding the area of the cavitation plate.

This principle is not new, in that airplanes have been using similar devices on their wings. The only difference here is that instead of air flow there is water flow passing across the winged area. The outcome is a more effective response when adjusting the power-trim angle and a drastically improved response time for getting onto a plane with little or no hindrance as to where the weight is positioned in the boat.

Thus far, we have considered the power trim device and the hydrofoil stabilizer. There is yet another planing device that is most useful and will do what the previous are unable to accomplish.

This device is called trim tabs. They, like outdrive trim, come in either manual or power type;
diagram.jpg (18660 bytes) the latter is of more benefit and convenience.

Theoretically, like airplane flaps, are attached to the lower edge of the transom area at or near each side of the hull. The power type are controlled from a switch at the cockpit and can move the tab flaps independently, either up or down.

Ironically, more than a few people have seen this device as unnecessary if already possessing either power trim and/or hydrofoil stabilizer. Perhaps this is due to not fully understanding the purpose of each device.

First of all, power-trim tabs trim the hull, while outdrive trim trims the prop. It is true both will move the bow up or down. But, the trim tabs will move the lateral position of the boat as well as positioning the bow up or down. Outdrive trim can only do the latter. So, if passengers or gear are disproportionate, you do not have to keep shuffling everything to balance the ride.

Trim tabs also enable you to plane your boat at lower than normal planing speed. This means that when you take off and the trim tab flaps are down ("bow down" setting), the back of the boat will rise instead of lugging down, hence, producing an almost instant plane. Very beneficial when trying to get onto a plane in shallow water areas. 

This device, like outdrive trim, needs to be finely tuned both to achieve the best possible ride, including lateral stability, by moving the switch to affect either port side or starboard side levelness.

For example, on take off, when the flaps are down ("bow down" setting) and the boat is brought to a plane, it will be necessary to readjust the flaps to adjust the bow up or the boat will have too much water drag which will cut fuel economy.

A quick and easy way to get a basis for proper trimming of both trim tabs and outdrive is to first move the trim flaps to "bow up" position (flaps to the extreme up position) and adjust outdrive trim in all the way. Then, accelerate the boat onto a plane and proceed to adjust the outdrive trim to a comfortable planing position prop path to or near 90 degrees to water-flow level. Next move both trim tab flaps down ( "bow down" position) until stability is achieved without nosing the bow too low. The boat should always be trimmed fore and aft before correcting a list.

If there is any listing to either side, try adjusting the bow up by pushing the trim tab switch to "bow up," either port or starboard side accordingly. If not affected by that, push the trim tab switch marked "bow down" on the side that is too high up, until the bow laterally levels.

Extreme caution should be taken when moving the flaps of the trim tabs to maximum position while underway, since steering maneuverability may dangerously be impeded.

The advantage of trim tabs over just having outdrive trim is that if your boat is encountering a choppy sea condition which is causing the waves to pound either off the starboard or port bow, you are able to laterally lift the encountering side slightly to reduce pounding and over spray.
    
There is perhaps another misconception about the employment of trim tabs on boats without outdrives that is inboard motor boats that utilize rudder(s) and shaft driven prop(s).

Some boat owners having non-outdrive type vessels have unfortunately thought that the addition of trim tabs would give them the ability to lift the bow of their boat beyond that which it previously had before trim tab installation.

This is not true! In such cases, the only thing trim tabs will do is to trim the boat down from the previous position and to give more lateral stability where needed. If more bow up is needed, the weight distribution of the vessels load could be placed more astern, thus giving a broader range of trim tab angles to deal with.

Likewise, boats with outdrives, power trim, and hydrofoil stabilizer can only expect to trim their boat up only as high as they could prior to trim tab installation.

Certain hull designs are more influenced by trimming laterally than are others. Trihull boats normally are broader in the front, which gives them more buoyancy, making them less apt to nose dive in rough seas. Though this is an advantage, it also can make for a rougher ride and, with trim tabs improperly adjusted down too far on one side, along with having rough seas pounding it cattycorner, may cause stress cracks and/or structural damage to this type of hull.

Deep V boats are also affected by extreme trim tab lateral adjustment, in that the hull will almost tilt to such a degree to either side causing passengers as well as gear to perhaps fall overboard. And, as previously mentioned, helm control loss with unexpected direction change.

Nevertheless, having planing devices on your boat will give you added advantages. Getting use to adjusting them properly takes some practice, but generally becomes second nature after a short while.

The bottom line is that trim and hydrofoil devices are the easiest and quickest way to dramatically improve your boat's performance at a price that's well worth it.             

 

Picture Captions:

Hydrofoils: Easy to install, and it will get your boat onto a quicker plane.
Water Flow Diagram for Trim Tabs Devices: Water flows across the planing device area and causes the stern to left upward when tab is extended downward.




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