Hydrafoils versus Trim Tabs

[hwsiii]

It seems there are many questions asked about Hydrafoils and Trim Tabs on a regular basis, so I tried to compare the two with each other.

There are tremendous amounts of Hydrafoils sold every year under different brand names, they serve a purpose or they couldn't sell them in those amounts. Some of the reasons people buy them is to try to raise the stern of the boat up to improve hole shot performance and to keep the bow down during takeoff to planing speed and/or to help carry heavier loads in boats that are either underpowered or have the wrong prop installed.

For small slow boats they do work very good and are usually about $ 50 to $ 70, but you an buy Smart Tabs SX for very little more money and they are much more efficient and can actually increase speed as well as help keep the boat balanced fore to aft as well as port to starboard, and a hydrafoil does not do this because it is in the center of the boat and thus does not help balance the boat from starboard to port.

To use a hydrafoil and not have problems the outboard needs to be mounted so the hydrafoil is actually out of the water at cruising speeds and in most cases smaller boats are not setup this way but since they are slow it doesn't usually matter as it does help pick the stern of the boat up. But in larger faster outboards when it is trimmed up for maximum speed and the motor is not mounted high enough on the transom the hydrafoil actually adds excess drag and slows a faster boat down. The hydrafoil can also trip on itself in higher speed turns and certain wave action directions as well as it can and does increase chine walking on very fast boats, whereas trim tabs help reduce chine walking and the bad behavior of port to starboard listing.
Stern drive boats usually should never use hydrafoils, because the anti ventilation plate never rides out of the water and is always immersed so they will in most cases create more drag at high speeds and actually slow the boat down at higher speeds. Whereas with either smart tabs or trim tabs, if they are set in the right position, only lift the stern of the boat and create nominal drag or can actually help a boat attain higher speeds than before their installation.
With a drawing of Trim Tabs versus Hydrafoils you can see very easily why trim tabs are much better in most cases, because you can tell that the stern is being held up at the outer chine areas of the boat and NOT the center, which makes the boat more prone to leaning either to the port or starboard like a seesaw.

Trim Tabs and Hydrafoils

Hydrafoil

In this picture you can see why a hydrafoil that is not mounted high enough on the transom creates extra drag and can actually slow a faster boat down because it is not on top of the water but in the water flow.

Trim Tabs

Whereas with Trim Tabs you can see they do not create this extra drag unless they are deployed deep in the water column to either plane at very slow speeds or to push the bow of the boat down so the boat an cut the waves with the bow instead of hitting the boat further back than the bow and causing it to slam and have a bad ride in head on seas. A hydrafoil can possibly do this also with the engine trimmed all of the way down, but that means it can't be out of the water and it is creating extra drag on the boat at all times and slowing it down.

Advantages and Disadvantages

H

 

[sleeper]

Re: Hydrafoils versus Trim Tabs

This needs to be a sticky since this question gets worn out over and over. Good information!

 

[Bondo]

Re: Hydrafoils versus Trim Tabs

Stern drive boats usually should never use hydrafoils, because the anti ventilation plate never rides out of the water and is always immersed so they will in most cases create more drag at high speeds and actually slow the boat down at higher speeds. Whereas with either smart tabs or trim tabs, if they are set in the right position, only lift the stern of the boat and create nominal drag or can actually help a boat attain higher speeds than before their installation.

Ayuh,... Another Great post, H....

I'll only add 1 more Issue I have with the drive mounted Foils,....

They're lifting the Whole hull at a point it's Not supposed to be lifted from....
(probably an Over-Weight hull at that)
Outdrives,+ outboards are designed to Push a hull,.... Not Lift it.....

 

[hwsiii]

Re: Hydrafoils versus Trim Tabs

Thank y'all very muh, and I agree it normally wouldn't be there if there wasn't a problem with weight or low HP and it is putting every bit of that load on the anti ventilation plate which is not good either.


H

 

[ondarvr]

Re: Hydrafoils versus Trim Tabs

Good post, but there are problems with you comparison list.

The list is accurate if all you do is bolt on a foil and head for the water, but if you actually spend time on the setup, then many of the short comings listed aren't true. I say this if the boat you're using it on can take advantage of what one will do though, it may not help at all in many situations.


Foils are advertised to do the same thing as tabs, and that's how you're comparing them, they actually do totally different things, but both can be used to improve performance with little or no down side.

Loss of speed..... you can actually increase your top speed with a foil. As you raise the motor gearcase drag is reduced, which may get you a couple more MPH.

MPG improves with the above also.


Chine walk.... again incorrect set up... chin walk should not increase.



Tripping in turns..... again incorrect setup increases the chances of this, so do other factors, but if setup correctly it shouldn't be an issue.


Stern lift.... If your using a foil (incorrectly) for stern lift it is adjustable. As you trim the engine you're changing the angle of the foil, so its very easy to get a change in stern lift. Set up correctly it shouldn't affect it one way or the other at speed.

Bow rise can be improved not because the foil is dragging in the water, but because if the motor is raised higher it will have less leverage on the hull and this keeps the bow lower.

A foil allows you to raise the motor higher which is where all of the benefits come from, it should not be dragging in the water, if it is then the setup is not correct, or the hull/motor combo can?t take advantage of what a foil ?might? be able to do.

On an I/O there?s not much you can do on the setup, so the down side of a foil may far out weigh any possible improvements. It just depends on what the owner is looking for.


While the two products can achieve some of the same results, they go about doing it in a completely different way and 95% of the time tabs are the correct tool needed for making the improvements the boat owner desires.

 

[hwsiii]

Re: Hydrafoils versus Trim Tabs

Ondarvr, my experiene with most people who install hydrafoils is 95% of them don't understand the priniples you are talking about and they do not raise the motors. Just my opinion.


H

 

[ondarvr]

Re: Hydrafoils versus Trim Tabs

That's true, but I don't rate a product on the short comings of the user, I rate it on what it can actually do when used correctly. There are so many misconceptions about foils that I like to at least give a little info on how they?re supposed to be used.

 

[joe009]

Re: Hydrafoils versus Trim Tabs

a very good post.i use both on my 204 fx angler,with a 150 yamaha 4x and the boat run better.

 

[wca_tim]

Re: Hydrafoils versus Trim Tabs

Good post, but there are problems with you comparison list.

The list is accurate if all you do is bolt on a foil and head for the water, but if you actually spend time on the setup, then many of the short comings listed aren't true. I say this if the boat you're using it on can take advantage of what one will do though, it may not help at all in many situations.


Foils are advertised to do the same thing as tabs, and that's how you're comparing them, they actually do totally different things, but both can be used to improve performance with little or no down side.

Loss of speed..... you can actually increase your top speed with a foil. As you raise the motor gearcase drag is reduced, which may get you a couple more MPH.

MPG improves with the above also.


Chine walk.... again incorrect set up... chin walk should not increase.



Tripping in turns..... again incorrect setup increases the chances of this, so do other factors, but if setup correctly it shouldn't be an issue.


Stern lift.... If your using a foil (incorrectly) for stern lift it is adjustable. As you trim the engine you're changing the angle of the foil, so its very easy to get a change in stern lift. Set up correctly it shouldn't affect it one way or the other at speed.

Bow rise can be improved not because the foil is dragging in the water, but because if the motor is raised higher it will have less leverage on the hull and this keeps the bow lower.

A foil allows you to raise the motor higher which is where all of the benefits come from, it should not be dragging in the water, if it is then the setup is not correct, or the hull/motor combo can?t take advantage of what a foil ?might? be able to do.

On an I/O there?s not much you can do on the setup, so the down side of a foil may far out weigh any possible improvements. It just depends on what the owner is looking for.


While the two products can achieve some of the same results, they go about doing it in a completely different way and 95% of the time tabs are the correct tool needed for making the improvements the boat owner desires.

Some good points overall. and excellent post overall! My impression is that due to the lower amount of flexibility for most stern drive set-ups (primarily not being able to easily change propshaft height), the use of addons like tabs and foils are more commonly tried as a way to offset things that are much more difficult to change in other ways (ie. heavy "car" engine in the back of the boat). I am not aware of foils that allow an angle adjustment and so for a sterndrive boat, the angle is fixed by the trim angle that the set up works best for. Two examples from my own recent experience. On one boat / set up, a heavier, deep v, the hull works best with neutral or even slightly trimmed in. A foil would probably work great if I were having problems. On the other, a much lighter, (somewhat "overpowered") padded v-hull with lifting strakes and steps in the hull, the hull was designed with a good bit of hook near the very rear of the hull so the boat runs best fastest with a good bit of positive trim. It would be positively dangerous with a foil because if the leading edge of the foil catches the water wrong or is dragging... and angled downward, life is not so good. where with tabs, the faster the boat goes, the further out of the water the hull is and the less the tabs hit the water and you're not worried about them catching anyhow.

so i suppose for a foil on a large heavy boat that doesn't require a lot of positive trim to carry the bow, or on outboards where the fin would be far out of the water and set up was kept so that the fin was not tilted down wher eit could catch the water... could be a reasonable use.

Overall, I really feel like even cheap trim tabs give all the benefits without any of the potential drawbacks. and I liked / agreed with Bond-o's comments as well.... good information overall.

 

[wwcj101]

Re: Hydrafoils versus Trim Tabs

i use a foil on my bass boat when i pull the kids around on a tube it gets it up on a plane faster.
when im running the foil comes out of the water about an inch or so. the boat runs the same wot speed with or without it.
maybe the motor is too small (150 johnson), maybe the prop is too big (13 1/4d x 25p 4 blade stainless), but hole shot definitely increases with it.