Re: Hydrofoils on lower units?
I had typed a long response on my phone and then fat fingered a key and deleted the whole thing, so I’ll try it one more time.
“Foil” is a bad name for the product and has more to do with marketing than how it’s supposed to function. The foil shape is almost irrelevant in what it does, but when they started making them out of plastic, which needed to be thicker than metal, it was a good marketing ploy. All it does is help prevent the prop from sucking air from the area right above it during hard acceleration, no different from the existing AV plate (anti ventilation plate) on every outboard. This defeats the argument that if foils were a good thing outboards would come stock with them, because they all do.
Trims tabs and foils are not designed to do the same thing in the same way, but there is some overlap in the final results. Tabs are designed to exert a downward force on the water to raise the stern and achieve many beneficial results. A foil stops the prop from sucking air…but when used incorrectly may also do a bit of the same thing. This misuse is where the disappointment and negative comments come from. A person bolts it on thinking it’s a cure-all for every issue they have with the boats performance and finds out it may actually make things worse in some ways. To use a foil and find out if your hull and motor combo can benefit from one you will need to raise and lower the motor on the transom to see if you can get the motor higher with the foil. If it makes no difference in performance then there is no reason to use it, but in some cases you can mount the motor an inch or more higher and make good gains in performance. This can include a better hole shot, not due it dragging in the water, but because there is now only water around the prop, not air. It can also reduce the amount of bow rise because with the prop up higher it has less leverage on the hull. A higher top speed and better MPG’s because there is less of the gearcase in the water so there is less drag (again…the foil should not be dragging in the water at speed). Trim tabs can do none of these things, they just push down on the water.
Now if you have an IO, little or no adjusting can be done, so you need to live with the good and bad, there is potential for both, many times the trade off on the bad side is worth some possible improvements in other areas, it’s up to the owner. But IO’s can have some real handling issues with foils (so can OB’s), that foil dragging in the water at 45+ MPH can have a huge affect on the boat, sometimes it can be downright scary. This is where the exact size and shape of the foil can make a big difference. But for the most part trim tabs are a better bet on an IO, plus there is rarely a downside to installing tabs, it’s almost universal that you will see at least some benefit from them.
In a thread a long time ago I compared tabs and foils to hammers and crescent wrenches. Everybody knows a hammer is good at pounding on things, it’s what it was designed to do. Many people also use a crescent wrench as a hammer to pound on things, it’s a poor substitute for a hammer though, and is totally the wrong way to use it. This is where I find it frustrating when people try to use a foil in place of trim tabs and then complain about the results.
op2:
