Hydrofoils on lower units?

[alldodge]

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.

Opinion

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 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.

While I agree with some of your comments but not all. Your previous post were good when it came to folks claiming they had first hand knowledge when in fact it was opinion, you to appear to have an opinion side. My I/O tops out around 55mph and I have never felt anything when driving it give me any reason of concern. let alone scary.

 

[ondarvr]

Re: Hydrofoils on lower units?

Did I say every IO is going to respond horribly to a foil...no, I only said it was possible and that if it does, there's not much adjusting that can be done to remedy the problem.

Because your boat responded well doesn't mean someone else's will, this can be very hull specific, it can also be very dependent on how the owner uses the boat.

 

[JimS123]

Re: Hydrofoils on lower units?

So let's see....the same company that owns Bayliner, SeaRay and Mercruiser aren't optimizing their performance by adding $5 worth of plastic to their sterndrives? Just think what a huge marketing advantage they would have if they could do that.

No you missed the point....probably because you are so fixated on the belief that they are no good, so you didn't read the intent of the post.

If the hulls were designed for performace, they wouldn't need the "fin". The fin's purpose is to correct what the hull designers missed. Fins were not around in the old days because old boats performed very well with low HP.

I'm so hyped about fins because in my personal experience, its one of the best marine inventions in recent decades.

This forum is for expressing opinions. My data backs up my opinion. If your data is contrary, so be it. But it would be a shame for a novice to not try sliced bread because some didn't like it.

 

[bruceb58]

Re: Hydrofoils on lower units?

No you missed the point....probably because you are so fixated on the belief that they are no good, so you didn't read the intent of the post.

If the hulls were designed for performace, they wouldn't need the "fin". The fin's purpose is to correct what the hull designers missed.

The hull designers are the same company as the sterndrive designer in many cases. You don't think they could get together and figure out a way to add something to their drive to boost performance?

Maybe you missed my post about my Doel-Fin experience and why I removed it. If I was fixated about it being bad, I wouldn't have run with it at all.

 

[ondarvr]

Re: Hydrofoils on lower units?

Those points you refer to as opinions are more like facts.

A foil's real purpose is to stop the prop from sucking air, that it may also help by dragging in the water is a side affect that people feel they can depend on. Sort of like beating on something with a crescent wrench, for some applications the hammer/wrench may work well, I've done it many times in the past.

Some IO's and OB's respond terribly to foils, just do a search on the subject, I've had negative results myself when the setup wasn't correct.

This is why I say people need to understand what it's designed to do before making a judgment on whether they work or not. Because one person had a great result or terrible one needs to be examined closer as to the exact reason why, and that no blanket statement can be made.

 

[thumpar]

Re: Hydrofoils on lower units?

If your prop is sucking air you have a other issues.

 

[V153]

Re: Hydrofoils on lower units?

Fwiw nobody I know runs a doelfin, etc on an IO. To no ill effect ...

 

[rallyart]

Re: Hydrofoils on lower units?

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.

Although the foil is in the same location as the AV plate most actually act as a 'foil' by providing a hydrodynamic lift with water flowing over and under the shape. This happening at the stern causes lift to contract the water pressure on the bow and torque affect of the prop being below the center of mass for the boat that lead to a very bow high pitch. By maintaining a flatter pitch on the hull the lift has the net result of a faster hole shot. Once on plane most foils will still be primarily immersed in the water. At high planing speeds some will be running on the surface.

If you were to ride in the Grew 185 that I had a Stingray foil on you would find that at a fast idle of 1100 RPM the foil was creating enough lift that the pitch of the hull would be more bow down than while it was floating at rest. The pitch at below planing speeds would be as much as 20? less than without the foil on it. None of this is in anyway affected by any cavitation condition with or without the foil. On this particular hull trim tabs would be effective and also provide some list control, but the foil was an inexpensive remedy that dramatically improved the capability of the boat for all tow sports and it's safety and speed when travelling in storms. I spent $50 to achieve that and it has been repaid by fuel savings many times over due to the lower time I have spent running full throttle while trying to get up on plane.

I believe that helm adjustable trim tabs are the most effective means of controlling the attitude of a boat and often recommend them. However most foils do act as a hydrodynamic lifting device and are relatively cheap. They are a patch however. Some people do see less cavitation at high speed with foils but as stated by others here it might have a negative affect. On my Grew it eliminated cavitation on high speed turns.

With a foil on the Grew my starts are much quicker with a 3 blade 21" SS prop than they are with a 5 blade 17" SS prop and no foil. I use a 4 blade ventilated 21" SS prop on the boat as the optimum prop for what the boat is used for. The negative I have with the foil on the Grew is that steering is stiffer when towing at and high speed. It would be better if I had Bennett tabs on but it is a 34 year old boat and I don't wish to spend the money on that boat.

 

[ondarvr]

Re: Hydrofoils on lower units?

IO's are designed to work on many different hulls, with many different motors, for many different purposes, so they can't be designed around one hull and motor combo. The hull designs are also a compromise, people want the most space they can get in a boat of XX length, so the best length to width ratio for performance is rarely used, this can results in poor characteristics when getting on plane, plus poor handling in many other areas.

If you want a very good hull for top speed, you can get it, if you want very good handling, you can get it, if you want to get on plane quickly, you can get it, if want to carry 10 people in comfort, you can get it, if you want to crash through waves at 60mph you can get it, if you want great MPG, you can get it, if you want perfect fit and finish, you can get, if you want an affordable price, you can get it. You are not going to get all these things in one package though.

 

[JimS123]

Re: Hydrofoils on lower units?

Those points you refer to as opinions are more like facts.

A foil's real purpose is to stop the prop from sucking air, that it may also help by dragging in the water is a side affect that people feel they can depend on. Sort of like beating on something with a crescent wrench, for some applications the hammer/wrench may work well, I've done it many times in the past.

Some IO's and OB's respond terribly to foils, just do a search on the subject, I've had negative results myself when the setup wasn't correct.

This is why I say people need to understand what it's designed to do before making a judgment on whether they work or not. Because one person had a great result or terrible one needs to be examined closer as to the exact reason why, and that no blanket statement can be made.

Well come on now, don't add logic and common sense to the discussion,.....LOL.

Yeah there are some cases where they don't work...I had an overpowered tinny that got squirrely with one on it.

But there are many cases where they can significantly improve the boating experience. The naysayers won't agree to that though.

I just wish I had the patent rights - I'd be retired now and driving a Rybovich....and I guarandamtee you THEY don't need a fin!!!

 

[thumpar]

Re: Hydrofoils on lower units?

IO's are designed to work on many different hulls, with many different motors, for many different purposes, so they can't be designed around one hull and motor combo. The hull designs are also a compromise, people want the most space they can get in a boat of XX length, so the best length to width ratio for performance is rarely used, this can results in poor characteristics when getting on plane, plus poor handling in many other areas.

If you want a very good hull for top speed, you can get it, if you want very good handling, you can get it, if you want to get on plane quickly, you can get it, if want to carry 10 people in comfort, you can get it, if you want to crash through waves at 60mph you can get it, if you want great MPG, you can get it, if you want perfect fit and finish, you can get, if you want an affordable price, you can get it. You are not going to get all these things in one package though.

And a $5 piece of plastic is not going to make it all things.

 

[ondarvr]

Re: Hydrofoils on lower units?

Although the foil is in the same location as the AV plate most actually act as a 'foil' by providing a hydrodynamic lift with water flowing over and under the shape. This happening at the stern causes lift to contract the water pressure on the bow and torque affect of the prop being below the center of mass for the boat that lead to a very bow high pitch. By maintaining a flatter pitch on the hull the lift has the net result of a faster hole shot. Once on plane most foils will still be primarily immersed in the water. At high planing speeds some will be running on the surface.

If you were to ride in the Grew 185 that I had a Stingray foil on you would find that at a fast idle of 1100 RPM the foil was creating enough lift that the pitch of the hull would be more bow down than while it was floating at rest. The pitch at below planing speeds would be as much as 20? less than without the foil on it. None of this is in anyway affected by any cavitation condition with or without the foil. On this particular hull trim tabs would be effective and also provide some list control, but the foil was an inexpensive remedy that dramatically improved the capability of the boat for all tow sports and it's safety and speed when travelling in storms. I spent $50 to achieve that and it has been repaid by fuel savings many times over due to the lower time I have spent running full throttle while trying to get up on plane.

I believe that helm adjustable trim tabs are the most effective means of controlling the attitude of a boat and often recommend them. However most foils do act as a hydrodynamic lifting device and are relatively cheap. They are a patch however. Some people do see less cavitation at high speed with foils but as stated by others here it might have a negative affect. On my Grew it eliminated cavitation on high speed turns.

With a foil on the Grew my starts are much quicker with a 3 blade 21" SS prop than they are with a 5 blade 17" SS prop and no foil. I use a 4 blade ventilated 21" SS prop on the boat as the optimum prop for what the boat is used for. The negative I have with the foil on the Grew is that steering is stiffer when towing at and high speed. It would be better if I had Bennett tabs on but it is a 34 year old boat and I don't wish to spend the money on that boat.

A foil has nothing to do with cavitation, it's ventilation that it helps to prevent, which is what you are talking about.

The foil shape has little to do with raising the stern, it has more to do with a large flat plate being force through the water at an angle. And again, the foil should not be dragging in the water at speed, the fact that many are dragging means there are opportunities for negative results. When a crescent wrench, I mean foil is dragging in the water at speed it means it's not being used to it's best potential, which is difficult to do on an IO. Yes the crescent wrench, (oops, sorry) foil is working in some fashion to achieve the desired result, but it doesn't mean the tool is being used as it should be, or that a different tool may not perform much better. In fact if the foil was just a flat piece of metal there would be less drag and the possible loss of speed would be lessened.

 

[ondarvr]

Re: Hydrofoils on lower units?

And a $5 piece of plastic is not going to make it all things.

And did I ever at any time recommend a foil on an IO?? I think I have been saying trim tabs are a much better option.

 

[ondarvr]

Re: Hydrofoils on lower units?

If your prop is sucking air you have a other issues.

There can be many reasons for ventilation, we are talking about one reason, the prop being close to the surface. Other reasons deserve a different thread.

 

[thumpar]

Re: Hydrofoils on lower units?

And did I ever at any time recommend a foil on an IO?? I think I have been saying trim tabs are a much better option.

I never said you did. I was adding to it.

 

[ondarvr]

Re: Hydrofoils on lower units?

I never said you did. I was adding to it.

OK, the responses have been coming rapid fire and I've been ducking and returning fire as fast I as I can, sometimes not aiming at the correct target.

 

[rallyart]

Re: Hydrofoils on lower units?

Sorry, the tabs are flat pieces of metal, or plastic that the water pushes up on. The foils that I have used work as foils at speeds where they are completely immersed in the water, which on the one that I used as an example includes 15 MPH driving through 3' waves and at 8 MPH on flat water and in many other situations. On that boat there is no ventilation to the prop from the surface while it is climbing, laboriously, up on plane with or without the foil.

Oh, and it also makes a handy and sturdy step to help climb back into the boat. Overall I like my inboard better as it does not need the foil to get out of the water. (But it's hull has no design issue for the use I put it to)

I do admit to using a Crescent wrench as a hammer but I also have a wide selection of actual hammers in my tool box that, I should mention, have cost me much more than my plastic crescent wrench foil. Crescent is doing a lot of brand expansion right now. Perhaps they will come out with a C-foil as a quick fix for hulls used for more than they were built for.

 

[NYBo]

Re: Hydrofoils on lower units?

A hydrofoil gave me male pattern baldness.

 

[ondarvr]

Re: Hydrofoils on lower units?

Sorry, the tabs are flat pieces of metal, or plastic that the water pushes up on. The foils that I have used work as foils at speeds where they are completely immersed in the water, which on the one that I used as an example includes 15 MPH driving through 3' waves and at 8 MPH on flat water and in many other situations. On that boat there is no ventilation to the prop from the surface while it is climbing, laboriously, up on plane with or without the foil.

Again. The foil at that same low speed does the exact same thing, the foil shape has little to do with it. Although the marketing department has done a good job of convincing you otherwise. Look at the stock AV plate on any motor, it's flat.

Plus you seem to think that from what I said it has to ventilate without a foil, it doesn't, most props don't because when rigged new the motors are rarely mounted for best performance, which means they tend to be on the low side, IO's included. slightly low is better in a one size fits all world.

Take that same motor and mount it higher and the foil comes into play in preventing ventilation. Mounting the motor higher may also increase performance.

 

[rallyart]

Re: Hydrofoils on lower units?

You seem to think that a foil is primarily an anti-ventilation plate and I've tried to show you that it's not. Oh well, I guess you'll keep your beliefs and I'll keep mine. It's all a good discussion.