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Author Topic:   Hydrofoil on outboard
baileyoconnell posted 06-07-2004 02:19 PM ET (US)   Profile for baileyoconnell   Send Email to baileyoconnell  
I have a 1989 Outrage with a 130 Yamaha.

I presently do not have trim tabs but am considering hydrofoils (sting ray) and stabilizers on my outboard.

Any suggestions or comments - pro or con?


JohnJ80 posted 06-07-2004 04:51 PM ET (US)     Profile for JohnJ80  Send Email to JohnJ80     
Tell us what problem you are trying to solve with this.

Is your boat porpoising? Lack of trim range? What is the vertical height of the motor (what hole is motor mounted in)?

All depends on what you are trying to do.


acseatsri posted 06-07-2004 08:08 PM ET (US)     Profile for acseatsri  Send Email to acseatsri     
You'll be to get on plane faster and at a slower speed.

In rough seas, you won't take as much of a pounding.

You'll lose about 3-4 mph of top end

You'll have to drill holes in your anti-cavitation plate.

If the motor is mounted too high, the anti-cavitation plate and fin will be out of the water and will be ineffective

kingfish posted 06-07-2004 08:56 PM ET (US)     Profile for kingfish  Send Email to kingfish     
acseatsri (et al), with all due respect:

While I am not personally familiar with Stingray hydrofoils, I am very familiar with DoelFins, and I believe strongly that the concept regards the following is the same.

1. You won't lose top end if the motor is mounted at the correct height, because the foil is mounted on the anti-ventilation (cavitation) plate, and the a. v. plate should be above the surface of the water at high planing speeds.

2. That is why foils don't do anything for porpoising; what they do accomplish is to help get up on plane more quickly and easily, and to maintain plane at loere speeds.

3. If the motor is mounted low enough that the foil adversely affects top end, or has any effect at all at high planing speeds, then the motor is mounted too low, and that in itself would be the likely reason the boat porpoises (if it does).

acseatsri posted 06-07-2004 09:52 PM ET (US)     Profile for acseatsri  Send Email to acseatsri     
The hull has a "V" to it, and the Doelfin has a slight "V" in the opposite direction of the boat bottom, so the foil may or may not be in the water at higher speeds. It all depends how high the engine is mounted and what you're trying to accomplish with the fin. If most of your boating is in flat water, then the engine can be mounted higher without adverse effects. If you frequently run in heavier seas, mounting the engine higher will give you more ventilation and loss of power in such conditions. There's no right or wrong here- it's a matter of personal preference and how and where you primarily operate. As JohnJ80 said, it all depends on what he's trying to do.
I recently replaced my Doelfin and transom wedges with trim tabs. My primary objective was to be able to run on plane at slower speeds, which can be accomplished with either choice. The oversized M120 tabs are MUCH more effective than the fin and wedges. The wedges were added due to a late-80's OMC which does not have enough negative trim range to suit most needs.
JohnJ80 posted 06-07-2004 10:47 PM ET (US)     Profile for JohnJ80  Send Email to JohnJ80     
A foil on a lower unit can absolutely effect porpoising ESPECIALLY if it is mounted so it runs at the surface of the water. It will then provide active damping on the porpoising oscillation for the portion of the cycle in which the bow rises (consequently pushing stern down). In this case, as the bow rises, and the stern is pushed down, the fin then generates lift which resists this effort (damps it) and trys to move the fin back to the surface.

The causes of porpoising are many and widely vary from boat to boat. One common reason - but by no means the only one - is a very stern heavy boat. Some other reasons can be flex in the hull, improper hull shape, too much weight in the bow, improperly mounted vertical height of the motor (usually too low, but can also be too high), improper trim angle of thrust to direction of forward travel (lower unit trimmed up too high), and improper prop choice.

There are a number of ways to solve this problem - if this is the problem that the original poster actually has (they didn't say).

Varying the vertical height of the engine also can dramatically change the hyrodynamic drag. So, if too low, it can cause big performance losses, difficulty in planing, porpoising, harsh ride etc..

So, we need to know more before we can recommend what to do with his motor in terms of height placement and adding fins etc...


kingfish posted 06-07-2004 11:45 PM ET (US)     Profile for kingfish  Send Email to kingfish     
I'm not aware of any Doelfin installation in which there is a "V"; upwards or downwards. The bottom surfaces are basically flat extensions of the a. v. plate and the upper surfaces are foiled like the leading edges of an airplane's wing and consequently provide lift when they are below the surface of the water.

I suppose theoretically Doelfins could be set to run right at the surface, and theoretically could affect porpoising, but at what speed? The rise of the water behind the transom is a function of the speed of the boat, the higher the speed, the less rise. If the motor height is set such that the fin as at the surface at WOT, then it is penetrating the surface at all speeds below that and under those circumstances, it certainly would affect speed. The problem is that when the fin is running below the surface at high speeds, besides adversely affecting top end, it carries a great potential for instability if it "hooks up" at the wrong time, especially if the motor isn't trimmed exactly correctly at any given time.

Doelfins are great for getting up on and keeping plane and I've used them successfully on five different boat and motor setups I can think of, but I learned that they are not the tool to try to use at and for higher speeds. You are then into trim tabs, which are a lot more money, but in the progression of effect, trim tabs do everything foils do, plus control porpoising at high speeds with no potential for instability, plus offer lateral trim. I have tabs on my Outrage 22 and my Parker 2520, and I love them.

Bailey, we need to know more about the size of your boat, the speed at which you feel you need some adjustment, and why (what don't you like about your boat's current performance?).

And if you are differentiating between hydrofoils and stabilizers, what are stabilizers?

Tom2697 posted 06-08-2004 12:42 AM ET (US)     Profile for Tom2697  Send Email to Tom2697     

I run an 1989 Outrage 18 with a Suzuki 140 on the back. I had a Hydrofoil on the 150 Evinrude that I replaced and decided to transfer it to my new motor. I didn't NEED it on my motor for most conditions but it does help the boat plane faster when waterskiing or with a heavy load. I installed it mainly because I run in a lot of rough water. With conditions such as this, it helps immensely because it will help keep the bow down and it allows for a smoother, more enjoyable ride for everyone aboard. Rarely do I run top speed but I did notice a slight difference (1-2 knots) at WOT. If you are looking for a product to improve the ride at cruise speed, I would highly recommend installing any one of the above mentioned products.


JohnJ80 posted 06-08-2004 11:00 PM ET (US)     Profile for JohnJ80  Send Email to JohnJ80     
There is a very interesting thread in the performance forum on exactly this subject.



kingfish posted 06-09-2004 07:55 AM ET (US)     Profile for kingfish  Send Email to kingfish     

Are you out there??

JayR posted 06-09-2004 02:19 PM ET (US)     Profile for JayR  Send Email to JayR     
I've got one and I love it! The ride in rough seas is so much better than without it. In fact, I would have sold the boat if not for the Doel-fin. It made that much of a difference.
JohnJ80 posted 06-09-2004 04:12 PM ET (US)     Profile for JohnJ80  Send Email to JohnJ80     
JayR - tell us about the vertical positioning of your motor on the transom.


JayR posted 06-10-2004 07:42 PM ET (US)     Profile for JayR  Send Email to JayR     
I have a 1968 Madetauk. The motor is a 1988 88SPL. It was installed 16 years ago by the dealer and it sits right on top of the transom. I've always been pleased with the performance. Never any issues except for running in big seas (5-10 feet). When coming over the to it would slap on the way down and rattle my teeth. It no longer does this since installing the Doel-fin. When the hull hits the apex of the wave, the bow would pivot forward. Now the back is held in the water and cannot pull loose. We are talking very slow speeds here.

When on plane, the fin is just above the water surface.

My ribbed sided '71 Outrage will get one before I even put it in the water for the first time. I love it that much.....

JohnJ80 posted 06-10-2004 09:49 PM ET (US)     Profile for JohnJ80  Send Email to JohnJ80     
I would still look at raising the motor. Sounds like it is down pretty far.

With that old of a motor, you probably are not heavy in the back end and the fin can generate enough lift to get to the surface.

Just raising the motor reduces the drag by a 3/4" high cross section of you lower unit (very significant amount). Might also help with that harsh ride in waves.


Boston Blackie posted 06-12-2004 12:47 AM ET (US)     Profile for Boston Blackie    
I have a 1989 22' Outrage with a Suzuki 200 on it and researched the hydrofoils extensively. I had one of the Doelfins on another 20' Outrage with a 150 Johnson and it worked well in heavy seas and got it on plane very quickly. I installed a Permatrim on my 200 and did not tell anyone. My wife and I went fishing off San Diego in a 2 to 3 chop and she commented how well the boat was riding. A few wweks later my son took the boat and told me the same thing. Before you make a decision go to the Permatrim website and research. I bought a model M9 and it was about $110.00, but well worth it. I immediatley noticed by fuel consumption diminished slightly also.
kamie posted 06-12-2004 01:00 AM ET (US)     Profile for kamie  Send Email to kamie     
correct URL for permatrim

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