DIY hydrofoil/stabilizer for small outboards on underpowered boats...... success!

[redfishsc]

Motor-- 1986 Johnson 6hp, clean carb, now running quite well aside from an exhaust leak.

Boat- 14' Starcraft SS (1974) loaded with me (240lb) at the transom, son (50lbs) up front, and maybe 120 pounds in gear/cooler.

Problem-- Boat won't plane out at WOT, but this is to be expected on such a small motor with such a huge rear end sitting in the back.

Solution-- Really just for fun, fabricated a DIY hydrofoil/stabilizer (whale tail).

Material-- offcut of 1/4" Starboard HDPE plastic.

Cost-- $1 for the screws. Starboard stuff was free.

Time-- 1 hour.

Tools--- bandsaw (or jigsaw would work), power sander, drill and associated bits.




I'm posting this at a bit of a risk, because this is a fairly hasty and half-cocked idea that I just threw together yesterday morning before an afternoon fishing trip. I took a scrap of HDPE 1/4" thick, bandsawed out a random whale-tail pattern of a random size, and used an air sander to sand a very steep bevel (but otherwise a random pitch) along the underside of the whale-tail curve (for proper lift, like on airplane wings). Also note, the motor is a long-shaft but it sits on a jack-plate. It needs to be up another 2 inches or so, but I'm not moving the jackplate because I'm very soon upgrading to a larger (and more reliable) motor.


I hastily notched it out to fit around the foot over the AC plate, not worried about "exact fit" by cabinet maker standards (my trade). Shot a couple self-tapping screws through it, left them long (didn't bother to clip them short).


Result--- Worked like a champ, much to my utter shock.



Before I installed this, the motor (once I had all the throttle and carb issues worked out) would not plane the boat unless I sat in the middle with a really long tiller extension. Even then I only got 8.3mph (GPS). Transom ran fairly deep in the water as it plowed through, and the bow was lifted a little more than I prefer. The gunwales in the rear were only a few inches from the water surface, and if I came to a sudden stop, I risked washing a wave onto the back of the motor and perhaps (in windy conditions) even getting some over the transom.


After I installed this, the transom was FAR higher in the water, essentially sliding across the surface on plane even at about half throttle (6mph on the GPS). Max speed went from 8.3 to 10.0, which isn't exactly fast, but is a huge improvement when your favorite fishing hole is 3-4 miles from the boat launch. The boat didn't chine walk in the least, and no porpoising. This was in optimal conditions (no wind, flat water) and in slight wind/chop (5-10mph cross wind). Those of us with small underpowered boats would benefit greatly with one of these.

I also notices that steering was a bit slower (didn't seem to turn as sharp of a radius) BUT it turned much more stable without jacking the boat off to one side as steep as it would without the stabilizer. At slower speeds (fast idle) it turned as sharp as you needed it to (no change).



Here are the pics. Please note that this particular version was done in a bit of haste, and I didn't even bother removing the sharpie marker drawings on the back where I had used the plastic as a template to lay something else out. Since this worked, I'm going to pull it off, clean it up, and install it using stainless steel bolts.

http://i347.photobucket.com/albums/... Pics 2012 and later/CAM00190_zps49b4df8f.jpg

http://i347.photobucket.com/albums/... Pics 2012 and later/CAM00191_zps88353c17.jpg




One final question. I had the trim set to the lowest setting (closest to the transom). I am assuming that if I lift the trim up a couple of notches, it changes the angle of this whale tail and will probably cause me more headache pulling down the transom (ie, the opposite problem). What do you think trimming it up a little would do?

 

[redfishsc]

Re: DIY hydrofoil/stabilizer for small outboards on underpowered boats...... success!

 

[kfa4303]

Re: DIY hydrofoil/stabilizer for small outboards on underpowered boats...... success!

Hi red. Neat little project. I like anything DIY, but "whale tails" are kind of a gimmick and when they do "work" it's only because they're masking a bigger problem. There's a reason you'll never see a pro using one. You could probably get just as good, if not better performance from the motor be setting it to the proper height and trim setting. Ideally, the anti-vent plate that the whale tail is currently attached to, should be level or slightly higher than the keel of the boat. From the looks of the pics, the motor appears to be sitting too low which creates excess drag and impairs its performance. I would try to raise the motor, if at all possible. I put my old '66 20hp Johnson on a transom riser and it made a huge difference. You'll also want/need to experiment with different trim settings. The motor should not be all the way in towards the transom, or all the way out. Most folks have their motor at the 2nd-3rd position out from the transom, but you'll just have to experiment to find the best set up.

 

[redfishsc]

Re: DIY hydrofoil/stabilizer for small outboards on underpowered boats...... success!

kfa, I am aware of all of what you said, and I do appreciate your input here. Thanks for the JPEG about boat trim also.

However, do these statements actually work in regards to underpowered boats? For example, I'm fairly skeptical that I will ever get this boat to plane off with my 240lb beef sitting in the back, regardless of how high the motor sits and how I trim it. Right now, the boat won't come anywhere close to planing off with me in the back and without the whale tail. Add the whale tail, I gain almost 2mph and I go on plane at around half throttle.

Right now the anti-vent plate sits about 1.5 to 2" below the keel, which I am aware is not optimal. The pic exaggerates how low this is sitting because of the angle of the picture and the angle the motor is kicked out. Note, I am using a jack-plate, but if I raise this any higher, I will have too few bolts holding the jack to the transom and I wouldn't trust it. I need to rebuild the transom in the next year anyhow and I will remedy this, fitting the transom to my motor.

I will ask you this (honestly, not sarcastic) do you think I could actually plane out a semi-V with a 6hp, with my weight in the back at 6mph like I am currently getting with the DIY tail? I honestly think (though I cannot promise) that the addition of the whale-tail is doing something that I cannot do without it on such a small motor.

Hi red. Neat little project. I like anything DIY, but "whale tails" are kind of a gimmick and when they do "work" it's only because they're masking a bigger problem.

If it solves the problem as-is, and fixing other issues would take a lot more time, and the solution (whale-tail) doesn't cause other problems, is there any need in reconfiguring how I have it (other than the risk of hitting a rock/stump with the prop 2" lower than it needs to be)?

There's a reason you'll never see a pro using one.

Well largely because you won't ever see a pro using an undersized worn out motor on a beat up old aluminum Starcraft lol


I do appreciate your help.

 

[JimS123]

Re: DIY hydrofoil/stabilizer for small outboards on underpowered boats...... success!

"whale tails" are kind of a gimmick and when they do "work" it's only because they're masking a bigger problem. There's a reason you'll never see a pro using one. View attachment 186194

Oh how I hate to hear about that "problem". Sounds like you need to correct a "problem" and then the whale tail won't be needed.

So take a small tinny with the gas tank and battery installed right on the transom (where they are supposed to be because that's where the manufacturer puts the wiring and the hold-downs for the tank) and then add a fat guy (like me) that goes out by himself and now we got a "Problem". OK, go on a diet, extend the wires and put all the crap up in the bow where they are in the way. Viola, the boat planes OK now. Or, put on a DoelFin and the "problem" is solved for 39 bucks and no holes in the boat!!!

Funny thing, though, in the old days wooden boats didn't need add-ons and they ran fine without them, even though they were set up with a tiller and all the weight aft as well. So, maybe the "problem" is the way they design the boats today. I know from my personal experience, I never "needed" a whale tail until I bought my first fiberglass boat.

Ohhh nooo, of course you never see a "pro" using anything like it. They got a $50,000 boat for free, with all the bells and whistles and HP and tabs and power trim and jack plates, ad nauseum. The reason they got all the bells and whistles is because the Bennetts and Rangers gave them away just for the advertising. Poor old Mr Doelscher wasn't rich and had to expand his business by word of mouth alone.

I have nothinga against manual or electric tabs. They have a place for sure. But if you're a fat guy with a tiller that likes to sit next to the tank, you can't beat the tail!

PS - to the OP, ignore any negative comments.....you done good!

 

[redfishsc]

Re: DIY hydrofoil/stabilizer for small outboards on underpowered boats...... success!

Lol Jim I didn't take kfa as being negative (at least I didn't take it as being snide). Criticism of the stabilizer won't hurt it's feelings and keep it from working.


I can only restate that I'm very skeptical that I will ever get this boat to plane with a 6hp motor as predictably and quickly, using only trim and height, as I can with the stabilizer on it.

I'll bust the budget and buy $6 worth of stainless steel bolts to hold it on next week and I'm taking it offshore to troll for marlin. Or not. But there are catfish waiting to be caught and they won't wait for me to fiddle around with fine tuning the exact position of the outboard when I already have it planing at half throttle, when before I couldn't even plane it at full throttle.

 

[jestor68]

Re: DIY hydrofoil/stabilizer for small outboards on underpowered boats...... success!

Best invention since the mouse trap!

I discovered the Doel Fin in 1985 and have used it on every boat(4)since, including the current 23 ft boat you see to the left.

 

[ondarvr]

Re: DIY hydrofoil/stabilizer for small outboards on underpowered boats...... success!

One of the arguments I hear against Fins, foils, or whatever you want to call them, is that if they were of any value motors would come stock with them…..well they do, all a fin is is an oversized Anti ventilation plate (it’s not cavitation plate). Although in this application the OP is relying on it to drag in the water and force the rear of the boat up (and it is working), the real value is no different than that of the stock AV plate, it just helps prevent the prop from sucking air from the surface. This possibly allows you to raise the motor higher than without it and you may gain a little in performance.

 

[fishrdan]

Re: DIY hydrofoil/stabilizer for small outboards on underpowered boats...... success!

Right now the anti-vent plate sits about 1.5 to 2" below the keel, which I am aware is not optimal.

Is that a long shaft motor (20") on a 14' tinny, that's most likely part of the issue. I agree that moving weight forward (fuel tank and batteries) will help revolve planing issues.

I have a Stingray on my jon boat's 7.5hp outboard, love it.

One of the arguments I hear against Fins, foils, or whatever you want to call them, is that if they were of any value motors would come stock with them?..well they do, all a fin is is an oversized Anti ventilation plate (it?s not cavitation plate).

The same could be said for trim tabs, not many boat manufacturer's have those as original equipment, but they do work well. I think outboard manufacturers don't advise the use of foils as it's added stress on the AV plate, a part that's not designed for the additional stress of a foil. It could be quite expensive for them if they had to warranty broken AV plates, by saying it was OK to use a foil on an outboard.

 

[JimS123]

Re: DIY hydrofoil/stabilizer for small outboards on underpowered boats...... success!

View attachment 186380 Best invention since the mouse trap!

I discovered the Doel Fin in 1985 and have used it on every boat(4)since, including the current 23 ft boat you see to the left.

That's my story as well. In fact, I've put them on both my son's boats and they love them too.

 

[JimS123]

Re: DIY hydrofoil/stabilizer for small outboards on underpowered boats...... success!

One of the arguments I hear against Fins, foils, or whatever you want to call them, is that if they were of any value motors would come stock with them…...

First of all, some boats just don't need them, while others can't live without them. The latter being stern heavy boats, cuddies and tillers. If the manufacturer's put them on very motor, then the boats that don't need them would be stuck with them.

Or could it be that Mr. Doelscher just invented a better mousetrap before anybody else thought about it? Or, could it be that his patents prevented early copying? Or, is it that the motor manufacturer's won't admit that somebody else perfected their product?

The originals were advertised as having an aerodynamic shape, or custom CAD designed by an Engineer, etc. Seems to me if a simple flat plate works too, then maybe the fact that its a baffle is the key, not how its shaped...

 

[redfishsc]

Re: DIY hydrofoil/stabilizer for small outboards on underpowered boats...... success!

Is that a long shaft motor (20") on a 14' tinny, that's most likely part of the issue. I agree that moving weight forward (fuel tank and batteries) will help revolve planing issues.

Yes, it is, but I do have it on a jackplate as high as I can safely move it at the moment. The transom could use a rebuilding and the motor could use a replacing, so both will happen in the near future. If I put a 15hp on here, I probably won't need a fin like I do now.

The motor needs to be about 1.5" to 2" higher to get the anti-vent plate even with the keel. Don't let the angle of the pics fool you, it looks WAY lower, but it isn't.

Seems to me if a simple flat plate works too, then maybe the fact that its a baffle is the key, not how its shaped...

Pretty much. However, it's hard to see in the pics, but I have a bevel sanded around the bottom (underside) of the leading (curved) edge of the fin to provide lift.


I cannot compare this to a Doel-fin since I've never used one. I expect the Doel would work a bit better since it's actually designed by some form of research. Mine is literally just a near complete shot in the dark. I suppose even blind squirrels find a nut once in a while because I'm thoroughly pleased at how this boat performs with the fin. I'm NOT impressed with it without the fin.

 

[saumon]

Re: DIY hydrofoil/stabilizer for small outboards on underpowered boats...... success!

To solve your problem, you should have got a new 15hp short shaft for $3000 but, for $1 and some spare time, I said "well done"!

I, too, love those DIY projects. I doubt that, in that "speed" range, the shape and beveled edge add anything and I suspect that, since you're probably tilted fully in, it simply act as a giant trim tab. A real Doel Fin is out of the water at planning speed but, since you don't plane, yours, being at an angle when underway, provide stern lift.

If you can take 5 mins, simply cut a square or rectangular shape out of 1/2" plywood with the same notch and bolt it to the AV plate. I bet it will work even better cause I'm pretty sure the one you make, being only 1/4", flex a lot, thus reducing his efficiency.

 

[redfishsc]

Re: DIY hydrofoil/stabilizer for small outboards on underpowered boats...... success!

To solve your problem, you should have got a new 15hp short shaft for $3000 but, for $1 and some spare time, I said "well done"!

True, except that I'm not spending $3k on anything on this boat lol. Maybe $1K for a good used one (very soon).

I, too, love those DIY projects. I doubt that, in that "speed" range, the shape and beveled edge add anything and I suspect that, since you're probably tilted fully in, it simply act as a giant trim tab. A real Doel Fin is out of the water at planning speed but, since you don't plane, yours, being at an angle when underway, provide stern lift.

Probably true, I wouldn't doubt it.

If you can take 5 mins, simply cut a square or rectangular shape out of 1/2" plywood with the same notch and bolt it to the AV plate. I bet it will work even better cause I'm pretty sure the one you make, being only 1/4", flex a lot, thus reducing his efficiency.

I thought about that but won't use plywood because it wouldn't last long. We do have the occasional scrap of 3/4" thick Starboard I could use if needed. I probably won't bother though because I'm pretty sure 10mph with my heft in the boat is about all I can really ask for anyhow lol.

 

[barrynfla]

Re: DIY hydrofoil/stabilizer for small outboards on underpowered boats...... success!

Well, I have a SE Sport 200 fin on my boat and I'm happy. You did a great job with a DIY project. Now get out there and catch some of those bull reds your state is known for! :joyous:

 

[redfishsc]

Re: DIY hydrofoil/stabilizer for small outboards on underpowered boats...... success!

Well, I have a SE Sport 200 fin on my boat and I'm happy. You did a great job with a DIY project. Now get out there and catch some of those bull reds your state is known for! :joyous:

I have every intent of it. Except I don't really want the bull reds, you have to release them. Slot reds, 15-24", now you're talking.

 

[pecheux]

Re: DIY hydrofoil/stabilizer for small outboards on underpowered boats...... success!

redfishsc
This is the best I have seen with a 6 hp and a 14 footer since slice bread ... LOL I own 2 boats and 3 OB's One boat is a 12 footer alu LUND with a 6 hp mounted on it. It wont plane with me in it sitting out back even with the gas tank way up front. It's will only plane if I move to the middle seat with while motor is at WOT. So now that I have read your story I wont sleep tight until something like this is mounted on both my 6 hp and my 9.9 which I use on a 14 footer fiber since it hardly planes with 2 passengers LOL But not being equiped for this sort of DIY jobs I wondering if I can buy one already made and if it will get me the same results. I have seen one for the 9.9 and it seemed a bit smaller than what you have on that motor. Tx for the infos.

 

[redfishsc]

Re: DIY hydrofoil/stabilizer for small outboards on underpowered boats...... success!

Yeah there are versions of this you can get online but I cannot say anything about their performance. The Sting Ray Jr and also I think the Doel Fin.


You do have to screw them/bolt them down to your anti-vent plate, so have a good drill handy. I prefer to pre-drill and lube the drill bit with a touch of random oil (WD40, 2 cycle or 4 cycle oil, any flavor will work). It's not hard, most plates are fairly thin metal anyhow.


Mine is way bigger than most of the jr-sized hydrofoils I've seen and it was a complete shot in the dark. If you know anyone that has any skills whatsoever, and can find a reasonably affordable source of 1/4" thick polyethylene (Starboard), then you could easily replicate this. But maybe check out the Stingray Jr.

 

[pecheux]

Re: DIY hydrofoil/stabilizer for small outboards on underpowered boats...... success!

REdfishsc: I checked both the Sting junior and the Doel fin. Ordered the Doel fin because is seemed to be narower and thus would cause less drag. I"ll have a chance to compare with the Junior that they allready had in stock ... and choose then. Tx

 

[pecheux]

Re: DIY hydrofoil/stabilizer for small outboards on underpowered boats...... success!

Final choice = the Junior. Many know already I own 2 OB's an OMC 9.9 and a Force 15 hp. Mounted on a 14 footer fiber and with 2 passenger onboard the 9.9 will hardly plane an will labor keeping on plane with a head wind. The 15 hp cures all that but is a noisier motor and will not idle smooth with fishing. Now I installed the hydrofin Jr. on the 9.9 and the result is this: Takes the boat on plane almost as fast as the 15 hp, top speed has noticebly increased, and will keep the boat on plane with reduced trottle. Basicialy if behaves like the 15 hp. It is important to have the trim ajustement fully inward to get those results. I could not be happier. Cheers.
Nb: I know the post is not old, but thought my feedback could help those wondering about the impact of installing a hydrofoil on a smalll OB"