Does a hydrofoil improve performance in small outboard application?

Jerry_NJ

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My recently purchased 14' Discovery Aluminum boat with a 9.9 hp 2-cycle Yamaha is slower than my 14' Aluminum Grumman with a 9.9 hp Evinrude. The Discovery while the same length is wider by 6" and deeper (20" stern) so it weighs more. The speed difference is not known as I have only a "eye ball" guess at speed, but I also note the Discovery at full throttle has a lot of cavitation churn behind the boat while the Grumman has more of a "rooster tail", of course there is no rooster tail but the Grumman develops the appearance of one forming.

I spoke with a fellow fisherman a couple of days ago at a local lake who has hydrofoils on his 9.9 hp 4 cycle Yamaha on a larger 17' Bass boat. He said it improves the performance including speed on his boat.

These are not expensive, under $100 for self installed. Are these devices really of value in the operation of low horse power outboard engines?

I am concerned too about having to drill 4 holes in the anti cavitation plate on the existing engine to install the hydrofoils... I have seen some that don't require holes as they are somehow clamped on.

Ideas and experience sought, thanks.
 

Bondo

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Re: Does a hydrofoil improve performance in small outboard application?

My recently purchased 14' Discovery Aluminum boat with a 9.9 hp 2-cycle Yamaha is slower than my 14' Aluminum Grumman with a 9.9 hp Evinrude. The Discovery while the same length is wider by 6" and deeper (20" stern) so it weighs more. The speed difference is not known as I have only a "eye ball" guess at speed, but I also note the Discovery at full throttle has a lot of cavitation churn behind the boat while the Grumman has more of a "rooster tail", of course there is no rooster tail but the Grumman develops the appearance of one forming.

I spoke with a fellow fisherman a couple of days ago at a local lake who has hydrofoils on his 9.9 hp 4 cycle Yamaha on a larger 17' Bass boat. He said it improves the performance including speed on his boat.

These are not expensive, under $100 for self installed. Are these devices really of value in the operation of low horse power outboard engines?

I am concerned too about having to drill 4 holes in the anti cavitation plate on the existing engine to install the hydrofoils... I have seen some that don't require holes as they are somehow clamped on.

Ideas and experience sought, thanks.

Ayuh,.... In some limited cases, ya they might help, but not as well as hull mounted tabs,....

This new boat has a taller transom, is the motor a longer shaft motor,..??

A whaletail won't fix a poorly set up boat,...
 

scipper77

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Re: Does a hydrofoil improve performance in small outboard application?

On small aluminum boats with no weight in the bow the advantage of a "whale tail" is that it helps you get on plane. I have found that in my 12' a 50# sandbag in the tip of the bow is all I need. In my 16.5' Sea Nymph I have a hydrofoil. The only way I could see a hydrofoil increasing speed is if your boat isn't able to plain out properly without help. Adding 50#'s of weight makes my 12 footer both faster and more stable because the 5.5 hp motor can't plane it without help. More stable because lowering the bow with weight raises the stern which sits too low with no ballast.

I'd say get to know the best way to set up your boat based on deficiencies and decide the best way to correct them. There is no universal answer as all boats are different. I hope my experience helps.
 

JimS123

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Jul 27, 2007
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6,152
Re: Does a hydrofoil improve performance in small outboard application?

It will likely help in your situation. Don't buy a clamp-on model. And don't worry about the holes - they can be filled if you dont like it.

Buy a DoelFin for $39.
 

roscoe

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Re: Does a hydrofoil improve performance in small outboard application?

Correct motor mounting height, and correct prop pitch, will do more than a foil.
Check those first.

How much hp is the boat rated for?

A foil will do more harm than good, if you don't have the motor mounted at the correct height.
 

Jerry_NJ

Petty Officer 1st Class
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Re: Does a hydrofoil improve performance in small outboard application?

The engine is a 1990 Yamaha 9.9 HP (the limit on the lakes I use) 2-cycle with a 20" shaft. The anti cavitation plate is about even with the bottom of the keel. The boat is rated at 30 HP max, the previous owner used a 25 HP, thus the old Yamaha has very few hours on it...runs great, and the impeller looks good.

My reference Grumman was operated with a deep cycle battery and a 40# thrust motor at or near the bow. I have not yet made arrangements to put the motor on the Discovery, but when I do that will add at least 50 pounds to the bow. I will try that before making any other changes. If I could get my wife to come along that would add a few more pounds forward, the amount is not a published number : )
 

ondarvr

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Re: Does a hydrofoil improve performance in small outboard application?

There are huge misconceptions of what a foil is actually good for and what it can do, people have preconceived ideas based on what some body's brother's second cousin repeated from what he could remember when talking to some drunk at a bar. Or they bolted one on with no adjustments done and headed for the water, used it for an hour and decided they are evil and have no right to exist.

Yes a foil can help increase top speed, not in all cases, and maybe very few, but used correctly on the right setup it may. They work by not allowing the prop to ventilate when mounting the motor higher, that’s it. When a motor is mounted higher (to a point) there are a few possible benefits, higher top speed, better MPG, better hole shot, better handling, less porpoising, etc. These are just possibilities and may not be the best way to get there, but can work. Sort of like a poor man’s hydraulic jack plate.

In this application, used as a large flat surface dragging it the water to possibly increase speed…. no…it possibly could, but only if it was raised to where it wasn’t dragging in the water at speed. It may help get the boat on plane whether raised or not, but would possibly decrease speed because of the drag if not mounted high enough. Dragging in the water at speed is where most of the negative comments come from, they can create odd handling on some boats, and at times can be scary.
 
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Jerry_NJ

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Re: Does a hydrofoil improve performance in small outboard application?

Thanks, I read another vote not to install a hydrofoil with a zero knowledge of all the particulars of performance.. heck!! I'm running a 9.9 HP, not a 150.. so the whole idea of putting the boat on a plane never entered my mind as even possible. I just noticed the Discovery didn't seem to put out the same top speed (maybe 10 mph?) fishtail as the Grumman. Again, what I mean is the Grumman also produced a lot of froth behind the boat but as the speed went up to its top the froth changed to more of a center with a wake departing from each side. I'll describe it as a weak fishtail.

I'll try moving some weight forward, this will also help stop water from splashing in the transom drain holes when the boat is still in the water - and there is some mild wave action. This has been discussed in a tread I started on "about construction of the transom".
 

Scott Danforth

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Re: Does a hydrofoil improve performance in small outboard application?

The hydrofoil improves the performance of the cash intake flow from the company that makes the unit you buy. That is the only guarantee on performance improvement.

regarding your two boats. you have 7 more square feet of wetted surface assuming the transoms were the same. since the new boat also has a taller transom, my guess you have closer to 14 square feet of wetted surface along with about 100 more pounds.

that being said, your new boat should be slower than your old boat with the exact same motor on it (neglecting any long-shaft vs short shaft issues with transom height).

Add on top of that Brand E vs Brand Y, plus not knowing if you have the right length lower on the new boat. you are trying to compare apples to pecans and expecting a sales gimick like a hydrofoil to help.

without us knowing the facts such as proper placement of the lower to the keel, etc. the best we can say is no it wont work given the larger and taller boat with the same hp. I would recommend a picture from the side showing the placement of the lower to the keel and transom.

ondarvr nailed the "potential" improvements that are a possibility as well as the reality that the extra wetted surface area will not help.
 

ssobol

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Sep 3, 2010
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Re: Does a hydrofoil improve performance in small outboard application?

Need to check that the engine is trimmed properly for the kind of operation you want. Adding a "whale tail" to an improperly trimmed motor set up will just make things worse.
 

JimS123

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Re: Does a hydrofoil improve performance in small outboard application?

The engine is a 1990 Yamaha 9.9 HP (the limit on the lakes I use) 2-cycle with a 20" shaft. The anti cavitation plate is about even with the bottom of the keel. The boat is rated at 30 HP max, the previous owner used a 25 HP, thus the old Yamaha has very few hours on it...runs great, and the impeller looks good.

My reference Grumman was operated with a deep cycle battery and a 40# thrust motor at or near the bow. I have not yet made arrangements to put the motor on the Discovery, but when I do that will add at least 50 pounds to the bow. I will try that before making any other changes. If I could get my wife to come along that would add a few more pounds forward, the amount is not a published number : )

You add extra weight to a pickup truck in the snow to keep traction on the rear wheels. Add weight to a boat only slows you down. If weight DISTRIBUTION is the issue move things around.

My 2 tinnies (one with a 9.5) suit me quite well to have the tanks and batteries aft where they are out of the way. No clutter forward or midship to get in the way. As a matter of fact, the Starcraft was designed that way, with battery box and tank platform right on the transom. Add to that my ever gaining weight and I'm stern heavy.

A Doelfin solved the problem in both boats, with no issues. Yeah, some adjustments we required, but a piece of cake. Due to the transom design (only 1 thin layer of aluminum on the bottom, tabs were not practical.
 

jestor68

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Jun 12, 2012
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Re: Does a hydrofoil improve performance in small outboard application?

If you'd like advice from someone who has actually used a Doel Fin, I'd say stick one on there and give it a try.

There as web sites(no this one) containing hundreds of testimonials by those that actually used the Doel Fin and swear by it. :)00440.jpg
 

fishrdan

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Re: Does a hydrofoil improve performance in small outboard application?

The 14' discovery is rated at 30HP and you're running a 9.9, that right there could be the problem as as it's a rule of thumb to run at least 2/3 of the boat's rated HP. The best you can do is try to get the boat's load perfectly balanced and try the foil.

A Doelfin solved the problem in both boats, with no issues.

If you'd like advice from someone who has actually used a Doel Fin, I'd say stick one on there and give it a try.

^^^ Agree, I'm happy with the Stingray JR, on my 14' jon boat.

Before rigging my new 14' jon boat, I test ran it with my son, 2 outboards and a couple of batteries to figure out how the boat needed to be balanced. I picked up a decent looking 7.5HP before finishing the project and slapped it on the boat, this outboard came with the StingRay installed. This combo turned out to be very quick out of the hole and nice top-end speed, 20+mph.

I heard all the negativity about foils and knew I needed to get that thing off my outboard as it was ruining my boats performance!?!?!?

My first trip with the foil removed, the boat was a pig out of the hole and the top-end speed was a lot lower, probably 15MPH,, and the bow was sitting a lot higher on plane. Fiddling with the tilt pin resolved nothing.

There wasn't a second trip with the foil removed......

I don't think foils are right for every boat, but do think they will help in certain situations.
 
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JimS123

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Re: Does a hydrofoil improve performance in small outboard application?

The 14' discovery is rated at 30HP and you're running a 9.9, that right there could be the problem as as it's a rule of thumb to run at least 2/3 of the boat's rated HP. The best you can do is try to get the boat's load perfectly balanced and try the foil.
.

That "rule" was initiated by the engine manufacturers....LOL. I know a guy with a glass tri-hull rated for 115 and he runs a 9.9. H's happy! All he does is fish! Its all what floats your boat.

In my experience, the only time a foil was a bit squirrelly was when the boat was OVER powered.
 

JimS123

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Re: Does a hydrofoil improve performance in small outboard application?

If you'd like advice from someone who has actually used a Doel Fin, I'd say stick one on there and give it a try.

There as web sites(no this one) containing hundreds of testimonials by those that actually used the Doel Fin and swear by it. :)View attachment 212658

There are 2 fishing clubs near me. Each have maybe 40 docks, and 99% of them are fitted with lifts, so the lower units are all in view. The boats range from 14' OBs, to about 20' I/Os. Without actually counting, I would guess that about 60% of them are fitted with a fin of one type or another. Now, how's that for a testimonial!!!

These guys are all finatical about their equipment and fish with a passion. Funny thing is that none of their AV plates broke off and none of their boats sunk because of the fins.
 

roscoe

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Re: Does a hydrofoil improve performance in small outboard application?

There are 2 fishing clubs near me. Each have maybe 40 docks, and 99% of them are fitted with lifts, so the lower units are all in view. The boats range from 14' OBs, to about 20' I/Os. Without actually counting, I would guess that about 60% of them are fitted with a fin of one type or another. Now, how's that for a testimonial!!!

These guys are all finatical about their equipment and fish with a passion. Funny thing is that none of their AV plates broke off and none of their boats sunk because of the fins.

Sooooo, why don't all engines come with one from the factory?

How many of those in your club are on motors that aren't set up properly, and could have been fixed by raising or lowering the motor an inch, or using the correct prop, or adjusting the trim?
 

ondarvr

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Re: Does a hydrofoil improve performance in small outboard application?

Sooooo, why don't all engines come with one from the factory?

Every motor does, just a smaller version. a foil is just a larger AV plate. Just like adjustable trim tabs, the vast majority of boats don't come with them even though they may help a great deal.

I'm not saying they are as useful as trim tabs, only that they have their place.
 

jigngrub

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Re: Does a hydrofoil improve performance in small outboard application?

Correct motor mounting height, and correct prop pitch, will do more than a foil.
Check those first.

Proper trim setting is a must as well.
 
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