Hydrofoil vs. stainless steel torque tab

[Scoot]

As some of you may remember, my bow of my boat is bouncy as soon as I trim it up at all. It's in the bottom holes of the plate in back, so I can't go any lower. I'm told moving it up will not help matters. I've also been told that my problems can be fixed by a stainless steel torque tab. If I understand right, this is simply a stainless steel version of a hydrofoil- is that right?

If this is true, how much more advantage will I see with a stainless steel torque tab vs. a hydrofoil? Is it worth the extra cost? Hydrofoils are relatively cheap and the torque tab is pretty pricey.

Valv, Marine Man, someone... please advise.

[countryboy9799]

So is the bow going up and down constantly even on a calm day? On my boat if I trim it up to far it will have this porpoising effect. You can cut the throttle down a little, doesnt take much, or you may have to put some flat washers in the top holes to change the angle of your motor to keep the front end of the boat down a little.

[Whoaru99]

The hydrofoil and torque tab are completely different things.

The hydrofoil mounts horizontally on the anti-ventilation plate (the flat part above the prop). The torque tab mounts vertically on the skeg (the vertical part on the bottom of the gear case).

The hydrofoil probably will not help with this problem once you are up on plane as it's likely just skimming across the surface of the water, if not out of the water completely.

The torque tab probably will not help either as it's purpose is to reduce the amount of pull you feel in the steering wheel.

If trimming the engine up or down does not fix it and the engine is already low mounted, then the next most common cause is too much weight in the back. Alternately, some configurations of propeller can affect porpoising if there is too much or too little rake to work properly with the boat hull style and weight.

Where is the position of the anti-ventilation plate with respect to the bottom of the boat right in front of it? Is it level with the bottom of the boat, or above or below?

Have you called the boat maker to ask for advice? Doesn't hurt to see what they say...

[Scoot]

I spoke to the mechanic again and he tried to clarify what he meant by "torque tabs". He said he was referring to a product made by Happy Troller that is very much like a doelfin (hydrofoil), but is stainless steel and has a cupped front. According to him, this helps to keep the bow down and does a much better job of it then a standard hydrofoil. Anyone know what this product is called and if it's much better than a hydrofoil?

[Whoaru99]

Regardless of the brand or what you call it, I believe the purpose is to keep the bow down during initial accelation to help you get up on plane easier/faster.

Someone else please comment if I'm wrong here, but I don't believe the purpose or effect of a hydrofoil is to reduce/eliminate porpoising.

[united jigsticker]

Ok...

The "Ventilation Plate" is correctly termed as CAVITATION PLATE.

And what was mistaken for as the "Torque Tab" is correctly termed as the TRIM TAB.

The trim tab will not affect the porpous effect. It affects the pull in your steering.

The cavitation plate should be just slightly below level with the hull of the boat at the transom.

If you have issues with porpousing, try distributing your weight (gear, batterys, occupants) different so the weight is more forward in the boat.

How large of a boat and what HP are we talking?

[united jigsticker]

I read your post again.

If I interpret this right, the boat doesn't porpous when the motor is all the way down; it only begins once you start to trim it up any amount..Correct?

So, my opinion is this: The only purpose for trimming up while on plane is to reduce drag on the bow of the boat, thus increasing fuel economy, reducing stress on the engine, and increasing speed.

If you are trimming up as little as possible and immediatly starting to "hop", then your "drag" is already at the least possible amount it can be, providing, of course, that your weight is distributed properly.

So, if you are not having trouble achieving a plane with your boat, then my honest opinion is, the hydrofoil will benefit you minimally.

[Whoaru99]

Quote:

---

Ok...

The "Ventilation Plate" is correctly termed as CAVITATION PLATE.

---

Actually, prop ventilation and cavitation are two different things.

Ventilation is when air from the water's surface or exhaust is drawn into the propeller.

Cavitation is when the pressure on an object moving rapidly through the water drops so low that the water actually boils and forms bubbles of water vapor. When the bubbles find their way to an area where the water is slower moving (hence at higher pressure) they implode. It's this implosion of the bubbles that chip/etches away the surfaces on which it occurs.

So, back to the original point, the plate above the prop is technically an anti-ventilation plate because the purpose is to prevent the prop from drawing in air from the surface of the water.

[united jigsticker]

When a liquid is subjected to tensile stress above a certain threshold, it ruptures and forms vaporous cavities.

This is called cavitation.

When the ambient pressure at a point in the liquid falls below the liquid's vapor pressure at the local ambient temp, the liquid can take on a phase change, creating largely empty voids called cavitation bubbles.

The physical process of cavitation is almost exactly the same as that which occurs during boiling.

The biggest difference between the two is how the phase change is effected.

Boiling is when the local vapor pressure of the liquid rises above its local ambient pressure and enough energy is present to cause the phase change to a gas.

To be possible to have cavitation to occur, the cavitation "bubbles" generally need a surface on which they change.

With things like propellers and pumps, cavitation causes a great deal of noise, damage to parts, vibration, and a loss of power.

When the cavitation bubbles break down, they focus liquid energy to very small volumes. They create spots of high temperature and release shock waves which are the source of noise.

The break down of cavities involves very high energies, and could potentially cause major damage.

The pitting caused by the collapse of cavities produces great wear on components and can dramatically shorten a propeller's lifetime.

[Whoaru99]

Well, yes, I was a little frugal with the exact technical details. But, I think we are both of the opinion that a hydrofoil is likely not the solution to Scoot's porpoising problem.

[ChuckN]

I've kinda said this before on your other post, but the prop will make a difference. I have similar boat (Skeeter deep V), and tried a couple props. What is the prop? A different prop made a difference.

In my experience, trimming the motor up during mid-range RPMs my boat will porpoise. I can't avoid it either. But, 5,000-5800 RPMs I can trim as much as I like. There's enough torque to keep the bow up.

So, what RPM's is the porpoising bad? my experience with my boat:

Higher/Full throttle...if it's porpoising - you definity have an issue.

Mid-range....get used to porpoising with some trim on most days.

Lower speed...keep the motor down, all the way down, no way around it. Boat porpoises with any trim.

These boats like speed. My motor likes speed, and economy is the same at any RPM for my rig...sucks, but true...gas pig. Everyone thinks I'm in a hurry when I'm just trying to have a soft ride.

As far as trim tabs and hyrdofoil...all I believe is I don't think the hydrofoil is going to help. For trim tabs I don't know.

[marine_man]

I agree with everything said here regarding the hydrafoil not helping your problem Scoot...

The prop could be the culprit... you said that the motor was mounted in the lowest holes... do you also have a manual jack plate on this boat?

marine_man

[Scoot]

Fellas,

My appologies- I'm not trying to be dense here. I'm also not trying to ignore your feedback.

MM, yep, motor's in the lowest holes. Nope, no manual jackplate. Nothing I can do there.

Chuck, I'm looking for another prop or two to give a shot. I hate to go this route, given that I'm happy with hole shot and top end speed, but I'll mess around with other props and see what happens. Also, your other comments suggest it's sort of a "nature of the beast" sort of thing with this boat. Your description hits the nail on the head- bouncy at mid and low speeds, fine at higher speeds. I guess I'll just have to get more comfortable going faster than I'm used to.

Thanks again fellas!

[marine_man]

I'd definately try a different prop then, just to make sure you've covered that angle...

You're not going to be anywhere near Lisbon in the next couple of weeks, are you? I'd be glad to go for a spin with you...

marine_man

[Scoot]

I'll do that, MM. Nope, I won't be over your way anytime too soon. I'm headed up to LOW mid-week next week and I'll be putting the boat through the paces then.

Thanks again for your info and advice- much appreciated!

[Whoaru99]

If you are fine with the hole shot and top speed, then you would not want to select a prop with different pitch.

However, you can find a different style of prop with the same pitch that has a different rake angle. This would probably be the most significant factor to address your porpoising problem from a propeller aspect.

Rake angle is not a normally published spec, you can tell though which props have more or less by setting them side by side. Props that have the blades sticking out more perpendicular from the hub have less rake angle, and props with blades at more of an angle have more rake. I'll scan in a representation of this and post it here in a little while.

Uhhhh.... how do I add a file attachment? I must be overlooking the add file button...