Wake Softening by Aeration

[Dacon62]

I have noticed that various ballers have made reference to softening the wake via aeration.

The most common comment I have encountered is that MC has tested, fine, comb like fingers at the back of the transom to soften/aerate the wake.

Sounds like an interesting idea!

Why not use the motors exhaust and spent water to help with the proccess?

Either drive the spent gases and water through the bottom of the hull right at the end of the running surface (fig.1) or expel the waste through a transom piggy back box that has hollow comb like fingers (fig.2).

The holes/fingers could all be the same size or be variable? Maybe larger holes would be needed right behind the rudder with smaller ones starboard and port? As a side benefit exhaust noise would likely be reduced.

What other types of wake softening/reduction concepts are there?

[eleeski]

IO wakes and tables are soft and bubbly. Not always the best. The exhaust sent into the propwash is not magic - or toxic. While the idea has merit, there is a lot of engineering to make it work.

A lot of skiers trick, kneeboard or wakeboard. For that a crisp clean wake is best. Ideally a wake that can be aerated only for slalom and jump would be optimal.

Kudos to MC for so much creative testing of ways to improve the slalom wake while preserving a great trick wake. Better than advertising and Koolaid to push a draggy frothy one dimensional pos. Maybe I shouldn't say that - naah it's winter and the pot needs stirring even my crack isn't true.

Erici

[BraceMaker]

I do not believe the exhaust volume sufficient for the task.

 

Remember the skier is behind the boat. To be meaningful you must aerate the wake @ the skier. Not the hull.

[eleeski]

@Bracemaker Strategically placed exhaust vents would make a huge difference. There's plenty of volume. Controlling the effect is tricky.

As a trick skier, I really notice the difference between an inboard and an IO or outboard. All attributed to the exhaust bubbles. For me, it's a big noticeable effect.

As long as there is a valve to optionally offer the nice clean wake, great!

Eric

[Dacon62]

I would imagine that a switch to choose between full aeration for slalom and jump to no aeration for trick and a smooth wake would not be that difficult to accomplish. Also the deeper the aeration the further back in the wake the effect.

[BraceMaker]

@Dacon62 - the switching is easy, all the parts are sold for systems like "captain's call" where on inboard/outboards, particularly the "offshore racing" style like Baja's and Fountains, you can switch the exhaust between thru-hull and underwater - helps avoid noise violations in rivers and marinas and the like. I think Captain's Call is one brand.

 

Also many wake surf boats have a thru hull on the opposite side of where you are surfing - hence they have a switch for Left/Right exhaust.

 

@eleeski - have you been able to directly test one boat with different exhausts? Have you ever tricked behind a "FAE" fresh air exhaust "silent on the set"?

 

Which uses venturi suction to pull the exhaust down under the boat.

 

 

 

[DW]

Another aeration option would be the prop itself. Comb fingers at the trailing edge of the blade would be an interesting experiment. There are several good examples of the comb effect exist in nature, birds feathers, whale fins, etc..

 

At 5000 rpm a 350 CID small block puts out 600 cfm, so ~ 70% of that at slalom speeds and ~ 40% of that at trick speeds, so thats how much aeration the exhaust would add to the mix. Another option to D62's ideas are the infamous EL bubbler which injects to the leading side of the prop.

[mwetskier]

i no from when i was a kid blocking a tailpipe will stall a car. could forcing the exaust thru a nozle make back pressure the engine wont like?

[DW]

Adding to the backpressure would certainly not be a favourable engine option. The reality of having the exhaust exit in to fast moving water could / would offer a back pressure reduction if done correctly. It would take some development to get it right. From the initial pictures, I would envision the aerator to only cover about the center third of the hull or specifically the portion that affects the wake table or the elevated portion of the wake.

[BraceMaker]

All your inboards have the exhausts fully submerged when idled. Particularly when someone is standing on the platform.

 

So no issues there, it isn't plugged it just has a small pressure.

 

 

Fae gas videos of their device creating suction at speed.

[Horton]

some guy called me a few years ago all excited that he invented the next big thing. Was pumping air (bubbles) under the boat at about the pylon. Created lift or something. Guess he could not get a big boat company to care.

[DW]

@JTH: He sort of did, the longitudinal strakes along the new LXi / TXi hulls are supposed to entrain some air under the hull and create some lift.

 

One interesting comment on using exhaust: the F1 guys do it in the diffuser to add downforce, the challenge there is off throttle effects on downforce, lots of development work to make it work well. It really alters the flow field under the car and has a big effect on downforce.

[Dacon62]

As we know the big boat companies are busy taking ideas for making a bigger or more surfable wake not the reverse. And as times are tough they do not have a lot of R & D time and money left over to chase every idea that comes along.

However @Horton posses an interesting and I believe possibly better senario.

Have some or all the exhaust gases introduced just behind the water break line on the hull on the outsides of both strake lines. Introducing gases in the center section may cavitate the prop.

The exhaust gases would break up the surface tension of the water reducing drag and aerate the outer sides of the wake.

Maybe you use a portion of the exhaust to aerate both sections outside the strake line and use the other portion to aerate the section inside the strake lines, just behind the rudder. Whole wake aerated, no cavitation.

[ForrestGump]

Mastercraft found that they could aerate the boundary layer water passing under the boat and create enough lift that they destroyed all tracking ability of the hull. Therein lies your problem. You're not going to know until you destroy a boat trying it. Eric Lee found that his bubbler modification which only added a little air underneath the boat made a difference.

[eleeski]

Note that the bubbler improved the 04MC but not the 11MC. There are lots of ways to tweak a wake.

Eric

[DW]

@Dracon62: Keep up the idea's. From my wake analysis studies, the portion of the wake you would aerate via that method would not include the table, which I think is the key to softening the wake. Also, if you watch, a significant portion of the water is jettioned (for lak of a better term) outward from the midsection of the outer hull sections, so most of the exhaust bubbling would be entrained in the side spray particularly of the midsection.

 

On a separate note, we are talking about some cfd (computational fluid dynamics) studies on boat wakes. No idea if we can get anywhere with it, but it could be a fun project, not simple to model by any stretch.

[MCskiFreak]

The Underwater exhaust is already available, MC uses it on its 300 day yacht to reduce noise emission.

 

Dacon Your second picture is a pretty close illustration to what a stepped hull does to the flow aft of the steps. Like Shane said though it does have side effects that a good design must eliminate.

 

The reason that there are problems with ventilated hulls is they reduce the effective lateral area of the boat to be able to support a turn without slipping or spinning out. One of the best design to date has been a Micheal Peters designed hull that incorporates a central tunnel to give more lateral area to the hull. The two attached pictures show the ventilation problem and the Peters hull.

[Dacon62]

Wow, some good stuff here gentlemen!

@Bracemaker - Thanks for stearing us in the 'fresh air exhaust' direction. I remeber hearing about this product years ago. This video link shows exactly what you explained. Very interesting that the guys that recorded this video were (assumably) doing it to prove that their underwater exhaust did not exert back pressure. Quite the opposite as the vid shows...massive suction and exhaust evacuation. Something else they weren't looking for but may have discovered by accident...notice how aerated the wake becomes in the middle portion! Position 3 of these tubes (port/center/starboard) behind the transom and I wonder what would happen??

 

 

@DW - Perhaps these drawings below will better explain what I had in mind yesterday (with some updates).

All 3 areas would receive some volume of aeration.

Fig. 3 If tracking is adversely effected by the port/starboard air induction the amount could be controlled via a damper.

If that idea is more complex than neccessary and/or any amount of frontal air induction kills the tracking or handling then fig. 4 provides a simpler solution. Not sure how having 3 slender oval pipes dragging in the water would effect handling?

 

If the picture below is studied it looks like the table is made up of 2 'types' of water.

1) The water pushing back up after passing under the boat, including prop wash(easier to aerate).

2) The water converging again after being pushed outward to both sides(more difficult to aerate).

As we know these all come together usually around the 22 off rope length while slaloming to give the wake that 'rooster tail' bump we all dread. Not sure how to deal with point 2 unless the port/starboard portlet aerators (fig. 3) contribute.

 

The obvious ultimate goal would be max. aeration to provide wake softening and possibly reduced wake height by providing a bit of lift to the hull without adversely affecting tracking and handling beyond acceptable limits.

 

[mwetskier]

what about the water driven down under pressure by the prop i always thought thats where the bump came from.

[6balls]

and if you didn't have water down from prop and down from hull coming back up off the back, how much sooner would the side displacement waters come back together, perhaps hurting shorter rope lengths.

[BraceMaker]

Being a fluid shouldn't the energy of the displaced water continue to travel away from its source?

 

The return would be other water attempting to return to the area and become level.

[Dacon62]

Yeah...I guess that makes sense.

Making a row of these 'suck tubes' and mounting them under the swim platform might be an interesting winter garage project. Shouldn't cost too much and don't really have to drill holes into the hull. Maybe a few into the swim platform or swim brackets.

Question is do you make 3 large tubes of the size in the YouTube video or half a dozen or more that would maybe be half the size or smaller?

 

 

[thager]

Buy PVC and heat mold the shape.

[Dacon62]

Shedule 40...80? Probably easy to work with just not sure if it would be strong enough at the mounting point to withstand the torque? What about squeezing muffler tubing in a large vise?

[DW]

The muffler tubing would work. You could also drill a small hole at the submerged end and run some safety wire slightly forward to resist the force. Things in tension are usually quite strong.