Tunnel Question . . . Know anything about Transverse Flow?????

[home made tunnel]

I came across this picture on BoatDesign.net and it got me really thinking...
This user called "Baeckmo" was talking about Transverse flow on a tunnel hull with pads... saying the hull was very un-stable. It's exactly what I have built into my little 13.5' tunnel... Will I have a very unstable hull? Anyone have any experience with this?

Here's the link, and a picture.

http://www.boatdesign.net/forums/hydrodynamics-aerodynamics/tunnel-hull-pads-47138.html
"We studied pads on a 4 m asymmetric cat with similar proportions to your hull. Bad result; it tended to plane slightly yawed and was not stable. The reason we found was that the transverse flow into the tunnel increased, resulting in an unstable wetting of the aft tunnel sides. The boat yawed and heeled slightly until the flow along one tunnel side or the other was stable.

Course instability in planing, asymmetric catamarans often occur due to too low trim (too much lift aft) and absence of even the slightest keel or fin lateral area aft. So, I can't see the pad as a solution to your problem. Rather a short, additional step aft might do the trick."

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[FMP]

You don't know yours will be an issue? Maybe Jim will chime in.

[home made tunnel]

Well find out come springtime of mine will be an issue. But still wondering if anyone else has input.

[FMP]

So cornering skidding aside , you're thinking the spray "TF" is going to cross to the other side. How much of a difference is the 90* corner vs a sharper<* in TF ? Both side produce and at high speed consider the air flow under the hull at the spot where the two TF will meet. Will the force of the air flow deflect the spray out the stern before any results are felt? Would a small tunnel in the profile of a half circle or NIKE swoosh on the the pad to the inner 90* help?

[XstreamVking]

In my limited experience, the main change I would make is to put a wedge in the tunnel . The deepening of the tunnel going aft might make the boat porpoise. But, the flats may alter that entirely. Test and tune is gonna shake it out. I built a 13'x5' tunnel with angled tunnel sides just to be different. Same depth tunnel front to back. It works real good. Also has a small wedge on it to give more lift as speed increases. Faster you go, the more the bow drops and it ended up running flat. On turns it slides by letting air escape from the angled tunnel sides versus the need for anti-trip chines on some vertical wall tunnels. I have had 40hp as a max on it, now has a 25 merc and is a happy boat that does mid 30's.

[FMP]

Was thinking the same depth profile front to back making an additional tunnel hull out of the pad. No skidding , even in your shorts! LOL

[home made tunnel]

The deepening of the tunnel going aft might make the boat porpoise. But, the flats may alter that entirely.

The photo makes it look that way, but it's an 8" deep tunnel front to back. It doesn't deepen in the aft.

We'll see come spring time I guess haha. Go for broke !

[Mr. Demeanor]

Run it!

[home made tunnel]

Run it!
Crap load of work to do before spring, but should have the ProXS 115 purchased by the end of march. She'll probably be ass heavy as all heck, but we'll see how she goes!!!.

Mr.demeanor... does your tunnel have pads on the bottom?

[Mr. Demeanor]

home made tunnel;2928709]Crap load of work to do before spring, but should have the ProXS 115 purchased by the end of march. She'll probably be ass heavy as all heck, but we'll see how she goes!!!.

Mr.demeanor... does your tunnel have pads on the bottom?[/QUOTE]

Yes. Almost flat.
367188

[Mr. Demeanor]

367189

[Mr. Demeanor]

I think we could help you better if you included pics of wife/girlfriend . :D

[home made tunnel]

367188

Do those plastic Fins on your lower really help you plane easier? I always thought they were a gimmick.

And here's the girl haha.

367190

[Mr. Demeanor]

Do those plastic Fins on your lower really help you plane easier? I always thought they were a gimmick.

And here's the girl haha.

367190 My motor is so high it was out of the water before it could help but it was there when I bought it. Thats a very old pic.

[FMP]

Do those plastic Fins on your lower really help you plane easier? I always thought they were a gimmick.

And here's the girl haha.

367190

Ahh summer !

[kelvin523]

This thread reads like obfuscated bullsh!t, just saying...

[XstreamVking]

That's from an expert!

[CUDA]

The gimmick is transverse flow, your going WAY too S L O W. ..................

Yeah don't put a plate on your lower unit It will slow you down:rolleyes:

[FMP]

This thread reads like obfuscated bullsh!t, just saying...

Please elaborate, and mind the French.

[FMP]

Lots of info and plenty of links http://www.google.com/patents/US5458078

[MN4V]

I've worked on many race tunnel boats since the late 1970's and have tried many different ideas for sponson shapes to help turning and acceleration/speed.
The transverse flow (water spray) doesn't seem to happen, like your sketch, on the race boats. This water spray comes up almost vertical and hits the lift strake
on the vertical tunnel side.
I'm not sure why you've added the pads to your sponsons, but we always like to test one design change at a time. Test the boat as designed/built and then decide
if the boat needs changes to run correctly. There are a lots of things going on in tunnel boat design. Even prop design can make a huge difference.
Mark N

Here is a SST-60 boat I designed and built. I used Jim Russell's TBDP to design the wing section of the tunnel and used a new technique
for the other parts of the boat.
https://youtu.be/_hJf8Sigpt0


https://www.youtube.com/watch?v=_hJf8Sigpt0&amp;feature=youtu.be

[Jimboat]

The transverse flow (water spray) doesn't seem to happen, like your sketch, on the race boats. This water spray comes up almost vertical and hits the lift strake
on the vertical tunnel side. Here is a SST-60 boat I designed and built. I used Jim Russell's TBDP to design the wing section of the tunnel and used a new technique
for the other parts of the boat.
Agree with Mark's comments. There is a "Whisker Spray (http://www.aeromarineresearch.com/whisker_spray_drag.html)" that can generate some drag in some circumstances, and effectively designed spray rails (http://www.aeromarineresearch.com/tbdp6/strakes_and_sprayrails.html) can reduce this.

[WillySteve]

My little 10' Cougar Cub tunnel needs a fin to come out good! I installed one from Bob's and it now leaves really nice. Before at the high engine height it blew out really bad!
Actually on N2o it initially clears the water!
This little boat wasn't made to run 60 but it handles extremely well at that speed. IMO it needs to be a few more feet longer though!
The tunnel is 8" deep in the front and 5" in the back with no pads!

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