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TOPIC: Extend tunnel?

Extend tunnel? 19 Sep 2007 06:56 #1

  • dmcb
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This may have been covered before but it seems to me there is a lot of energy from the props directed upward.
A lot of water shoots up where it seems to me it would be better directed back.
I wonder is a tunnel extension would do anything?
Not the complete tunnel of course but a foot or more on the top 1/2.
Doug

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Started boating 1955
Number of boats owned 32
Bayliners
2655
2755
2850
3870 presently owned
Favorite boat. Toss up. 46' Chris Craft, 3870 Bayliner

Extend tunnel? 19 Sep 2007 07:49 #2

  • Aussie
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One member posted a note about adding an extra trim tab over the tunnels and claimed a 1 knot advantage.

It makes sense as it increases the effective water line length a little when under way

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Extend tunnel? 19 Sep 2007 09:58 #3

  • rbmitchell
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I did the extension thing Doug, and it pushed the bow down too much even though they extended only a foot. After one trip from NJ to FL and backI took them off. Didn't see a huge advantage.

I think the extra trim tab would be the way to go. I would set them up so they could be lifted up above horizontal.

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Extend tunnel? 19 Sep 2007 16:54 #4

  • Randy Rowboat
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Doug,
Those two small geysers that followed my 3270 got me thinking the same thing. That thought and the incline of the boat at intermediate speeds prompted me to build contoured tunnel tabs using Bennet cylinders and the existing pump.
I dreamed with this addition I could reach warp speeds… but, 1MPH at cruise RPM is more like it. The new tabs help the boat’s pitch and acceleration thru intermediate speeds. They are 16” long, follow the full tunnel arch and move from parallel to the prop tunnel to angled to approximately 4” down at the trailing edge.
They’ve been my primary pitch control for 4 years now. No geysers.
I’m a do-it-yerselfer so the only significant costs were for stainless and having the tabs arched. The rest was a chop saw, welder common hand tools and second-hand cylinders. Cost was about $700 and a chunk of spare time ...and near frost-bite (winter project).
I can’t recall whether you have a 38 or 32. On a 32 encompassing the rudders, which are mounted thru the transom, make the job a little different (some hurdles – some built-in solutions) but the job is much cleaner on a 38.
I have ACADs and pics. Let me know if you are interested.

Randy

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Extend tunnel? 19 Sep 2007 17:40 #5

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I do have a 38 Randy.
Thanks Bob. Guess the extension isn't the way to go.
I run mostly trawler speeds so it likely isn't worth it for me to do anything.
My mind starts to wander when the boat is out of the water.
Or is it my age?
Doug

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Started boating 1955
Number of boats owned 32
Bayliners
2655
2755
2850
3870 presently owned
Favorite boat. Toss up. 46' Chris Craft, 3870 Bayliner
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