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Old 08-20-2013, 10:01 AM   #1
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Trim Tabs, Stern Wedges, Etc

This subject of wedge shaped additions onto the sterns of hulls came up today, so after I finished trying to remember why I had chosen to add them to the sterns of my sportfishing catamaran design, I went looking on this forum for the subject. Either I did not see much discussion about these objects, or it was hidden in some other 'titles' of subject threads.

So I thought I would start a new subject thread on these 'attitude altering' devices. I'm sure there are many opinions.

Here's one link that starts out speaking of such devices on sailing hulls but quickly gets to power hulls:
Stern Wedges - Sailing Anarchy - Sailing Anarchy Forums

And if you google the term 'stern wedge' you will find a lot more links.
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Old 08-20-2013, 10:08 AM   #2
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Now those are some "trim" tabs.
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Old 08-20-2013, 10:16 AM   #3
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They could even alter my trim.
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Old 08-20-2013, 11:25 AM   #4
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And they sure were "attitude altering" too!

Now, what were we talking about again?
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Old 08-20-2013, 11:47 AM   #5
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What speeds are you talking about? I don't think trim tabs and wedges are very effective depending on displacement and speed till you get up into it's planning/transition speeds.
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Old 08-20-2013, 12:52 PM   #6
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My tabs make a surprising difference on my Mainship 400 twin, but only above 12 knots or so. Trimming the bow down at 2800 rpms adds between 1/2 and a full knot, without an increase in throttle.

I would never own a small planing boat without tabs.

On a displacement hull? Meh.
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Old 08-20-2013, 01:55 PM   #7
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What is this 12 knots you spread of?
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Old 08-20-2013, 01:57 PM   #8
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I had trim tabs on my smaller, faster boats years ago and always found them useful. I think designing a boat slightly bow-up and then using tabs can be a good thing not to mention the ability to correct list due to improper loading or to add list in beam waves.

Wedges always seem like a patch on an improper design and/or loading, are not adjustable and add drag in situations where they are not needed. Wedges and tabs when they are deployed seem to make a boat "feel the bottom" squat & drag when in shallow water.

I would like them on any planing boat
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Old 08-20-2013, 05:41 PM   #9
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The new to us boat runs really happily at 15 to 16 knots when we ask it to, which is about 15 to 20% of the time. It is perhaps the main reason we sold our old 8 knot boat, there are times we need extra speed. When we go slow we burn darn close to what we burned in the old boat, perhaps a gallon an hour more (not enough to care).

Case in point, on our Bahamas trip this year we got stuck over there about four or five days longer than we wanted to (had to get back to work) waiting on weather. When we finally got a window we ran all the way from West End, Grand Bahama to Sebastian, Florida (about 130 miles) in one 9.5 hour day. We burned about 120 gallons.

Also, the stream was safe but sloppy, and would have rolled our guts out in the old boat. Much easier to pound through at 15 or 16 knots.

Did we burn more fuel? Yes. Was it worth it? Oh God yes.
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Old 08-20-2013, 06:06 PM   #10
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A trick used on New England lobster boats that are contemplating installing wedges, is to use rope. A piece of 1/2" or 3/8" line is run from stern cleat, under the transom and tied tightly to the other stern cleat. get line as tight as possible.
Speed trials will reveal whether wedges would work. YMMV
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Old 08-20-2013, 11:27 PM   #11
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...from another forum

Quote:
Originally Posted by mydauphin
These destroyer have some strange ways to get that speed

Stern Flaps
A stern flap (Figure 3) is a relatively small plate that extends behind a ship's transom, lengthening the bottom surface of the hull. A stern flap alters the water flow at the stern in ways that reduce the ship's resistance and increase fuel efficiency by a few or several percent. A stern flap for a Navy surface combatant in 2000 cost about $170,000 to fabricate and install.11 Preliminary tests of stern flaps on DDG-51s showed an annual fuel reduction of 3,800 to 4,700 barrels, or about 6.0% to 7.5%,
--5--
per ship.12 As of November 2004, the Navy had installed stern flaps on 98 ships (primarily surface combatants) and planned to install them on an additional 85. The 98 ships equipped as of November 2004 had accumulated 403 ship-years of service and saved $44 million in fuel costs.13 The Department of Energy stated in 2003 that by 2005, stern flap installations on Navy ships would save 446,000 barrels of fuel, or $18 million, per year.14
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Old 08-20-2013, 11:53 PM   #12
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Sailor of Fortune, Thanks for that tip. I've been wondering what the effect of trim tabs would be on my boat. I just might try that. Sounds easy enough.
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Old 08-21-2013, 05:33 AM   #13
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Hopcar.
Your boat is just the kind of rig it works on. Semi displacement DE hulls with some turn of speed above displacement speeds. No commitment necessary. No $170,000 navy re-fit necessary. Of course you won't see the $18 million in fuel savings per year either.
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