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Old 08-14-2019, 07:37 PM   #1
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Sizing trim tabs. Is really big bad?

Hi folks. Has anyone experimented with trim tabs? I have a Sabre 34 which runs nicely up to 22 knots with two yanmar diesels each pulling 365 hp, but it would be nice to get the boat to plane flatter. The boat has the original Bennett trim tabs, but I was wondering if I upsized them would it pull the stern more out of the water. My mechanic suggested that changing props with different cup ratios can also flatten the ride. I also think it would be more fuel efficient not dragging the stern. I would love your thoughts if I can change this or if this is just life with a fast trawler. Thanks.
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Old 08-14-2019, 07:42 PM   #2
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Hi folks. Has anyone experimented with trim tabs? I have a Sabre 34 which runs nicely up to 22 knots with two yanmar diesels each pulling 365 hp, but it would be nice to get the boat to plane flatter. The boat has the original Bennett trim tabs, but I was wondering if I upsized them would it pull the stern more out of the water. My mechanic suggested that changing props with different cup ratios can also flatten the ride. I also think it would be more fuel efficient not dragging the stern. I would love your thoughts if I can change this or if this is just life with a fast trawler. Thanks.
Call Bennett, their customer service is second to none! When I was having their tabs added to my Silverton Aft Cabin the yard ordered tabs that were much too big (even though it was a big heavy boat). Bennett stepped in to fix the issue, sending the right tabs before the originals were returned and worked closely with the yard to get everything right.

Highly recommend you call them. They will also probably have the information on many previous installs on your boat, so can immediately tell you what you need.
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Old 08-14-2019, 08:18 PM   #3
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I’d say making the tabs bigger will work better. Wider would deliver much more benefit. Make sure your mechanicals can handle the increased load.

No comment on the “cup ratios”. HaHa
I usually say to forget cupping props on trawlers but your boat is faster than a trawler so there may be some benefit.
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Old 08-14-2019, 08:50 PM   #4
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Thanks guys I will give Bennet a call.
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Old 08-15-2019, 05:08 AM   #5
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Boat manufacturers will usually size tabs on the smaller side of the recommended range, which is fine when the boat is new and light, but as boats age and the owner brings "stuff" aboard the tabs are asked to do more with marginal size.
Larger is better and more efficient. A larger tab deployed less to get the desired effect will be more efficient with less drag than a marginally sized tabs deployed a lot to get the same effect.
Bennett standard tabs use a single actuator (ram/cylinder) up to size 12 x 42, so in many cases you can use your existing ram and just install bigger tabs. As long as the ram is centered on the tab you may be able to leave it where it is, another option if a larger tab will have the existing ram land offset from center is to add a second ram equally spaced from the existing one and drill new holes in the tab for the non-standard spacing. The second ram just plumbs into the existing plumbing with a "tee" Bennett will supply, the pump will handle the extra ram just fine.

Your mechanic is correct that prop cup can increase lift, and that may be a viable fix if you only need a little more lift, but it sounds like you need more than that.

Bennett is at 954 427-1400, ask for Thomas and tell him I sent you

http://bennetttrimtabs.com/wp-conten...stallation.pdf
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Old 08-15-2019, 05:36 AM   #6
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"I also think it would be more fuel efficient not dragging the stern."


The usual trim angle is 4 deg, hull surface to water surface at speed.
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Old 08-15-2019, 07:01 AM   #7
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When I had my old Mainship model I (same boat as seen in Keysdisease avitar) I used 42 x 12 tabs. I mounted them on mahogany blocks which I fit to the hull to make the tabs square at the stern. Effectively they were 16 inches at the outside edge and 13 inside.
Most of the width of the boat was tab.
They lifted the stern so much that at 7 knots the stern was "on plane" as much as a semi planning hull could get.
At WOT they needed to be retracted some as they could push the bow too far into the water.
In summary, BIG tabs work.
They also acted as "stopper floppers" while at anchor. Bonus!
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Old 08-15-2019, 09:33 AM   #8
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I donít see a problem with bigger, you can always retract them if not needed. As long as you have room, go for it.
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Old 08-15-2019, 09:33 AM   #9
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Originally Posted by jleonard View Post
When I had my old Mainship model I (same boat as seen in Keysdisease avitar) I used 42 x 12 tabs. I mounted them on mahogany blocks which I fit to the hull to make the tabs square at the stern. Effectively they were 16 inches at the outside edge and 13 inside.
Most of the width of the boat was tab.
They lifted the stern so much that at 7 knots the stern was "on plane" as much as a semi planning hull could get.
At WOT they needed to be retracted some as they could push the bow too far into the water.
In summary, BIG tabs work.
They also acted as "stopper floppers" while at anchor. Bonus!
Next haulout I am putting my new ones I made out of 316 rivets and 316 sheet steel on.
They are replacing the 42 by 9's. The new ones are 42 by 12's and I bent down the edges to form the wings pointing down which makes them more effective. I have not had tabs on since 2014 and miss them.

I made them myself, and painted with what was it, por15 silver.
They have an angle since the transom is curved and are bottom mount style.
I also bought a 5 foot SS chain to chain them to the transom, in case the delrin plastic pin breaks, I dont want to loose them to a 1/4" pin on the bennet cylinders.
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Old 08-15-2019, 06:09 PM   #10
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That is great information. One of my issues is I don’t have much on the boat so all of the weight is 240 gallons of water under the cockpit and the engines (prior owner oversized)/260 gallons of fuel near the 1/3rd point. The rest is just boat.
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Old 08-15-2019, 06:18 PM   #11
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Just reading the comments again to get out the detail. Are you saying your tabs are 42 inch wide? I didn’t picture them that big. I am hauling the boat next month to paint the topside and I will check them out them. Anything magic about building your own? Seems like just aluminum plate and hardware. Do you think there is a ton of load on them or more of “holding you hand in the water” to turn a surfboard?
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Old 08-15-2019, 07:45 PM   #12
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Trim Tabs for real boats are stainless steel, 304 and usually 12 guage. If you have Bennett tabs you can just buy larger Bennett Tabs and everything will play nice. The tricky part is the hinge and making the tab so it wont corrode, Bennett has that down pretty good.
The rule of thumb for tab sizing is an inch of tab width for each foot of length. Add extra for slower boats or ass heavy boats that need some extra lift. So base size for you would be 36 x 12 +
For the 34 Mainship Jay Leonard mentioned they only had 160 to 200HP and needed 42" x 12" tabs.
And yes, there is quite a bit of load on them, they are lifting the entire ass end of your boat as it travels 20 knots



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Anything magic about building your own? Seems like just aluminum plate and hardware. Do you think there is a ton of load on them ?
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Old 08-16-2019, 09:18 AM   #13
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And to the original poster and anyone else contemplating adding tabs....
You want them full UP in a following sea.
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Old 08-16-2019, 10:06 AM   #14
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I owned a boat that the previous owner had added an extra piece of SS and riveted it on top of the original tab. Worked perfect and looked OK if you didn’t look carefully. It was a cheap DIY fix.
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Old 08-16-2019, 10:51 AM   #15
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I don’t see a problem with bigger, you can always retract them if not needed. As long as you have room, go for it.
In following seas you would want a fast system that could trim down and up as fast as you go from trough to crest. Much of the time depending on location that could be quite fast.

Fish,
One could put the extension in a brake and have a “tab on a tab”. Extra load on the system though.
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Old 08-16-2019, 11:01 AM   #16
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When installed properly trim Tabs will have no effect on the boats attitude when fully retracted. Per Bennett instructions the tabs are installed at a "negative" retracted angle above the plane of the bottom of the boat.

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Old 08-16-2019, 11:51 AM   #17
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Thanks for all the comments and suggestions. I am going to call the Bennett folks on Monday since I am hearing that swapping up in size with the current system is the way to go. I am intrequed by the adjusting as heading down a wave. Is that to surf or keep the roll to a minimum?
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Old 08-16-2019, 02:37 PM   #18
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I don’t see a problem with bigger, you can always retract them if not needed. As long as you have room, go for it.
In following seas you would want a fast system that could trim down and up as fast as you go from trough to crest. Much of the time depending on location that could be quite fast.

Fish,
One could put the extension with a brake (put a and have a “tab on a tab” .. like down flaps. Extra load on the system though.
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Old 08-16-2019, 03:47 PM   #19
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Thanks for all the comments and suggestions. I am going to call the Bennett folks on Monday since I am hearing that swapping up in size with the current system is the way to go. I am intrequed by the adjusting as heading down a wave. Is that to surf or keep the roll to a minimum?
In a following sea you won't have time to adjust the tab position. They do not move that fast. It will quickly become a disaster IMO.
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Old 08-16-2019, 04:32 PM   #20
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In a following sea, just put in the retracted position. They will not be fast enough to put them up and down on each wave.
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