Larger trim tabs and/or drop fins

Interesting. I got a set of dropfins last winter but didn’t put them on because of my curved transom and slight angle on my tabs.
Bennett cautioned me about installing the drop fins before talking to someone with a diesel 410 who had done it to see what their results were.
How did you enlarge your tabs? Did you get them readymade or custom made? Did you get wider hinges too?
 
Last edited:
JVM225 said:
Interesting. I got a set of dropfins last winter but didn’t put them on because of my curved transom and slight angle on my tabs.
Bennett cautioned me about installing the drop fins before talking to someone with a diesel 410 who had done it to see what their results were.
How did you enlarge your tabs? Did you get them readymade or custom made? Did you get wider hinges too?
Click to expand...

I used a flat pice of stock stainless cominserate to the material Bennett uses. Had a ridge worked into the very back to give it strength, avoid flexing Mounted it directly to the Bennett tab. Mathematicaly the surface area was increased 30%. That was the amount of addition surface the Bennett techs said I could do with out issues. Drop down fins were added to the side.
 
JVM225 said:
Interesting. I got a set of dropfins last winter but didn’t put them on because of my curved transom and slight angle on my tabs.
Bennett cautioned me about installing the drop fins before talking to someone with a diesel 410 who had done it to see what their results were.
How did you enlarge your tabs? Did you get them readymade or custom made? Did you get wider hinges too?
Click to expand...

By the way, the Bennett guys were saying the fins increased effecency by up to 10%; least that's what they said back then. They kind of backed off that number. I'ld install the fins next time she is out of the water. Might notice a difference. It's a simple instillation. Sure won't hurt anything.
 
stephens013 said:
By the way, the Bennett guys were saying the fins increased effecency by up to 10%; least that's what they said back then. They kind of backed off that number. I'ld install the fins next time she is out of the water. Might notice a difference. It's a simple instillation. Sure won't hurt anything.
Click to expand...

I’m sort of leaning that way. If there does wind up being any adverse effect I could always remove them in shallow water.
The one doubt I have though is if I remove them I’m going to have all those holes drilled in my factory tabs and they’re not an “off the shelf” shape so replacement would be a problem.
Worst case scenario, I guess I could always just put the pan head screws and nylock nuts back in the holes once the fins are off.
Unfortunately, I can’t have anything made up to attach to the existing tabs to make them any larger because the outboard side of each tab has a downturn of about 1” or so in it.
Oddly enough, they have an equally sized upturn on the inboard side.
9037DCED-5C86-409D-B7E9-D432A966AD2C.jpeg
 
Last edited:
JVM225 said:
Unfortunately, I can’t have anything made up to attach to the existing tabs to make them any larger because the outboard side of each tab has a downturn of about 1” or so in it.
Oddly enough, they have an equally sized upturn on the inboard side.
Click to expand...

My tabs also had the same upturn/downturn which I had ground off when the enlarged tabs were welded to the originals.
 
juggernaut1 said:
My tabs also had the same upturn/downturn which I had ground off when the enlarged tabs were welded to the originals.
Click to expand...
The other issue I would face is that the outboard sides of the existing tabs go right up to the point where the transom starts to curve so I’m out of flat space.
Rather than trying to enlarge what I have I may be better off seeing if I can add a second set of tabs/actuators over the prop pockets.
Bennett mentioned that as a possibility when I spoke to them on the phone.
I’d want to get some feedback from someone with a similar boat that has made that modification before going down that road.
 
JVM225 said:
The other issue I would face is that the outboard sides of the existing tabs go right up to the point where the transom starts to curve so I’m out of flat space.
Rather than trying to enlarge what I have I may be better off seeing if I can add a second set of tabs/actuators over the prop pockets.
Bennett mentioned that as a possibility when I spoke to them on the phone.
I’d want to get some feedback from someone with a similar boat that has made that modification before going down that road.
Click to expand...

The added set of tabs between the pockets has been done many times on the older 390ECs with much success. I added mine 8 years ago and made mine fixed with a 5 degree down angle. They changed the entire characteristic of the boat in a very good way. I also enlarged my outer tabs. Now I can keep the boat on plane with 10 passengers and 2/3 tabs at 15mph and 2600rpms. Find the official 390EC thread and search trim tabs.
 
JVM225 said:
The other issue I would face is that the outboard sides of the existing tabs go right up to the point where the transom starts to curve so I’m out of flat space.
Rather than trying to enlarge what I have I may be better off seeing if I can add a second set of tabs/actuators over the prop pockets.
Bennett mentioned that as a possibility when I spoke to them on the phone.
I’d want to get some feedback from someone with a similar boat that has made that modification before going down that road.
Click to expand...


Just putting tabs won't effect the hinge or water flow all that much. They make the flow more concentrated aft of the boat . Really noticed a difference on the 400DA. Of course that was 30% increase in over all tab area plus the fins. I could actually take some tab out once on plane. Put fins on the 510DA. What I’m noticing is the boat stays on plane all the way to 16 knots. That ain’t the norm for that big a boat.

Like I said; I’ld slap them on. Just gives you more authority over the effect of tabs. Got to be a good thing.
 
hottoddie said:
The added set of tabs between the pockets has been done many times on the older 390ECs with much success. I added mine 8 years ago and made mine fixed with a 5 degree down angle. They changed the entire characteristic of the boat in a very good way. I also enlarged my outer tabs. Now I can keep the boat on plane with 10 passengers and 2/3 tabs at 15mph and 2600rpms. Find the official 390EC thread and search trim tabs.
Click to expand...

Interesting. My boat has prop pockets. Do those older 390’s have prop pockets.
 
stephens013 said:
Just putting tabs won't effect the hinge or water flow all that much. They make the flow more concentrated aft of the boat . Really noticed a difference on the 400DA. Of course that was 30% increase in over all tab area plus the fins. I could actually take some tab out once on plane. Put fins on the 510DA. What I’m noticing is the boat stays on plane all the way to 16 knots. That ain’t the norm for that big a boat.

Like I said; I’ld slap them on. Just gives you more authority over the effect of tabs. Got to be a good thing.
Click to expand...

Yeah, that’s what I’ll probably do this winter. Was holding off to see if someone had done it on the factory tabs on a diesel 410 like Bennett advised, but I’m probably just going to roll the dice.
Worst case scenario: I remove the drop fins, put the panhead screws back in the holes, secure them with Nylocks on the top and call it a day.
 
JVM225 said:
Interesting. My boat has prop pockets. Do those older 390’s have prop pockets.
Click to expand...

Yes they have very large pockets. They may be bigger than those on your 410
 
JVM225 said:
The other issue I would face is that the outboard sides of the existing tabs go right up to the point where the transom starts to curve so I’m out of flat space.
Rather than trying to enlarge what I have I may be better off seeing if I can add a second set of tabs/actuators over the prop pockets.
Bennett mentioned that as a possibility when I spoke to them on the phone.
I’d want to get some feedback from someone with a similar boat that has made that modification before going down that road.
Click to expand...

Understand. I'd add some chord of around 3 inches and 3 inch drop fins. I only added 1 inch to each side of mine, but 3.5 inches of chord and 3 inch drop fins.

I can now keep the boat flat at all speeds and it will semi plane down to 12 knots. The original tabs were useless at 12 knots and the boat just plowed with big bow rise.
 
Resurfacing this old thread.
I’m thinking about trying to add size to my stock tabs like some of you have on this thread to my 420DB. I see some of you welded and some bolted the add on. If I can’t find a local shop to weld stainless was there any issues with those of you that bolted the extra size plate to original? And how many screws used to take that lid on?
Did anyone have fairly accurate numbers before and after mod for fuel burn at their normal cruise speed?
 
No issues with bolt on. I think I used 8 bolts. Make sure your design has your downfins run parallel with the water. As far as speed and efficiency, not sure. Definitely worth the control. SeaRay was notorious for installing underwhelming tabs on the boats
 
Matt Michels said:
No issues with bolt on. I think I used 8 bolts. Make sure your design has your downfins run parallel with the water. As far as speed and efficiency, not sure. Definitely worth the control. SeaRay was notorious for installing underwhelming tabs on the boats
Click to expand...
How thick a piece of stainless did you use?

Agree on the underwhelming tabs. My 420 is stern heavy and at cruise always have to use a lot of tab. Figured bigger tabs would help be a little more efficient.
 
I can’t remember the gauge, but less than 1/8” thick. Here is the drawing. The angle was off a little, but I made it work. Mine is a 380, so a little different. The single ram can handle this size tab
 

Attachments

  • 83CDFF17-5CC1-46BD-B3F8-2DAA50DBC06E.png
    83CDFF17-5CC1-46BD-B3F8-2DAA50DBC06E.png
    192.4 KB · Views: 104
Matt Michels said:
I can’t remember the gauge, but less than 1/8” thick. Here is the drawing. The angle was off a little, but I made it work. Mine is a 380, so a little different. The single ram can handle this size tab
Click to expand...
ok thanks. Was thinking 1/8” for good rigidity.
 
Matt Michels said:
I can’t remember the gauge, but less than 1/8” thick. Here is the drawing. The angle was off a little, but I made it work. Mine is a 380, so a little different. The single ram can handle this size tab
Click to expand...
I was looking closer at your drawings and saw a n angle of 103 degrees between the front leading edge and the sides where it folds down. How did you come to that angle?
 
You must log in or register to reply here.

Forum statistics

Threads
113,163
Messages
1,427,599
Members
61,072
Latest member
BoatUtah12
Back
Top