Adding Drop Fins - Good or Bad Idea

[Gypsy]

I've read several positive comments about adding drop fins to the trim tabs. I decided to add them to my C28. However, after I installed the first tab, I noticed that the transom is not straight across so the tabs are not in line with the keel and so the drop fins would not be in line with the flow of water. It looks like the drop fins would actually disrupt the flow of water and slow the boat down. Any comments would be appreciated.

 

[Chriscz128]

I just installed drop fins on my C-28 and noticed the same thing about the alignment. The trim tabs "toe out" when mounted flush with the stern. That being said, they work great, provide more lift, increase speed by 1-3 knots, and increase WOT rpm 150-200. All of these results are good for my situation.

Chris

 

[scross]

There’s been a lot of discussion on the installation of drop fins.
Try this thread to get started on the topic.
viewtopic.php?f=5&t=17040&p=111440&hilit=Drop+fins+angle#p111440
Lots of other threads if you search.
Ours were installed parallel to the keel, not parallel to the trim tabs per instructions I received directly from Bennett. On our boat, a R25 Classis, that’s about an 8 degree offset from the trim tabs themselves.
I’m glad I did the project. Didn't magically change my performance to that of a Cutwater 28 but it definitely improved getting on plane.

PS: This is a good thread too:
viewtopic.php?f=24&t=9262&hilit=bennett+trim+tabs

 

[BB marine]

You will not regret installing the drop fins on the C26 or C28. There is an offset to deal with but this in my opinion does not effect the performance or control of the boat. The boat C26 and C28 are not fast enough to make a difference with the offset from the angle in the transom. I did notice a dirty wake but honestly the Cutwater with the tunnel has a dirty wake anyway. I used the trim tabs with drop fins installed without adjusting the fin angle for a year. I was concerned about the offset. The Cutwater C26 and C28 have 12X12 planes so there isn't a lot of area to make adjustments to have the drop fins running parallel to the keel or boat center line. The R25 and R27 tugs have wider planes so it is easier to adjust without loosing the drop fins added lift. After running with the drop fins off set from centerline of the boat for a year. I decided to make a custom set of drop fins that I could adjust moving outward instead of inward adding wetted surface to the planes. This worked well but I really noticed no difference in handling or performance. I did gain more trim when needed because of added weight from passengers in the cockpit or our dingy mounted on the swim platform. Eventually I did install larger planes only because I planned on the possibility of rack storing the boat and having Fork truck lifting it out of the water. The drop fins will be an issue if you plan on lifting the boat with a Fork truck. When I installed the larger tabs 18X12 I had the same trim as the 12X12 with drop fins. I had slightly more trim with the custom made drop fins when compared to the 18 X12. IMO install the drop fins edge to edge on the planes with the angled offset. You will not notice a difference in control or handling. You will have more trim and the ability to bring the bow down.

This is a link to the trim tab photo album I have on TugNuts. The photos tell the story.
gallery2.php?g2_itemId=48595

 

[CruisingElvinRay]

I’m happy with them on my R-23, but AutoGlide made a much bigger impact on performance for me.,


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[Rocky]

I added drop fins to my R29. Obviously a different hull, but it is the best under $50 mod I have done to the boat. It feels more stable at high (relative 😀 ) speed and the tabs have more authority.

I would recommend them.

Rocky

 

[S@LTD]

Okay I finally got around to adding drop fins and fabricated them to eliminate the offset and align them straight with the strakes per the suggestion from BB Marine. Admittedly I was not expecting much of a difference and figured what the heck for about 50 bucks.

Perhaps it’s the extra couple feet of LOA for the C28 vs. the C26 combined with the the tabs but it’s a dramatic improvement. Feels like I have an extra 50 hp coming up on step and the motor just seems to breath easier and not bog down. I picked up a solid 1 kt increase in speed at 2900 rpm. Overall the fatigue factor of pushing current, wind and coming up on step is way down. It just feels like have have a bit more hp capacity for added weight. Add me to the plus column for this modification. I couldn’t be happier.

Brian, I really appreciate your testing and details for this modification.

 

[Severum88]

I would almost tout the addition of the drop-fins, as game changer on my C26 as well. I can finally bring that bow down a bit as she should.

 

[Submariner]

I’m happy with them on my R-23, but AutoGlide made a much bigger impact on performance for me.,

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I've often wondered this. Was glad to see your response as I knew you did both. As I watch the autoglide run the tabs, it rarely keeps my tabs at 99%... I'll often see 85% or substantially less which is why I've questioned the need for drop fins on an RT27-OB.

 

[Brian98133]

As I watch the autoglide run the tabs, it rarely keeps my tabs at 99%... I'll often see 85% or substantially less which is why I've questioned the need for drop fins on an RT27-OB.

An interesting bit of info... I only have the stock rocker switches, so there's no percentage. Mine is more "seat-of-the-pants/timing/remembering", so I don't know for sure if I've gone past 85%.

However, it does seem - deduced by others' input - that the drop fins increase the performance of the trim tabs... so I might offer that this would equate to the actuators needing to do less adjustment on autoglide. Less actuator adjustment = longer actuator life? This is my only concern regarding installing my now-purchased-but-not-yet-installed Autoglide: If it's 'constantly' adjusting, will my actuators wear out more quickly? But, again, as others pointed out: A nice, level ride outweighs the potential for slightly less life of the actuators (IF that is even the case). I would agree. 😀

 

[briandunnington]

Ugh you guys - am I gonna have to get my wallet out again and spring for drop fins and AutoGlide (and a 4-blade prop while I am at it)? This forum continues to be very good at separating me from my money. :lol:

 

[Submariner]

so I might offer that this would equate to the actuators needing to do less adjustment on autoglide. Less actuator adjustment = longer actuator life? This is my only concern regarding installing my now-purchased-but-not-yet-installed Autoglide: If it's 'constantly' adjusting, will my actuators wear out more quickly? But, again, as others pointed out: A nice, level ride outweighs the potential for slightly less life of the actuators (IF that is even the case). I would agree. 😀

I would counter with... if the tabs aren't adjusting, then the boat is riding crooked with a list or a the wrong pitch, both which affect speed, fuel efficiency and comfort.

As far as would they wear out more quickly? Take a look at most marinas and you usually don't have to look very long or hard to see all the boats that never move. I'd consider it an honor to wear out my boat. UV rays, and lack of use age a boat too.

 

[RichBirk]

When I installed drop fins on my R29 I had to shorten my trailer bunks slightly first to prevent the full weight of the boat from setting down on the trim tabs.
Now the stern thruster blasts right into the side of the drop fins one direction and into the rudder the other making the stern thruster somewhat less effective. No big deal as moving that big keel sideways is a heavy push anyway.
Bear in mind, the lift trim tabs give goes up exponentially with speed. The R29 is a 15kt boat. The effect that drop fins have at 15kts is dramatic but quickly goes mushy as you drop down from there. I suspect that Martin is correct in that an outboard boat traveling at 25-30kts probably doesn't need drop fins.

 

[CruisingElvinRay]

My only minor complaint about the drop fins + AutoGlide is that it basically gives me four more rudders which, at times, gets the AutoGlide into an argument with my autopilot. Autopilot adjusts, then AutoGlide adjusts, then autopilot adjusts, etc…

I didn’t notice it until I was editing one of my videos for YouTube and sped up the video of a long run down a channel to 10x speed. At that speed the camera was sweeping back and forth and it looked like I was drunk at the wheel.

Minor complaint.


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[Submariner]

As I watch the autoglide run the tabs, it rarely keeps my tabs at 99%... I'll often see 85% or substantially less which is why I've questioned the need for drop fins on an RT27-OB.

An interesting bit of info... I only have the stock rocker switches, so there's no percentage. Mine is more "seat-of-the-pants/timing/remembering", so I don't know for sure if I've gone past 85%.

You can see how the trim tabs work with Autoglide. I video recorded it.

With a 3 blade prop (factory)
https://www.letsgochannelsurfing.com/autoglide?pgid=kwpreupu-418ab816-ec91-4815-8fb6-ad8edb268fb0

With a 4 blade prop
https://www.letsgochannelsurfing.com/autoglide?pgid=kwpreupu-2456bed3-1000-47e4-be1e-601dcb4d8e68

 

[BB marine]

For the owners of the outboard R23, R27 or C24 I don't see why a drop fin design trim tab would benefit unless there is a lot of equipment stored aft. Theses boats are true planing boats. Honestly the amount of trim tab used while planing should be minimal unless the boat is running at the speed the boat is just getting up on the pad. 18mph to 20mph. Anything above that, the hull, engine and prop combination should maintain planing speed without tabs if the hull design is efficient. The only time I used trim tabs on any boat I owned (planing hull) was to aid in a faster hole-shot ( getting the boat on plane) and to keep the boat at a level port/starboard attitude. I normally would pull my tabs up once on plane and add a little stern drive or outboard angle to give a slight bow lift for better ride and handling performance. A well designed planning hull with properly sized engine should hold plane with no need of trim tabs. If the planing hull is stern heavy a good stern lifting prop and proper engine mounting elevation is all that should needed.

The drop fins? I started researching drop fins as a way to get my Cutwater C26 to plane properly. When we went on our C26 sea trial I could not believe the bow high attitude this boat experienced. I thought there was something wrong with the boat. The dealer said this is how it runs. What?? Why?? I had never operated a planing boat that cruised with the bow so high. I talked to the Fluid Motion customer service about the issue and I discussed the drop fin idea. The trim tabs on my C26 were mounted to high so I was not getting full travel. The Bennett customer service rep that I talked to said the 12X12 tabs were to small for that size boat. He suggested a 18" X 12" plane would be the minimum size for a 26' boat. He said the drop fins would give the same lift mounted to a 12"X12" Plane comparing it to a 18" X12" with no fin. He went on further to say if the boat had a 18" X 12" he would still install drop fins because 18" X 12" is still marginal for that size boat.

He stated .... "You know Bennett made the drop fin trim tab design for the Bayliner hulls that Dave Livingston designed in the 80's. The Bayliner planing hulls would not plane with the standard tabs installed and there was not room for larger tabs to be installed. We designed the drop fins specifically for the Bayliner designed hulls. It is interesting that there is another Livingston design that needs to use the drop fins"

The inboard Rangers and Cutwaters will benefit from the drop fins if you cruise at 12kts + If you spend most of your time at hull speed or 8Kts or lower don't bother. If you cruise with a light boat all the time don't bother. We cruised heavy and I would not pilot a C26 or C28 without drop fins or larger tabs. I could hear a difference in the load on the engine when I installed them. My radar was now not picking out the stars!!! 😀

I have never piloted a 30' Cutwater outboard but I have seen them out on the water. I would highly recommend trying drop fins if you own one of theses boats. They run way to high for my liking when on plane. I can only image that dental work will be needed if you get in a heavy chop in one.

The drop fins are not for everyone. The people that I have talked to that have installed them have given praise when installed on a inboard Fluid Motion Ranger or Cutwater.

 

[cliffm]

Feb 28 picked up our boat from CSR Marine South (in Des Moines WA) . New CuKote bottom paint and installed Bennett DF12 drop fins to our 2022 R27OB. With our current TP Mach3 OS 15.6x13P 3-blade prop, running north to Elliot Bay Marina, in a minor headwind, incoming tide and in the fog I got some "rough numbers" captured.

I can say it runs flatter, I barley had to bump the helm starboard trim down to correct the typical list back to level running at 30mph.
I didn't touch it after that. Backing off the throttle slowly there was no noticeable stern squat till I got >4k/20mph. I didn't adjust trim to compensate, as I was busy watching the radar in the fog.

Overall, it would appear my GPH dropped slightly with MPG increased 2-3 tenths. The "attitude" of the boat is very stable, less perceived roll, and the slow speed wander is all but gone. I understand the R23, R25 and R27 are planning hulls, but the before and after performance difference is noticeable, and to us worth the $50 in parts plus labor to install.