Bennett DF12 Drop Fin upgrade

[cliffm]

Currently we have our 2022 R27OB out of the water at CSR Marine in Des Moines WA, getting 2-coats of SeaHawk CuKote applied. While it's out of the water, we are addressing the under water lights mounting screws corroding away, drilling out and installing 318SS 8/32x1.25" with nylocks.

We decided we would go ahead and install Bennett DF12 drop fins and see if that helps with the stern squat of our R27OB with Factory dinghy lift and outboard. We had swapped out prop's to a Turning Point Express Mach-3 OS in 16x13P and have been tracking MPG numbers. Fingers crossed we pickup some more midrange MPG with the drop fin stern lift?

 

[80Ramey]

Currently we have our 2022 R27OB out of the water at CSR Marine in Des Moines WA, getting 2-coats of SeaHawk CuKote applied. While it's out of the water, we are addressing the under water lights mounting screws corroding away, drilling out and installing 318SS 8/32x1.25" with nylocks.

We decided we would go ahead and install Bennett DF12 drop fins and see if that helps with the stern squat of our R27OB with Factory dinghy lift and outboard. We had swapped out prop's to a Turning Point Express Mach-3 OS in 16x12" and have been tracking MPG numbers. Fingers crossed we pickup some more midrange MPG with the drop

We’re running a turning point as well. Mach 4 express 15/15. It’s a solid 1.9mpg prop above 4200rpm. Got forced into it as a quick replacement but it will be a good spare. Really interested in seeing your drop fin improvements.

 

[Cutwater28GG]

drop fins helped my C28 get its butt out of the water

 

[cliffm]

We’re running a turning point as well. Mach 4 express 15/15. It’s a solid 1.9mpg prop above 4200rpm. Got forced into it as a quick replacement but it will be a good spare. Really interested in seeing your drop fin improvements.

We had tried that Prop, same MPG, but were concerned when couldn't get WOT rpm past 5300-5400 (its due to having allot of cup acting like a 16P rather than 15P) but the slip numbers were lower than the SWSII 15.75x15P and way better than OFS4 15x15P. Our Mach 3 OS is slightly less MPG but it's also 13P providing 600-6K rpm range.

 

[cliffm]

drop fins helped my C28 get its butt out of the water

fingers crossed it will do the same for us!

 

[Submariner]

Here's a great thread on the topic.

G

Thread 'Adding Drop Fins - Good or Bad Idea'

Feb 10, 2022

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.

• Gypsy
• Replies: 16
• Forum: General Technical Discussion— Ranger Tugs

 

[Rocky Lou]

+1 helped my C302c

 

[Rebel112r]

I installed them on our R27, can’t say that I have noticed much difference either good or bad.

 

[Submariner]

I installed them on our R27, can’t say that I have noticed much difference either good or bad.

Brian's got a Sharrow on his R27, and I'm sporting the SCE5 Modified prop. With the Lenco Autoglide, I see the position of the tabs on my chartplotter (0-100%).

Because the R27 is a planing hull, both Brian and I have found that the best fuel economy comes by using the least amount of tabs (the boat rides on the hull). It's rarely possible to run with 0% tabs on both sides due to the torque of the motor twisting the boat to port. Brian spends a lot more time balancing his boat than I do. But moving more weight to the starboard side to compensate for the port-torque is one way. I generally drag the starboard tab (which is really the port side tab that's down). Going faster than 20 knots, I'm usually between 20 and 60% deployed, and the port tab (starboard side) is at 0%.

Drop fins, as I understand them, are meant to act like you have more tab surface area than you actually do. For example, if you needed 16" tabs, but only had space on the transom for a 12" tab, then drop fins are a way to improve the tabs.

Since I'm usually not near 100% usage on my tabs, I've always felt the R27 has enough tabs to do their job. However... there is one use case I'd be interested in hearing more about... The mid-range.

When I slow down and enter the mid range (11-19mph), that's when I would expect my tabs to be deployed, and seeing near 100% deployed wouldn't be unusual, depending on the prop I'm running. Usually my fuel efficiency remains good, but I suspect that's more attributed to the 5 blade prop or the Sharrow, being able to bite and hold the stern up.

Some examples I've seen in my testing using the SCE5 Modified (PowerTech)
5653 RPM @ 36.3 mph, 27.1gph = 1.34mpg, port tab 0%, stbd tab = 0% (WOT).
4800 RPM @ 31mph, 18.5gph = 1.67mpg, port tab = 0%, stbd tab = 86%
4100 RPM @ 24mph, 13.0gph = 1.84mpg, port tab = 0%, stbd tab = 62%
3700 RPM @ 17mph, 10.6gph = 1.60mpg, port tab = 0%, stbd tab = 63%
3100 RPM @ 13mph, 7.60gph = 1.71mpg, port tab = 0%, stbd tab = 66%

For comparison:
Some examples I've seen in my testing using the OFS4 Modified (Power Tech)
5805 RPM @ 34.5mph, 26.7gph = 1.29mpg, port tab 0%, stbd tab = 34% (WOT)
4900 RPM @ 26mph, 18.4gph = 1.41mpg, port tab = 0%, stbd tab = 78%
4000 RPM @ 20mph, 11.8gph = 1.69mpg, port tab = 12%, stbd tab = 98%
3600 RPM @ 16mph, 9.80gph = 1.63mpg, port tab = 0%, stbd tab 97%.
3200 RPM @ 15mph, 7.60gph = 1.93mpg, port tab = 20%, stbd tab = 96%.

On the factory 3-blade (Yamaha SWSII)
5600 RPM @ 38.5mph, 26.0gph = 1.48mpg, port tab = 0%, stbd tab = 43% (WOT)
4700 RPM @ 29mph, 17.3gph = 1.67mpg, port tab = 0%, stbd tab = 100%.
4600 RPM @ 29mph, 17.3gph = 1.67mpg, port tab = 0%, stbd tab = 68%.
4100 RPM @ 16mph, 12.1gph = 1.32mpg, port tab = 27%, stbd tab = 94%.

I have screenshots (photos) of my dash, which is where the above data came from.

It'd be interesting to see what happens to a R27 with a 3 blade prop, and drop fins installed, on how slow the boat can roll before fuel efficiency really plummets. Get the boat up on plane doing 30 mph... lower both tabs with drop fins, and slow down and watch fuel efficiency to see how the fins can help the boat stay up on plane at slower speeds. (11-19mph, specifically).

I've finally got the SCE5 Prototype off Channel Surfing and am running a production SCE5 Modified from Power Tech (the 3" extension with 1" diffuser in annodized black). We went to the Bremerton Heavy Weather cruise hosted by Bremerton Yacht Club two weeks ago. I captured this screen shot crossing the VTS in Puget Sound, south of Edmonds, east of Agate passage.
4100 RPM @ 24mph, 13.0gph = 1.84mpg, port tab 0%, stbd tab = 62%.

I'm not sure if I'm going to wear out my starboard tab from excessive use, or if I'm going to wear out my port tab from lack of use. I may try moving some weight to the starboard side this season to use neither tab like Brian does.

 

[Rocky Lou]

My 5 year old Lenco were getting worn and drop fins fell off, so I ordered: 12 x 24 Bennett: https://www.fisheriessupply.com/bennett-trim-tabs-drop-fin-trim-plane-assemblies/6bt500700700

 

[cliffm]

Picked up our boat from CR Marine south today. New CuKote bottom paint and Bennett DF12 drop fins installed. With our current TP Mach3 OS 15.6x13P 3-blade prop, running north to Elliot Bay Marina in a minor headwind and incoming tide and in the fog I got some "rough numbers" captured.

I will say it runs flatter, I barley had to bump the starboard trim to bring the starboard high list back 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 below 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 bumping up 2-3 tenths. The "attitude" of the boat is very stable, feels like less roll, and the slow speed wander is all but gone.

See attached comparison