Modification of trim tab,Bennett Drop fin and spacer

B

BB marine

Well-known member
Joined
Feb 1, 2016
Messages
2,905
Fluid Motion Model
R-21
Non-Fluid Motion Model
Mainship 34 Pilot
Vessel Name
PORT-A-GEE
Our 26 Cutwater has always had a bow high attitude. From 2250 RPM the rest of the way up the rpm ranges the bow just rose. Great ride just bow high attitude. When it was just two on board it was ok. Add gear ,cooler and 4 extra people it was really digging a hole at the stern. Full tabs all the time 2250 rpm and up. I never raised the tabs up because of speed loss. I became concerned about taking our extended trip (LOOP) adding extra equipment ,RIB and outboard, portable generator, paddle-board and tools. Extra gear, cooking, clothing, grill, bikes, paper charts the list goes on. Then provisions it all adds up. I wasn't looking for more speed just more control of the trim attitude to maintain the boats designed performance.

My goal was to make modifications that could be reversed if they didn't work. I machined two spacers to drop the trailing edge of the trim tab down an additional 7/8" This puts the trailing edge even with the bottom of the boat (7/8" of the 2 1/2" trim ram travel was above the bottom of the boat, active trim travel was actually 1 5/8") now I have use of the 2 1/2" travel.





Next I spoke with Cutwater about using Bennett Drop fins with the understanding that if it didn't work the fins could be unbolted. I also talked to a technician from Bennett about the application. He ( Oliver) very informative, said that adding drop fins is equivalent to going to the next larger size trim plane. In my case 26 cutwater 12"x12" Trim Plane, add a drop fin =18" X 12". He stated that 18" X 12" is the size trim plane that Bennett recommends for a 26' boat. Another advantage to the Drop fin is less spray when full trim, the fins hold the water in . Thats how they work. They trap the water and that adds lift. I removed both trim planes from the boat so I could drill the mounting hole using a drill press. I used 1/2" 10/24 stainless screws washers and lock nuts and reinstalled on the boat using life caulk polysulfide sealant.




I have logged three sea trials since modifications all showing positive results. My WOT is the same as last year but the bow is down and I can actually over trim. At WOT I can reduce trim and maintain speed. My cruse speed is the same as last year but the bow is down and I am leaving a flatter wake. The difference is I am comparing last year with 2 people and less equipment no bottom paint ( we didn't leave the boat in the water last year). This year I have bottom painted , 8'10" aluminum RIB ,6 hp outboard, 2000watt inverter, 10' paddle board, Catalina Magna grill, approx 300lbs extra weight in interior modifications.One of the sea trials was done with 4 on board.Three in the cockpit Me at the helm. 1/2kt different compared to my delivery sea trial speed.
2017 with modification 1' to 2' seas Lake Michigan
4130 RPM +/- 20 rpm 20 kt +/- 1/2kt. WOT. bow down full tab but I can lift tabs and maintain speed and bow
comes up
3200 RPM 12kt +/- 1/2kt. cruise speed bow down great ride flatter wake full tab
2250 RPM. 7 kt +/- 1/2 kt. full tab boat runs level

We are happy with the modification and feel that it is helping to maintain the designed performance of the boat. I did post some sea trial numbers in another topic on tug nuts (26 cutwater Performance)

Brian Brown
26 Cutwater
PORT-A-GEE
 
What material did you use for the spacer?
 
304 stainless, same material that Bennett used on their planes. 3/8" thick angle machined to fit drilled and tapped so I could use all same mounting hardware.
Brian
 
I really, really like your post. I am going to look at this for my "bow-up" C30. Good work, thanks.
 
BB Marine,

I really like your post also. On my R31S, with only two people aboard with both forward in the cabin, it runs fairly level at higher speeds. Also, with only two of us, it is somewhat bow high at slower speeds but above displacement speed. But with passengers in the cockpit and only me in the cabin, it sits fairly bow high.

Since my R31S is in storage, does anyone know the size trim tabs it has? What is the chord (fore to aft)? It appears that the Bennett "Drop Fin" works on 12" chord (fore to aft) tabs only.

If I can confirm that the "Drop Fin" kit will fit, then I will have the boat storage place install this kit while the boat is out of the water.

See this link for info about the specs for this kit: http://www.defender.com/product.jsp?pat ... id=2393734

Thanks,
 
This post really should be a *STICKY* or at least included in a list of "top" modifications... or better yet, the factory should incorporate this modification as part of the factory configuration!

I recall seeing this post back in JUNE and completely agreeing with the assessment that our Cutwater rides bow high and that we would be at max tabs all the time in order to try to get the bow down...

Had boat hauled out earlier this month as we enter our 3rd year of boating to refresh the bottom paint. 1st priority was following BB Marine's modification to get the Bennet Drop Fins installed (the hardware itself is only about $30-40 delivered... a bargain in the grand scheme of boating expenses! Have I mentioned the factory should consider installing them from the onset? ) 😀

Could not be happier with the outcome... have gained 1-4 knots in speed depending on conditions compared to prior to haulout. Whereas previous speeds were cruising 16-18knots with top speed approaching 20 knots, now seeing cruise 17-20 knots and WOT speeds up to 24 knots.

Some of it may be attributable to fresh bottom paint but the trim tab mod allows for more control over trim as it is now possible to slightly over-trim instead of always having to max out the trim tabs.

Thanks BB Marine for posting this! Fluid Motion/Cutwater, please do future owners a favor and make this standard! 😀
 
To be clear, you made two modifications? First one dropped the stock tabs the distance of the spacer, and the second added the Drop Fin supplied by Bennett? This mod has hit the top of my list of priorities. I complained about the bow high attitude of my 28 from the get go. The other thing I wish the factory would do is move the swim deck up flush with the door sill for the entry into the cockpit (or as close as possible to minimize flooding risk when combined with a gap around the hull where the door sill is located.) The lower lip at the back also needs to be removed or filled solid and tapered to the front of the platform. The platform currently adds huge drag as the boat accelerates and carves a big hole in the water with attendant spray going everywhere. Seems a little computer modelling could solve these issues.

Can't wait to talk to Bennett about this. I did phone them in my first year of ownership and they confirmed that I could not upgrade to larger tabs because of prop and rudder interference and hydrodynamics, but this sounds like those clearances are not affected.

Thanks so much for posting this. Really tired of trying to climb out of a big hole every time throttle is above hull speed!
 
Years back a friend made his own version of the drop fins for the undersized tabs on a C-Dory 22. He experienced the same dramatic improvement described in the OP.
 
pleeson said:
To be clear, you made two modifications? First one dropped the stock tabs the distance of the spacer, and the second added the Drop Fin supplied by Bennett?

Yes, first modification was to make a spacer to lower the tab down. My tab fully retracted was 7/8" above the bottom of the boat. Bennett trim cylinders have 2.5" of travel so I was only getting 1 5/8 useable travel. The standard is 5/8 above the bottom and 1 7/8 useable travel. I machined a saddle for the cylinders plastic mounting bracket to sit in. This lowered the trailing edge to be even with the bottom of the boat with the trim ram fully retracted. This gives me the full 2.5" of travel all workable. I don't need all that travel but I would rather have more travel than not enough. Then installed drop fins. After season of having the modification I am happy with the results. We had our RIB mounted on the swim platform, 6 hp outboard, Three adults sitting in the cockpit and cruising at 3200 RPM bow down plowing threw a 2 foot chop on Lake Michigan smooth ride !
Brian Brown
26 Cutwater
PORT-A-GEE
Click to expand...
 
Thanks very much Brian! Looks like I have a winter project! Sure be good to get one irritant off the table. Really great boat but it does have a couple of silly shortfalls that grind on you... Happy cruising!

Cheers, Paul.
 
as a cutwater 28 owner I understand the bow high attitude of the cutwater. that being said when I trim the bow down it really makes steering that much more volatile. in that if a wave catches you from the side slightly there is s tendency for the bow to role a little catch the wave and spear off in a random direction. this is solved by keeping some bow high attitude.

I am not a trimming expert but it seems to me that not putting stress on the autopilot trying to control these tendencies is beneficial so a little bow high is not bad. also I dont see any speed improvement when dropping the bow so its not like its more or less efficient and the boat takes waves much more comfortably with the bow up slightly. yes there is slightly less visibility. but I dont find this a problem anyway.

what am I missing in keeping the bow as low as possible?
 
Cutwater28GG,

I don't think the goal is to "[keep] the bow as low as possible". The trim tab modification with spacer and drop fins really improves the ability to adjust the trim tabs to match conditions above and beyond the factory configuration.

I've mentioned that I now have the ability to "over-trim" whereas before, I generally always maxed out the tabs and sometimes struggled to stay on plane when fully loaded with no chance of actually getting the bow "down".

Lots of articles out there on boat trim... here're a few from a quick search:
http://www.sportfishingmag.com/boats/bo ... s-attitude
https://www.clubmarine.com.au/internet/ ... +technical
http://www.floridasportsman.com/2014/07 ... oper-trim/

For example, when meeting a steep head sea, many boats run more comfortably and efficiently with the bow down slightly, allowing the sharper forward-V to knife through the waves. Conversely, in a following sea and when overtaking waves, it is often safer to run with the bow up a bit to help prevent stuffing into the backside of a roller.
Click to expand...

Prior to modification, couldn't get the bow down (trim tabs maxed)... now I can... and can adjust tabs so that boat attitude improves handling/efficiency.

How much this is or isn't an issue for you likely depends on boating habits (factory tab configuration was inadequate for us when fully loaded with 3-4 scuba divers, 6-8 tanks, weights, gear, etc.) and local conditions (directly into wind/swell kept bow high and tabs unable to compensate prior to modification).

Just my 2 cents and hope that helps!
 
Thanks ASM777 - most useful.

I absolutely notice when i have people in the cockpit how it effects trim so if you have people and 8 scuba tanks thats a serious change to the balance of the boat that would need to be trimmed out and more effetive tabs would be needed
 
I installed a set of drop fins after talking with Brian. I also lined them up as much as I could with the hull strakes, which makes them look like I did a poor job lining them up with the trim tab edges. The drop fins made a noticeable difference on how far I could trim the bow down. I have to be careful not to overtrim, but I wanted a little extra trim when I am carrying extra passengers in the cockpit and trying to make some time. Also, In the past, I normally had to use full trim at 18 kts+ to get the best performance. Now I use about 2/3 trim at the same speed.
 
Brian,
Thanks for your post and in sharing the details of the two trim tab modifications you made. I’m getting ready to make both mods to our 12” by 18” tabs.
Our R-25 Classic also has a problem getting the bow down when we are fully loaded for a long cruise.
The trailing edge of our trim tabs are a full inch higher than the bottom of the boat when fully retracted so our problem was slightly worse than yours in that respect.

gallery2.php?g2_itemId=76522

The transom on our boat is not a straight line perpendicular to the keel but curves forward by 7 degrees at the mounting point for the trim tabs. The trailing edge of the trim tabs move down and out as they are extended. It’s my understanding that the drop fins should be installed parallel to the keel. Bennett Marine customer support confirmed this on 11/6 so I will be mounting the drop fins at a 7 degrees offset to the trim tabs themselves.
As to the SS spacer, Bennett offered a different solution. They make a 1/2” spacer (part number A1124) that goes between the upper actuator bracket and the transom. I ordered two of the spacers from Fisheries Supply and will try going that route. The one thing they were absolutely against doing is repositioning the lower actuator bracket higher up on the vertical trim tab flange. Doing so could lead to the flange cracking and breaking.
I will post photos of this project in my album when it’s completed.

EDITED On 11/6 to add photo link and to include feedback from Bennett Marine Customer Support
 
BB marine":1kkelof8 said:
304 stainless, same material that Bennett used on their planes. 3/8" thick angle machined to fit drilled and tapped so I could use all same mounting hardware.
Brian
Click to expand...
Thanks! I found a website who will sell by the inch of 304 3/8" thick in various lengths.
What size did you use, 1"x1" or 2" x 2" i.e. the length of each side of the "L".
How long was each piece on each tab, 4" or length of the entire bracket?
Thank you.
Cutwater C30.
I already ordered the drop fins.
Also what technique did you use to get the fins aligned with the keel vs. edge of the trim tab which is apparently wrong.
Did you use a cobalt drill bit in the Stainless?
 
serpa4":3kczap90 said:
What size did you use, 1"x1" or 2" x 2" i.e. the length of each side of the "L".
How long was each piece on each tab, 4" or length of the entire bracket?
Click to expand...

The spacer shims I made were 2" H 3" L 3/8" SS . I found no need to make them any longer. There is no change in the load applied, so increased area in not needed. The 3/8" dimension that I used was the needed spacing to put the trailing edge of the plane even with the bottom of the boat.


serpa4":3kczap90 said:
Also what technique did you use to get the fins aligned with the keel vs. edge of the trim tab which is apparently wrong.
Click to expand...

There is a lifting strake on the bottom forward of the trim tab plane. This lifting strake is running parallel to the keel or center line of the boat. I used a straight edge laid up against the strake. I had 1' of the straight edge extending past the transom and just below the trim tab plane. Using the straight edge has a guide and a permanent marker draw a line on the bottom of the trim tab plane. This line is your reference line for the angle the drop fins need to be mounted to make them run parallel to the boats center line.


serpa4":3kczap90 said:
Did you use a cobalt drill bit in the Stainless?
Click to expand...

Yes, Don't use cheap ones either. The extra$$$ on the bit is worth it.


Here is a photo of the Shim and there are several pictures of the complete Trim tab project.
gallery2.php?g2_itemId=77008
 
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