Enlarging Trim Tabs

MasterFab

MasterFab

Well-Known Member
TECHNICAL Contributor
Aug 17, 2010
1,160
Wallkill, NY
Boat Info
2005 Cruisers Yachts 500 Express
Zodiac Yachtline 340DL w/ Yamaha 40 hp
Engines
Twin 715 hp Volvo D12s with V-Drives
As many on this site have already lamented, the trim tabs on the 1999 - 2002 340s, and some other years as well, have been pretty well recognized as being undersized. They sure don't meet Bennet's recommendations! So, since I had them off the boat to apply the barrier coating, I though I would tackle the problem.

I am posting a few pictures here of the finished result. There is a lot more info concerning the modification on my blog, for those of you who want more details. For the rest, I am not going to retype it all here - my fingers are getting tired! Basically, I expanded the surface area of the tab about 60%, with 1/8" thick 304L stainless steel. The details can be seen in the pics. I hope to have the engines in the boat in the next couple weeks, and splash sometime in July. I can provide performance details then. PDR_1974.jpgPDR_1980.jpgPDR_1975.jpgPDR_1979.jpg
 
View attachment 19812I extended mine 3" on each side and it made a huge difference. Yours will to I'm sure. The only problem I have now is trying to keep the paint on them! Its not the shear speed of my boat either lol... I think the paint is getting fizzed off by electrolysis? Dont know how to cure this? I have zincs fitted.
 
Wish I could buy some tabs ready to bolt on
 
annie said:
View attachment 19812I extended mine 3" on each side and it made a huge difference. Yours will to I'm sure. The only problem I have now is trying to keep the paint on them! Its not the shear speed of my boat either lol... I think the paint is getting fizzed off by electrolysis? Dont know how to cure this? I have zincs fitted.
Click to expand...


What type of paint are you using? Have you tried Trilux 33?
 
Mittens said:
I have not, but the boat is being pulled in feb-march for bottom paint, and I plan to address tabs at the same time. weather it be drop fins, or larger tabs with fins...
Click to expand...


i have also read where you can add plastic spacers between the bottom of the actuator and the tab to increase performance of the tabs....this works by pushing the tabs a little deeper into the water....you could use some remnant 1" starboard for the spacers....you can find remnant starboard on e-bay pretty cheap....

cliff
 
I had extended tabs put on my boat when I bought her in 2010. This picture isn't the best but in it you can see the tab extensions as well as an extension of the hull which increases the running surface and therefore increases the hull speed.

The tabs almost doubled the surface area and have really made a difference in how the tabs affect the attitude of the boat. Small adjustments of the tabs have a big effect on the boat.

Trim Tabs and Hull Extension.jpg
 
Nice My boat has prop tunnels so only so wide you can go. Could do spacer and the fins.. maybe that would make a bigger difference and be easy. currently only use the tabs all the way down.. so could space them so I have more room to push them... that is if it does not ruin the rams
 
Gofirstclass said:
I had extended tabs put on my boat when I bought her in 2010. This picture isn't the best but in it you can see the tab extensions as well as an extension of the hull which increases the running surface and therefore increases the hull speed.

The tabs almost doubled the surface area and have really made a difference in how the tabs affect the attitude of the boat. Small adjustments of the tabs have a big effect on the boat.

View attachment 61912
Click to expand...

Cool hull extension.

MM
 
I would think a Stainless spacer would be easy to cut, and bolt in... and add the Down Fins. maybe that would work, worse case is I run them up some and then its back to its normal position..

but I read you can not change to much angle with out moving the mounting of the rams position... so woul dbe nice to know from someone in the know how much spacer would be acceptable... will do that and the Down fins and see how it helps.
 
Glad you did... I will have a project for one of my machine shop customers...
 
This thread causes me to wonder how a major boat building company would send a product out the door with something as seemingly easy to fix as a set of trim tabs that is too small for the boat. Don't they run a test boat to see how it works before they commit to the final spec? Do these boats work well when the engines are fresh but performance falls off as engines age? It seems like such a simple issue to address proactively. Having said that, I'm sure there are many engineering questions to answer. But this is a big company with lots of expertise.
 
Nautical Engineering is a science unto itself. Making anything is a series of compromises whether small or large. Ironically, "improvements" can hurt or have no impact on performance.

In Sea Ray's case most of their larger boats have custom Bennett tabs which shows that Sea Ray engineering put some serious thought to the size and shape they were working with. Boats do not get lighter after they leave the factory which I believe is why trim tab size and structure can make a performance difference on older boats.

The problem is isolating any one improvement. If you haul the boat to modify the trim tabs, you have the bottom of the boat power washed. Then in my case, I added Bennett drop fins and extended each tab 3" to accommodate the drop fin length (12") rather than reducing the size of the drop fin to the trim tab (9"). So....which "improvement" worked? My speculation is that the drop fins probably added the most benefit by adding greater lift. The clean bottom initially also helped but throughout the season it gets normalized.

Why do I believe the drop fins helped? Because the boat has a different wake at speed with the drop fins which I don't attribute to lengthening the tabs or cleaning the bottom. If you look at the original tabs, they weren't designed to channel the water aft. Water could easily slide off the high and low side of the tab allowing the tab to "sink" in the water. The drop fins channel the water aft which clearly improved the lifting action of the tabs on the stern of the boat.

The boat now jumps up on plane which it hasn't done since I added a hydraulic swim platform 14 years ago. The overall cruise speed is still 1-2 mph better throughout this past season. It also handles better at cruise.

I'm sure someone with nautical engineering expertise could explain the physics better.
 
PlayDate said:
Nautical Engineering is a science unto itself. Making anything is a series of compromises whether small or large. Ironically, "improvements" can hurt or have no impact on performance.

In Sea Ray's case most of their larger boats have custom Bennett tabs which shows that Sea Ray engineering put some serious thought to the size and shape they were working with. Boats do not get lighter after they leave the factory which I believe is why trim tab size and structure can make a performance difference on older boats.

The problem is isolating any one improvement. If you haul the boat to modify the trim tabs, you have the bottom of the boat power washed. Then in my case, I added Bennett drop fins and extended each tab 3" to accommodate the drop fin length (12") rather than reducing the size of the drop fin to the trim tab (9"). So....which "improvement" worked? My speculation is that the drop fins probably added the most benefit by adding greater lift. The clean bottom initially also helped but throughout the season it gets normalized.

Why do I believe the drop fins helped? Because the boat has a different wake at speed with the drop fins which I don't attribute to lengthening the tabs or cleaning the bottom. If you look at the original tabs, they weren't designed to channel the water aft. Water could easily slide off the high and low side of the tab allowing the tab to "sink" in the water. The drop fins channel the water aft which clearly improved the lifting action of the tabs on the stern of the boat.

The boat now jumps up on plane which it hasn't done since I added a hydraulic swim platform 14 years ago. The overall cruise speed is still 1-2 mph better throughout this past season. It also handles better at cruise.

I'm sure someone with nautical engineering expertise could explain the physics better.
Click to expand...

Yes, but........why didn't SR come up with this in the first place, saving you the hassle of refining the design to get your improved performance? The company built a lot of 370s. BTW, I never felt the need to modify tabs on my 370 Express. They worked just fine, so I'm wondering if this is an issue that develops with age as engines begin to produce less power?
 
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