Swim platform digs in

T

tcdillard

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Joined
Feb 26, 2026
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8
Hi, I'm real close to a deal on a RT 31 CB and when we did a sea trial during a professional survey, the portside corner of the swim platform was digging into the water while underway, more so the higher the speed. It is a 2021 model and has a Seawise 2 stage manual dinghy lift with a Highfield CL290 BL and a Yamaha 9.9 outboard and 3 gal fuel tank. There were 3 adults on board during the trial. Fuel and freshwater quantities unknown. The surveyor seemed to think that having most of the batteries on the port side of the boat might be the issue, but I'm wondering if it might be something else? Trim tab issue? Thanks for your help!
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I had the same problem on my 2021 C30CB. With the same electric lift 9ft Zodiac hard bottom and 5hp outboard. It was al solvable. If you want to pm me with your phone number i can explain what my issue was and the solution is came up with.
 
Personally, I think it's expected behavior.

I have the same Seawise setup but with a Highfield 310 and a 20 HP Yamaha on my R29...and yes, the port corner of the swim grid is frequently just above the water and will catch the wave. It's a big weight right at the back corner.

Funny enough it was most noticeable when I did the sea trial as the boat was light. Adding our gear, filling the tanks, etc. adds a fair bit of weight, and we found we could trim the boat better with the gear. I tend to put heavier stuff in the starboard lazarette to balance out the batteries in the port lazarette.

We typically find that at lower speeds (< 1600 RPM) the boat runs fairly flat so no problem with the swim grid catching the wave. At the higher end (2800+ RPM), there's a trough behind the boat and no problem with the swim grid. In between those two, where we're not running flat but also not pushing enough water to generate the trough behind the stern, it gets wetter back there and the stern wave might bounce the dinghy a bit.

When we leave the dinghy at the dock, the swim grid rides 6" higher!
 
ShaneM said:
Personally, I think it's expected behavior.

I have the same Seawise setup but with a Highfield 310 and a 20 HP Yamaha on my R29...and yes, the port corner of the swim grid is frequently just above the water and will catch the wave. It's a big weight right at the back corner.

Funny enough it was most noticeable when I did the sea trial as the boat was light. Adding our gear, filling the tanks, etc. adds a fair bit of weight, and we found we could trim the boat better with the gear. I tend to put heavier stuff in the starboard lazarette to balance out the batteries in the port lazarette.

We typically find that at lower speeds (< 1600 RPM) the boat runs fairly flat so no problem with the swim grid catching the wave. At the higher end (2800+ RPM), there's a trough behind the boat and no problem with the swim grid. In between those two, where we're not running flat but also not pushing enough water to generate the trough behind the stern, it gets wetter back there and the stern wave might bounce the dinghy a bit.

When we leave the dinghy at the dock, the swim grid rides 6" higher!
Click to expand...
That may end up being the case and I'll just have to live with it. Thanks for responding.
 
We have a RT31 CB and use the Weaver Davits to carry a West Marine RIB-275 which is an aluminum bottom RIB. We also have an 8hp Yamaha on the swim platform bars and strap the fuel tank to the swim platform. I have the outboard on a bracket on the starboard side of the platform to help trim out the boat.

At slow-boat speeds, we have no issues with the swim platform getting close to the water.

At 14-16 knots, we do get some splash. I find using the trim tabs to bring the bow down helps somewhat. To adopt the belt and suspenders approach, in addition to the stainless struts holding the dinghy, I also have a web-strap at the bow and stern of the dinghy pulled tight to the transom.
 
Adding drop fins sure helped my 2021 R31S

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Like Steve says add drop fins.
You will be happy with the results.
 
jacrider said:
We have a RT31 CB and use the Weaver Davits to carry a West Marine RIB-275 which is an aluminum bottom RIB. We also have an 8hp Yamaha on the swim platform bars and strap the fuel tank to the swim platform. I have the outboard on a bracket on the starboard side of the platform to help trim out the boat.

At slow-boat speeds, we have no issues with the swim platform getting close to the water.

At 14-16 knots, we do get some splash. I find using the trim tabs to bring the bow down helps somewhat. To adopt the belt and suspenders approach, in addition to the stainless struts holding the dinghy, I also have a web-strap at the bow and stern of the dinghy pulled tight to the transom.
Click to expand...
Thank you!
 
First, make sure your trim tabs are the correct size for your boat. Search Bennett trim tab sizing on the web. You will see this:
Boat Length Tab Size Recommended
25’-31’ (7.6-9.1 m) 12”x24” or 9”x30” or 12”x30”

Then add drop fins parallel to the keel. Not parallel to the tabs themselves as mentioned above. FM has a history of providing undersized tabs on stern heavy boats. Not a good combo.
Also helps to move as much weight forward in the boat as you can.
 
it4llc said:
Different boat (C-28) but drop fins helped a lot with trim. Easy to install. I just aligned mine to the edges of the tab but Brian Brown had a good post about installing them so they are inline with the water flow when the fins are down. https://www.tugnuts.com/threads/mod...-bennett-drop-fin-and-spacer.9262/#post-68056
Click to expand...
Wow, super helpful. The boat I'm looking at already has spacers on the trim tabs, but neither the broker nor the surveyor knew why. Looks like they got started on fixing the issue, just need to finish it with down fins. Thanks for the help. Much appreciated.
 
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