Hydrodynamics of integral swim platforms

[rapscallion]

My boat, a recently-acquired 1982 Carver 3007 aft cabin, has a factory-installed integral swim platform. While I believe the platform is bolted onto the boat's transom as a stand-alone part, it definitely continues the hull bottom by roughly 30 inches. The platform has a built-in bait well on the centerline as well as two inspection hatches, one to port, the other to starboard. The platform's vertical aft surface has large holes in it that allow it to fill with water. I'm curious about that. Why is it meant to fill with water? I would think that by extending the hull bottom the swim platform would provide additional buoyancy in the stern, perhaps compensating for the weight of the aft cabin that also serves as the location for the fuel tanks, which differs from the more conventional cockpit which would presumably be significantly lighter and would not require any additional buoyancy. Does the platform actually provide that additional buoyancy if it's full of water? What is the hydrodynamic purpose of the platform if the boat drags around the additional weight of water? My boat sits relatively trim in the water while at rest, but not perfectly level; it has a definite sternward tendency. Perhaps the platform has to be full of water or it would provide too much buoyancy, forcing the bow down and prevent lifting onto plane? Then why extend the hull bottom? Why not just add a cantilevered swim deck with no bottom? Perhaps it's less a question of hydrodynamics and more one of structure: the platform must be capable of supporting the weight of people, a dinghy, outboard motor, etc, and therefore requires the additional structure that only an integral platform can provide?

Do other Carver owners here have factory-added integral swim platforms that are water filled?

Any naval architects or boat builders here who can answer these questions?

Thanks.

 

[ChefJunk]

I was seriously considering a like boat and am intrigued by your description of the platform. Any photo's available?

Jim

 

[Carver 3007]

I have a 1982 3007 also with the extended swim platform. However, the top surface on mine is solid, no inspection hatches or bait well. The openings in the vertical surface on mine are 1.5" +/- diameter holes with hoses inserted. I am the 4th owner of the boat and I bought it in 2006. The survey suggested that I would need to monitor the condition of the bolts that attach the platform to the transom. I can inspect the bolts by removing the drawers in the aft cabin cabinet.
At my last inspection of the bolts, it is time to plan a day to change out the bolts, washers and nuts and rebed. The only way to do this is for me to remove the top deck of the platform,,,,I can see attachment screws or bolts with washers under the top deck rim and assume this is where to start,,,by your post, it sounds like you could replace your attachment bolts without removing the deck.
I asked the same question about the reason for the waterfill but no one in my circle could answer why. The hoses that I mentioned on mine were replaced, according to the log, in 2004.
ChefJ,,,,I will try to post a picture later tonight.

 

[rapscallion]

There is a great post in the Carver Owners forum called Dinghy Davits (posted 22-Apr-09 by awboater) that shows photos of how to cut circular-shaped holes into the top surface of a swim platform in order to get to the underside/inside for service work, or, the poster's case, to install a set of davits. Off-the-shelf inspection plates are available to cover up the holes. My boat had these access holes with inspection plates when I bought it.

Given that you have the same year and model boat as I have, I will certainly be pulling out those drawers in the aft cabin to have a look at the swim platform mounting bolts. Thanks for the tip.

In reply to ChefJunk's post, I do have some photos of the swim platform taken during a haulout. I will look for those and try to post one or two later tonight.