Hydrodynamics of integral swim platforms

[bcboater09]

Hello. I'm new to this site, and have a few questions about the hydrodynamic properties of the integral swim platform on my boat. My boat is a recently-acquired 1982 Carver 3007 aft cabin. The platform is bolted onto the boat’s transom as a stand-alone part; it 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. This 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 the hull from 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.

Any thoughts on this would be appreciated.

[daiquiri]

Could be some openings left for maintainance reasons... If you say they are pretty large, it means that they might be there to allow a person to insert his hands or tools inside.
They might have been placed there by the manufacturer for the maintainance of warping winches (if you decide to install them), for example.

[bcboater09]

There are inspection hatches in the top horizontal surface for accessing the interior of the swim platform, but my question relates to the openings in the vertical aft surface, of which there are several.

[daiquiri]

Do you have any photos?

The builder of the addition wanted to increase planing area (probably to lift more weight to plane as the boat got older) without adding (much) weight or buoyancy and wanted a fishbox that bled the fish well. You're slightly stern-heavy now at rest. You would be very stern light without allowing water in. Another benefit is increased stability at rest - another thousand pounds down low, or so, while at rest. If it is well done, you have a nice addition to your boat.

[baeckmo]

In addition to Mark775: you're spot on, plus that the water pumping in and out with the boat moving at displacement speed adds considerably to the damping of both roll and pitch.

[scotch&water]

to bcboater09 as a newbee to this forum Ithink your fish box extention is great, may help with fuel burn? Iam planing extentions on both sides of the IO to get lift. A photo or so would be nice more comments on this subject would help.

[marshmat]

bcboater,
mark775's explanation appears logical. Your extension adds some bottom area while on plane, while having little effect on trim while at rest.
Such a design is not unique to your boat. Zodiac had (has?) a little thing they called a Ribster a few years back; it was an inflatable with a segmented hard bottom. The bottom was hollow- it filled with water when at rest (thus providing ballast, of a sort, to improve the boat's roll characteristics) but quickly drained out of big holes in the transom to make the boat lighter when you accelerated.

[TollyWally]

This is very interesting. My thread of a week or so back regarding bottom loading was sparked by an interest in something similar to lower the bottom loading of my boat in an effort to decrease fuel burn. Mark's comments about the weight of the water within make a lot of sense. Sort of like using the additional weight as a trim tab to vary trim angle with out the drag associated with tabs. Knowledgeable comments expanding on the subject of a swim step after plane to decrease bottom loading and fuel burn would be appreciated.