Wave slap under boarding platform

[harrytnz]

We own a 48ft trawler style motor vessel, built in steel in 1999.
Our marina has strict length regulations, so we hinged the boarding platform to limit our overall length to 45ft. Because of a favorably placed pile, we have never had to lift the platform while at our berth.
However, when she was launched we found that she floated lower in the water than anticipated (not really the designer's fault). That causes us some problems when anchored in a busy place with waves slapping under the platform. In fact, it can get so bad that the platform (which is quite heavy) is lifted by the water and slams down, shuddering the entire boat. That's when the hinging is an advantage and we lift it with the mooring winch in the cockpit.
We have lived with the disadvantages of this setup for many years and are spending a small fortune every year to attack the rust around the hinge caused by all the flexing (the hinge itself is stainless).
We have contemplated welding the boarding platform down permanently and extending the hull. This seems perfectly feasible and would have the added benefit of extending the waterline, but it is something we cannot afford right now.
As an interim solution we are now considering welding the platform in position and mounting a simple flooded square fibreglass box under the platform to stop the effect of the waves. We thought this could be done by getting the welder to weld some tags onto the stern and under the platform to hold this box. The box may need to be spaced away from the stern near the waterline because of a few bilge pump skin fittings mounted there.
We thought that a flooded box would be less stressful on the fastenings than a box that wants to float. On the other hand you would not want to lift a box full of water on a few fastenings every time the boat heaves.
Has anyone any relevant experience or can anyone give me some advice on whether this idea is viable in some form?
Would a box like that still give me the benefits of an extended waterline?
I look forward to your input.

[diwebb]

Hi,
it looks from the photograph as if the boarding platform is about six to nine inches below the cockpit floor. If this is the case why not just raise the platform level with the cockpit floor?? The additional height above the water should not be a problem for boarding and the additional clearance should alleviate the wave slamming problem, also it would be considerably cheaper than the options that you are considering.

[harrytnz]

Thanks for your suggestion.
We looked at that possibility but, as you can see from the attached picture taken during construction, the boarding platform hinges some distance away from the stern. The support structure attached to the hull is integrated with the stainless belting. To shift it probably involves as much work as leaving the platform where it is and filling in underneath.
Moving it up would use less steel but would not give us the benefit of the extra hull length.

[eponodyne]

Torch it off and hang a ladder over the side.

[ASM]

Harrytnz,

I have expererienced this phenomena as well and had a lot of reports from captains on the same thing. But, by adding an empty 'hollow' box, you will get the same problem, only if that box is well submerged with its body then it will not be that big of a problem. The shuddreing of the boat could well be the excitement of the ship's hull natural frequency. If you go for the box solution, make it stiff and if possible, fill with fine DRY sand or at least add a sandwich damping system on the bottom plating. THis will dampen the structure dramatically, thus preventing any possible problems with this hollow box construction, which can act as a drum.