Cockpit Extension for a Sundeck Trawler ???

The title of this thread is the basis of a question my wife proposed to me while looking at affordable Sundeck trawlers (42' - 50') at a recent brokerage show. Sundeck type trawlers offer a great deal of living space. However the spacious aft sun deck is often quite high and transitioning to the swim platform and on/off the boat via the transom area is precarious by stern & boarding ladders. So her question was could one not just add an extension to the boat that offers a more graceful transition from the sun deck to the swim platform while incorporating a small cockpit area as well? I thought that was a great question!

So I know that just extending the entire hull form aft is nothing new. However doing that has many consequences some of which are both very expensive and negative. These are mostly related to the added buoyancy, propeller/rudder and negative handling issues.

My question is what if the added buoyancy was minimized by using a modified rounded transom or boat tail concept in the extension design? In other words for support structure taper the bottom of the extension from the a point at the hull transom up to the aft end of the extension? And how would this concept affect the potential negatives of just extending the hull form an additional 4 - 6 feet? Would this negate/alter handling issues? Reduce affects of the extension on propeller and rudders?

I feel that 4 - 6 feet is adequate for this type of extension, understanding that loading may be limited and that the existing swim platform would be moved the the aft end of the extension.

Anyone who would like to partake in this design thought is greatly welcomed. Also I am new to this forum and look forward to your reply's

 

You will be shifting the center of gravity aft which will affect the attitude of the boat. Secondly, the geometrical relationship of the thrust (propellers), steering and pivot point will all change. Another major change will be the reserve flotation when the boat pitches, and particularly running which may induce broaching.

 

Welcome to the forum, Skip.

I think a cantilever type of affair would work, though you'll possibly have to move a few items to balance out her trim again. Placing this lower platform in the water will bring up a host of new issues, that may not be necessary, particularity if the prop(s) has to blast into or around it. Maybe post a simple sketch, to get an idea of your thoughts.

 

One of the problems with extensions is reversing. There is a research vessel here that was modified like that and it ships water when going hard astern.

 

You may be right but you should never go hard astern except in case of imminent collision (crash stop). In this case the most important thing is to stop the vessel as soon as possible.

 

Transom extensions seem to be a routine procedure around here. If they're only going 2-3 ft they don't move the running gear. Not sure at what point you'd think about moving the gear. It seems like a lot of fish boats have quite a bit of distance from gear to transom, so they're less chance of getting the nets in the prop, with no bad side effects. I'm sure a couple of hours with a NA would get you sorted out.
From an added displacement standpoint, it seems like an awful lot of boats are overloaded and may actually benefit from some extra buoyancy aft.
From a doing it standpoint, if you're doing more than extending the bottom under the swimstep, and are adding to the topsides and building a cockpit it's not a cheap project.

 

Cockpit extensions are a pretty common modification here in Ft Lauderdale. I would bet that at any given time there is at least one happening at one of the local yards.

That said, if I was shopping for a boat I would find one that suites my needs without a major mod like a cockpit extension. Its an expensive mod and not without potential issues regardless of how carefully and skillfully its executed.

Steve