Hull Strength

I am going to make some alterations to my boat, and although what I want to do will not weaken the structure (I hope) what I am wondering as a general rule: Would the hull of a boat designed with a cabin be just as strong if the cabin was left off.

I am altering the cabin but would like to use the boat during the alterations, wise or not?

To anyone who wants to pick on me, I know I haven't posted a pic of my boat with the details of the structure and I know that a definate answer can't be given, but as a general rule. She's 6metre.

Poida

 

True Poida - without a picture it's a difficult one to answer. All I can suggest is that before you remove cabin (or part of it) check that the cabin cross-beams are not integral to bracing the hull. If so you must fit alternative bracing in the shape of a bench or splash shield or some such or you may find the hull closing around you or buckling under heavy wave pressure.

 

Generally, the trunk cabin (structure sticking up through the deck) is a weak link in the athwart strength of the decking. Much effort must be made to reinforce openings and any non-full width beams or load bearing pieces. In wooden structures, it's pretty easy to see how much you have there, often the deck and cabin roof beams are larger in dimension then the frames, even though they may share on center lengths. In 'glass boats, it's more difficult to see what's going on, but I can assure you, if you cut the cabin off the boat, there will be considerably more hull twist vulnerable (at the very least), regardless of construction type.

Some build methods are less prone to "racking" or other distortion forces, but not knowing what you have only permits some guessing. A flush deck, or raised deck structure is much stronger then a deck with houses poking up through it.

If you want to use your boat, without moving things around, unintentionally after the cabin has been removed, then install some temporary braces to accept the loads while you use it. These can be removed as you continue your upgrades. They will also serve to maintain the original shape of the boat. Any time you consider cutting off a major component of the boat, thought should be employed about the likelihood of things moving around when the pieces are removed.

This is a common issue during restoration and repair. The decking and beams needs to be replaced, for example, but in removing this, the hull sides (possibly the keel too) will relax and the boat shape will become distorted. You could remove and repair sections at a time, but this isn't very cost effective nor efficient use of time and materials, so you install temporary braces to keep the hull sides from falling outboard, removing them as the new beams get installed. Can you use the boat in the mean time? Sure, but you'll have a highly weakened boat that will be seeing loads it wasn't designed to accept. You can brace it, hope for the best and not over work her, or . . .