How do motorboats sink?

Mike k said:
Hi thanks for replies but....
do you mean 'deck level' or water line- the top of the compartment is above the waterline?
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I did mean deck level. Going up that high just keeps the water on the other side until it finally reaches the level of the deck. Obviously stops water flooding the rest of the boat until that point.

Your boat won't be built that way as you'd need a water tight door or lose a huge amount of internal space. More a reference to Titanic.

The top of the compartment may start out above the waterline but may end up below it after you get holed. It all depends on design and location of holes in hull. Titanic was meant to be OK with bulkheads built up well above the waterline.
 
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Interesting topic for me as I'm rebuilding a 21' 1980's Fletcher and done quite a bit of reading up on this (and a few threads on these forums).

As a bit of background - please bear in mind that this is a small vessel (open sportsboat with cuddy and inboard V8) and thus will have less options than perhaps a 30-40' Mobo.

Currently: I have almost stripped back to bare hull (just removed engine) with just engine bay stringers, engine bearers and transom left to remove. Thus the hull will be a blank canvas.

Original format: The bilge ran through the hull from the doorway of the cuddy to the drain plug at the base of the transom (limber holes quite large - basically being the triangle of space between the keel and the framing so 4-5 sq inches). The bilge pump was in the engine bay. Either side of the engine bay were two compartments with the bottom section filled with buoyancy foam and the top section (above waterline) for battery and storage. There was no bilge pump in the cuddy, but the entire section below a molded floor was filled with buoyancy foam. The cockpit floor was on three levels - lower in the centre walkway and stepped up along the sides with the voids beneath this (formed by the frames and stringers) also filled with buoyancy foam.

You will see a picture forming already - all the 'unusable' voids filled with buoyancy foam! This won't necessarily stop the boat from sinking, but will importantly slow the speed it does - you can find examples in the news if you look where boats of this type have been towed in after 'sinking' and this is usually because the bow remains above water. If your vessel is swamped or capsized, no practical amount of foam will keep you above water, but enough will stop the boat from descending into the deeps! A partly submerged boat is a bigger 'target' for search and rescue and can also be salvaged.

Sinkings?: As to why they'd sink in the first place, most of my findings have suggested mechanical failure - engine or hose etc, or a forgotten bung, seacock not closed or corroded etc. Aside of running aground, accidents seem to be the exception rather than the rule. One thing certain is that if you break down in rough water, the boat is at the mercy of the elements - especially with no pumps when battery goes flat etc and an open boat will eventually get swamped.

Revisions: As part of my project, I'm looking at a separate bilge for the cockpit/cuddy and sealing off the engine bay bulkhead with a valve/tap/sluice of some kind. I figure from research that failure of a hose, clip or similar is the most likely source of water ingress so want to limit this to the engine bay. I will also raise the height of this bulkhead - although this needs to be partly removable for access/servicing. I will also reinstate the buoyancy foam in the previous areas (and fill better!) with extra in sealed pockets along the sides of the cockpit, inside the gunwales and right up under the topside moldings. For this, I will use preformed, closed cell sheets. This will help create positive buoyancy which could help right the vessel in the event of a capsize.

Essentially, the more empty space you fill with something lighter than water, the less room there is for water and the longer you will stay afloat. The downside however that above water, even the foam has a weight and will thus have an impact on a planing boat which needs to lift in free air... The challenge for me will be where to compromise as I'm already looking to add other stuff like auxiliary outboard, diesel heater and electronics... :)

My rebuild thread is here if anyone wants to see pictures etc to get a better idea.
 
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JimC said:
Like many boats mine has a triangular double berth in the forecabin, beneath which is a large locker which is divided into 2 compartments by a transverse half bulkhead and accessed via loose plywood covers which fit into recesses in the berth top under the mattress. I've often wondered whether it would be worth replacing the plywood covers with watertight access panels such as these. I would think that if the yacht is going to be at risk of sinking due to being holed, the impact is most likely to be in the forward part of the ship.
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To answer JimC question. Yes those water tight hatches may be worth having. However they would have to be on the side of the bunk because they would I think protrude above bunk enough that a thin mattress would not cover the bump. Simplest is to screw and seal the existing covers if you are prepared to sacrifice storage space or put items in there only accessible with screw driver.
IMHO opinion the areas under bunks of a small boat are ideal for sealing to make the boat unsinkable. Further with the boat mostly full of water the buoyancy will be at the sides so making the boat more stable.
good luck olewill
 
hi
haveing rescued a few boats from sinking i dont think you are close to saving the boat,
a wooden yacht 25ft long takes four 45 gallon drums and fifteen 25 litr drums just to hold it at deck level with the water.
a small cabin cruiser 23 ft took five 45 gallon drums and fifteen 25 ltr cans to keep it just above the water
the bigest problem i have is once it is full of water most boats just roll over thay dont want to stay up right
best bet dont let the water in...simmbals
 
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