Originally Posted by DeeB
In fact I'm hoping to hear back from him on the technical aspects of my project. I'll make sure to ask him whether he thinks his boat can undertake such a journey
I apologize that this is really long and that I wrote this for another discussion on the math of cruising weights and so some of the detail may not specifically apply to this discussion. I still think that this may provide useful background on my comments on carrying capacity.
There has long been a tendency for conservative cruising sailors to talk about needing some arbitrary length boat that seems to be ideal for their personal needs, and then to argue that the heavier the boat the better. But as coastal cruisers and even distance cruising designs have become progressively lighter for their length, I continue to suggest that the way to find the right boat for a sailors needs is to start with how much displacement is needed and then back into the length and type of boat really makes sense. This exercise is predominantly aimed as distance cruisers because their choice of displacement ultimately controls the comfortable range of the boat.
So let’s start with the basics, when you talk about the displacement of a boat (its weight), it is comprised of the weight of the boat with all of the fixed and operable components needed to sail and motor which is often referred to as it’s ‘dry weight’. This is often the weight that is published for racing and performance boats since most racing rating rules use this weight as a part of the rating calculation. But in real life the actual sailing displacement will vary, and vary widely depending on how much crew weight, gear, supplies, spares, consumables and so on happen to be aboard.
Cruising boat designers typically design to a load level that they anticipate as being something of the intended normal loading that would be expected for one of their designs, and that is sometimes referred to as the ‘design weight’. Often the published displacements for cruising boats are actually their design weights. Because performance boats publish their dry displacements and cruising boats tend to publish their design displacements, this difference in the chosen published numbers leads to the frequent misconception that relative to cruising boats, racing and performance cruisers are relatively lighter than they really are.
All boats can tolerate some additional loading beyond their dry weight, but at some point there is a weight limit after which the safety, motion comfort, and performance of the boat will be harmed. The difference between this fully loaded weight and the dry weight is generally what is thought of as the carrying capacity of the boat. I suggest that determining how much carrying capacity for the type of cruising you are considering needs to be one of the very first steps in selecting the right boat for distance cruising.
The amount of required carrying capacity will be directly related to the number of people on board, the anticipated passage distance and resultant time at sea of the planned voyages that are being contemplated, the climates involved, variations in a particular crew’s comfort level preferences, the level of conservatism, and discipline.
These last items are purely determined by the idiosyncrasies of the individuals involved, but the some of the first items are relatively easily quantifiable. The most obvious weight driver is the number of people on board. Each person needs their personal gear; clothing, foul weather gear, PFD’s, sleeping gear and so on. The weight of these items are somewhat controlled by the ethos of the crew and the distance and number of climates that they are likely to encounter. Warm weather cruisers need less clothes than cool weather cruisers, but a boat that rounds one of the major capes needs both warm and cold weather gear. A crew which is diligently disciplined about the weight that they drag aboard will get by with less gear than a crew who is more relaxed and so might bring more souvenirs, heavier personal affects and contingent or redundant clothing.
A disciplined crew in a warm climate might take as little 30 lbs of personal articles per person. A cold climate can easily double that. A more relaxed crew might take 300-500 pounds of ‘stuff’ per person. Think of the weight of a library of pulp fiction, the right outfit for all occasions including dress clothes for nights ashore, gel foam mattresses and ‘proper lines’, dive tanks, wet suits, diving weights and you can quickly understand why there is such a large range. There is no one right answer on this issue, but its important for you to understand where your venue and crew fall in the continuum of possible crew types.
A normal person eats roughly 2-3% of their weight per day. For a typical healthy weight American man that is somewhere around 3-4 lbs of food per day without containers. But sailing burns a lot of calories, simply sitting on a boat at sea causes flexing and un-flexing of muscles just to accommodate to wave motion. Even a laid back crew will burn nearly double the calories that the do on shore. A performance oriented crew can increase that daily consumption dramatically, literally nearly doubling the food requirement since they are burning lots more calories making more frequent sail adjustments, and hand steering a larger portion of the time. Canned containers can add as much as 25% to the weight of the food, whereas bulk containers may add as little as 4-8%. Refrigerated packaging tends to be lighter, but it comes at the price of added weight in compressors, battery capacity, fuel, and or solar collection, wind gen equipment, resulting in minimal, if any real savings. Vegetarians, who eat fish, probably get by with the lightest food weight and cost. Serious carnivores, can greatly increase the weight and cost of voyaging. Again, there is no one right answer here, but its important for you to understand the realities of your sailing style and crew preferences.
Taken as a whole, including containers and packing fluids, the average weight of the food consumption per crew member per day should be seen as falling in a rough range of 5 lbs. to 10 lbs per day.
And that does not include potable liquids. The most efficient liquid to carry is simply water. Cruising with tight discipline, planning should assume that a person will consume around a half a gallon of water per day including cooking. But most cruisers use more than that, especially in warm climates where minimum consumptions can quickly exceed a gallon per day. Performance crews in warm climates can easily drink that much without considering water use for cooking or cleaning. Allowing for the weight of tankage that comes out to an approximate range of 5 to 15 lbs per person per day.
Combining the weight of food and water, containers and tankage, you end up with an approximate allowance of 10 to 25 lbs per person per day. Surprised? I know I was.
And this is where the anticipated cruising range comes into play. Coastal Cruisers might think in terms of partially restocking (perhaps in the form of eating at restaurants) while transoceanic voyagers need to plan to have everything that they need on board for their longest leg. But even amongst distance cruisers, there are big differences between an approximately 3,500 mile great circle trans-Atlantic passage from England to New York for example, vs. a jump across the Pacific from Panama to Hawaii of roughly 5,300 miles.
But there are also further differences between oceanic cruisers even in the same ocean. One trans-Atlantic crossing may jump from Portugal to the Canaries to the Caribbean which cuts the hops in half, while another trans-Atlantic might be Cape Town, South Africa to Florida of around 7,800 miles. It is in this way, that the choice of boat with its carrying capacity, and reasonable average daily distances that this boat can sail, as well as the consumption rates of your crew might limit your choices for possible passages that can safely be made.
Of course, it is entirely reasonable to buy a boat with less carrying capacity planning to be more comfortable on short distance voyages but with the implicit understanding that there will need to be more intense rationing on longer passages.
But assuming the capacity to make reasonably safe and comfortable passages, and assuming average daily passage rates between 100 miles a day (normal sub-40 foot cruiser sailed moderately aggressively) to 200 miles for a bigger faster cruiser, a cruiser making the leap from straight from Panama to Hawaii might expect to spend between 25 days (fast passage) as much as 40 to 80 days depending on luck and boat speed. For the fast and disciplined cruiser, allowing no reserves that means roughly 300 lbs of food and water and another 30 lbs in gear for the passage. Not all that much really.
But more realistically, even with a comparatively fast boat and a sharp crew, and reasonable safety factors; the food, water and gear weight per person jumps up to being much closer to 700 lbs, plus their body weight.
As a broad rule of thumb, in addition to that you need to carry approximately 1 to 2 times the weight per person for fuel, loose gear and spares. Obviously, this last category is highly variable. As the displacement of the boat increases, the required amount of fuel and the weight of gear will increase independent of the weight of food, water, and personal gear that each person needs.
But as a broad generality for a couple making long passages and planning to restock when they are in port, that comes out to a minimum carrying capacity of roughly around 3,500 -4,000 lbs.
And after all that, this is the point at which differences in individual hull design and boat type come into play. The carrying capacity of a boat is generally proportionate to the displacement of the boat and its water-plane (footprint area of the boat in the water at an average loading). Boats which have long waterlines for their weight, tend to have comparatively large water planes and so have a larger carrying capacity as a percentage of their displacement. Boats which are short for their length tend to have a smaller carrying capacity as a percentage of their displacement. Where this often gets confused in the ‘court of public opinion’ is that a boat which is short for its length will tend to have a greater carrying capacity than an equal length boat that is light for its length.
Also as a broad generality, monohulls tend to have larger water planes relative to their length, and also tend to have greater displacement for any given length and so have substantially larger carrying capacity for any given length. But also, monohulls tend to be more weight distribution tolerant than multihulls, which as a broad generality do not handle weight in the bow and stern as well as a monohull due to the comparatively small reserve buoyancy in multihull ends.
Trimarans are even more capacity challenged than Cats because they start with very small displacements and tiny water planes when sitting level (which in part is where their speed comes from). But as they heel the main hull and leeward ama has to support the weight of the windward aka and ama. The safe carrying capacity of the Trimaran is thereby reduced by the weight of the lifted aka and ama. As a broad generality, this makes Trimarans far less suitable for longer distance cruising which include passages such as the South African to Caribbean or the major Pacific hops.