Originally Posted by black_sails
If the catamarans were even longer, does the power to reach 30 knots actually go down? If I added more power to the displacement catamaran (say doubling it) do I get more speed or do I 'hit a wall' like with nonplaning monohulls?
Curious what happens if taking the long and narrow philosophy even further. Like say I knew I wanted a cabin
of around 10,000lbs - whether that's mounted on 40 foot cats, 80 foot cats, or 160 foot cats is up to you. At what point does the extra weight (of the cats themself, obviously this depends upon what theyre made of in part/toughness) and the drag from wetted area to hold up that load become counterproductive?
In terms of efficient sailing vs efficient power catamaran, are those hull shape changes changes mostly in relevance to trying to sail into the wind? What if I were content to just go with the wind when under sail and slowly diesel
my way when against the wind..
There is a lot to unpack here... And limiting the conversation to only displacement hulls....
1) for a given weight the longer the hull the easier it is to drive at a given speed. So yes making a boat longer, but the same weight will require less power for a given speed. BUT making a boat longer and retaining the same weight can get very expensive very quickly.
Practically length is also just expensive to own. Haulouts cost more, slips cost more, painting costs more.
Ideally length is something you design too, and are not given. So if you want a hull speed of 30kn (with the load carrying you need) then you go long enough to hit that speed and no further.
2) adding more power to a displacement cat doesn't help much. Once you hit the hull speed of the hulls any further hp has incrementally smaller effects on speed. The major difference with cats is that if designed correctly their hull speed is far higher than on a monohull.
3) is the 10,000lbs total ship weight, or spare parts
? The better way to think of it from a design point is the all up weight including gear
, people, the hull, because it is this weight that the hulls have to support. If you want 10k pounds of load carrying in excess of the boat itself then you need to go on a diet.
But for the sake of conversation... In a powerboat you would probably need to be in the 65' displacement range to handle it. With a top speed of 10kn or so. For a power catamaran probably in the 80' range, but I am really not sure. A performance 55' catamaran (sailing) will have a maximum capacity of around 2-2.5 tonns. Trying to stuff 5t on board is going to be difficult.
4) cats are typically lighter for the same length when compared to a monohull not heavier.
5) about the only change between power and sailing cat hull shape has to do with the amount of rocker you would expect. Sailing cats need more to provide a pivot point around which to turn, while power boats don't need it so much, since they always have excess power. A power cat hull is best designed as a power cat, but switching a sailing cat to power isn't a terrible idea either.
6) there would be almost no gain in giving up windward performance on a sailing cat. You might be a touch faster off the wind but I doubt it. Modern performance cats are fast enough to pull the apparent wind in front of the beam even when sailing down wind. About the only major design change would be to eliminate daggerboards, which almost all cruising boats have done anyway. It costs you a few degrees of pointing ability, but maybe 5 degrees, not 30.
Here's the thing, I think you are going about this all wrong. Define what you want to do, this becomes the SOR (statement of requirements) then you design the boat around this. Being able to do 30kn is part of that, being able to carry 10,000lbs of stuff is part of that, having sleeping accommodations for 2 people or 20 people is part of that. Price
, and operational budget
are part of that. These then become the pivots around which the hull design is formed.
Trying to handle them in isolation is tricky because there are too many undefined issues.
Crossing an ocean under power for instance is almost impossible for small powerboats, but trivial for most sailboats. Because the weight of the fuel kills your range until you get pretty long (there is only one power boat
I know under 65' with truely trans-pacific range).
Worse the initial SOR is often in conflict. Crossing oceans on a small catamaran powerboat is going to be very difficult because you need a pretty long boat just to carry all the fuel required. While crossing the ocean on a sailing cat is pretty easy. So if the goal is to cross oceans almost everyone is going to wind up in a sailboat.
Speed is also a critical issue. Crossing an ocean at xkn is one thing, crossing at 2x is entirely different, requiring about four times as much fuel/hr, and substantially more than that in fuel to move the fuel... It's like the rocket/fuel problem. Something like 90% of the fuel a rocket carries to get a payload to space is just there to get the fuel off the ground.