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Old 05-05-2016, 16:27   #1
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Hullshape compromises for efficient passagemaking vs high speed (ie 30kts plus)

Hello, hoping someone with more knowledge than me on marine engineering can point me in the right direction.


ASSUMING that the primary design purpose of a catamaran or ILAN hull was for efficient passagemaking with minimal drag (say maybe intending to cruise at 12-18 knots or so, though that number isn't locked in stone) but that you wanted the ability to seriously push the upper limits of performance on a few rare occasions by mostly throwing horsepower at the equation, at what speed does the hullshape start to become a serious impediment?

I'm under the understanding with monohulls it's mostly about being light enough to get up on plane, and then you can just keep throwing almost all the power you want at it.

I am not sure whether the best way for high performance in a catamaran is to get up onto plane, or whether long thin slender wave piercing hulls can work more efficiently up to a certain speed, especially if you aren't the lightest possible catamaran either.


Suggestions on where to find engineering texts showing mathematical charts and graphs of different hullforms welcome.
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Old 05-05-2016, 16:40   #2
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Re: Hullshape compromises for efficient passagemaking vs high speed (ie 30kts plus)

Different principles at work. Displacement monohulls are limited in speed by their wave making. Planing is a requirement for high speed for them. When a hull is very narrow in relation to its length (high length to beam ratio) then wave making is greatly reduced, along with its speed limitations Multi hulls can go very fast without planning. Take a look at some high performance cat hulls. They are not planing hulls.
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Old 05-05-2016, 20:49   #3
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Re: Hullshape compromises for efficient passagemaking vs high speed (ie 30kts plus)

If you want to go fast then you have two options,

1) stack a load of horsepower on board, for a planing catamaran you need about the same amount of power as for a monohull going the same speed... Possibly a little less, but in the same order of magnitude.

2) make the boat as long as you can manage. A 65' displacement catamaran has a hull speed in the 30kn range. With a fraction the amount of power that the planing cat needs.

See http://www.multihulldesigns.com/pdf/powercatslt.pdf for a more detailed view.
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Old 05-05-2016, 21:09   #4
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Re: Hullshape compromises for efficient passagemaking vs high speed (ie 30kts plus)

If you aren't trying to plane long narrow hulls are the answer but you also need to keep the weight low.

Build a 60' cat with the accommodations of a 25'mono and it will be fast and efficient.
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Old 05-05-2016, 21:25   #5
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Re: Hullshape compromises for efficient passagemaking vs high speed (ie 30kts plus)

Quote:
Originally Posted by Stumble View Post
If you want to go fast then you have two options,

1) stack a load of horsepower on board, for a planing catamaran you need about the same amount of power as for a monohull going the same speed... Possibly a little less, but in the same order of magnitude.

2) make the boat as long as you can manage. A 65' displacement catamaran has a hull speed in the 30kn range. With a fraction the amount of power that the planing cat needs.

See http://www.multihulldesigns.com/pdf/powercatslt.pdf for a more detailed view.
He would need at least a 60ft cat. An efficient sailing vessell will have a different hull shape to an efficient displacement power catamaran and certainly not a planning cat.

Seek out cat designers such as Richard Woods http://www.sailingcatamarans.com/ or Dereck Kellsall http://www.kelsall.com/.

Most Multihull designers have had eqperience with both sail and displacement powerhulls. Others include Kurt Hughes, Tony Grainger, Roger Hill, Ron Given, Morelli and Melvin and so on.
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Old 05-05-2016, 21:43   #6
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Re: Hullshape compromises for efficient passagemaking vs high speed (ie 30kts plus)

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He would need at least a 60ft cat. An efficient sailing vessell will have a different hull shape to an efficient displacement power catamaran and certainly not a planning cat.
There are some minor variations but not much difference in displacement hull shapes. The vast majority of miles on most sailboats is under power anyway. (yes, plaining hulls are completely different animals)

The early trawler cats were basically the sailboat hulls without a mast and slightly larger engines.
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Old 06-05-2016, 05:39   #7
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Re: Hullshape compromises for efficient passagemaking vs high speed (ie 30kts plus)

That Powercats pdf is exactly the kind of information I was looking for, thank you very much. Though i'm curious how the efficiency of the displacement cat compares to the efficiency of the monohull/can they both travel on the same fuel load provided the cat just goes slower too? (and does that have to be as slow as the monohull)

I'm also curious what happens when weight control cannot be quite as rigorous. Lets say I HAVE to have a payload of 10,000lbs. (say it's a bit of an explorer or/and I like to bring alot of junk) Is that handled most efficiently by simply sizing up the displacement cat to a larger and longer size? Is there a point of tradeoff where one ever gets steered back towards a monohull? (like if you need a 90 foot cat to move the weight a 40 foot monohull handles) I'm sure for sheer weight the best solution is just 'get a barge' but i'm wondering about sweet spots of handling some weight combined with some performance.
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Old 06-05-2016, 05:44   #8
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Re: Hullshape compromises for efficient passagemaking vs high speed (ie 30kts plus)

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He would need at least a 60ft cat. An efficient sailing vessell will have a different hull shape to an efficient displacement power catamaran and certainly not a planning cat.
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..
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Old 06-05-2016, 05:53   #9
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Re: Hullshape compromises for efficient passagemaking vs high speed (ie 30kts plus)

With twin 1650 hp motors you can go as fast as you wish north of 180 miles an hour
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Old 06-05-2016, 09:13   #10
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Re: Hullshape compromises for efficient passagemaking vs high speed (ie 30kts plus)

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Originally Posted by black_sails View Post
That Powercats pdf is exactly the kind of information I was looking for, thank you very much. Though i'm curious how the efficiency of the displacement cat compares to the efficiency of the monohull/can they both travel on the same fuel load provided the cat just goes slower too? (and does that have to be as slow as the monohull)

I'm also curious what happens when weight control cannot be quite as rigorous. Lets say I HAVE to have a payload of 10,000lbs. (say it's a bit of an explorer or/and I like to bring alot of junk) Is that handled most efficiently by simply sizing up the displacement cat to a larger and longer size? Is there a point of tradeoff where one ever gets steered back towards a monohull? (like if you need a 90 foot cat to move the weight a 40 foot monohull handles) I'm sure for sheer weight the best solution is just 'get a barge' but i'm wondering about sweet spots of handling some weight combined with some performance.
For cargo, monohulls are they way to go. Other than high speed ferries virtually all cargo is moved by monohulls.

It really is more the housing and feeding that stops people from building crazy long hulls. If you have the accommodations of a 30' cat, are you going to be willing to pay for slips, haul outs, bottom painting for a 90' cat?
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Old 06-05-2016, 09:31   #11
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Re: Hullshape compromises for efficient passagemaking vs high speed (ie 30kts plus)

Nobody considered yet, just *where* do you want to go so fast? I mean, if you are going that fast you must be considering long distance cruising, if you are posting on this forum. That means you will be on the open ocean most of the time, and that means you must be able to comfortably handle 10' swells and 5' wind waves, at a minimum.

Nuclear subs are a good option - they travel very fast beneath all surface turbulance. Assuming you can't afford one of those, you could fly the boat. I refer to waterfoils. Look up info on this - some very fast speeds have been acheived by multihulls with waterfoils. A large enough catamaran, probably need 70' or more and beam of 40', can fly on foils such that average ocean wave conditions are not too violent at high speeds.

Speed costs money.
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Old 06-05-2016, 09:42   #12
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Re: Hullshape compromises for efficient passagemaking vs high speed (ie 30kts plus)

Oh, I forgot to mention if you go with any high performance multihull you will need one or two very experienced helmsmen and trimmers on watch whenever you are sailing at high speed.
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Old 06-05-2016, 11:24   #13
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Re: Hullshape compromises for efficient passagemaking vs high speed (ie 30kts plus)

Quote:
Originally Posted by black_sails View Post
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.
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Old 10-05-2016, 18:51   #14
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Re: Hullshape compromises for efficient passagemaking vs high speed (ie 30kts plus)

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Originally Posted by black_sails View Post
Hello, hoping someone with more knowledge than me on marine engineering can point me in the right direction.


ASSUMING that the primary design purpose of a catamaran or ILAN hull was for efficient passagemaking with minimal drag (say maybe intending to cruise at 12-18 knots or so, though that number isn't locked in stone) but that you wanted the ability to seriously push the upper limits of performance on a few rare occasions by mostly throwing horsepower at the equation, at what speed does the hullshape start to become a serious impediment?

I'm under the understanding with monohulls it's mostly about being light enough to get up on plane, and then you can just keep throwing almost all the power you want at it.

I am not sure whether the best way for high performance in a catamaran is to get up onto plane, or whether long thin slender wave piercing hulls can work more efficiently up to a certain speed, especially if you aren't the lightest possible catamaran either.


Suggestions on where to find engineering texts showing mathematical charts and graphs of different hullforms welcome.


read 'seaworthiness, the forgotten factor' by c.a. marchaj. you can get it used on amazon for a very reasonable price. a bit of math to slog through but, you can understand it even if you totally skip over the math.
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