Power Trimaran vs Power Catamaran efficiency

Im interested in building a power aluminum yacht, and I'm curious about the efficiency of 3 thin hulls (trimaran) vs 2 wider hulls (catamaran).

I'm assuming that the reinforcements on the bridgedeck could be lighter with a trimaran since there would be less flexing between the hulls, as well as reduced reinforcements in each hull, possibly resulting in simpler overall build.

However, I dont know if 3 thin hulls would result in more drag than 2 hulls, due to total wetted surface. Also, would there be an issue with wave interference if the 3 hulls are too close? Appreciate any help.

I should add, my design concept is along the lines of a Neel trimaran, where the deck extends to all 3 hulls, rather than the amas just being stabilizers

 

You first need to define what you mean by efficiency. For example, if it is fuel per mile or fuel per ton of displacement. Efficiency may be measured on how much time it takes to arrive to the destination if time is the most expensive commodity.

 

As this will be a solar/generator hybrid liveaboard trawler moving at displacement speeds, I'm referring to fuel efficiency. Im wondering if a boat with 3 light hulls requires less power to move at hull speed than 2 thick hulls. The concept itself is somewhat hybrid, with the center hull being longer than the outer hulls, but not as long as typical trimarans. All 3 hulls are there to provide buoyancy, not just stabilizing

 

It seems that most power yachts are either monohulls or catamarans, but I'm curious why exactly this is

 

Cost is the first reason. The more hulls you build, the higher the cost. Liveaboards are usually heavy, so multihulls with low displacement are probably not the best choice. There are minimalists that can keep their possessions small. Multihulls have large decks which is one of their main appeals. You need to start by creating a Statement of Requirements (SOR). Write all the things you need the boat to do, operating conditions, amount of crew, time expected at sea, climate it will be used on, maximum draft, maximum beam, accommodations, etc. Some items are constraints, that means that they have to be included, so marked them as such. Other items are optional or can be modified/negotiated. That is the start of the design spiral, where you go around and around. When you change one thing, it may interfere with another or make possible a new item. At one point you will call it good. Whether the boat ends up as a multihull, catamaran or trimaran is irrelevant. The SOR will drive the decision.

 

Right, I understand that multihulls work best when their hulls aren't deep in the water. But is there a difference between a 10000 lb monohulls and a 10000 lb catamaran? By all accounts it seems that the catamaran always wins out due to hull shape. But I honestly don't know if that's true or just anecdotal

 

To carry the same weight (equals volume) on the same waterline length, the cat will use 1.41 times the hull surface as the monohull, plus the tunnel structure, and its wetted surface (ie the friction surface) will also be 1.41 times the monohull friction surface. If you go to a tri, then we're talking about a factor 1.73 instead.

For a power tri, the connecting structure has to be strong enough to carry the complete center hull with its load in the case when the side hulls ride on top of waves and the center "hanging free". So, you can easily come up with a basic structure (hulls plus bridge) of double the amount of material required for a monohull of the same carrying capacity. Investing a fraction of the surplus material will thus give you much more boat in a monohull. Multihulls have their place when you need high speeds with light load, combined with specific stability conditions.

For the conditions you mention (displacement speed, liveaboard, electric), cats or tris are bad choices.

 

Indeed, but as Gonzo notes:

Since at higher Fn a catamaran with a higher LD ratio, has approximately 50% less resistance, than an equivalent monohull.

As always, horses for courses!

 

The thing that is also important is mooring costs. Most marinas won't have a slip for a 24' wide tri, so they make you pay for two fourteen footers. On the west coast of the US, this can mean slip fees similar to a small apartment.

You can pretend efficiency is all about fuel economy, but there are realistic hurdles to meet first.

 

Thanks, this is the information I was looking for. I might just look at a displacement monohulls trawler than. I think I had this assumption that catamarans were inherently able to move through water with less power requirements than a monohull due to their slimmer hulls, but that is clearly only when little of that hull is touching the water.

 

This 'can' be so, but again it depends on how you are comparing them.
Have a look at (for example) the Silent range of electric catamarans - no monohulls (or trimarans) here.
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These folk have an electric monohull as well as electric cats -
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The trimaran can have less wetted area than a catamaran. The reason for this is that the outer hulls (amas) can be made to do little more than act as training wheels, to keep the narrow main hull from tipping over. Double outriggers work on this principle, as do most modern sailing trimarans. At this point, one comes to a decision on whether or not one wants the amas to be "full flotation". What full flotation means is having enough buoyancy in one ama to support the weight of the whole boat (as it is with most sailing trimarans). To have this much buoyancy the amas have to be rather large in both length and enclosed volume. And, if they are only partially immersed, the main hull must support most of their weight (as is typical with most modern sailing trimarans).

With "low buoyancy" amas, the amas are still usually a high fraction of the length of the main hull. But their enclosed volume is a lot less. With a powered trimaran there is very little downside to this, as far as I can see. The low buoyancy amas will be both lighter and will impose far less loads on the connecting beams. Sailing double outriggers use this approach, but usually go with a greater beam over all to compensate for the lower ama buoyancy. With a power trimaran, you don't have to do this.

Also, with a power trimaran, it is a whole lot easier to get by with a single engine (almost always more efficient than multiple engines).

 

Even if the sidehulls of a tri has zero wetted surface, ie just touching, they represent a considerable amount of construction material; "dead weight" which has to be carried by the buoyant hull. If it's worth the trouble or not can only be evaluated versus the SOR; a specific shape or configuration should not be the starting point of a design spiral.

 

What is your definition of efficiency ? The principal difference between a motor trimaran and a motor catamaran is that the trimaran is more stable than the catamaran. If you don't want to be rock and roll or if you intend to built swiss watches onboard, land an helicopter, then a trimaran will be a better fit.

Yes and no. Having to manage the built of three hulls rather than two is indeed a complication, somehow. Also, AMAs slenderness give less room than in catamaran's hulls.

The total drag of a motor catamaran compared with a motor trimaran are roughly the same, for a given weight. The fore shape of the AMAs could indeed produce an higher drag interference, if the bow waves generated are too big and affect the wave train of the main hull. The distance between the AMAs and the hull, as well as the volume and shape of the bow of the AMAs should be taken into account.

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I have make the hydrodynamics and structural engineering of the LEEN56 and LEEN 72. Although the weight quote has been exploded, from the design to the final built of the number ones, I may say that
- your AMAS should be asymetrical, for the wave interferences to be minimized
- your weight quote should be as low as possible, unless you may prefer a big fat monohull for a motorized yacht.

 

This thread is really remarkable. OP asks which boat cat or tri? ...okay mono.

But we have no sor as pointed out