Are Two Hulls Better Than One? Multi-Hull Design Considerations

Walking the docks at a show and seeing the wide variety of vessels our industry offers is exciting. I’m sure automotive designers feel the same way at car shows. But one could argue that there’s even more variability with boats. Most cars/trucks I have owned have four wheels with tires. But boat hulls? There are many different forms, even for a specific market segment.

Multi-Hull Design Considerations

One question I often get concerns the merits of multi-hulls. Here are some design considerations with regards to catamaran hulls. This is not meant to be a comparison with monohulls, as this comparison almost always comes down to operator choice depending on several variables. But multi-hulls have certain features and will exhibit many common traits between them worth noting.

In the powered and ultra-competitive fishing market, catamarans typically make up no more than about 10 percent of the total—so they remain a niche. But they have existed for a long time, so why hasn’t that number grown? I would say it’s due to some inaccurate information, unreasonable expectations, and, possibly, simply aesthetics. Cats do look a bit different, although styling has gotten better and better.

Why Catamarans Are So Beamy

Why are cats so beamy? Mostly, this is due to the need to have enough separation between demi-hulls to minimize wake interaction effects, which will increase drag. So, what are some benefits of multi-hulls? Two skinny hulls can exhibit lower resistance than one wider hull. Ride can also be somewhat better due to higher hull loading, resulting in lower vertical accelerations. Due to their wider overall beam, low-speed handling can be enhanced because of larger thrust moments created by the greater distance between propulsion units, not to mention great transverse (roll) stability. Cats also typically run with lower dynamic trim so the need for tabs or interceptors is lessened.

Just a reminder though, an odd number of engines is basically a no-no. Be it outboard motors or inboard diesels, triples or quints are off the menu.

Hull Forms for Different Speeds

It is worth noting that, like monohulls, different cat hull forms can be used depending on expected speeds. For slower displacement hulls, round bilge symmetric demi-hulls would be in order. Hard chined symmetric demi-hulls are great for a wide range of speeds including low to mid-range planing speeds. Higher speed applications, especially those that are focused only on these high speeds, like patrol craft or race boats, will typically have asymmetric hulls. To be clear here, I’m talking about the demi-hull itself being asymmetric, but port and starboard hulls are the same. These different hull forms and speeds mostly have to do with minimizing resistance.

Less-Than-Ideal Running Characteristics

But multi-hulls also offer some less-than-ideal running characteristics. While bare hull resistance can be lower, cats are often heavier than a similar length monohull, so some of this benefit is lost. With cats, the wet deck, or top of the tunnel, should stay mostly dry. The more water interacts with the tunnel top (especially solid water), the more resistance increases and ride quality can suffer. This is the main reason why cats are normally taller in the water than monohulls.

The higher beam values also mean at higher speeds, turning radius is greater. The cat hull won’t lean into a turn as much as a monohull, so you’ll need more space and time for high-speed turns. In really heavy seas, that wet deck can slam hard, negatively impacting ride. This is why we often use wave knockers on centerline to provide some amount of deadrise to help soften heavy wave impacts. Finally, cats behave differently in a seaway with motions unlike those of monohulls. A sort of corkscrew motion can be seen in following or oblique seas. This is not necessarily bad, but it is different and some find it off-putting.

Arrangement and Space Considerations

From an arrangements standpoint, cats provide generous deck space due to their wider beam, so salons on main deck can be quite spacious, and mezzanines can be inviting. Cockpits can also provide more than enough real estate. However, for larger boats, which typically provide significant amounts of interior accommodation spaces including staterooms and heads, the narrower demi-hulls provide limited options. This alone is enough for many owners to stick with monohulls.

Cat Hulls: Are Two Hulls Better Than One for Fishing Applications

For our fishing craft, cats offer pros and cons. The roll stability can be great while trolling. And the low-speed maneuverability mentioned above is beneficial while fighting fish. But the main drawback of the multi-hull arrangement with fishing is that high tunnel height. Unless we’re talking a large yacht with a cockpit on each demi-hull, the high main deck results in the fishing cockpit being higher off the water than optimal. And there’s really no way around that, minus getting super clever, and expensive, with actuated decks, etc. To me, this is the number one reason we don’t see more cats that are true sportfishing vessels.

Multi-Hull Evolution and Applications

Catamarans absolutely have their place in our marine world. Their benefits are real. But there are negatives which should be understood, especially with regards to angling. Multi-hulls have become somewhat more prevalent with the advent of new outboard power levels on small-to-medium craft. The key is knowing what the operator wants and to have reasonable expectations. The designer must take these considerations into account as well as applying sound engineering practices in developing a superior multi-hull design.

Key Catamaran Characteristics Summary

Advantages:

• Lower resistance with two skinny hulls
• Better ride quality due to higher hull loading
• Superior low-speed handling and maneuverability
• Excellent roll stability for trolling
• Generous deck space and cockpit area
• Reduced need for tabs/interceptors

Disadvantages:

• Often heavier than comparable monohulls
• Higher fishing cockpit due to tunnel height
• Larger turning radius at high speeds
• Different motion characteristics (corkscrew motion)
• Limited interior space in demi-hulls
• Wet deck slamming in heavy seas

Design Considerations:

• Even number of engines required
• Hull form varies with intended speed range
• Wake interaction must be minimized
• Tunnel height critical for fishing applications

The choice between catamaran and monohull designs ultimately depends on the specific application, operator preferences, and understanding the inherent characteristics of each configuration.

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Originally featured in InTheBite Magazine April/May 2025 Issue.

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