Dear all, What would be the impact of a projected planning cat hull if we set up engines for lower speeds? Is there loss of efficiency? Confort? Any other points? Roger Yes. Great losses. Planing hull shapes -- monohull, catamaran or any other -- incorporate large flat horizontal surfaces on the bottom and power plants many times larger than those found useful in displacement hulls. They use this great power to gain enough lift from those flat hull surfaces to raise the whole boat up to the surface of the water and then climb it up over their own bow wave crest, commonly termed 'getting up on the step' or 'getting up on plane.' Once this planing is achieved they are free to operate at any speed they can afford above the wave trap speed limit imposed on displacement hulls. They literally skim or fly along on the surface of the water. Purely planing hulls achieve tremendous speeds up there on the surface, but of course they depend on having very little weight to lift, great whopping engines to lift it and a small shallow hull to put it all in. This small shallow hull shape must be carefully optimized for stable performance on plane, given the high speeds involved, and the usual presence of high testosterone levels in the operators. These design tradeoffs sacrifice almost every other design consideration deployed in displacement boats to achieve the highest possible speeds and adequately thrill their owners. The propellers are particularly optimized for performance at high speed. Since these designs rely on keeping the flat hull surfaces cleverly oriented just so to the surface of the water they cannot plane in rough water -- where their other hull shape compromises, such as low freeboard, short length, sleek lightweight hulls with heavy engines perched way too far aft for balance when not up on plane, etc. etc.-- their off plane performance is appalling. The only speeds that are worse than displacement speeds are seen on overloaded or under powered planing boats, where they are struggling to almost but not quite get over the step. At displacement speeds they are frequently threatened to be swamped by their own stern wake. They squander fuel, rock and roll, and are inordinately threatened by rough water. They have little room for living amenities and seem to be best suited to water skiers and bow wenches. That of course is just fine for them, and they have exactly the same rights to be on the water as anybody else. I sense that those priorities are not particularly important for us Power Catamaran folks. Purely displacement boats from canoes to supertankers can be designed today with considerable freedom to provide fuel economy, rough water capabilities, weight /cargo capacity, etc., but must always operate nicely below the wave trap hull speed. There is one other practical way to minimize the hull speed limit, frequently termed 'slender ship theory' performance. These designs rely on exceptionally long slim hulls with power increased somewhat over the common displacement model, but not nearly as much as the planing hull. These hulls have in-the-water shapes with lengths anywhere from ten to twenty or more times their beam. They work by minimizing the depth of the so called wave trap trough so that they can climb over the diminished bow wake crest to a considerable degree. Catamarans are particularly well suited to this ploy, as they have two parallel hulls, and the beam of each is less than half the beam needed in a monohull to carry it's weight (weight capacity, or displacement goes up at a rate like the cube of its linear dimension, such as beam). Of course, modern boats, particularly catamarans, are very seldom pure displacement hulls (like ancient Polynesian paddling/sailing cats) and almost never pure planing hulls (example racing hydroplanes running with only two sponsons and the prop in the water). These are compromises between the extremes, often referred to as semi displacement hulls. Power catamaran hulls are almost always going to have some slender ship stuff going for them. They are usually a varied mix of these design approaches, beginning with a displacement look, adding some flat bottom aft and employing widely spaced slender hulls. My beloved PDQ is a particularly good example of this approach. We get pretty much equivalent performance and stable behavior at any speed up to about three times hull speed, without any discernible sweet spots and with no difficult speeds. Thanks to very sophisticated modern designs the displacement behavior seamlessly transitions into the planing and slender hull boosted high speed behavior. We are not penalized by needing huge thirsty engines and have lots of usable space available in a stable (and to some eyes at least, attractive) hull. There is another approach common enough in power catamarans today, one I call the tunnel hull boats. These hulls much more closely resemble familiar planing hulls with a slender tunnel down the middle. They get some fine advantages from this tunnel, much softer high speed performance, somewhat wider deck plans, better roll resistance and such. They do not harvest all the advantages of the slender hull power cats, but they don't take all the hits of the modern monohull planing boat either. They have much higher top speeds than their slender hulled brothers, but at the cost of higher fuel consumption, much heftier engines and of course the advantages and costs of the shape and size of the hull. Selecting any of these kinds of boats is entirely a matter of personal preference. Fishers seem to like to get to and from the fishin' spots quickly. Coastal cruisers want comfort and economy. Party patio pontoon boats and lake houseboats appeal to folks with socially powered agendas. Your particular choice is unlikely to line up perfectly with mine. That's fine, there's room for us all. Returning finally to your question Roger, I want to suggest that to the extent that any boat is designed for real high speed planing it becomes very difficult to recapture very many of the low speed displacement or semi displacement boat attributes, and impossible to get them all. You have to have big, heavy, thirsty engines to plane, and props optimized irretrievably for high speed. The real planing hull has so much flat horizontal surface on its bottom that there is no hope of getting slender ship advantages, and at displacement speeds it will have to shoulder out of the way huge amounts of water to proceed. The shape of a planing catamaran offers less space for the open amenities coastal cruisers want. I cannot claim that I entirely escape my own preferences and prejudices but having said that I will say that I think the slender hull option offers the best of all possible worlds. YMMV of course. Warm regards, Gary