The RNLI are also locked into the search of this holy grail, unlike the Dutch who have adopted full planning for their boats. I myself am not convinced that the difference in rough weather performance is clear cut between the semi and full planing hulls.
Location: Flattop Islands KC Barnaby's Basic Naval Architecture of 1954 did not mention semi-displacement or semi-planing.
My personal view is that a Semi-Planing hull is one designed to lift substantially, and it's probably hard chined aft for maximum lift. Most of us can associate to the type of vessel he is looking for, so why are you flipping his nose with a dozen of irrelevant notes on linguistics? Location: Milwaukee, WI Any hull that moves through the water produces waves. BTW, it is completely possible to design a hull that is producing zero or negative hydrodynamic lift in the vertical plane, even if there will be dynamic forces acting on the hull as a result of movement.
Location: Milwaukee, WI This is the first time you define what semi-displacement is for you. The regression methods mostly used for displacement speeds fail when it comes to speeds where dynamic lift is coming into play, and the empirical algorithms for planing are of little use in the intermediate speed range. Location: Milwaukee, WI Which comes to the original discussion, what is semi-displacement?
Location: Japan Not sure why stating basic facts is seen as a pissing contest by others?
One of my very good friends and I often debate this simple questions, what is a the definition of a semi-displacement hull.
Originally Posted by baeckmo To be precise, the prefix "semi" per definition says that the boat is operated in such a way that its weight is carried 50 % by buoyancy, and 50 % by other means (normally, but not necessarily, by hydrodynamic lift), end of discussion! If i were to have seen these lines and asked for my opinion, i would have suggested that it may be, but depending upon several other factors may be not too. Pushing a boat through the water or riding a bit on top, both can be achieved in totally different ways, with both operating at the same speed.
The person i referred to above, having this endless debate, simply because it amuses us, has been referred to here already. Laurent Giles' Woodpecker and Arthur Mursell of TT Designs and their Nelson range of pilot boats are all quoted as being the definitive examples. When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Location: Eustis, FL There are many variations on this theme, but mostly it's a gimmick, unless extra drag and decreased turning ability are your goals.
Location: spain My interpretation of tri hull is that all three hulls are in the water when underway . Location: Eustis, FL You can attempt to justify these drag inducing protuberances anyway you want, maybe even inventing some new syntax to describe them, but they're what they are and it still doesn't matter, as they're not especially efficient.

Location: NW Washington State USA The longer bottom is like a longer wheelbase on a car.
Location: Australia I have long thought that tri-hulls have the potential to be very good offshore boats, but not the cathedral hull types with terminating tunnels, or shallow tunnels that bottom-out easily. The intended purpose would be shallow river operations AND, retaining the outboard, not changing the lower unit over to jet drive. Location: Texas 12' mini Eliminator pickle fork tunnel hull I recently came across a 12? Eliminator pickle fork tunnel outboard hull. Originally Posted by dutchtat I have the same boat as you.Have a yamaha 50 on it that is no longer working. As long as the rest of the outboard is in good shape it would be a cheaper option and there are many more like that on the web. Location: Minneapolis, MN lakelife, I just stumbled upon this thread and I love the boat! My thinking is that a formula along the lines of the one below could be used to establish whether a hull is limited to a hull speed or whether it is able to break free and continue accelerating in displacement mode e.g.
The waterplane coefficient, Cw, could be used in place of angle of entry of bow, ideally just Cw of forward half of hull would be used. For hulls that do not exceed the critical k value, factors other than bow wave formation dominate drag, so the concept of relating the hull's maximum speed to the square root of its length is of little practical importance. Alternatively you could take the opinion that there is no such thing as hull speed, hence trying to calculate it is pointless.
Location: mallorca Displacement Hull Speed calculation Juan Baader the writer of the book?Cruceros y Lanchas Veloces? introduced a very simple way of Displacement hull speed calculation.
Location: mallorca The displacement hull is a hull with narrow stern and the speed is limited to the waves the hull is producing.
Location: USA I tend to think it's probably impossible to come up with a single, simple equation that would be accurate enough for all hull forms and sizes.
The predictive value of such equation would tend to considerably diminish in extreme cases e.g.
The problem seems to be that numerous factors affect R, and under different circumstances they don't always interact exactly in the same way with other factors.
A more useful approach could be to find for each boat category of general size and form an equation that best describes empirical findings, and therefore would have a good predictive value for design purposes. Location: Dubrovnik, Croatia If it would be that easy to calculate resistance what would be doing all these towing tanks and institutes. Location: mallorca Using the R-Value formula it is very easy to calculate boat speed for Displacement hulls. His dismisses faster boats as only being suitable for the calm waters of the USA and the Riviera. There are many other factors that seem to be more important and can be incorporated into hulls of both types.

The towing tank archives are full of reports on these hulls, and there are certainly numerical attempts galore, so come on with the real stuff and skip the "semi********"! And what do you call a hull that has 50 % aerostatic lift assistance from a "lighter-than-air" vessel?
The problem with this analysis is that we are depending on different tools for the two (at least for the moment). The solent can be rough, short sharp chop at times, so a fully planning boat is out especially in the winter times.
It is in incredibly mint condition except for a little bit of gelcoat fade which I have already found, buffs out real nice.
I want to know if anyone has any idea about motor height and setback specs for this type of hull. I am interested in talking or listening to anything anyone has to say about this boat and possible project. For the background to this thread see Humpless Planing in the Sailboat section of this forum. It may have been used on numerous occasions for this purpose but to say it was invented solely for this purpose is to dismiss all the real work that went into refining and exploring the semi-displacement hull form. This association of true planing vessels with being un-seaworthy for UK waters persisted for years (still persists?) and lead to thousands of hours being spent refining the idea of semi-displacement hulls, predominantly for the work boat market. However there are thousands of Pilot boat skippers and Lifeboat coxswains running their semi displacement hulls in all weathers, that are convinced that their boats are the way to go, and are not using these boats because they were fooled by the marketing strategy by some poor boat builder. There have been some quite successful boats with deep tunnels built, but generally speaking they have too much wetted area to be really efficient boats, but offsetting that they require only one engine unlike cats. I am not talking naval architecture again, I designed (looks at my gallery the 50' motor boat) it is a semi displacement she goes to 20 knots and do not plane. Fifty years of daily service in ports around the world would have rumbled these boats if they were nothing but an illusion.
I would probably just make the boat longer by putting the transom at the trailing edge of the bottom. Then he said that a boat can have 10 hp for every foot, and that I could put a 120hp on it.
At that point, I thanked him and hung up.Apparently he doesn't understand what kind of boat it is.
A semi-displacement, or semi-planing, boat is supported by a significant proportion of each.