semi displacement hulls?

suggest you check grand banks as a start. there are also several english pilot boats like aquastar.

The Independence 45 is a semi-displacement boat. It was built by Independence Cherubini Company, now out of business. I did the lines extension for the 50' version, the Independence 50. You can find both sizes in single screw and twin screw versions. They are pretty nice boats.

Except for speed claims, what clues should we look for to tell if a boat design is full planing or semi-planing? Obviously you can throttle back a planing boat and run it at semi-planing speeds. But I would think that the true semi-planing designs are optimized in some way?

Thanks,
Kelly Cook

Another Semi displacement yacht (also my personal favourite) close to your size range is the Skagen 50,

www.skagen50.dk

they are designed by some of the worlds best boat designers (something like 60 boat of the year awards) and as you will see from the photos on the web site, they rightfully claim very good sea keeping and i think she does 21kts,

Kelly,

There are five things to look for to determine the capabilities of a semi-displacement vessel. Speed-length ratio, of course, is the obvious one. V/Lwl^0.5 (speed divided by the square root of the length on the waterline) would be between 1.4 and 2.5. Below 1.4, the boat is in full displacement mode, and above 2.5 it is planing.

The second thing to consider is the displacement-length ratio, which is Displacement/(0.01*Lwl)^3 (displacement in long tons divided by Lwl/100, this quantity cubed). A long ton is 2,240 lbs. If the displacement-length ratio is between 100 and 300, the boat probably has the capability to travel at semi-displacement speeds, if enough horsepower is installed. Below D/L ratio of 100, the boat can plane, and above D/L ratio of 300, she is never going to get out of displacement mode.

The third thing to consider is the weight-power ratio, Displacement/Horsepower. Here, Displacement is in pounds, and HP is the total installed shaft horsepower. If the D/HP ratio is between 150 and 450, the boat probably will be able to cruise at semi-displacement speeds. Below 150, she'll probably be able to start planing, and above 450, she won't have enough power to get out of displacement mode.

The fourth thing to consider, and this is harder to determine because you most likely don't have any lines plan of the hull, is the flatness of the aft end of the hull and the angle of the buttock lines to the horizontal. Buttock lines are the traces of lines running fore-aft along the hull, parallel to the hull centerline. The quarter buttock (trace taken at the quarter beam off centerline, i.e. halfway between the centerline and the extreme beam to one side) should have an angle off horizontal no greater than about 7-8 degrees. If the angle is 2.5 degrees or lower, the boat will likely plane, given enough installed power. Above 8 degrees, the shape of the bottom is too steep for the hull to start planing. Again, you really need a lines plan of the hull to make this judgement.

One last thing to consider, in a general way, is the strength and seakindliness of the hull. Shallow and wide hulls can go fast, but they can pound badly in a seaway. The structure has to be taken into account--can the boat structure handle the pounding? Adding more structure means adding more weight, which makes the boat heavier, and therefore, slower. In a proper semi-displacement design, all this has been taken into account so that the boat works the way it should. Narrower and deeper hulls usually handle better and require less horsepower to move through the water.

Wow!

Thanks so much for such a thoughtful reply Eric. That helps a bunch!

Kelly

Your choice of hull type really depends on what you want to do with it, and how fast do you want to get to where you are going. Planing hulls are built for speed, and if I recall correctly, Vikings are sport-fishing boats, planing boats designed for speed.

If you are going to be living aboard, chances are you are not going to be moving very much out of the marina. Therefore, I would think time and speed are of little importance. The faster the boat goes, the more expensive it is to run and maintain. So why pay for speed when you don't need it? If you intend to really enjoy the trip to wherever you are going, and don't need to get there in a hurry, then I would opt for the displacement hull, first, and a semi-displacement after that.

Eric

And then there's the question of round-bilge or hard-chine hull forms.
A displacement hull is usually a round-bilge type hull, while a planing hull is usually a hard-chine type of hull.
In the semi-displacement range both forms occur, and the choice is a matter of priorities (speed or seakeeping). Note that a semi-displacement round-bilge hull has it's volumetric coefficients and hull lines optimized for the speed range (same goes for a semi-displacement planing hull). Most typical are the flatter buttocks in the aft ship. There is no way you can drive a conventional round-bilge up to semi-displacement speeds. The horsepower needed would probably sink your boat.

For a live-aboard that still has to go faster than hull speed (upto aprox. 2x), a round-bilge semi-displacement hull may be just the right choice...
But then again, it's harder to build and the interior space is more difficult to exploit (less boxy).

more on round-bilge vs planing hulls

To add to the above, the main reason why a round-bilge hull is better in a seaway, is that it's volume is concentrated much more around the midship, whereas a planing hull has more volume in the bow and the stern.
As a wave moves from bow to stern, you can imagine that a planing hull will be pitching more, while a round-bilge hull will move up and down and pitch less.

Because speeds are usually only calculated and tank-tested for the (dare I say hypothetical) mirror-flat sea, the round-bilge hull form has been given an unfair disadvantage (but even there, the speed may be equal to or even supersede the planing hull).

You have to be very careful in comparing the motions and performance of hard chine and round bilge hull forms. Certainly, for the same length of hull, and the same position of vertical center of gravity, the amount of rolling (angle of heel) in a round bilge hull form will be greater than that of a planing type of hard chine hull form, but its rolling speed and start-stop accelerations will be much less and, therefore, easier to withstand. That is, the rolling motions on a round bilge hull form are generally more comfortable than in a hard chine vessel.

Don't forget, however, that displacement and semi-displacement hull forms also come with hard chines but are not as severely shaped as a planing hull form. Hard chine hull forms are much easier to build with metal or plywood construction than are round hull forms, and that is probably the singe reason why hard chine displacement hulls are so common.

But how does this affect their motion in a seaway? That is a hard question to answer because the six degrees of motion in a seaway (angular motions of roll, pitch, yaw, and the linear motions of sway, heave, and surge) are coupled, and it is very difficult to separate and analyze the different motions (amplitude, speed, and acceleration) between the different hull forms. Many studies have been done separately to study the motions on hard chine and round bilge hull forms, but very few have been done to compare and analyze their motions directly and to characterize what elements of shape and volume distribution give rise to certain motions characteristics. The last published study that I am aware of, published in 1992, was severly flawed. It is an extremely complex problem.

The general belief is that a round bilge hull form is more comfortable in a seaway. It's amplitude of motion may be higher, but its speed and accelerations are lower. It is acceleration, particularly, that leads to seasickness, and the lower the accelerations, the less likely one is to be seasick, particularly with regard to rolling motion, and somewhat with pitching motion.

First, I am not a naval architect, but I still like to design a preliminary hull as I think it is an integral part of a yacht concept.

My idea of a good hull for a semidisplacement yacht in the lower speed regions is a combination of hard chines and roundbilge.

A canoe stern on this 30 m yacht would limit the realistic cruising speed to about ten knots or less. Just a roundbilge without chines would increase speed but still allow the yacht to roll quite a lot. With the hard chines the speed will increase a little further and both rolling and pitching will be less.

I think the cruising speed will be 12-13 knots without much more power needed.

Another advantage is that with increased speed the stabilizers will be more efficient. In following seas and a canoe stern, the waves will almost pass the hull from behind and the stabilizers will lose all effect. With the hard chines you will increase the speed and course stability enough to keep away from most followong seas and the stabilizers will keep up better.

At least this is what I predict with this design...

/Lars