where should the widest part of a sailboat be?

8 frames on a 17 ft boats is quite a lot. You could build it with 4 or 5.

 

When studying the shapes of a hull it is normal to use 10 cross sections, even more. It depends on the rigor and professionalism of the designer. Another very different thing are the building sections. I wonder if the drawing he has shown us is the construction plan or just a sketch of the boat. Of course, there is no frame to be seen in it.
To build this boat you will need 4 or 5 frames or more, it depends on many things.

 

Given that the same amount of force is required to move the mass of water to the given width and depth and back over the vessel's given length, I suspect the simple application of the F=MA formula wouldn't yield any information about where to place the beam. The production of turbulence and skin friction as well as understanding the underwater shape changes under various sail and wind conditions would be a separate problem.

A wide beam forward would mean more resistance to sinking the bow under sail power with its CE located high over the deck, while a wide beam aft would offer better trim for a crew located primarily aft in light winds. Then there is the balance around the CG were the boat's own weight moves with the beam, but likewise, so does its center of flotation to counter it.

Mostly, it sounds like, for moderate changes around the waist of a vessel, there's are always counter balances working to make little difference.

-Will

 

Accelerating too a given speed over a shorter period of time requires more energy. Acceleration is not just a change in speed. It can be a change in direction too.

So, with the max Beam further forward, the water has to move aside over a shorter period of time. Therefore, it requires a greater amount of accelerateion.

But, we must keep in mind that the water must change directions again (hence another accelerateion) once it passes the max Beam of the boat. But this acceleration is of less importance, so moving the max Beam aft of mid-ship does more good than harm. A sharper bow also parts waves better.

An interesting note: John C. Hanna thought that Slocum's SPRAY was too beamy to be safe on the ocean. So, he endeavored to "improve" it. He drew a version with about the same displacement, but a narrower Beam. He also set the max Beam back in proportion to the max Beam, and not in proportion to the Length.

This created hulls with cavernous interiors, that were seaworthy, but slow as snot. I guess he never figured that, with the proportionately shallower SPRAY, about as much water went under the the max Beam section as went around it.

This being said, a boat with the max Beam further forward is going to have greater form stability than one with the max Beam further aft. It's also going to have more deck area and more interior volume.

It will almost certainly have better manners going down wind, and may even have better course-keeping qualities.

But it will pay for all this when going upwind. While doing this, it will almost certainly be parting waves. Then, it will be pushing much more water asside. This is why, with the exception of narrow boats (B/L < 0.30), we rarely see racing sailboats with the max Beam forward of admidship.

But max speed upwind is just one quality that makes a good boat. And it often comes at the expense of other qualities.

 

I had no intention of making a big deal out of the OP's question. When I proposed that the beam and maximum bottom depth should be compatible it was in good faith. The whole idea is to have the displaced area of a section in a supposedly advantageous position....complete with the inevitable compromises. If the max beam is in one position it is obvious that the max depth of the bottom can be in a different position. All that fiddling around is to make the curve of areas as smooth as one can make it with the least difference in the change of areas in consecutive sections.

Beam is not an independent variable and it is entirely alright to have the beam forward of the middle if the max bottom depth is farther aft. Some one, above, mentioned Sunfish and force 5 configuration with max beam forward. That is accurate. The difference is that max rocker aft makes the boat less likely to plane on account of the rapid rise of the bottom aft. Be that as it may, I can assure you that the Sunfish will plane, however it takes a near gale to make it do so.

In the case of the crab skiff it is of little consequence since it is not intended as a performance boat.

 

Does it make for a pitchier boat to put max beam and max depth together? I would think doing such means most of the flotation is concentrated at the beam and the ends are more likely to pitch around it.

 

Good question Will. As in so many boaty things; it all depends. Depends one whether or not the ends are fuller or skinnier. A boat like a garvey with box like configuration could have considerable buoyancy out toward the ends. In that case an approaching wave would be more likely to cause pitching than if the end sections were skinny like a kayak. Drawing a curve of displacements would give us some clues about behavior in lumpy water conditions

Speaking of Kayaks there are spirited arguments over the merits of the so called fishform and swedeform layouts. One has the max beam forward and the other has it aft. Back to the "it depends" deal. It depends on the intended purpose of the kayak. Whitewater layout is much different from a touring layout.

 

One thing worth considering is that if max beam is aft then the dinghy will tend to become very tippy as you shift weight forward e.g. when the crew moves to the mast to raise and lower sail.

Presumably the inverse is also true, which in the extreme could make shipping the rudder tricky.

 

On this hull the point of maximum beam shifts from the floors to the Gunwales. On the floors, the maxiumum beam is much closer to center, and should give really good performance and handling. The deck swells out forward, which will tend to deflect water coming from the front. This will make a slightly drier boat. This boat is most most likely designed to skim over the water on nearly even keel, so the design on the bottom is much more important. The V bottom is used to gain displacement and beam without ruining sailing qualities. It works well, but will likely have a higher displacement than you need. In this type of design, in order to maximize performance, you need to find out what the weight of your finished boat will be. Then you need to figure out how much you need to carry. Redraw the lines so the displacment is very close to your load plus hull weight plus sailing gear. Remember, this was designed as a working boat that lugged around huge amounts of seafood. If you aren't planning on lugging around seafood you can get rid of displacement. With a boat like this you are able to easily trade displacement for performance. I would be very interested to see the stations for this design to see if there is any twist in the sides. With this design, I would aim to cut displacement, increase flare, and make sure the flare increases slightly from stem to stern. I would aim for stability to carry sail on nearly even keel, since the extra width will increase weather helm as the boat gets over powered. A hiking crew will help greatly with stability. Flare especially where the crew is likely to be will assist with this, since it gets the crew farther to windward. Decreasing the bottom width will also cut down on wetted area. Building this boat as light as possible will pay off huge in speed. Traditional methods are heavy. With stitch and glue plywood construction and a careful trimming of the lines, you should be able to develop a really fast boat from this.

In regard to the advantages of placing beam forward or aft, it depends on what you need to do. Maximum stowage means beam carried far forwards. Maximum speed on a planing dingy will see a shift rearward with a fine entry. Fine entries with flat runs tend to be very fast on planing boats. The finer entry can cause stability problems though. If your bow submarines then you aren't going to go very fast. On the other hand, a broad bow will keep you from pointing well in rough water and cause your bow to be swatted around by waves. So everything is a trade off. A thin hull can get away with more beam forward and still be fast. A beamier hull needs the max beam closer to center. The beam can also shift as it does in this boat to get some combination of the best of both. The old timers in Europe used to say it is more important to consider the flow around the stern than the bow. They built bluff bows with fine runs. As a result, their boats got thinner and thinner until they were unsafe. On this side of the pond, the opposite happened. We built wide shoal schoooners with fine entrances that sank by the dozens in every storm.

Racing rules created some real freaks.

As a general rule, if your maximum beam is within 5-10% of center on the bottoms, you will not get into too much trouble. You have to look carefully at your displacement currve to determine where you need to put weight to settle on your lines. More beam near the ends will cut down on pitchpoling. Of course more beam on the ends also means more bouyancy in a seaway. This can lead to being thrown off course. Flat water planing boats can have maximum beam carried almost to the bow. Boats like this are very fast, but only in strong winds and flat water. See inland racing scows. Look at 49ers for a fast boat used in rough water. Max beam is very far back, and carried very far back. The bottom profile is very different from the topsides, even without hiking racks.

All in all, the exact location of maximum beam is far less important than making sure the beam location works with the overall design.

 

David Dodd's last statement hit the nail on the head. Beam location has to work with the overall design. Beam is a measurement in only one dimension. Depth of the underwater part of the section is the other salient dimension. The area of the section, wherever it occurs, with respect to all the other section areas is the determinant of how and where the water is displaced or accelerated. That is why the construction of a curve of areas is an essential part of the trial drawings.

The boat has to push water out of the way if it is to move or even to float. The manner in which we push the water out of the way is the determinant of whether the boat is as efficient as it could be or not. Bottom line is that beam is not the most important dimension. . The dimensions of the rest of the submerged body has to be considered as well.

Beam alone does not entirely determine how the boat is likely to pitch. Displaced volume of a pair of adjacent sections, at whatever location, will suggest how "floaty" some part of the boat may be.

Applicable to the mini rant, above, is the disclaimer that we are not talking about all out planing boats. Nor are we talking about big stuff like tankers, bulkers, or RoRo beasts. Big ships are the territory for more knowledgeable contributors like Ad-Hoc and a few others on this forum. (Virtual hat doff to the big ship guys and girls)