On Changing Stock Boat Plans

Here’s an interesting article from the Glen-L archives. —Eds

Article by Ken Hankinson

Changing length can be easy

The most common change requests involve size. A most important thing to remember is that a boat hull is not a box—it has shape and form, and must do certain things; not just sit still. A seemingly simple matter of adding or subtracting an inch here or there can have complicating effects that the novice may not realize.

For example, while adding or subtracting length is usually easy, changing beam and depth is usually not, and the effects can be serious. As a rule of thumb, most (but far from all) boats can be varied up to about 10% plus or minus (see Fig. 1). In other words, a 30' boat could probably be built as short as 27' or as long as 33' with little problem. But there are some exceptions as I’ll explain later.

Fig. 1 shows a hull with equally-spaced transverse stations. Changing the length of the boat is simply a matter of adding or deleting an equal number of inches from each station space. Often these stations correspond to frames thus making length changes on most framed boats straight-forward. Note also how the boat’s center of buoyancy (CB) stays in relatively the same position (you’ll see why this is important later).

When and why length changes should NOT be made

There are some cases, however, where boat length should not be altered, at least without professional advice. First is sailboats. The problem here is that there is a definite relationship between a sailboat’s center of lateral resistance and center of effort of the sail plan in order to maintain helm balance (see Fig. 2). Also, the ratio of wetted area of a hull (drag or resistance) to its sail area (the "horsepower" of the boat), as well as stability, must be taken into account. Thus, if you alter length, you may also need to alter the rig and ballast to compensate.

However, as also shown in Fig 2, it might be possible to add length to a sailboat hull ABOVE the waterline by increasing the overhangs, in effect, increasing on-deck length but not waterline length. Although not always practical or as simple as it may seem, theoretically an addition to the bow and or stern as shown by the broken lines would have little, if any, effect on the relationships just discussed.

Another case where changing length is usually not recommended is with "stitch-&-glue" and some other designs where hull panel shapes are pre-determined, and where typical frames are not evenly spaced or even used in the boat. In these cases, there is no easy way to expand all the panel shapes proportionately so that they will match up exactly during assembly short of redesigning them which is much like creating a new hull.

Altering length - how NOT to do it

With boats based on pre-determined panels, some might want to add a foot or two to the aft ends of the panels, which seems simple if the boat is relatively parallel here. But the danger is that the designer may have planned the shape of the hull (especially the bottom) to have the boat’s hull entry area at a certain point, or have volume distributed along the hull in a certain way for balance, etc. If you simply add to the aft end of the panels, you could change the boat’s proportions and volume distribution in ways that could detract from the boat’s ride, balance, and performance. In other words, results could be questionable.

Generally, adding or subtracting frame spaces should be avoided on framed boats too. Again, a boat is not a box - it has curves and shape. If you attempt to add or delete a frame space - say in the middle of the boat - unfair lines are likely (see Fig 3). Not only will this look ugly - assuming the boat can even be built - but could also affect performance.

Of course, if a boat has parallel lines, such a change may be possible, which is often the case at the stern of many powerboats (see Fig. 4). But such changes must be approached with caution since hull lines may be subtle and not as parallel as they seem to the untrained eye.

Then too, adding or subtracting a station at the stern will shift a boat’s CB, and if the underbody volume is not recalculated, it will be a guess where the CB has shifted, making it perhaps difficult to balance the boat. And the same problems as mentioned for the "stitch-&-glue" design could occur because you’ve altered the distribution of the hull’s volume and various proportions that affect performance.

Changing beam? DON’T!

As for beam—or boat width—the best advice is DON’T, especially if it’s added below the waterline. Doing so could change a boat’s characteristics completely including all the hydrostatics (displacement, CB, CG, stability, etc.) as well as the boat’s performance characteristics. In effect, these changes result in a virtual new design of potentially different qualities (see Fig 5). While structure-wise it may be easy to increase beam in, for example, flat bottomed boats such as dories, the statements about changing characteristics above still apply.

S ome builders attempt to change beam by altering the angle of the topsides while leaving the underbody unchanged. This may work on some boats, assuming the builder has some experience. But usually it means re-fairing the boat’s lines and redesigning frames, structures, etc. Such changes can be fraught with pitfalls. Of course, popping out the beam this way may also effect a boat’s stability, so such a change should be discussed with the designer in any case.

Changing freeboard (hull depth) can get complicated

Some may also want to change the depth or freeboard of a hull (often in the fallacious belief that more freeboard automatically makes for a more-seaworthy boat). But because most boats have some flare to the sides, hull lines will again need re-fairing in the area of the sheer ("gunnel"), and there may be other considerations.

While adding an inch or two of freeboard may seem innocuous, such a change will affect the sheer line - the most important line defining the aesthetics of a boat. Designers often spend hours working on this line to get it "just right" in all views, and nothing can spoil a boat’s appearance quicker than a botched sheer line.

To complicate matters, this line typically has curvature in both plan and profile views. If you project the topsides along their angles (which are often ever-changing), the sheer in plan view will change. If you keep the sheer plan view fixed, then the topside angles will change instead. You have to pick which way to go and then re-check everything for fairness, and most beginners will find either approach complicated.

Then too, on boats using sheet material for planking or plating, adding freeboard may add just enough inches to preclude using sheets of a standard or maximum available width (see Fig. 6). In such cases an extra joint may be required which can be unsightly.

Superstructures often easy to change but...

What about changes to superstructure and cabin arrangements which may have nothing to do with boat size or shape? If such changes don’t alter location and/or size of major weight items such as tanks or motors, they can usually be made at will.

But remember, a boat is like a teeter-totter in the water. The fulcrum is the boat’s CB. Weight located on either side of the CB when multiplied by their distance from the fulcrum point results in moments, and the sum of these moments in both fore and aft direction, should be equal, or nearly so. If not, the boat will trim (go down) by the bow or stern to some extent.

So if the changes you want involve relocating such equipment and tanks, reconfirm hull balance first on paper by recalculating the moments. But take care when relocating tanks. Since their contents will vary between full and empty, their moments may vary considerably, especially if located at transom or bow. For this reason, tanks are usually best located at, or as near as possible to, the CB so that moments won’t vary as much between full and empty conditions.

What about changing motor location?

Hull design involves the distribution of volume below the waterline in such a manner that there is enough to float the boat at it’s waterline, while allowing the boat to float level with all fixed items aboard - such as engines - at their designated positions.

Consider boats with motors at the stern. Because motors are a major weight component, a designer must design the hull so there is ample volume below water at the stern to support the weight without undue trim changes. Conversely, boats with centrally-located motors will have their underbody hull volume arranged with more volume amidships to support motor weight here in a level attitude.

As a result, it is usually NOT recommended that motor positions be altered in boats. For example, a plan intended for outboard or stern mounted inboard (I/O, jet, etc.) should not have the motor relocated amidships. Likewise, a centrally-located inboard boat should not have the motor shifted to the stern.

Often builders want the simplicity and reliability of a in-line propeller shaft set-up rather than an I/O or outboard. When this is the case, the motor should still be located at the stern and coupled to the shaft through a v-drive rather than relocating the motor amidships. You will then have the advantages of the in-line shaft/propeller set-up without throwing the hull out of balance which would happen with the motor amidships in a hull without enough volume here to balance out the load. •SCA•