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Transom is an old word deeply rooted in tradition, like many nautical terms. The word is also used in architecture in a similar function. So what is a boat transom, and why are they important?

Here’s a quick look at the origins of the word and the different types of derrieres sported by all the sorts of vessels on the Seven Seas.

boat transom

What is a Transom?

Like all other things nautical and on boats, the boat’s backend has a fancy name. It’s called the transom.

The word derives from as far back as 1300s Middle English and can trace its roots even further back to Latin and Old French words that meant transverse or crosswise.

This definition is important to understanding the transom in boat design since it’s usually a vertical panel of the boat’s hull set crosswise to the sides.

A similar word in nautical lingo is “stern.” Stern is more of a direction, though, as in setting the “stern lines” when docking or “powering astern” when driving in reverse. In contrast, the transom is a part of the boat located on the stern.

Depending on the type of boat in question, the transom might support some important gear. For example, small powerboats often have their outboard motors mounted on their transoms. Many sailboats mount rudders on the transom. Larger vessels may support swim platforms back there.

Sailboat Transom Designs

Nothing brings a sailboat design together quite like the transom does. Of course, there are many different designs out there, but to some extent, the transom is a trendsetter.

For example, in the 1970s, it was fashionable for bluewater boats to have canoe-style sterns. Sometimes called double-enders, these boats were sturdy and solid. They’re trademarks of the Valiants and Hans Christian fleets.

Today, the sleeker looks of the reverse or flat transom have taken over. This is mostly due to the changes in yacht design over the last few decades. Today, emphasis is placed on wide cockpits with walk-through transoms and big swim steps.

The transom is just one feature in the overall design of a yacht. Therefore, it’s impossible to draw any big conclusions from this one feature. It’s even impossible to declare one better than the other.

What we can say with confidence is what we like. Some love the salty looks of those canoe sterns, while others find them cramped and dated. Some sailors want that sugar scoop-style transom with a big swim platform, and others want the curvy lines of a heart or champagne flute-shaped rear end.

Canoe Sterns or Double Enders

Bringing the stern of a sailboat (and sometimes a powerboat) back to a point like a canoe lends a salty and traditional look to any vessel. Canoe sterns are often compared to transom stern sailboats. It implies seaworthiness, although whether or not it provides any real benefits is up for much debate.

Canoe sterns are common on many of the cutters from the 1960s and 1970s, including famous designs like the Westsail 32, Valiant 40, or Hans Christian 32. On smaller vessels, it’s also often combined with a hung rudder, such as on the Bristol Channel Cutter, Pacific Seacraft Moriah, or the Morris Francis.

Canoe stern with transom-hung rudder

Raked – Classic Transom Stern with Overhangs

The rake of the transom refers to the angle at which it extends aftwards. If a transom is raked, it angles away from the vessel. If it is flat, it is vertical. And if it angles in toward the boat, the transom is said to be reversed.

Raked transoms, combined with long overhangs, were popular on racing boats for most of the 20th century. This resulted from limits on waterline length placed by racing rule committees. So designers added long overhangs that would add to the waterline as the boat healed.

Flat Transom Sailboats

Flat transoms have a distinctive look and allow a boat to carry her beam farther aft. This is great for having bigger cockpits and more hull volume in the rear of the boat.

Flat transoms are an iconic feature of Island Packet sailboats, but they’ve been used in many others as well. The Passport 40, a well-regarded Bob Perry design, has a particularly graceful one.

Flat transoms are very popular in modern yacht design, with Beneteau, Catalina, and Jeanneau offering yachts with wide, flat transoms with rear door fold-down swim steps.

Flat transom with fold-down swim platform

Reverse Transom Designs

Reverse transoms angle forward toward the bow of the boat.

Several famous designs carried reverse transoms, like Sabres and Hylas yachts.

reverse transom

Other Transom Features

Sugar Scoop Transom

Sugar scoops are reverse transoms that are hollow in the center, allowing space to design in a built-in swim platform and stairs. This is very popular on catamarans.

Transom-Hung Rudders

Many boats include an externally mounted or transom-hung rudder.

Wineglass or Heart-Shaped Transom

One unique and eye-catching transom design is the heart-shaped transom, which is usually raked and sometimes includes a prominent overhang.

A few examples of boats with beautiful behinds like this include the Shannon 38 and the Cabo Rico 38 and 42.

Power Yacht Transom Designs

Enough about sailboats, it’s time to get somewhere already. Power vessels have transoms too, and they’re just as valid and appreciated.

Flat Transom Powerboats

Flat transoms are likely the most common powerboat feature. For one thing, this suits the planing hull form well. Where sailboats come to a fine exit at the rear, a planning boat will need a broad wetted surface to support the boat’s weight at speed. The result is a flat and broad transom sported by all sorts of vessels.

Flat transoms often support an added-on swim platform. Many smaller boats have a flat transom where outboard motors are mounted. An outboard bracket is used if the motor doesn’t attach directly to the transom.

Fantail

A fantail is a semi-circular aft deck, so the transom is curved. It’s a classic look that you’d find on the back end of the HMS Titanic or other classic wooden yachts of the early 1900s.

Outboard Motor Transom

Powerboats and fishing vessels with outboard motors will hang them on the transom.

The traditional and simplest way to do so is to mount them directly on the transom. An outboard transom will usually have a notch cut out of the top of it, which will allow space for the motor to be mounted. Unfortunately, this reduces the freeboard of the transom itself and could present a problem in following seas or when operating astern. Some offshore designs combine the cutout with a self-bailing well to keep the rest of the boat dry.

The molded outboard bracket is a modern method of preserving transom height while allowing outboards to be mounted low. These are mounted onto the transom and allow for standard outboard shaft lengths to be used, no matter the boat’s size or the actual transom’s height.

outboard motor transom

Outboard Transom Mounting

Sizing your outboard correctly is critical to getting your boat’s performance right. The goal is to get the anti-cavitation plate, located just above the propeller, roughly in line with the bottom of the boat. It needs to be the right height, not too far above the keel and not too far below. It also needs to match the hull’s angle so that the thrust from the engine propels the boat forward at the most efficient angle.

While the transom is an important structural component in any boat, a transom that supports an outboard needs to be extremely strong and structurally sound.

How to Measure Transom Height

As described above, transom height can describe two things–one is the actual freeboard provided, and another is the outboard mounting height. The boat’s design will dictate what length of outboard motor shaft you should use.

What is Transom Height?

The boat transom height is measured from the lowest point on the keel along the center line to the spot where the outboard motor’s mounting bracket rests. In the case of a dinghy or johnboat, this point may be the top of the transom. In the case of a larger offshore boat, it may be to the top of a cutout on the transom. If the boat has a bracket, the measurement is made from the bottom of the keel to the top of the bracket.

Depending on the design, this number may or may not represent the freeboard available from the transom. Freeboard is the height of the sides of the boat above the water. Generally, the more freeboard you have, the better. Extra freeboard provides a drier ride and keeps waves and splashes out.

Boat Transom Angle

Height isn’t the only consideration when measuring a boat’s transom for an outboard motor. You’ll also want to consider the transom angle. A flat transom will be vertical when the boat is at rest. Many powerboats have transoms that angle aft, meaning that the outboard needs to be trimmed up slightly to sit correctly.

Outboard brackets allow for adjusting their mounting angle to neutralize the effects of an angled transom. But the brackets can only correct for so much, so in some cases, you may have to use an angled backing block when you mount the motor.

Outboard Motor Transom Height

Motor manufacturers have standardized a few basic lengths for their outboard motors. Of course, the precise measurements will depend on the engine’s power rating. But motors are usually available in short, long, and extra-long shaft lengths.

When looking for what size transom for a short shaft motor, you’ll need to consider a few things. Don’t just assume that a “short shaft motor” means the same thing for every engine–one manufacturer may believe short means 15 while another uses 17. Some may have an XS extra-small shaft option available. Additionally, a short shaft five hp engine may be 15 inches, while a short shaft 300 hp engine may be 25 inches. While 15, 20, and 25 inches are the industry standard, you should double-check the specifics of your engine before making such a big commitment.

Here are some of the standards, but remember to double-check these numbers for your make and outboard model.

Outboard Height on Transom

Type of OutboardShaft Length/Transom Height
Transom height for short shaft outboard15 inches
Transom height for long shaft outboard20 inches
Transom height for extra long shaft outboard25 inches

Boat Transom FAQs

What is an engine transom?

The transom is the vertical section of a boat’s hull that is perpendicular to the sides of the hull and located at the back. For example, an engine transom is designed to hold an outboard motor.

Do all boats have a transom?

Yes, although various boat transoms can look different from one another. The transom is an integral part of the boat’s hull and an important part of the overall design. It serves both aesthetic and functional purposes.

The only exception to this rule is vessels designed with a canoe stern, sometimes called a double-ender. Instead of terminating in a flat transom, these vessels feature a rounded stern with no vertical surface.

Why is the transom important?

The transom of the boat is an important design feature of its hull.

In sailboats, the type of stern a boat has will dictate some of its design and performance characteristics, but the type of transom alone will make little difference. In general, the greatest advantages in performance come from out-of-sight below-the-watertight design qualities. The transom is, more often than not, designed for aesthetic appeal.

The transom will belay what sort of boat you are looking at in powerboats. Outboard motor transoms are functional. Planing hulls will tend to have broad, flat transoms. And finally, displacement hulls will have a variety of transoms, much like sailing vessels.