Transom Design For Powerful Outboard Motors

This article discusses factors that should be taken into account in the design and reinforcement of a boat’s transom, and by boat we mean any motorized small vessel, including microskiffs, motorized kayaks, and canoes.

The Outboard Motor Effect on a Boat’s Transom

An outboard motor is attached to the top part of the transom (or vertical mounting plate) by means of two screws. The lower part of the motor’s mounting bracket is pressed against the back side on the transom.
The motor’s propeller rotates at the bottom of a long shaft that can act on the transom as a lever, namely apply torsional forces on it (Torque), both vertically and horizontally, depending on the relative position of the hull and the direction in which the propeller is pushing –

  • Horizontally, when making a sharp turn or bumping against a wave hitting the hull on its side, and
  • Vertically, when a climbing up a big wave or coming down on the other side.
The following examples may clarify this –

1. A boat that makes a sharp turn

When a boat makes a sharp right turn, the propeller rotates the transom clockwise, while the water resisting the right motion of the hull applies a torsional force on the transom in the opposite direction, namely counter-clockwise.

2. A boat’s bow “dropping” on the back side of a big wave, or wake

In this case there is a sudden change, and there is no water to resist the hull’s forward motion and push its bow upward, so the bow drops abruptly. The result is a sharp change in the hull’s angle, with the bow now pointing downward and the stern being higher than the bow. In such case, the dropping bow pulls the transom’s top forward, generating considerable stress.

In both these cases, the torsional forces and the resisting forces create stress in the boat’s transom. The stronger the motor, the faster the boat goes, and the bigger and more sudden the change, the more stress.
If the transom and motor mount are not properly designed and built, this could lead to a structural failure in either, and to serious trouble.

Danger to the transom during transportation

The boat’s transom is exposed to powerful torsional forces even when it is not in use, particularly during transportation, be it on a trailer or a pickup truck bed.
In these conditions, the heavy motor hangs outside the boat from its transom, and it can be exposed to severe shocks when the transporting vehicle goes over bumps in the road, even at low speed. The heavier the motor and the higher the transporting vehicle’s speed, the stronger the impact on the transom, and consequently, the potential damage to the boat.
For this reason, using a motor guard is very much recommended.

Overpowering a boat

A boat is considered Overpowered when it is powered by a motor, or motors that can generate more power than the HP for which the manufacturer rates this boat. In some states, overpowering a boat is illegal, since it is perceived as hazardous, not just in terms of potential structural failure of the transom, or motor mount, but also a failure of the driver to drive the boat safely in speeds that exceed the maximum speed attainable with a motor whose HP falls within the HP limit for which the boat is rated.

Any boat owner considering overpowering their boat should be aware of the legal and technical aspects of such action. Another factor to consider is the attitude of companies that insure boats, which may not be favorable.

Physical factors

The physical factors that require attention are the motor’s additional weight, and the additional power that it can generate beyond the weight and power (HP) of a motor for which the boat is rated. The bigger these differences, the bigger the risk, and the more comprehensive the required modifications in the transom and stern.
In many cases, simply making the transom thicker is not enough, and more structural work may be required to firmly attach the transom to other areas in the stern, such as its sides, its bottom, and other rigid structures that may support it. Spreading the loads that the transom is required to sustain may be as important as reinforcing it, and in some cases even more so.
In case of a small twin-hull boat such as the S4 micoskiff or W720 kayak-skiff, these areas in the stern may include the rear hull tips, and the rear end of the saddle seat structure.

Note that Wavewalk recently updated the article Motor Power Rating For Canoes, Kayaks, and Small Boats, about rating for maximum motor power, and overpowering boats. However that article does not elaborate on structural issues discussed here.

The Ultralight W720 Kayak-Skiff Walkaround Video

The W720 is a successful embodiment of the Wavewalk Micronautical design.  It is the perfect blend between kayak and boat performance – The ease of paddling with the stability and control required for motorizing.
As a kayak, the W720 tracks like no other, thanks to its long waterline, and it’s more stable than any other kayak out there, including fishing kayak behemoths that are over 40 inches wide, and weigh north of 120 lbs.
As a motorboat, it is nimble and responsive, and the easiest to launch and beach.
On top of this, the W720 is comfortable to ride in, thanks to its high saddle seat that stretches along the center line of its cockpit, and allows for up to three passengers to enjoy paddling and/or motorizing on lakes, rivers, flats and estuaries.

It definitely stands apart from the crowd of electric motor kayaks that neither look nor perform well.

The W720 Kayak Skiff does not offer the amazing seaworthiness of the S4 Microskiff, not its speed, but it is the best solo and tandem kayak out there, and it works perfectly as an ultralight motorboat, including as a dinghy (boat tender).

This video shows the W720 outfitted with a 3 gallon fuel tank, This quantity is enough for a whole day of nonstop driving with an outboard motor up to 3 HP (practically 2.5 HP). This tank was too large to fit inside one of the W720 hulls, but it conveniently fits  on top of the saddle, between the motor and the driver.

Hands-on: DIY articles

Most people don’t design their own microskiff, or portable boat, but many love to outfit it, and build accessories for it  –

Wavewalk recently published several articles on subjects related to Do It Yourself (DIY) projects.

Kurt Raimer’s from WA state outfitted his S4 microskiff for fishing salmon and trout:

Bill Seay’s report on his Easy-Load for his S4:

Terry Wilkison’s new white S4 is loaded with DIY upgrades:

S4 microskiff top view

Read full article:

How To Trim My S4 Microskiff For Speed? »

Design, including boat design, is about choosing the formula that would best answer the requirements of the user. In the case of the S4 Microskiff, the requirements for extreme portability (easy car topping) and versatility (ability to paddle) led to a limitation on engine power, mostly due to its weight.
Many boat owners care about being able to reach the maximum speed that their boat allows for, so they spend much time and money on trimming their boat, and this article offers S4 Microskiff owners information and hand-on advice on how to trim their boat for speed.

One of the fist questions that future S4 owners ask is about motorizing, and more specifically, what size motor would best fit their needs –

3.5 HP or 6 HP outboard motor for my S4 microskiff? »
Wavewalk and its dealers sell the S4 microskiff non-motorized, and it’s up to the boat owner to choose what type and size of motor they will use. Wavewalk offers plenty of info and good advice on these subjects, and this is the latest article in this series.

It’s worth mentioning here that Wavewalk’s website offers dozens of articles including on subjects related to boat design and DIY projects.
Articles Directory »


Is Kayak Fishing Easy Or Hard?

According to this article, the answer to this question depends on who’s asking – It’s possible for young and fit anglers who are willing to compromise substantially to fish out of kayaks, but all others are effectively prevented from doing it, because typically, kayaks are neither stable nor comfortable enough for serving in the capacity of a fishing vessel.

Stability and comfort, namely ergonomics, are related directly to boat design, and kayaks don’t necessarily have to be unstable or uncomfortable. With the same size of a typical fishing kayak, Wavewalk’s W720 and S4 provide a much higher degree of stability, seaworthiness, and comfort. People who use these boats never suffer from back pain while they travel in them, or fish from them, and neither do they experience instability when they stand up, as they would in SOT fishing kayaks, .

W720 kayak skiff gif animation
W720 kayak skiff made by Wavewalk

And by “people” we mean everyone, not just persons who are young and fit.

Speaking of Boat Design, words are not enough, and one must use numbers: The above series of images shows the W720, a patented catamaran kayak skiff that weighs 85 lbs, and is 31 inches wide. These dimensions make it easy to paddle compared to the top tier SOT fishing kayaks who are always wider and in most cases considerably heavier, while providing less stability than the W720 design does.
85 lbs and 31 inches are low figures in this field of top-tier fishing kayaks, and they also make Wavewalk’s kayaks easier to cartop, transport, and carry than other fishing kayaks, which can weigh over 100 lbs (up to 140 lbs!…), and be as wide as 42 inches, namely “barges”.