We will be using the S4 as a kayak this summer with oars in Congaree National Park and in lakes in the NC mountains.
We live at the ocean and do some creek / tidal fishing so the engine is important. I read the article about overpowering the kayak, however I intend to carry 2 adults and gear typically (my wife and son or son and daughter) so I wanted to opt for a larger engine and got a deal on a Mercury long shaft (20″) which is the lightest 4 stroke 9.9hp I could find. It weighs just 84lb. I read in the Wavewalk blog that the transom may need to be reinforced to accommodate an engine of this size. I see adjusting trim and testing the performance is vital.
The S4 Microskiff is designed and rated for an outboard motor up to 6 HP. Overpowering it may be illegal in some states that forbid driving overpowered boats.
A 6 HP outboard motor fits the S4 perfectly, works perfectly with it, and driving the S4 with it is easy. However, powering this boat with a 9.8 HP motor must be done only after its transom has been reinforced, and both boat and motor outfitted in a way that effectively prevents spray shooting upward from the bigger propeller shaft from entering the cockpit.
Failing to perform either of these critical safety modifications is hazardous, and it could result in accidents and damage to the boat.
Inexperienced drivers should exercise caution when attempting to drive an overpowered S4, and take the time to learn how to drive it safely.
Does Wavewalk recommend trying to outfit an S4 with such a powerful motor?
No, we don’t, and for several reasons, the primary and most important of which are legal (see above warning) and safety related: The S4 Microskiff’s official power rating is 6 HP, and we’re not going to change it because if we did we’d have to offer a new version of this boat that fits such a motor, and we think the market for such a “muscle” microskiff doesn’t justify such a move.
Other problems with overpowering an S4 Microskiff?
Yes, and the first is that although the 9.8 HP Tohatsu outboard is the most lightweight in its class (together with the Mercury 9.8 HP), at 85 lbs it still weighs 25 lbs more than the 6 HP outboard from the same brand. This may not be a problem for some S4 owners, but it could be one for others. In other words, while carrying a portable 6 HP motor is possible and even easy for most owners, an 85 lbs is considerably less portable, and for many people it might be barely movable.
Other problems with such a project are related to the difficulty of reinforcing the transom, and of protecting the back of the cockpit from spray shooting up from the propeller shaft and splashing in. The former is pretty straightforward, but the latter is complicated and potentially frustrating, and only a skilled person with a good technical sense as well as patience may succeed in it. When we did it we relied on Captain Larry Jarboe’s multiyear experience in these matters, and the solid advice that he provided, but it still took us weeks before we achieved the required “zero spray in” point.
Note that the propeller shaft thickness is also a factor to consider, as a thicker shaft generates more resistance from the water, and therefore shoots more spray upwards, and some of this spray can find its way get into the rear part of the cockpit if this problem is not taken care of effectively.
And what are the advantages?…
More power means higher speed. But although a 9.8 HP motor generates 63% more power than a 6 HP motor, and it features a propeller with both a bigger diameter and a bigger pitch, it doesn’t drive the boat faster at the same rate, because of hydrodynamics. Well, not even close.
The degree to which such a bigger motor would improve the boat’s speed depends on multiple factors, and it is hard to assess accurately. Our educated impression is that the S4 is about 25% faster running a 9.8 HP motor compared to a 6 HP motor, and this is a noticeable difference.
In absolute terms a savvy 150 lbs driver is likely to go with their S4 at 20 mph on mirror flat water. Anyone who weighs more should expect to go slower than this.
This may surprise some people, but consider these figures:
The S4 weighs 100 lbs, and the motor plus fuel tank weigh 100 lbs at the minimum, a total of 200 lbs.
A US adult male weighs 200 lbs on average, which is equivalent to the combined weight of this boat plus a 9.8 HP motor, and the combined weight of driver + boat + motor is roughly 400 lbs. This is the weight that a 9.8 HP motor is required to push forward while gravity pushes this weight downward…
And by the way, a lightweight 6 HP motor is required to push forward 25 lbs less.
In comparison, a 150 lbs driver would reduce the total combined weight of boat + motor + driver by 12.5%, from 400 lbs to 350 lbs, a difference that thanks to hydrodynamics would produce a speed increase that’s bigger than 12.5%.
A different way to look at these numbers would be the following:
An S4 microsiff driven by 150 lbs driver and propelled by a 9.8 HP outboard weighs about 350 lbs in total. Change the driver to one who weighs 200 lbs, and the total weight of this compound object would increase to 400 lbs, which is ≈14.3% heavier. However, in such case hydrodynamics would work against the driver disproportionately, and the decrease is speed would be bigger than the 14.3% increase in total weight.
A driver weighing 180 lbs should be content with reaching 18 mph in optimal water conditions.
This said, if you like speed, driving a faster and more powerful boat is more fun.
Typically, 9.9 HP motors are somehow heavier than 9.8 HP motors, and some models are much heavier. The most lightweight 9.8 HP outboard motors are offered by Tohatsu and Mercury (85 lbs).
Cost
9.8 HP outboard motors cost about half more than 6 HP motors.
The W720 Kayak Skiff is a lightweight twin-hull (true catamaran) that’s 31 inches wide, and weighs 85 lbs. It is extremely stable for its size, and in fact it’s more stable than much bigger and wider kayaks, but still, it is a very small craft, which is why it is rated for motors up to 3 HP (effectively 2.5 HP) and not more.
A W720 outfitted with a bigger, more powerful motor is overpowered, which means that it is illegal in states that forbid overpowering boats, and potentially hazardous to drive everywhere, regardless of laws.
The overpowered W720 kayak skiffs that feature in demo videos are driven by experienced professionals whose job is to test new boat designs, including in special and even extreme conditions.
Many owners like to outfit their boat, and in rare cases this includes outfitting it with a motor that’s too heavy and powerful for it, and does not comply with the boat’s power rating.
There is no effective way to stop people from doing this except recommending that they don’t, but those who feel they must try this should be aware that the standard motor mount that comes with the W720 does not support motors bigger and more powerful than the 3 HP for which this kayak is rated. Therefore, they must build a special motor mount for their overpowering project, and attach it properly to their W720, or reinforce the standard unit that ships with this kayak. Failing to do so is hazardous and could cause serious accidents and damage to the kayak.
Any reinforcement must include making the mounting plate thicker as well as adding attachment points to the wooden saddle bracket and to the hull, including steel hardware. Special attention must be given to avoid unnecessary perforations in the hull, and to properly seal the ones that are needed for the project.
The picture below shows a reinforced W720 motor mount, for illustration purpose only – This is not a recommendation.
Smaller Motors Weigh Less; Stick To The 3 HP Limit
The professional boater in this video carries a W720 Kayak Skiff outfitted with a 6 HP Tohatsu outboard that weighs 60 lbs. Due to safety reasons, the W720 Kayak Skiff is rated for motors up to 3 HP that typically weigh around 30 lbs, so carrying it is easier for users who abide by this power rating restriction.
Note that overpowering a boat is illegal in some states, and you should always wear a Personal Flotation Device (PFD / Life Jacket) when using kayaks and small boats.
Reinforced, Non-Standard Motor Mount
The photo below shows the 6 HP motor attached to a non-standard, reinforced motor mount. The W720 is not designed for use with a motor stronger and heavier than 3 HP, and driving it with bigger motors can be hazardous. The standard motor mount that ships with the W720 does not allow for using motors above the boat’s 3 HP max power rating.
The reinforcement of this motor mount includes doubling the thickness of the mounting plate and adding steel brackets as well as additional attachment points to the twin hull.
Rear view of the W720 Kayak Skiff with a 6 HP Tohatsu outboard motor mounted on a reinforced motor mount
Attaching The Fuel Tank In the Front Of The W720 Cockpit
The picture below shows the fuel tank attached in the front of the cockpit, with the fuel line secured under the coaming. The advantage of this setting is that the weight of the fuel tank helps in keeping the bow down and the boat level, which improves its trim, namely its speed.
The 3 gallon fuel tank from Tohatsu fits perfectly at the bottom of an S4 hull, but since the W720 hulls are too narrow for this, the next best location for it was on top of the saddle.
W720 Kayak Skiff with 6 HP Tohatsu outboard motor on the beach, ready to launch
Carry Handles
The W720 is a catamaran that features a carry handle at each tip of its twin hulls, a total of four handles. Since the user who shot this video held a selfie stick in one hand, he had to carry the kayak with one just hand, holding a paddle that he inserted in the two front handles. Needless to say that holding the kayak from both its carry handles is easier.
I’ve been experimenting with propellers of various pitch, and it seems to me that when my S4 is fully loaded (electric motor, batteries, cooler, etc), the 9″ pitch propeller isn’t necessarily the best choice, and smaller pitch propellers work as well.
