Updated August 8, 2020
The purpose of this article to answer a frequently asked question (FAQ) from prospecting clients, which is “What outboard motor should I choose for my S4 skiff?”
The answer is that the outboard motor and propeller you should choose for your S4 depend mainly on two factors, which are Power and Propeller Pitch
- How much power you need: How fast you want to go, in what kind of water, and how many passengers you’ll have on board
- How important to you is the motor’s weight.
There is a trade-off between power and weight, namely that the more powerful the motor, the heavier it is, and the harder it is to carry it.
In any case, your S4 should have a 20″ long shaft (L) outboard motor. Do not buy a 15″ short shaft (S) or a 25″ extra-long (XL) shaft motor.
Recommended reading: How to measure an outboard motor’s propeller shaft length? »
2. Propeller Pitch
Another important trade-off that you should be aware of is related to propellers: High pitch propellers are designed to deliver more speed and less torque (thrust, push force), and they work best with lightweight boats such as the S4, while low pitch propellers are designed to move bigger and heavier boats at lower speeds, but they should not be used with lightweight boats such as the S4, since they might make the motor run at a too-high RPM.
Outboard motors manufacturers’ websites say that (quote) “UNDER NO CIRCUMSTANCES SHOULD YOU EVER USE A PROP WHICH WILL ALLOW YOUR OUTBOARD TO OPERATE ABOVE THE OUTBOARD’S RECOMMENDED MAXIMUM RPMs“. This means that you should avoid using propellers designed for use with very heavy boats, namely propellers in the low-pitch range.
Note that 20″ long (L) propeller shaft motors typically ship with propellers that are not necessarily optimal for very lightweight boats such as the S4. For example, the 6 HP 20” long (L) outboard from Tohatsu often comes with a medium range 8 pitch propeller, which is fine, but mounted on the S4, such a motor could drive the S4 12% faster if outfitted with a 9 pitch propeller.
Propeller pitch information is available on outboard manufacturers’ websites.
Since the S4 is a very lightweight boat, you should preferably outfit the outboard motor that powers it with the highest pitch propeller that’s available for it.
Diameter – Don’t try to use a propeller from a small outboard with a bigger outboard, or vice versa.
Hydrofoils (“Fins”) for the outboard
A hydrofoil is an add-on device attached to the outboard motor’s propeller shaft (a.k.a. “leg”). The hydrofoil generates lift at the boat’s stern – The higher the speed the more lift. Hydrofoils help keep the bow down and the stern higher, and a boat that travels this way, namely in a more level position, planes better than if its bow was higher and stern lower. Therefore, outfitting your S4’s outboard motor with a hydrofoil device can result in higher speed on flat water when driving in a solo mode, that is without a passenger in the front. The downside of using a hydrofoil is that when driving with a lowered bow in the ocean, the boat might generate more spray as it bumps into big waves, especially if there’s a passenger in the front. Bottom line: If you like driving solo at high speed on flat water, consider adding a hydrofoil, and if you need to drive in big waves with the hydrofoil on, trim the motor less aggressively by using its trim angle adjustment rod, or knob, especially if you have a passenger on board whose presence also works to lower the bow.
SMALL PORTABLE OUTBOARD MOTORS
For the S4, small size means a highly portable but under-powered outboard motor.
2 HP outboards
Outboard motors in this class weigh around 30 lbs, which makes them fully portable even for a user who’s not very strong. A 2 HP motor can propel an S4 skiff with one person on board at speeds up 8 mph, even in choppy water. This speed would decrease as the boat is required to carry more passengers on board. S4 owners who tested such motors with their S4 skiff reported that the boat felt under powered, which means that they felt like going faster, but the motor lacked the power required for this. For this reason 2 HP motors are not popular with S4 users. Another reason for their lack of popularity is the fact that being air cooled makes these small motors noisier than bigger, water cooled motors.
Propeller pitch –
The Honda 2.3 HP comes with a 4.75 pitch propeller
MEDIUM SIZE PORTABLE OUTBOARD MOTORS
For the S4, a medium size outboard motor means powerful enough but still portable. Motors in this 3.5 HP to 6 HP range are the ones we recommend for most users.
Tip: The optimal ratio of Power to Weight for this range of motors is 10/1, namely 10 lbs to 1 HP. For example, a 3.5 HP outboard should weigh about 40 lbs, and a 6 HP outboard around 60 lbs. If the motor you found has a Power to Weight ratio that’s very different from this, keep looking…
3.5 HP outboards
Outboard motors in this range weigh around 40 lbs, which makes them still portable, but less so than 2 HP motors. A 3.5 HP motor can propel an S4 skiff with one person on board at speeds up to 11 mph, even in choppy water, and it can propel the boat at 8 mph with three passengers on board in moving water. This size motor is popular among S4 and W700 users, as it offers a good trade-off between power and weight for people who must lift the motor or carry it by hand over long distances.
These motors are also less expensive than bigger ones.
Propeller pitch –
For their 3.5 HP motor, Tohatsu recommends either the 7 pitch (Plastic) or 6 pitch (aluminum) propellers, for lightweight boats such as the S4 (under 500 lbs). We say get the 7 pitch propeller, unless you think that you need the more durable aluminum propeller.
6 HP outboards
Outboard motors in this range weigh around 60 lbs, which makes them portable only over short distances, and not for everyone.
A 6 HP motor can propel an S4 skiff with one person on board at 17 mph, and it can propel the boat at 12 mph with three passengers on board.
6 HP is the second most popular motor size among S4 users.
For their 6 HP outboard used with lightweight boats up to 500 lbs (such as the S4), Tohatsu recommends their 9 pitch propeller. The 8 pitch propeller would work too, especially if you intend to have the boat fully loaded, so that it would weigh over 500 lbs in total.
BIGGER OUTBOARD MOTORS
8 HP to 10 HP outboard motors are too heavy to be carried by hand, and they may be too powerful for inexperienced drivers to use safely. However, some of them come with electric start instead on manual start, which is a nice feature. They also come with an alternator that can serve to power small electric devices on board.
8 HP outboards
Outboard motors in this class weigh between 65 and 80 lbs. At such weight, these motors can no longer be considered as portable, and the main reason to use them is the fact that are offered with electric ignition, which eliminates the need to start them by pulling a cord. People who like driving at high speed may prefer these motors to less powerful ones. The S4 outfitted with an 8 HP motor performs well in choppy water, but it could feel over powered for an inexperienced driver, which should avoid using such motors with their S4. Heavy users may benefit from driving with a U-jointed tiller extension, in order to move some weight forward from the stern towards the middle of the boat.
Note that these bigger motors don’t come with an integrated fuel tank, but this isn’t necessarily a problem, since external fuel tanks can fit inside the S4’s hulls.
9.8 HP and 9.9 HP outboards
We tested the S4 with a 9.8 HP Tohatsu and a 9.9 HP Yamaha outboard, and it worked well. Practically, this class of heavier and more powerful motors offers no advantage over smaller motors unless you’re a speed fan, and you’re also capable of driving small boats safely at such speeds. To those who want to take the risk of overpowering their S4 with such big motors, we recommend reinforcing the mounting plate.
This video shows an S4 powered by a 9.8 HP Tohatsu motor filmed from the deck of an S4 powered by a 5 HP Tohatsu motor:
Currently, the S4 is the world’s fastest designated kayak. The 17 mph kayak speed record was achieved by Captain Larry Jarboe, of Key Largo, Florida, in his S4 outfitted with a 9.8 HP Tohatsu outboard motor.
15 HP, 18 HP and 25 HP outboards
Incredible as it may sound, Wavewalk S4 owners have been experiencing with such extremely powerful motors as well –
- The 15 HP motor tested was a new, 4-Cycle Suzuki that weighed 135 lbs, and it was too heavy to be effective. In other words, the extra power did not add to the boat’s speed due to trim issues. The owner switched a 9.9 HP 4-Cycle that weighs 105 lbs, and they are satisfied now.
- The 18 HP motor was an old 2-Stroke Mercury that weighs less than 100 lbs. It was outfitted with an 11″ pitch propeller. The S4 powered by this outboard motor did not break the 17 mph speed record achieved with a 9.8 HP Tohatsu (same manufacturer), apparently since the motor was overheating at high RPM, and needed refurbishing.
- The 25 HP was an old 2-stroke Johnson weighing 110 lbs, and it proved to be too much for the S4. Upon reaching 20 mph the hulls started planing so high that practically speaking, the propeller shaft remained the only point of contact that this extremely lightweight boat had with the water, and the owner had to reduce RPM because of the risk of losing control over the boat, which is hazardous.
We recommend that inexperienced drivers avoid using high power motors with their S4. Instead, they should pick a motor within the 3.5 HP to 6 HP range.
OUTBOARD MOTOR BRANDS
After years of testing motors and getting input from our clients, we can recommend all Japanese brands, namely Honda, Suzuky, Tohatsu and Yamaha. Mercury is a Tohatsu brand, and the small, portable motors offered by Evinrude are made by Tohatsu. Motors from all these brands are good, and your decision about which motor to use with your S4 should be based on factors such as price, weight, features, availability, warranty, etc.
A kicker motor is a name given to any outboard gas motor used on a big boat for trolling, or as a spare motor, and for positioning and driving sailboats over short distances when sailing them is not possible. Being used on bigger boats, kickers often feature a 20″ long (L) shaft, which is the right size for the S4, but the propellers used with them are typically low-pitch models that are not optimal for lightweight boats such as the S4.
If you buy a kicker, check its propeller, and if it’s not a model with the highest pitch that the manufacturer recommends for lightweight boats, replace it with such a propeller.
UPGRADING A MOTOR’S HORSEPOWER
Many outboard motors, both 2 and 4 cycle, can be upgraded to generate more horsepower. For example, from 3.5 HP to 5 HP, and from 10 HP to 15 HP.
Upgrade kits are inexpensive, and installing them isn’t very hard.
The advantage of such upgrade is financial, I.E. buy a lower cost motor and get the power of a higher price one, and sometimes a better power to weight ratio, namely buy a lightweight motor and get the power of a heavier one.
However, before you decide to buy a low HP outboard motor and outfit it with an upgrade kit, you’d better make sure that your motor can take a propeller with the diameter and pitch that the higher HP motor is suitable for. If this is not the case, you might find yourself with a very powerful motor that rotates a small diameter and low-pitch propeller ineffectively, and possibly even at speeds that are too high, which means exposing the motor to a risk of serious damage.
In sum, this idea requires careful consideration.
MUD MOTORS – SURFACE DRIVES
Most motors feature a surface drive, and they offer a huge advantage to people who want to drive in very shallow water, mud, water with many underwater obstacles, and water with plenty of vegetation. Mud motors are bulkier and heavier than regular outboard motors of similar power.
3 HP motors
A 3 HP long tail mud motor weighs about 40 lbs, and as such it’s portable. It works well both in flat water and streams, although not very fast ones. The S4 can definitely use much more powerful motors.
6.5 HP motors
A 6.5 HP mud motor weighs around 80 lbs, and it’s not portable. The reasons you’d want to use such a motor instead of a regular outboard of similar power are if you need to go in very shallow water (skinny water) and mud, and if you fish or hunt in water with plenty of vegetation and underwater obstacles. The S4 performs very well with a such a mud motor. We do not recommend using less powerful mud motors because typically, these motors require more power than regular outboard motors do.
These outboard motors are cleaner than regular gas (fuel / benzine / petrol) outboards, and less prone to develop problems in their ignition and fuel systems. Their disadvantages in S4 terms is in the fact that getting a new bottle of propane takes more time than filling a fuel tank with gas from the pump, and storing large size propane bottles on board is more tricky.
4-Cycle or 2-Cycle motor?
In the US, outboard motors manufacturers are no longer allowed to sell 2-Cycle (2-Stroke) motors, but used 2-Stroke motors can still be found on the secondary market, typically on Craigslist.
Despite the disadvantage of having to mix oil with the fuel that these motors use, and the slight discomfort of inhaling more fumes, many Wavewalk owners favor 2-Cycle outboards over the new 4-Cycle design, due to their better power to weight ratio. Their reasoning is “For the price of a second-hand outboard, I can get one that’s both lighter and more powerful – clearly a win-win situation.”
But reality is more complex and often disappointing – Most used outbp0ards are not in great shape, and they often require refurbishing, or at least serious update. On top of this, since these motors have been used for years, and therefore all their sub-systems were subject to wear and tear, it is unlikely that they can still deliver the horsepower that they provided when they were brand new. The typical used 2-Stroke outboard is considerably weaker than what its HP sticker says.
So, unless you’re a good mechanic and you found a truly clean 2-Cycle outboard, your best bet is buying a brand new, or almost-new 4-Cycle outboard.
Electric outboard motors
Typically, electric trolling motors are not powerful, and they are designed to propel heavier boats than the S4 (e.g. bass boat, skiff, Jon boat) at a very low speed, namely trolling speed. This type of application requires high thrust (small pitch) propellers, and it’s also the reason why trolling motors are rated in thrust units (lbs) and not in horsepower (HP or KW) – When you buy such a motor, your primary interest is how big (heavy) a boat it can move, and not at what speed.
Some electric trolling motors are described by their manufacturers as “outboard motors”, namely comparable in performance to small outboard gas engines. If you’re interested in such electric motors, we recommend comparing apples to apples namely power units to power units, and not power to thrust, which is meaningless. Remember the laws of physics, and apply the formulae for Kilowatts to Horsepower conversion, which are:
- 1 KW = 1.34 HP
- 1 HP = 0.745 KW
No amount of words can overcome this reality.
Electric motors are further discussed in the Electric Trolling Motor or Outboard Gas Engine section of this article »
Mounting Two Motors On The S4 – Outboard Gas Motor and Electric Trolling Motor
Some S4 owners outfit their boat with two motors – A powerful outboard gas engine at the stern, and a bow mounted stealthy electric trolling motor. This setup is typical to bass boats, skiffs, and Jon boats.
avewalk S4 portable boat with two motors: 6 HP outboard gas engine and front electric trolling motor. Photo courtesy of Mike Silva, MA