How much HP for my S4 skiff’s outboard motor?

Updated February 3rd, 2018

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

  1. 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
  2. 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, the motor should be a 20″ log shaft (L) model, and not a 15″ short shaft (S).

Recommended reading: How to measure an outboard motor’s propeller shaft length? »

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 for it with the highest pitch propeller available for it.

Diameter – Don’t try to use a propeller from a small outboard with a bigger outboard, or vice versa.

 

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.

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.

Propeller pitch
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.

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.

 

KICKER MOTORS

 

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.

 

MUD MOTORS

 

Mud 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.

6.5 HP mud motors

Mud motors are bulkier and heavier than regular outboard 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.

 

ELECTRIC OUTBOARDS

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 discussed in the Electric Trolling Motor or Outboard Gas Engine section of this article »

 

6 Comments

  1. Bassman

    Yoav et al.

    In view of the Wavewalk transition to portable motorized twin hull skiffs (the S4 series), I have been finding and refurbishing small long shaft outboards for my customers to compare.

    No other kayak dealer that I know of will demo their product with a motor capable of planing the vessel.

    I also offer these motors for sale at less than half the price of a new long shaft outboard.

    In case anyone needed another reason to visit Key Largo…

    Larry J.

  2. fish

    Larry,

    I’m sure your clients appreciate this service.

    Yoav

  3. Pyt Rotary

    Good information.
    I just tested 2Hp Honda and indeed there is room for more power.
    Will get a 6Hp next year.
    Have fun and be safe.
    Canada.

  4. fish

    Thanks Pyt,

    Yesterday I drove the S4 with the 6 HP Tohatsu at full throttle at the beach.
    BTW, this motor’s user manual states that it can propel boats up to 3,000 lbs. I guess this means a sailboat maneuvering in a harbor.
    Anyways, driving was a lot of fun, both sitting and standing, and playing with waves and other boats’ wakes. I drove at full throttle all the time, including while turning.
    And this is where I need to make it clear that I’m not exactly and experienced driver.

    Following this fully positive experience, I let my older son drive the boat, although he had never driven a boat before. He went full throttle too, seated and standing, while shooting videos and pictures of himself, to show his friends 🙂

    Therefore, I am modifying our recommendations for the S4 to include 8 HP motors for experienced drivers, and I’m willing to accept the notion that very good drivers may use bigger motors – at their own risk of course 😀

    Pulling the 6 HP motor from the car and mounting it on the boat was OK, but I would have not made it with a heavier engine.

    Yoav

  5. Bassman

    My own opinion is that the S4 will easily accommodate as much motor as your back will. Thus, choose the most horsepower that you can sensibly lift.

    I look forward to enjoying my day seated inside my Wavewalk not lying on top of a chiropractor’s table.

    Been there. Done that. Will look out for my back to avoid going back.

    Larry J.

  6. fish

    Yep, this is good rule of thumb.

    Yoav

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