10 HP Outboard Motor Powering A 100 lbs Boat?

9.8 HP outboard motor powering S4 microskiff

This setup works perfectly, but to get it to this stage isn’t easy.

At 100 lbs, the S4 Microskiff is the world’s most lightweight high-performance microskiff, and its official power rating is up to 6 HP. So why try to overpower it? The answer to this is related neither to hydrodynamics nor to engineering, but to human psychology: Some of us like speed. We enjoy traveling fast, and the faster the better.
And while the S4 running a 6 HP is pretty fast, it’s faster with a 9.8 HP motor, providing you succeed in getting it ready for running such a bigger, heavier, and much more powerful motor.
Such project would include beefing up the boat’s transom, as well as blocking and plugging every way that spray and water can shoot up from the propeller shaft area and into the back of the cockpit, which isn’t a trivial project.
This post’s featured image (see above) shows parts of the structures added at the back of the transom, between the hull tips, and around the motor’s shaft itself.
Read more: S4 Microskiff Powered By A 9.8 HP Tohatsu Outboard Motor

Bear in mind that any boat that’s powered with a motor, or motors that exceed its official power rating is considered overpowered, and in some states overpowering a boat is illegal. Needless to say that anyone who wishes to overpower their boat should first check the legal status of such a project in their state.

Also, not all outboard motors in the 10 HP class are suitable for use with the S4. The two things you need to check are the weight of the motor, and the size of its propeller shaft. After this examination, two motors make it to the finish line: the 9.8 HP Tohatsu, and the 9.8 HP Mercury, each weighing 85 lbs. Remember that the S4 works only with 20″ Long (L) shaft motors.

Typically, small craft powered by 10 HP motors are limited to speeds of up to a little over 20 mph, at best, and this can be achieved only on perfectly flat water, and with a lightweight driver. In the case of the ultralight S4, the driver is the heaviest part of the trio comprising boat, motor and driver, and due to hydrodynamics that favor lightweight vessels, the driver’s weight has a most noticeable effect on the boat’s speed, for better and for worse.
Outfitting the motor with a propeller of the biggest diameter and pitch will certainly help, but considering the motor’s max RPM and propeller’s slippage, the chances of going faster than 20 mph are very slim, and for heavy drivers they are practically zero.

Pros and Cons of a big motor –
  • Cons – Potential legal issues in some states, technical issues including non-trivial ones related to safety, a bigger motor is much heavier than the recommended 6 HP motor, and for many people this could be a problem in terms of carrying it.
  • Pros – Driving an S4 microskiff powered by a bigger motor is more fun, at least for someone who likes speed and is capable of safely driving such craft.