HORSEPOWER CALCULATIONS FOR OUTBOARD MOTORBOATS WITH ENGINES GREATER THAN 2HP

Standards for Electric Propulsion boats, for Flotation, Safe Loading and Maximum Horsepower Ratings.

Electric Propulsion: For more imformation on Electric Propulsion Systems see https;/newboatbuilders.com/pages/electricity17_propulsion.hmtl

The U.S. Coast Guard has published a policy letter regarding standards for Flotation, Safe Loading and Maximum Horsepower ratings on boats with electric propulsion. Essentially the policy letter says that manufacturers of electric propulsion recreational boats should comply with the same standards for Safe Loading, Flotation, and Safe Horsepower as gasoline powered recreational boats and refers to ABYC Standard E-13 for Boats with Electric Propulsion. The policy letter can be seen at the Coast Guard's website at https://safeafloat.com/wp-content/uploads/2022/08/BSX-23-Electric-Propulsion-Policy-FINAL.pdf.  Also see the last paragraph on tnis page.

Reasons for Safe Horsepower ratings

Start-in-gear protection requirements for outboards.

The Rules are in 33 CFR Subpart D 183.51 - 183.53

Outboard horsepower ratings are based on a variety of factors.

These include;
Centerline length.
Maximum Transom width.
Transom Height.
Type of steering: remote or tiller.
Hard Chine Flat Bottom or Other.

There is a formula for computing Safe Horsepower based on various combinations of the above factors. See the table below.  This only applies to a monohull boat less than 20 feet in length (6.096 meters)

Canada uses similar formulas for computing outboard power in kilowatts. (1 HP = 0.745 KW)

The below formulas do not apply to: sailboats, canoes, kayaks, and inflatable boats, that are designed or intended to use one or more outboard motors for propulsion. It does not apply to boats that are true multihulls.  A multihull makes two or more separate footprints in the water. A pontoon boat is a multihull.

Taken from Title 33 Code of Federal Regulations Subpart 183.53

Example 1: Bow Rider - Length = 19' 6" Transom width = 84" remote steering and 20" transom height.

(Length x Transom Width X 2)-90 = HP

( 19.5 X 7.0 X 2) = 273

(273 - 90) = 183 (136 KW)

Horsepower = 185 (138 KW)

Example 2: Length 12' Transom width = 46" Tiller steering and 15" transom height.

( Length x transom Width) = 45.96    Horsepower from table = 10 (7.5 KW)

Horsepower is rounded to the next multiple of five.

Transom width : is to be measured at the widest point of the transom including permanently attached parts of the boat such as rub rails. Click on the image for the full size
Boat length: Length does not necessarily include such things as swim platforms. However if the swim platform is molded into the hull and contributes to the buoyancy of the boat, it is included. If you need some more information on this call the Coast Guard at  202-372-1077. Click on the image for the full size
Flat Bottom Hard Chine means: The boat is flat bottom if you can lay a straight edge across it, and there is no vee or curvature. A hard chine has no curvature.

Beware of steering changes:  If your boat is rated for remote steering, and the buyer wants you to reconfigure the boat from remote steering to tiller steering, the boat can no longer be rated for the higher horsepower allowed with remote steering.  You will have to change the labels to the lower horsepower rating.

Conversely: if the boat is rated for tiller steering and the buyer wants it to have remote, you will have to put on a  label reflecting the higher horsepower rating.

See Labels

Special Rules: For boats that meet the following qualification there is an optional test course method for determining horsepower:

13 feet or less in length.
Outboard Powered.
2 or less passengers.
Remote steering and 20 inch transom height.

See Horsepower page 2:

How did we get these rules for power on outboard boats? Is there a better way

Most of the horsepower (or kilowatts in some countries) ratings for monohull outboard boats, used today, were developed back in the 1950's by the BIA and the Yacht Safety Bureau.  They ran tests on various boats and came up with the formulas for different types of boats. Primarily the limits on power are due to two factors, one is simply handling of the boat. Putting too much power on the back of a boat can make it handle erratically, spin out, and difficult to steer. The other is weight.  Putting a much larger engine on the transom can make the boat sit stern down and make it easy for a following wave to roll over  the transom and swamp the boat. 

Also different types of hull shapes handle power in different ways.  Flat bottom boats with hard chines are much easier to skid sideways in a turn, or to catch a chine and trip the boat, possibly flipping it, than a boat with a vee or rounded bottom.  Boats with a shorter transom height sit lower and are easier to swamp than a boat with a full height transom (20 inch). Engines come in standard shaft lengths, generally 15 inch, 20 inch and 25 inch, i.e. short shaft, standard, and long shaft.

But, over the years both boats and engines have changed considerably, so shouldn't the rules have changed?  Well, probably, but with everyone required to use the same formulas, at least they present a level playing field. Everyone has to use the same rules. Tests conducted in 2003 indicated that many boats may actually be overpowered using these formulas. More testing needs to be done.

Is there a better way?  There may be. Way back when, ABYC developed a test course standard for boats that don't have to meet the US Federal regulations.  This course is basically the same one that was originally used to obtain the formulas and measures a boats ability to maneuver through a course with turns, without becoming unstable.  The Coast Guard used this test course when it developed a separate standard for boats 13 feet or less, with one or two passengers, a 20 inch transom height and less than 40 horsepower. These were popular in the 80's, and called thrill craft, before the advent of Personal Watercraft. 

In the 1980's the Coast Guard and Mercury Marine did a lot of testing on different types and sizes of boats instrumented with accelerometers. These instruments measured the acceleration sideways in a turn.  The basic theory was, the more power, the greater the sideways, or lateral accelerations. Unfortunately the data revealed that there was no correlation, between power and lateral accelerations in a turn. There was more of a correlation between acceleration and the boat hull type. 

The International Standards Organization (ISO) has developed it's own test standard, a collision avoidance test, or barrier test.  An imaginary barrier is set up on the water and the boat with rated power, run at full throttle, has to avoid "hitting" (actually crossing over) the barrier by turning at a specific distance which is calculated based on the length of the boat. The test is repeated with larger engines until it can't avoid the barrier. The highest power with which it could complete the turn is the rated power.

On the more technical and scientific side, some naval architects and engineers have done research into resistance on planing hulls and developed formulas based on weight, wetted surface at speed, resistance, angle of the vee and other factors.  These formulas are used frequently to determine power for larger inboard powered planing hull boats but aren't really applicable to small (under 20 feet) outboard powered boats.

So where does that leave us?  The Coast Guard has been doing some testing comparing the ISO standard to the US standard to see how a boat rates under each standard. Whether this will develop into a new standard is open to speculation.

Start-In-Gear Protection:  Another important safety consideration is to prevent engines from being started while the boats drive is in gear.  This matters because people get tossed out of the boat when the boat suddenly starts and jumps forward.  Statistics showed a significant amount of  accidents in which someone had gone aft to work on an outboard engine,  the engine started up in gear and the person was thrown over the transom of the boat.  So the USCG adopted a regulation requiring outboard motors with greater than 115 lb. of thrust (about 2 HP or about 1.5 KW) to have a device that prevents the engine from being started when in gear.  However, ABYC has a standard, P-14 Mechanical Propulsion Control Systems, which requires start-in gear protection on inboard and inboard/sterndrive boats as well.  So most power boats have start-in-gear protection. Not to be confused with Kill Switches.

Kill-Switches: Emergency engine stop switches that stop the engine if the device is tripped by the operator being knocked down or thrown out of the boat.  Usually this is a simple lanyard attached to the key, or a switch.  When the lanyard pulls the key out or trips the switch the engine stops. There are more sophisticated devices on the market.  This is now a USCG requirement.  ABYC does have a standard for it, A-33 Emergency Engine/Propulsion Cutoff Devices, and most boats produced today have them,  especially Personal Watercraft. The industry standard for PWCs requires them to have a Kill Switch.

Electric Outboard converting watts to horsepower.  

There are electric outboards on the market ranging from trolling motors which are less than 1  horsepower,  to much more powerful outboards such as Axopar 25 with the Evoy Storm Electric outboard, rated 300 Horsepower.  11,000 Watts is about 15 horsepower. 300 HP is in the range of 220KW.  So it takes a lot of watts to generate horsepower. 

As with gasoline powered engines,  how your engine performs is dependent on the size and weight of the boat, and the type of boat. Here is a link to a converter from Watts to horsepower. https://www.unitconverters.net/power/watts-to-hp.htm

See Electric Propulsion https://newboatbuilders.com/pages/electricity17_propulsion.html

Back To Top

© newboatbuilders.com 2007 All rights reserved Revised 01/08/2025