Outboard motorboat power calculations

Every year, mixed in with the usual chatter around the boatyards, I hear at least a few comments along the lines of "I beefed up the transom of my 14-footer so it can take 60 horsepower!"

Er, no. That's not how it works.

Maximum power ratings for small outboard vessels are calculated according to the size and design of the boat, taking into account the following factors:

  • Length of hull (longer = higher power rating)
  • Beam at transom (wider = higher rating)
  • Deadrise at midship (deeper V = higher rating)
  • Tiller vs. remote steering (remote station = higher rating)

It is assumed- or explicitly stated in the rules- that the boat must be structurally capable of handling its rated power. Your transom is designed to handle the largest, heaviest outboard that the boat's rating allows; that rating is determined by the boat's size and shape. Strengthening the structure won't change the rating (although, given the way some aluminum skiffs are built these days, beefing things up is often a good idea in any case).

So, where do these rating calculations come from? Empirical measurements, of course. The various Coast Guards and regulatory agencies have seen thousands of boating accidents, and they've run controlled tests in which hulls of many sizes were fitted with progressively larger motors until they became hard to manage at high speed. From all this data, they came up with a few very simple equations to say "For a boat of this length, beam and configuration, any engine larger than X horsepower is likely to be difficult or impossible to handle at top speed".

So yes, you CAN beef up your 14-foot skiff's transom and slap a 60-horse on her. But CAN is not the same as SHOULD. Overpowering a boat will usually make her squirrely at high speed, and often introduces dynamic instabilities such as porpoising that can be uncomfortable or dangerous. Unless you really know what you're doing- and can provide the math to back up your decisions- it's best to stay within the recommended limits.

I am emphatically NOT going to describe the complete calculation here, for a very good reason: If you need to do a maximum recommended power calculation, you also need to understand the rules associated with it. So, if you are so inclined, please proceed to the Transport Canada Construction Standards for Small Vessels here:

TP1332E s.4.3 maximum safety limits calculation for outboard monohull boat

TP1332E s.4.5 maximum safety limits calculation for pontoon boat

TP1332E s.4.6 maximum safety limits calculation for inflatable / rigid inflatable boat



Add new comment