There was a time when the formula, based on transom, beam and length dimensions was used to determine the maximum safe engine size for a boat and would have put about 100hp on the back of a six meter boat. Back then a 100hp engine was a “monster” and, despite this, there was a consensus that the best engine for your boat was the biggest the manufacturer recommended.
Then came ISO and RCD, and with that, a new formula that let us put engines twice this size onto the back of our boat. Construction techniques had move on, engines had got bigger and we weren’t going to rip the 4r5e out the boat with all that power. Still the standing advice was to fit the maximum or near maximum recommended by the manufacturer.
Moving on, ISO11592 now allows manufacturers to rip up all the formulas and basically, if a highly trained, very skilled and experienced stunt-helmsman (who likes living life on the edge) can pull some defined manoeuvres within given parameters, the manufacturer can recommend the engine they used as the “safe” maximum.
Now.....I’m not a killjoy. I’ve owned and played with boats that were outrageously overpowered but always recognised and respected them as being such. If you have a burning desire or need to do over 60mph (maybe you’re running drugs or fleeing a scorned spouse)...then hell, yes.... go for it, but we can’t keep advising novices that the best bet for them is the maximum sized engine the manufacturer recommends because that figure is just becoming a “d**k sizing” contest.
If you’ve read the MAIB report on “Milly” ask yourself this......had that boat been fitted with what would have been deemed a “safe maximum” 30 years ago (probably less than 200hp) do you think the engine would have produced enough thrust to “ hook-out” that hull.?
Maybe it’s time for both a “recommended engine size” and a “maximum engine size”.
Perhaps a scale where we could quote recommended engines for different applications based on hp/ton....a “lookup table”...ranging from light leisure use to “death wish”.
Rant over !!!