The late, great Thomas Colvin advised that one needed about 1HP per ton of displacement for one’s auxiliary engine. Note the emphasis on the word auxiliary. Using Colvin’s formula, that works out to an 8HP engine for the Islander 36.
Our current raw water cooled Yanmar 3HM is a 27HP.
Of course, Tom Colvin was more of a sailing guy and not inclined to use a motor unless absolutely required. As it happens, he was also not a man in a hurry when all is said and done. His last design that he did for himself didn’t have an engine and he refused to draw one in*.
In the event that I actually repower our Islander, it would be with an electric motor of around 8HP or a diesel around the same HP like the Bukh or Saab. In the meantime, the forty-year old Yanmar seems to work just fine.
* By the way, there is an excellent article about Colvin and his Antelope design over at Small Craft Advisor you really should check out.
It bugged me that Thomas Colvin, like so many titans of boat design, has had his work virtually disappear post death. He was a sailors sailor in the old school sense. He suffered my ignorance and my piddling purchases of a bunch of his study plans during the period I designed and built a 37′ sharpie and was actually kind to do so. So many designers works just silent now, their exquisite plans languishing who knows where? My little article just scratched the surface. The man really needs a biography and his works made public if possible.
So many great designs no longer available and, sadly, too many designers won’t be with us all that long.
A Bukh 10 was more than adequate in my last 8 ton ferro cruiser, and before that a Lister twin 15hp in a 5 ton plywood ketch.
A Yanmar 1gm10 with a 3-1gearbox will throw quite a prop. I have never been one to motor against tides (in the UK), at least not at their peak, a complete waste of time and diesel with a displacement boat that cruises around 5 knots.
I would dearly like a boat that could take a Perkins P3 152, but its a huge chunk of iron for its meagre output. I have one in my MF tractor, but i could listen to it all day. Low consumption figures in line with a Gardner, and being a tractor engine, parts are global. An entire rebuild kit can be had for the price of a Yanmar exhaust elbow and water pump.
I bought my aluminum Gazelle in 2013, a year before Tom’s death, and peppered him with numerous questions that he always answered although he could sometimes be a little gruff. I am sure the gruffness had to do with some of my stupid questions. My Gazelle originally had a 20 hp Sabb but was repowered with a 46 hp Lugger by the previous owner and has far more power than I have ever needed.
I often think it is time for me to get a smaller boat but then I go down to the dock and look at her beautiful lines and realize there is no way I am ready to sell her.
Just looking at her makes me happy. Tom sure drew some pretty boats.
Colvin himself wrote a article entitled something like “The Ultimate Blue Water Sailboat” in which he carefully explained why Gazelle was such a awesome boat. In a way self promotion but, as some folks say, it ain’t bragging if you can do it. The boat is aptly named…. so sleek and beautiful. You’re a lucky man to have a aluminum version.