A very good friend of mine had a new engine fitted recently. The boat was an ex hire boat and only had a 35hp engine which is fairly normal for a hire fleet boat. The hire companies do this deliberately as they want to control the speed the boat can achieve and they probably govern the engine as well.
The beauty of these boats is they can be bought for very reasonable price, they come well equipped with quality fittings to stand the riggers of hire and the hulls are well built.
So the internal skin tank was set up for a 35hp engine, a fact the professional boat company installing the new 52hp engine somehow overlooked? Hmm?! IMO the first thing to work out when fitting an engine is how to cool it. If you can't cool it you can't run it. Well you can but so long as you don't call up all the horses or want to destroy it.
The problem only manifested itself when Sue needed to punch against the flow on the Thames, which of course needed more horses than poodling on a canal which she had been doing since the new install.
There were 2 options to solve this, another internal skin tank linked to the original which would require the engine bay to be cleared of things like the errr! engine! The other option an external skin tank, or keel cooling which is really what a skin tank is requiring just 2 holes made and welded in the hull. No engine removal and just a few hours work in a dry dock.
There's a well regarded formula for working out the cooling area / engine hp. It's HP divided by 4, so a 52hp engine needs 13sq/ft of surface area. The most compact way of doing this is with tube, in this case 80 x 40 box section which gives 1sq/ft of surface area every 16" of length. To further compact this a serpentine patten is the norm.
This is it.
So this is what I came up with and made for Sue & Vic. See the links below as she takes up the story on her blog. Read backwards from here:- Sadly I wasn't able to fit it for her, but the local floating dry dock did a great job.
If you like boating blogs, you will love Sue's. Her's is constantly ranked in the top 10 UK boating blogs, its has a great balance of travelogue, opinion, and lifestyle showing the good and not shy of speaking about the not so good. Always punctuated with great photos and on the sidebars masses of excellent links, tutorials and resources as well as a massive list of fellow boater blogs.