Ask SAIL: Lawnmower Motor on a Boat?
Neal Sundhagen Williston, North Dakota asks:
I am the proud owner of a 1982 S-2. It is a great sailboat, fast in both light and heavy air, with a retractable keel that makes it an excellent gunkholer on Lake Sakakawea here in North Dakota. However, I do not get along with the boat’s one-cylinder BMW diesel engine. It is a bugger to start! Cranking and cursing usually does the trick, but once it does start it almost shakes the rigging down and then stops again. I have had it! I am considering replacing it with an air-cooled 18hp V-twin gas-powered lawn-mower engine from Honda. There is plenty of room to fit the Honda in and the transmission will mate up. What do you think?
NIGEL CALDER REPLIES
There are a number of potential issues here: 1) What is the peak RPM speeds of the BMW and the Honda? If they are dissimilar, whatever reduction gear you have may not be appropriate for the Honda. 2) Even if the RPMs match, diesel engines develop considerably more torque at low RPMs than do gasoline engines. You may find that your existing prop overloads the gas engine, in which case you will need to downsize the prop, which will make it less efficient. 3) The duty cycle of a lawn mower and a marine engine are very different! You may find the Honda engine works harder and gets hotter, especially in a closed engine compartment. Overheating may be a problem. 4) Lawnmower engines tend to be rather noisy, and if you try to sound-insulate it, you will exacerbate the heat issues.
I understand your frustration. BMW’s foray into the marine market was not a happy story. We had a friend who got so frustrated with his almost-new engine that he threw it overboard one day! However, I think you need to do a little more thinking before you do the same to yours.
Nigel Calder is an author and expert on boat systems and diesel engines