Blue View – Choosing the Right Propulsion for a Cruising Boat
/I've talked about the technical aspects of a cruising boat, and last week I expressed my thoughts about the rig type. This week I'll share my thoughts about some of the other basics of the boat – engines, props, and fuel tankage.
Engine Type. This isn't about whether a cruising boat should or shouldn't have an engine – almost every boat we know has one. Nor is it a discussion about diesel vs. gasoline vs. diesel electric. Although we do see the occasional small boat or trimaran that utilizes an outboard gas engine, most cruising boats use inboard diesel engines. And though diesel-electric boats, sort of the marine version of a hybrid car, are out there, we've yet to talk with anyone who has one on board. Instead, I'm going to assume the boats you are considering have an inboard diesel, and talk about the types and size.
Nine of Cups has a Ford Lehman engine, which is a marinized version of a Ford tractor diesel engine. The arguments for it are that it is a reliable engine that has been around a long time, and since Ford tractors are seen around the world, parts should be inexpensive and easy to get almost anywhere. In reality, most of the problems we've had have been with the marinized parts of the engine – the heat exchangers, raw water pump and wet exhaust system. We did need to replace the fresh water pump in Chile, and while we were able to find it at a local truck parts store, we still had to have the outlet pipe cut and re-welded to make it fit our engine. Even standard Fords parts, like the starter motor we needed to replace in Ecuador, were hard to locate – it turns out that parts for old Ford tractors aren't really that common in most places. My recommendation is to either find a boat with a common marine engine or plan to carry lots of spares if you plan to head to some of the more remote parts of the world.
Engine Size. Just as with a car, engine size is a trade-off between operating and repair costs – bigger engines cost more to run and fix – and the power that's sometimes needed to buck a headwind or strong adverse current, or to move the boat along at hull speed while still providing enough horsepower to power the alternator, refrigeration and other engine driven devices..
Nigel Calder has a great section on calculating the ideal engine size for any boat in his book, Cruising Handbook, a Compendium for Coastal and Offshore Sailors. A rough rule of thumb, however, is that a moderate displacement boat should have about 1 horsepower per 500 pounds of boat weight. Heavy displacement boats need more engine power, maybe 1hp/400lbs, while lighter boats need less engine, more like 1hp/650lbs. Add to this about 5 horsepower for every 100 amps of alternator output. If there are any other engine driven devices such as a refrigerator compressor or a watermaker pump add in a few more horsepower to drive them. Typically, a refrigerator compressor will require about 2 horsepower and a watermaker pump will require 3-5 horsepower, depending on its size.
Nine of Cups is in the moderate displacement category and weighs about 40,000 pounds. Therefore, the ideal engine size is as follows:
Horsepower for Cups' weight: 40,000/500 = 80
Horsepower for 200 amp alternator: 10
Horsepower for refrigerator: 0
Horsepower for watermaker: 0
Total: 90 Hp.
Cups has a 90 horsepower engine, which is right at the ideal.
Nine of Cups' 22x17 Propeller
Prop Size. Matching the propeller to the boat and engine size is as important as engine size. A prop that is too big will overwork the engine and reduce performance, resulting in overheating and even shortened engine life. An undersized prop will be inefficient and make the boat seem under-powered. Dave Gerr's book, The Nature of Boats: Insights and Esoterica for the Nautically Obsessed, (which is not only very informative about props, but a fun read on all things nautical), has several formulas and a handy nomograph for calculating prop size. There are also several online prop calculators – you enter your boat parameters like waterline length, engine size, transmission gear ratio, etc., and the calculator provides an estimate of prop size.
A New Fuel Tank in Process
Fuel Tanks. Here are my thoughts on fuel tanks:
Size. The tanks should have enough capacity for at least 500 nm, and more is better. You may rarely if ever plan to motor 500 nm, but it is nice to be able to stock up on fuel where prices are low, like Venezuela at 10 cents a gallon, and not need to buy fuel where it is expensive, like St. Helena Island at $15 a gallon. Cups has a total tank capacity of 160 gallons, which gives her a cruising range of about 800 nm. Except for our passage through Patagonia and Tierra del Fuego, which required motoring a lot and which didn't have many opportunities to refuel, our tankage has been quite adequate.
Number. Never rely on just one tank. Multiple tanks are better for a several of reasons: redundancy in case one begins leaking; an alternate source if one becomes contaminated with algae of other crud; when it's time to polish the fuel, it's much simpler to pump fuel from one tank to the other, so the first tank can be cleaned.
Material. Steel or black iron tanks were commonly used on sailboats. If properly cared for, they usually lasted about 20 years. Right on schedule, the two steel tanks on Cups began leaking when she was about 22 years old and had to be replaced. Aluminum tanks are a good material for fuel, but are subject to corrosion if allowed to come into contact with other metals, especially copper fittings. Stainless is subject to corrosion as well. We constructed one tank out of epoxy and it has held up quite well for more than 12 years now. Fiberglass can be used, but there is some debate as to whether the additives in the new low sulfur diesels attack fiberglass. Polyethylene is a good material, especially the rotomolded tanks, as long as the wall thickness is adequate.
Access ports. Fuel tanks should have access ports that allow periodic cleaning of the tanks. Ideally, they should be large enough and properly located so that you can reach all parts of the tank.
Switching. The fuel plumbing should be designed so the feed and return lines to each tank can both be turned on or off. (See the comments below for a better explanation - D)
So now we have a boat that will float, can sail and which can be motored when necessary. Next week we'll continue defining our ideal cruising boat.