Question: I attended the Miami Boat Show this year and saw a device there that really caught my interest. I sent along a photo from the company’s website to help identify it.

The Weego comes in a few different sizes and is something that looks like it could really help if the batteries on my boat die. The problem I have is deciding which of the three units to buy.

The larger Weego engine-starter models will supply enough amperage to fire up most small- to medium-sized gas engines and smaller diesels.

The larger Weego engine-starter models will supply enough amperage to fire up most small- to medium-sized gas engines and smaller diesels.

They talk about both gas and diesel engines in their literature, but they use engine displacement as the guideline for determining which unit to get. I’ve always been taught to refer to engine cranking peak amperage as the criterion for jumpstart power, not engine displacement as they seem to state. How can I get past all of this?

Answer:  OK, you are right that what really matters here is cranking amperage. That said, Weego is only trying to make things a bit easier for the consumer here. Engine cranking amperage specifications are almost impossible to find in any manual. What you really need to do is measure the current draw yourself with an inductive amp clamp around the largest battery cable connected to your engine’s starter motor with the engine turning over under fairly normal conditions. Write that number down and save it. It can be used to help diagnose other faults if there's a big variation from the norm.

The bottom line here is that Weego also provides starting and peak current values in the specification section of their data sheets. Understand that over the last 15 years or so, current draw values have gone down dramatically as starter motor design has improved and starters have gotten much more efficient. In the old days, it wasn’t uncommon for technicians to use a value of approximately 1 amp per cubic inch of engine displacement for gasoline engines and as much as twice that for diesels as a sort of “guesstimate” for how much current draw to expect.

Modern engines and their starter motors use far less current. Weego's model JS12 or JS18 units, with 200A crank/400A peak or 300A crank/600A peak, will work fine for most small-  to medium-sized gasoline-fueled engines and small diesels today. Larger six-cylinder diesels that I’ve personally tested will sometimes have a peak of just over 1,000 amps, with a cranking amperage of 800 amps. These would definitely fall out of the range for the Weego.

Written by: Ed Sherman
Ed Sherman is a regular contributor to, as well as to Professional Boatbuilder and Cruising World, where he previously was electronics editor. He also is the curriculum director for the American Boat and Yacht Council. Previously, Ed was chairman of the Marine Technology Department at the New England Institute of Technology. Ed’s blog posts appear courtesy of his website, EdsBoatTips.