Green Machine - Sail Magazine

Green Machine

David Kampenga of Houston, Texas, asks:"I’m thinking about converting my Hunter to an electric motor propulsion system, which would involve installing a 6kW 48VDC motor. I would like to keep the fixed two-blade prop that’s on the boat. I would also like to retain the existing gear reduction, which is 2.14:1. I don’t quite understand the reduction gearing and am curious how
Author:
Publish date:

David Kampenga of Houston, Texas, asks:

"I’m thinking about converting my Hunter to an electric motor propulsion system, which would involve installing a 6kW 48VDC motor. I would like to keep the fixed two-blade prop that’s on the boat. I would also like to retain the existing gear reduction, which is 2.14:1. I don’t quite understand the reduction gearing and am curious how I should connect the electric motor to the propeller shaft."

Nigel Calder replies:

If your present installation is a shaft drive, which I believe it is, you will likely want to connect the electric motor directly to the propeller shaft and not the reduction gear. If it is a saildrive, you will have to couple the input shaft to the saildrive. However, before following either course you will need to address an important issue, which is the operating speed of the electric motor.

To get you started, I’ll assume your current propeller is already well matched to your existing diesel engine, although my assumption could be wrong. If you look at the data sheet for your diesel engine, one of the graphs will show the power that is absorbed at different engine speeds by a “nominal” propeller. Find the 6kW (8hp) point on the propeller curve and read down that curve to find the engine’s rpm. If you leave the reduction gear in place, the maximum speed of the electric motor should be similar to that of the diesel engine when the propeller curve is at 6kW. If you decide to couple the electric motor directly to the propeller (i.e. remove the reduction gear), divide the engine rpm by 2.14 (your reduction gear) to find the speed at which the electric motor should deliver 6kW. In practice, for various reasons you will probably do best to have an electric motor that reaches the 6kW power level at a somewhat higher speed than that calculated.

Related

01b_WALKING-KEDGE-OUT-cmykpromo

Getting More Use From Kedge Anchors

If you are cruising, you need at least two anchors on board for the simple reason that you must have a backup. Imagine having to slip your anchor on a stormy night with other boats dragging down on yours, or having your rope rode severed by some unseen underwater obstacle, ...read more

SailAwayCharter

How-to: Navigating on a Bareboat Charter

So you graduated from navigation class where you practiced dead reckoning, doubling the angle on the bow and maybe even celestial nav, and you now feel well prepared for your first charter trip. Well, you won’t be doing any of that on vacation—not past the first day, anyway.Most ...read more

04-Turtle-rescue

Turtle Rescue in the Vic-Maui

Strange and often wonderful things can happen in the course of an offshore sailboat race, and one of the strangest and most wonderful things we’ve heard of recently took place during the 2,300-mile 2018 Vic-Maui race, from Victoria, British Columbia, to Lahaina, Hawaii.It ...read more

dorcap-open-blue

ATN Inc: Dorcap

COOL SLEEPYou’re fast asleep in a snug anchorage, forehatch open to catch the breeze, when you’re rudely awakened by a sneaky rain squall. Now you’re not only awake and wet, you’re sweltering with the hatch closed. Sucks, right? That’s why ATN came up with the Dorcap, an ...read more

HIGH-RES-29312-Tahiti-GSP

Ask Sail: Who has the right-of-way

WHO HAS RIGHT-OF-WAY?Q: I sail in Narragansett Bay, which is a relatively narrow body of water that has upwind boats generally going south and downwind boats generally going north. When sailboats are racing, the starboard tack boat has the right-of-way over the port tack boat, so ...read more

albinheaters

Albin Pump Marine: Marine Water Heaters

IN HOT WATERSweden’s Albin Pump Marine has introduced its line of marine water heaters to the United States. Complete with 130V or 230V AC electric elements, the heaters can be plumbed into the engine cooling system. They feature ceramic-lined cylindrical tanks in 5, 8, 12 and ...read more

03-squalls4

Squall Strategies

Our first encounter with a big squall was sailing from San Diego to Ensenada, Mexico. We left at 0200 to ensure we’d get into Ensenada before our 1300 haulout time. The National Weather Service had forecast consistent 15-20 knot winds from the northwest, which was perfect for the ...read more