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:
Updated:
Original:

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

210913-11HRT-SKIPPER-PORTRAITS-VC-122

11th Hour Christens Two IMOCAs, Hits a Snag

This week has been a big one for the American-founded, sustainability-centric ocean racing team 11th Hour Racing. In addition to christening their two new boats, the team also took them out for a quick test ride—against some of the most intense IMOCA 60 skippers in the world. ...read more

01-LEAD-DSCF3091

Clewless in the Pacific

Squalls are well known to sailors who cruise the middle Latitudes. Eventually, you become complacent to their bluster. But squalls vary in magnitude, and while crossing from Tahiti to Oahu, our 47ft Custom Stevens sloop paid the price for carrying too much canvass as we were ...read more

Nigel

SAIL’s Nigel Calder Talks Electrical Systems at Trawlerfest Baltimore

At the upcoming Trawlerfest Baltimore, set for Sept. 29-Oct. 3, SAIL magazine regular contributor Nigel Calder will give the low down on electrical systems as part of the show’s seminar series.  The talk will be Saturday, October 2 at 9am. Electrical systems are now the number ...read more

5ae5b8ce-3113-4236-927b-f795be4ae091

Bitter End Yacht Club Announces Reopening

Four years after being decimated by Hurricanes Irma and Maria, the Bitter End Yacht Club is set to reopen for the Winter 2022 season. Hailed as one of the best anchorages in the Caribbean and built by sailors, for sailors, this island outpost in the BVI has been a favorite with ...read more

01-LEAD-'21.05.01_Jay-&-Mira

Cruising: Bluewater Pollywogs

Bluewater sailing is 25 percent actually sailing and 75 percent learning how to live on a boat at sea, in constant motion and with no chance to get off the roller coaster. I cannot over-emphasize how difficult normal daily functions become at sea, even on nice, calm days. ...read more

01-LEAD-IMG_0078

Refurbishing Shirley Rose: Part 2

If you missed the first installment, click here. Thankfully, the deck and cockpit of my decades-old Santana 27, Shirley Rose, were in pretty good shape. The balsa core, in particular, was for the most part nice and solid. Nonetheless, there was still a fair bit of work to do. ...read more

orca

Orca Encounters on the Rise

This week’s confrontation between a pod of orcas and the Nauticat 44 ketch Tuuletar which left the boat rudderless is just the latest in a string of encounters with orcas off the coast of the Iberian Peninsula. In fact, over 50 of these encounters have been reported, half of ...read more

01-LEAD-Project-complete

DIY: an Antique Nav Station

Ever since the advent of GPS, I have not found much use for the chart table on my schooner Britannia. Most of our passagemaking navigation is done on a Raymarine multifunction display on the helm pod, which is then transferred to a paper chart on the saloon table roughly every ...read more