Engine for small sailboats , part two

Engine for small sailboats : how to choose it

part two

in the first part of this topic we pointed our attention on a general description of the three main engine categories (diesel inboard, gasoline outboard, electric engines, both inboard and outboard), and then we examined them form the costs and reliability points of view ; let’s now go on with this analysys:


Weight and room on board

weight and volumes savings on small sailboats are always an important subject to deal with; basing on pure weight, small outboards engines are simply unbeatable; a 4 strokes 5 HP engine weights 27 kg (about 50 lbs.) , a more powerful 10 HP weights about 45 kg (100 lbs.) including its own battery, exhaust system and so on; a normal 10-13 HP inboard diesel engine fitted with saildrive weights 130 kg , without battery and exhaust system, a more powerful 20 HP weight only few kgs more, since transmission, basement, cooling system, ancillary parts absorb most of the weight engine and are about the same in the 10-30 HP range ;  your 13 HP diesel inboard engine powerplant will weight around 160 kg with all related systems , that’s a really huge ballast for a 21 footer, but not so huge for a cruising 26 footer ;

let us focus now on electric engines weights: here the game is a little bit different since engine in itself although powerful is quite light, and 80%-90% of the weight of the whole system is given by battery pack and thus your powerplant’s weight largely depends on the required endurance ; just to crunch some numbers:

  • a small 1000-1300 W output electric engine gives similar performance to a 3 HP outobard engine, so we are in the lighter segment of sailboats , perfect for IDEA 19 or similar, its weight including a spare reserve battery is around 15+5 kg , so in the same range of an outboard engine, it’s bit more comfortable to manage since the engine weight without battery is around 9 kg, really light and easy to handle ; in this configuration we have about 2,3-2,5 hours of endurance that is not bad at all, for a normal “low intensity” usage;

  • if we go on the upper edge of our range, 24-28 footers, weights increase : an electric engine in the 4-5 kW class is what a Hirundo 750 will deserve ; the engine is quite light, 20 kg, but it requires a battery pack of around 230 kg to have a 2-2.5  hours endurance at full throttle, basing on AGM lead battery; you can, roughly speaking, half the weight and double-triple the price going for Lithium-ion battery pack; so electric engine are a little bit like a chameleon: they’re light as an outboard as long as you have to power really light boats for short amount of time, they’re heavy as a diesel inboard engine if you require more endurance; if you can accept a one hour endurance they’re a really interesting choice since their weight will be comparable to a 13 HP diesel engine

  • room on board: room on board is always an important feature on small sailboats, often definitively a pain in the ass, since you normally try to squeeze every inch to increase comfort …
    Inboard Diesel engines are quite big and noisy, they need to be enclosed in a properly sized engine room (basically a box…) with enough room around the engine to have a good air circulation and a decent soundproof insulation; in the smallest boat this is definitely hard to accept without sacrifice a good percentage of interior volume, while in a 28 footer engine box can be fitted without affecting too much the living volumes, berths, galley and so on; outboards engines packs a lot of thrust in very small engines, if you fit them on a transom bracket the virtually take no room on board, although they can affect the fore-aft trim on small boats (too weight on transom); things are a little bit different if you fit them in a well in the cockpit, since this solutions can be practically achieved in large cockpit; this will be probably a very good cost-effectiveness compromise on boat like Petrel 28 ; average sized electric engines are probably the smartest choice considering the room on board, for a simple fact: you can separate the battery pack from the engine , fit the engine (which is quite small) in its best position, depending on engine model can be on transom or under the hull as saildrive unit, and locate the battery pack somewhere else, paying the maximum attention to the center of gravity of the boat, that’s to say you can place the battery pack as low as you can ( typically in dedicated boxes in the bilge) and moving it fore/after to trim the boat properly, which is something that you won’t be able to do nor with diesel inboard neither with outboard engines.


48V battery pack


typical scheme of a 200 Ah 48 V battery pack for a 4 kw electric engine fitted on a 25-28 footer; it guarantees 2-3 hours of endurance (2 hours at full throttle, 4 hours at half throttle) , its weight is about 230 kg with AGM battery; its cost is around 2000 €













the lightest solution, probably the best for a small racing boat: an outboard electric engine, fitted on transom,  light and easy to remove from transom and store it on the center of the boat in cabin (no more stinking fuel leaks in the bilge, thank God !) right before the race starts








this is probably the most demanding feature for electric engines: at the actual state of the art on small boats it’s very difficult to achieve endurance longer than TWO-THREE hours with a reasonably expensive battery pack (plus one spare for small electric outboards), while you simply need a cheap 30 liters gas tank to run your endothermic engine (both outboard or inboard) for about NINE-TEN hours ; anyway if you’re often in need to run your motors for several hours, I’d doubtless go for an inboard engine. Of course the story is totally different if you have “green” charging system on board for your battery pack (solar panels, hydrogenerator, small eolic windmills and so on) and you can charge them significantly while going under sails , but we’ll explore this exciting new world and its real capabilities on a future detailed post.

All in all Endurance is the REAL Achille’s heel for electric engines here and now.

An electric future ?

There are few doubts that electric engines will gain momentum in the very next future; they’re reliable, they deliver a very good thrust, and they can have at least a decent endurance for very small boats and tenders; their actual limits are more linked to the battery packs features than to engines themselves, so I think that when automotive industry will start to produce electric cars in huge numbers, the energy charging and storing technology will move swiftly forward and boating industry will benefit from this situation also for medium and big sailboats; this kind of boats will became a complex energy producing , managing and storage systems , but we’ll talk about this in a new post ; high energy and high power diesel engines will be probably unbeatable in the next future only for high speed planing medium to big sized boats requiring a decent endurance.

Summing up :

Outboard gasoline engines:

  • PRO:

    • compact and light

    • cheap

    • good endurance related to tank size

    • easy servicing

  • CONS:

    • not really workhorses on heavy boats

    • safety at sea depending on fitting position

    • gasoline on board (fire hazards related to gasoline vapors in hot climates)

Inboard diesel engines:

  • PRO:

    • super reliable workhorses

    • can be fitted in the best position

    • good endurance related to tank size

  • CONS:

    • expensive

    • heavy , especially considering 20-25 feet sailboats

    • not so easy servicing (must be done on board)

electric engines:

  • PRO:

    • good thrust and high efficiency

    • reliable

    • can be easily handled (detached from their battery pack they’re really light)

  • CONS:

    • poor endurance and difficult to increase it

    • expensive

    • very heavy (as inboard engines with decent sized AGM battery packs)