Engine for small sailboats

Engine for small sailboats : how to choose it

part one

My projects are focussed on small sailboats, up to about 30 feet, boats on which the choice of the engine often has a heavy impact on the project from several points of view: costs, reliability, weights, effectivness of the propulsion system, safety at sea.

There are currently three main options to power a small (or not so mall, hirundo 750, Petrel 28) sailboat:

  • inboard diesel engine , both with shaft line transmission or saildrive one

  • outboard engine on transom bracket

  • electrical engine, both transom mounted as an outboard engine or fitted in bilge as a traditional diesel inboard one

until two decades ago this triple choice was simply not existent: small 2 strokes outboards were definitely unreliable (on J24 we used to go in and out of the harbour with sails mostly because we were SURE that our engine would have quitted working in the worst possible moment, providing us a plenty of adrenalinic moments…), electrical marine engines simply didn’t exist , so the only possible choice for a small cruising sailboat was a small inboard engine.

Nowadays things are definitely changed: small 4 strokes outboard engine are really sturdy and reliable , electric engine are becoming a reality year after year, and on the other side small diesel engines are stucked on models , technology , costs and weights not so different from 20 years ago, but nonetheless they are really workhorses;

Let’s now consider some of the choice parameters.


outboard 5 HP on transom on idea 19

idea 19: a 5 HP outboard engine fitted  on transom in central position, a good choice for smallest sailboats



engine has a dramatic impact on total budget on a small homebuilt boat that can be built with a budget of 20-23k euros (hirundo 750, very much less for idea 21 or 19)  , let’s see how much dramatic:

  • 10 hp 4 strokes outboard engine: it costs around 2700 € vat included , and can be purchased basically in each marina, additional costs to fit it on board are low: a plastic gasoline tank with pipes and plugs, a sturdy transom bracket, few bolts and nuts , and you have your powerplant ready to work ; if you want to go for a smaller 5 HP (perfect for Saltaspiaggia or idea 21), engine cost is about 1600 € ; there’s a wide market of second hand engines (pay attention, there are a lot of  “archeological” outboard engines on the market…)

  • inboard diesel engine : thing become serious for our wallet: a fresh new 10 HP diesel engine fitted with saildrive costs around 7500-8500 €, a shaft line version, just engine and transmission, is about 5600-6500 € , in this case you have to buy also complete shaftline, bracket, propeller, and fitting all the system on board properly requires a good amount of experience (or hiring a mechanic, other costs) , and you even have to buy and fit ancillary systems: fuel tank and filters, exhaust system, and a dedicated engine battery and related wirings ; upgrading from 10 to 20 HP (good choice for bigger boats like Petrel 28) engine cost raises of about 2000 €, both for saildrive models and for shaftline one;

  • electrical engines: small electric outboards , perfect to power idea 21, costs around 2000€, and other 600-800 € are required  for a spare battery pack, which IMHO a mandatory upgrade; for bigger boats, an electric engine that can be compared to a 10 hp endothermic one , in the 4kW class, can be purchased for around 3700 euros, with no batteries; if you add an AGM battery pack for at least 2 hours of endurance total cost raise to 5700-6700 euros , definitely more expensive than an outboard gasoline engine and not so far from the workhorse diesel inboard engine

  • use related costs: that is to say , mainly, fuel: given the fact that we’re talking about small engines, fuel rates goes in the order of few liters/hour (tipically 2-3 for 10 Hp diesel inboard and 1.5 for 5Hp outboard engine), so there are not big differences between diesel inboard and gasoline outboard ; of course as long as you have an electric plug in your marina, recharging an electric engine battery pack is free

  • servicing: small outboard engines can be taken away form the boat quite easily, so you can service them by yourself in your house’s garden or take them to a mechanic, same thing for electric engine, both outboard and inboard, since they’re quite light ; on the other side, removing a diesel inboard engine is a quite complicated task, and you’ll be forced to service it on board, they’re normally removed only for major overhauling or to be replaced



sailboats are made to set on sails and turn off the engine, any sailor know the magic of this moment , but there are several situations in which you have to turn on the engine, even for a good amount of time, and sometimes you have even to push hard your engine: strong tidal streams, strong upwind seas, total zero wind situations (and sometimes even no wind and big waves which is quite annoying), emergency situations due to failures on board and so on; so we need a reliable engine in which we can trust; I’m not considering here the normal situation in which you’ll turn on the engine for 10-20 minutes to sail away from your marina in calm-normal seas, for this standard purpose each engine is fine; in case of smallest boats, up to 23 feet, I’d anyway go for a electric engine (first choice) or a outboard 4 stroke engine, taking care that our battery pack for the electric engine gives us enough endurance, which may be a decent headache ; for bigger boats, from 23 to 30 footer, diesel inboard engine can be a good choice, and becomes the best choice if you normally need to motor for a quite long amount of time; in this last situation electric engine might be also a good choice but it’d be hugely compromised by the weight of a properly sized battery pack , or by its cost if you go for a light hi tech Litium-polymeric battery pack (they may cost a huge percentage of your entire boat) ; outboard engines often suffer from poor installation on a high placed transom bracket, that may lead the propeller to jump out of the water in short and steep waves with boat pitching, so the engine may be reliable in itself but its set up lead to a unreliable usage (a small well in the cockpit will make the outboard work veeeeery much better, see picture below)


in my humble opinion this is definitely the smartest way to fit an ouboard engine if you have a roomy cockpit : engine is on the centerline, propeller is in a very good position and it can work even with the boat heeled , and if you have to work on the engine you can do it in the cockpit, not hanging outboard in the weirdest positions

inboard diesel engine lombardini on petrel 28


inboard diesel engine: Lombardini 20 HP fitted with saildrive on Petrel 28,  an uncompromised classical choice for a mediterranean cruiser






a very neat and clean inboard electric engine fitted on a wooden boat : will this be the future ?


we’ll keep on with this subject in the next part of this post, stay tuned !