Advice on boat upgrade

[akis_t]

I have a small powerboat which I use in the summers to take us to the near beaches, it is an open 5.5m with an outboard.

I would like to upgrade to something better and one class of cheaper boats are the ones with those large inboard petrol engines (mercruiser 5.0L) with a stern drive.

I was told that boats like these have increased fuel consumption compared to boats with outboard engines (eg a Merry Fisher or a Scout Abaco), suffer from many faults on the stern drive and that they are unsafe as they do not have a backup engine.

On the last point my personal experience is that my outboard has cut out twice in 4 years and both times I had to use the spare engine to crawl back to port.

Any comments welcome.

 

[kashurst]

there is no black and white answer. A boats' reliability depends on age, maintenance, use, skipper etc. single engined boats are used right out at sea in fishing boats and all around coastal waters everywhere. To make any boat reliable takes time and effort and really good maintenance. Especially if its second-hand and older, you don't know how it has been looked after or used/abused. So when ever I have bought a boat I always assume it will take a year to get the bugs out of it and don't stray too far from home until I have gone through all the major parts. A well maintained outboard will be as reliable as a well maintained in-board and vice versa. Sea air and sea water are very corrosive, modern petrol can damage older fuel systems - that and infrequent use tend to lead to issues.

Single engined petrol engines with outdrives, have their weaknesses but they are not any more prone to reliability issues than an outboard boat - if looked after correctly.
The risk with any single engine boat petrol or diesel is that it will fail at some point - hence the popularity of twins. You have a back up outboard at the moment you could do the same with a bigger single inboard, engined boat. you can get hi-lo brackets etc. However you still have to make sure the back up is looked after just as well as the main.

 

[rafiki_]

There are tens of thousands of boats with petrol engines and sterndrives performing fantastically week in and week out. There are also some that are not well maintained, which suffer reliability problems. I have had 2 petrol engines sterndrive boats, and one diesel sterndrive. Never let me down. In terms of fuel consumption, yes, a 5 litre V8 is going to consume its share of fuel if you open the throttles, probably 12 to 15 gallons an hour. Your outboard equivelent will be a smaller engine, and consume a bit less. Not a lot less for a similar sized boat. Outboards can be easier to maintain, as the transmission is less complex than a sterndrive, and of course, you can take it off the boat.

 

[akis_t]

When we say gallons we mean US or UK gallons? A US gallon is 3.8lt, but a UK gallon is 4.5lt ?

I saw a picture of a boat for sale it had a VW TDI engine in it, when most/all other boats have either a Volvo Penta or a Mercruiser engine. Out of academic curiosity, could a good mechanic fit a car's diesel engine or must they be made different?

 

[[2068]]

The requirements for a marine engine are quite different to a car engine (car engine only uses lots of power for short periods of time). Also, there is the cooling system to consider ... most boats don't have radiators at the front! Having said that, some marine engines are loosely based on car engine blocks (e.g. Volvo Penta D3 and some VW Marine engines, now Mercruiser) but it's still the case that all the ancillaries are different.

 

[BruceK]

Not so much a comment but an observation. Having an inboard on that size boat frequently chews up a lot of deck space. As this size / class of boat is also in the domain of the recreational fisherman, lost deck space is an evil and so the negative rep trend starts. As said above. Maintenance and reliability go hand in hand and there are pros and cons to both I/O vs OB

 

[rafiki_]

When we say gallons we mean US or UK gallons? A US gallon is 3.8lt, but a UK gallon is 4.5lt ?

I saw a picture of a boat for sale it had a VW TDI engine in it, when most/all other boats have either a Volvo Penta or a Mercruiser engine. Out of academic curiosity, could a good mechanic fit a car's diesel engine or must they be made different?

I quoted Imperial gallons, not US.

Many marine engines in this category are based on automotive engines, the V8's you quote are a good example. The VW Tdi engine is also automotive based, but all these need to be marinised. The V8 gasoline engines take raw sea water and cools the engine directly. The diesels have an indirect cooling system, with a sealed system that circulates around the engine and then uses a heat exchanger, which draws raw sea water to cool the sealed system. This is more reliable, but takes more engineering. Engine ancillaries tend to be specific for marinised engines like pas pumps and sometimes alternators. There are companies in the UK that will marinised most auto engines.
Mid size Diesel engines, 4 litres and above will be mainly based on truck engines. The really large marine diesels will be mainly shared with power generation sets, or really large off road (eg mining) vehicles. Some are marine specific, but of course the volumes are small so economies of scale are a challenge.