Big engines or small engines

[asteven221]

I am planning my next boat which will be a 40 to 45 foot flybridge cruiser. Much of the time, in fact probably the majority of the time we intend to plod along at displacement speeds. Logic seems to suggest that we would be better of with the smallest engine option fitted to any given boat. In my mind the advantages would be that the engines will be happier at slow speeds, physically smaller hence easier servicing and perhaps better fuel consumption at a slow speed. My observations are that the market wants the biggest engines for any given boat. If I went small, does than mean that the boat would be more difficult to sell when the time comes? Am I misguided thinking that smaller engines are happier at slow speeds compared to big ones? Any advice would be welcome.

 

[Adrianwool]

Much depends on what your proposed use is going to be. For example are your primerarly going to use on the Thames or Broads?, where you will only achieve low speeds which are worse for larger engines than smaller ones?. Or are you proposing to do much sea work where you will be better off with larger engines (especially if you are going for a planing hull and associated speeds).Smaller engines does not mean necessarily better economy (again dependent on what you are going to do). This needs to be your prime driver before making a choice.

 

[volvopaul]

In the early years, most uk builders offered small engines in there medium size models, ie the Fairline 50 circa early 90s was only available with 357hp engines, which in todays league would now have twice that hp and a bit more, all for another 10 knots of speed, however a small hp rated engine running flat out to give say 20 knots will use more fuel than an engine running at half the rpm with twice the hp to give the same speed.

A classic example was the fairline turbo 36, when new it was fitted with 2x 158hp engines, it would barely get on the plane when loaded, when the model was phased out it had 2x 340 hp engines as an option, a friend has one that had 2x 200hp volvos in when new but was later re engined with twin yanmar 350 s , what a diffence the boat is with modern engines fitted.

I would personally never buy a model with small engines in, as it will be difficult to sell on in the future, unless your going to keep it for a good number of years.

 

[BartW]

Steven,
perhaps you could find a "under powered" boat
at a very attractive price,
if this is what you're after for your usage

 

[Divemaster1]

First of all define your expected use:

1) plodding along at displacement speeds at sea
2) plodding along at displacement speeds in the river / inland water

Two very different things indeed...

Also, you need to tell us your intended age (no need to talk about price yet) of what your budget appears to afford... Then we can talk about good /bad / indifferent of various hulls, build quality, layout and so on.

Remember that a hull requires X HP to be pushed forwards at Y speed .... so therefore large engines,used lightly, may not use more fuel (as X is pretty constant ...weight plays in here... but only marginally at displacement speeds). Mechanical friction (and subsequent HP loss at lower RPM's) in a larger engine is of course higher than on smaller ones, but once again only marginally. A modern four stroke whould run forever at low RPM's ...but once you put turbos etc., on, these will need to be run up now & then etc.

Please provide more information and I am sure that advise will be forthcoming..

 

[MapisM]

Any advice would be welcome.

Since you ask...
In my simple mind, I can't see why for the usage you have in mind you're considering an underpowered fast hull.
Isn't a slow(ish) hull with proper engines a more straightforward choice?

 

[oldgit]

On this forum and elsewhere,it does not appear blantantly obvious,that the smaller engine options,when installed in larger boats,has any more propensity to blow up or wear out faster ,than their more powerful bretheren.
In fact when the more powerful engine option is merely the same block a year or two down the line with its turbo on steroids,the reverse could be true.
The first owner from new may have ordered the smaller option knowing full well the limitations of his power units and used them accordingly,whereas the big option WOT man will use his engines to the limit.
Would interesting to compare the failure rate of the 158 hp engine against the 235hp in the Fairline Turbo for instance or to take another example the range of engines fitted to various Brooms considering the number of very low powered old Perkins still chugging about.

 

[blueglass]

can I ask why you want a planing boat if you don't intend to plane. Surely a displacement boat would suit your usage better or if you want the occasional blast, perhaps go for s/d


I once had a fairline which I used almost exclusively at displacement speeds at sea or inland mainly because at the time I just couldn't afford the fuel for planing. It taught me what kind of boat was best for me and it wasn't that fairline. now much happier with an s/d boat which actually LIKES going slow.

 

[asteven221]

I agree with everyones thoughts and yes a semi displacement would seem the sensible option BUT there aren't any I know of that look like say for example a Princess 420 with the same layout/features/finish. Well not for £150k that is!

I will be using the boat in the sea and despite what some say I don't see going slow in a planing hull as an issue for the most part. I am aware that planing boats aren't too keen on going slow in certain conditions, but that's not a big deal as I would just alter speed in those conditions. I am convinced it's not just me thinking along the lines of the "slow lane". The vast majority of cruisers I am seeing now are do just that. In my 25 years of boating, I have never seen so many 40ft plus flybridge boats doing 7 or 8 knots as I have this year. Frankly, the idea of bombing around at 25 knots burning £130 per hour is not my idea of value for money. BTW I have had a 40ft flybridge in the fairly recent past so I am not completely new to all of this.

We want to be able to walk from the cockpit on to the bathing platform on the same level, so that rules out aft cabin boats including Princess 435 and suchlike. Some may cringe but I have to tell you that my SWMBO hates boats with lots of traditional furnishings (i.e. built by craftsmen to you and me!) so that completley rules out Traders/Grand Banks and the like - even if you could walk off the back. As a way to making the point - my mate has nearly new Grand Banks which is a beautiful boat to most of us, but my SWMBO thinks it looks like something from ancient history - oh and a tug! Hmmmm.....

So the criteara is. The boat must have the same look and feel as a Princess 420 or similar and ideally displacement or semi displacement for £150k ish. I really don't think such a boat exists. Maybe someone will tell me otherwise. That would be great.

Going back to my original post and given that I will likely end up with a Princess 420 or similar and the fact I will be for the most part going slow, in the sea, what are the pros & cons of two big engines versus two smaller ones? Please assume that whatever engine installation I am likely to end up with, the boat will still be capable of planing speeds in all conditions.

As always thanks to everybody for taking the time to put in their tuppenceworth.

 

[BartW]

my mate has nearly new Grand Banks which is a beautiful boat to most of us, but my SWMBO thinks it looks like something from ancient history - oh and a tug! Hmmmm.....

yes that sounds familiar, my SWMBO thinks exactly the same

 

[mainshiptom]

well i used to think that small enignes is the way to go but now got big lumps, I am quite happy with them allthough most of the times we cruise at 8-10 knots consuming anything between 10-20 lph for both engines, it is also nice for say 5 minutes to go up to 35 knots, Swmbo hates it but the number 1 son approves.

I would buy any size engine as long as soild boat, I have had underpowered boats so as long as the hull is good I am happy to slog along at a slow pace.


Tom

 

[rafiki_]

If you are buying a used boat, it is unlikely that you will get so much choice on the engine spec. I think you will find a boat that you really like, and accept a few compromises, including engines large and small.

As for fuel consumtion at low speeds, I don't think you will find a huge difference. You will only find this at WOT, where the powerful engines will move the boat faster, and generally consume more fuel.