Yanmar: why does anyone buy the 2YM15 instead of the 3YM20?

Greenheart

Greenheart

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If you're buying/replacing a small auxiliary, what would you prefer? A 14hp 2cyl unit or a 21hp 3cyl?

The bigger Yanmar engine is only 7kg heavier and costs only £700 more, not much difference when they both cost over £5000.

Naturally the bigger engine drinks a little more at maximum revs, but it gives a lot more in return.

Why does anybody buy a smaller engine which, per kilo and per quid, is so much less efficient?
 
Depends on use age intended, space aboard, And next time you spend 5k you can gift me 700 as it's not important when spending that much money. You can send new a check;)
 
Alright, I certainly don't have £700 to burn, but if I was about to spend over £5000 on 14hp (£365 per horse-power), I'm sure I'd prefer to spend £5750, for 21hp -£273 per hp!

Having spent an awful lot of cash for a small amount of power, an extra 15% wouldn't seem much if it buys 50% more power, with only a 7kg weight penalty...relatively good value.
 
You might well be able to buy a new sail 50% bigger than one that fits your boat for a lower cost per sq metre. Not much of a bargain, though, is it?

If you only need 14hp to do the job you want doing (or have only room to swing a prop that can transmit max 14hp), the additional £700 is a complete waste, as is the additional 7kg, and the (presumed) slightly increased service costs. You might also only have room for the 14hp, or the additional size means taking up some cabin space, and/or reducing the space around the engine reducing ease of access for servicing.
 
I was mainly thinking of the thousands of rather elderly yachts whose original auxiliaries' output was modest (or even inadequate) when the yacht was new...and now that it needs replacing, the owner could pick between another low-power, high-price motor, or from the same manufacturer, 50% more grunt for about the same weight and a little more cash.

However hard I try, I can't remember any skipper ever saying he wished he'd bought a boat with a less powerful engine! :rolleyes:
 
dancrane said:
I was mainly thinking of the thousands of rather elderly yachts whose original auxiliaries' output was modest (or even inadequate) when the yacht was new...and now that it needs replacing, the owner could pick between another low-power, high-price motor, or from the same manufacturer, 50% more grunt for about the same weight and a little more cash.

However hard I try, I can't remember any skipper ever saying he wished he'd bought a boat with a less powerful engine! :rolleyes:
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Very simple answer. The size of engine depends on the size of prop you need to push the boat, which in turn depends on its waterline length and displacement. So you start from the prop and work back to the shaft speed and then the horsepower, revs and reduction ratio. If your boat only needs 14hp to achieve displacement speed at full power, you do not need a 21hp as you cannot use the extra power. There is clearly a need for engines in roughly 5hp increments from 10 to 30hp reflecting the differing power requirements for a range of boats from around 24' to 36'.

Just think about it 21 is 50% greater than 14 so a massive increase in power!

In the case of the two engines you are talking about there is a significant difference in physical size which is often critical in a small boat. For example, on my boat I can fit (and have fitted) a Nanni 14 even though a 16 might have been better, but the extra length would mean a major rebuild of the engine compartment with a knock on effect on the galley. A Yanmar 14 would have been my first choice, but that is even bigger physically, both length and height.

The seemingly small price differential reflects the fact that the 20 is the most popular size and very competitive sector compared with the nominally 15hp.

Such decisions are always a compromise and there are many boats where the power requirement falls between 2 sizes, and it was even worse a few years ago when there was often a jump from 10 to 20hp, then 20-30 in many manufacturers' ranges so you often had boats where the standard (cheap) engine option was underpowered and the other overpowered.

As for re-engining, you are right, there is a lot of choice. My old boat originally had a Stuart Turner 8hp petrol. Changed to a 9hp Yanmar. Little change in performance, except that the Yanmar worked when I asked it to not when it wanted to like the ST. 2cyl engines at that time were physically too big until the Kubota based Beta and Nanni came along which are physically virtually the same as the Yanmar. Most replacements will be with a more powerfu, smoother, quieter engine, but there is no need to install too much power.
 
Thanks Tranona, that sounds very reasonable.

I often read the Westerly Owners' Association reviews of the old models, some of which seem to have been only lightly engined; and when replacements are discussed, it's usually with something considerably bigger than was originally judged sufficient.

Personally I like the idea of keeping auxiliary use to a minimum and I'd prefer that the engine was a lightweight occasional 'helping-hand' for tight spaces, rather than a much more purposeful alternative to the mast and sails.

Just the same - however unnecessary or unhelpful it may be to have more power than is required to reach hull speed, I reckon people will be tempted by Yanmar's scale of economy which almost looks equivalent to "buy two, get one free". Especially when the bigger unit is just a few kilos heavier and only 3" longer.
 
dancrane said:
However hard I try, I can't remember any skipper ever saying he wished he'd bought a boat with a less powerful engine! :rolleyes:
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Well now you can say you have heard of one - ME! - In the dim and distant past I had a Jaguar 27 which had been re-engined with a 20hp beta - "bargain" says I oodles of "grunt" but lo and behold it came to pass that the engine was cursed from here to doomsday because it had far too much "grunt" - so much so I nearly sank her one fateful day. Any more than ½ throttle and she would squat - more throttle the deeper she squatted until the open drain from the lazarette would be submerged with the inevitable result - the lazarette began to flood adding more weight on the stern causing the stern to squat even further causing the water level in the lazerette to rise further etc. - "No problem" I hear you say - "just back off the throttle" Uh Huh - drain below water level ~10" of water in the lazerette and the main bilges flooded. Salon floor floating up the companion way to greet me. Fortunately I managed to stuff a bung in the open drain and with the aid of two electric bilge pumps, a manual pump and very frightened crew with a couple of buckets we managed to bail out enough water to get the drain above water level.
she originally had a 13 hp and that extra 7hp was really too much for her. I did seal the lazarette bulk heads to stop any water entering the drain subsequently draining into the bilges. Had I not had 3 very able bodied crew on board she would have been lost.
Naw, too much power is no good.
 
dancrane said:
Thanks Tranona, that sounds very reasonable.

I often read the Westerly Owners' Association reviews of the old models, some of which seem to have been only lightly engined; and when replacements are discussed, it's usually with something considerably bigger than was originally judged sufficient.

Personally I like the idea of keeping auxiliary use to a minimum and I'd prefer that the engine was a lightweight occasional 'helping-hand' for tight spaces, rather than a much more purposeful alternative to the mast and sails.

Just the same - however unnecessary or unhelpful it may be to have more power than is required to reach hull speed, I reckon people will be tempted by Yanmar's scale of economy which almost looks equivalent to "buy two, get one free". Especially when the bigger unit is just a few kilos heavier and only 3" longer.
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Well, cliff has given you a good answer. If you cannot use the power, it is pointless installing it. The limitation is that it is the prop that drives the boat, not the engine and the prop will demand the power it needs. Another big downside of having too much power is that the boat goes too fast at tickover which can be a nightmare in close quarters situations. You need power delivery that is smooth and progressive.

Older boats are often re-engined with slightly larger engines than the original, but that is because the original engine was often marginal and the new engines are smaller and lighter. So quite possible, for example to fit a 25hp where the original was a 20 or a 25 where the original was 10. But if the boats were built now they would be fitted with the larger engines as standard.
 
Tranona said:
The limitation is that it is the prop that drives the boat, not the engine and the prop will demand the power it needs. Another big downside of having too much power is that the boat goes too fast at tickover which can be a nightmare in close quarters situations.
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Ah-hem. If it's the prop that drives the boat, why would maximum power matter at tickover? Does a three cylinder Yanmar tick over significantly faster than a two cylinder one?

Actually, I think "the prop drives the boat" is a bit simplistic. It's the engine/prop/boat/conditions system which results in movement, and no one of these matters any more than the others.
 
JumbleDuck said:
Ah-hem. If it's the prop that drives the boat, why would maximum power matter at tickover? Does a three cylinder Yanmar tick over significantly faster than a two cylinder one?

Actually, I think "the prop drives the boat" is a bit simplistic. It's the engine/prop/boat/conditions system which results in movement, and no one of these matters any more than the others.
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Because the prop is larger to absorb the power, so gives more thrust at lower revs. If you have ever tried an overpowered boat you will understand how awkward it can be.

I was making the point about the prop driving the boat because that is the starting point for deciding how much power you need. The prop does not care where the power comes from. The amount it demands depends on its size and shaft speed.
 
Tranona said:
Because the prop is larger to absorb the power, so gives more thrust at lower revs. If you have ever tried an overpowered boat you will understand how awkward it can be.
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Ah, so you're now saying that choice of prop will depend on the engine and not the other way round?

I was making the point about the prop driving the boat because that is the starting point for deciding how much power you need. The prop does not care where the power comes from. The amount it demands depends on its size and shaft speed.
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Same comment.
 
Quite clearly black is white and white is black, we have one thread suggesting that using an engine at the minimum practical size for a boat is foolish and you should go as big as possible, then another which tells us going for the maximum is equally foolish and small is beautiful.
 
JumbleDuck said:
Ah, so you're now saying that choice of prop will depend on the engine and not the other way round?



Same comment.
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No, I am not saying that I am saying if you match the prop to absorb the power that the bigger engine will produce it will be "bigger" - usually pitch. So for each revolution will move the boat further - so for a given engine speed the boat will move faster.

Suggest you get a copy of The Propeller Handbook by Dave Gerr and read up the theory for yourself.
 
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