Engine replacement with another of increased horse power: Is it worth it?

Wing Mark said:
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I was looking at re-engining a prospective 28ft boat from a 8HP 1GM to 16(?) 2GM.
Hopefully I'm going for a 14HP Beta.
The online prop calculators suggest 7 knots flat out, which is more than plenty, but hopefully gives confidence of sustaining over 5knots in adverse conditions when the weed has started to grow?
Do people think that is about reasonable?

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Should be fine. I had an old 12hp Petter on a 32 ft boat for many years and was quite happy.
To some extent it is a Zen thing, I am quite content if the boat is moving and the engine works, other folks are incandescent with rage if they can't move at hull speed +, and tow a stern wave you could surf on.
Sometimes you do see silly things like a Sigma 33 with 48 hp engine, they mean to impress but most folks just think "well, that's buggered that". It generally means someone got the engine at a good price.

OTOH if motoring is the priority, fit a large 3 bladed prop and an engine to suit.

.
 
Spanjaard said:
What should I look on the prop? Higher efficiency?
What on the gearing?

Thanks
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I don't know what prop you currently have, but some people go for a 2 blade folding prop to reduce drag when sailing at the cost of motoring performance.
3 blade can vary widely, blade profile, diameter, pitch, not to mention feathering props. A prop specialist would give you good advice, but first inspect your prop and note the markings stamped into it giving pitch and diameter details.
Gearing ratio is normally calculated wrt the engine and boat but if someone has replaced the engine without consideration to the gear box it could be like driving a car in 3rd gear because you cant get to 4th (or vice versa).
 
Spanjaard said:
What differences or impact would have a sailboat if its engine is replaced for another one with increased horse power?

I don’t think speed would be too much difference from 20 HPto 36HP on a 28 foot boat as this would be limited by the hull speed.

Are there any advantages at all if installing a more powerful engine, or not really?

Thanks
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1982 I bought a 33 ft Nauticat which had a Perkins 72 hp diesel. Which I beleived was about right.
Years later I bought a 24ft Colvic Watson with a 50hp engine which I believed was just about right.
When these boat manufacturers make a boat they fit the smallest engine they can get away with as it keeps the cost down.
So if I had the choice of a larger engine I would go for it, and when you have the wind on the nose punching against a foul tide and the weather is rubbish you will be glad that you bought the bigger one.
 
LadyInBed said:
Have you dismissed a 1GM10? It's the same block so same weight and fittings.
2.5 increase in hp as 1GM is 7.5hp.
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I was using 1GM vaguely, meaning any variant. Wouldn't a 1GM that's not a 1GM10 be pretty old by now?
A 1GM 10 is 9HP, according to Yanmar.
But that's at 3600rpm, and my experience of single cylinder diesels is that I don't want to be within 50ft of one doing 3600rpm for any length of time.
I may be being unfair, because the yachts I spent time on years ago with 1GMs were likely amateur installations, and we mostly thrashed them to and from the start area, but if you wanted to cross Lyme Bay in no wind, anything more than 2500 rpm is going to be unpleasant and then you're only talking about a few HP if the prop is pitched for max power?
From what I've seen looking at boats over the past couple of months I feel that although the Beta is only ~50% more peak power, the power you can use while cruising in a civilised fashion is in a higher league?

As a sometime motorcyclist, I'd have thought parallel twins were nearly as bad as singles for NVH, what you want is a V twin or a triple, but a Kubota is maybe not much like a Triumph Bonneville.... Are they 90 degree cranks?
 
Who ever complained that their boat's engine was too powerful? Too heavy yes, but modern engines are nothing like as heavy as the traditional old lumps, unless you had a Dolphin that is!

I replaced my old Centaur's MD2B with a variety of engines, but the last one, a VP2003 at 27hp-ish, was far and away the best choice. I now have 50hp in a 34 footer, but would quite happily have something 50% more powerful as it gives the power to punch into anything without fretting about over stressing the motor.

Modern multi cylinder engines are definitely the way ahead too - Far less stress on every element of the boat, though I do miss the comforting old thudding.

The worry about under-stressing the engine is, IMHO, grossly exaggerated. But then maybe I have a leaden left hand. :)
 
Can we roll this back a bit and talk about numbers?

The two single and most important numbers regarding required HP for an engine are DWL and displacement.
As we are still talking displacement hulls here, max speed, for all intents and purposes, is a relative speed of 1.34. Light boats or boats designed with semi displacement hulls might reach 1.4 or even 1.5, but in this context of auxiliary propulsion that is irrelevant.

To reach hull speed you need: 1 HP per 500lbs. Actually, most of us do not travel at hull speed, but at which one might call cruising speed. This should be somewhere between 45% and 65% of full throttle and at a relative speed of 1 for most economic operation.
A relative speed of 1 makes perfect sense as this is the point where resistance rises exponentially.
To reach a length/speed ratio of 1 requires 1HP per 1100lbs. To note: these figures for HP are actual HP at the shaft, i.e. engine HP minus 4-6% for loss through gearbox, bearings etc.

In practice 4-4.5hp per ton is recommended, which fits in nicely with what I previously stated. To be sure, the recommendation for free sailing, i.e. non-trawling, fishing vessels is 4-6 hp/t with 6hp considered the upper limit for economical operation. Motorsailers have on average 6.8hp - 8hp/t.
Displacement should be the real displacement and not what it said in the brochure or on Sailboatdata.

Much is made of the idea of a yacht being able to motor against strong winds and seas and therefore a boat needs to have excess power to do so. Has anyone here actually ever done that or know what that looks like in reality? We have had the opportunity on several occasions, in open water and against a F8 and F9 with steep, breaking seas. It takes a special type of boat to do that. We have a motorsailer with extremely buoyant bows that are 1.9m tall on a 31.5' hull with the aerodynamic qualities of a brick. It was a rather sobering experience.
Our engine delivers 58hp on a displacement of 8.5t or 6.8hp/t. We normally cruise at 5.5kts (relative speed of 1.03) and for this we need 16.5hp at the shaft at 1500rpm. To reach hull speed of 7.12 we need 36hp and 2200 rpm in flat water. This equates to 70% max rpm. You should not run an engine at more than 80% capacity for long periods at a time. At this rpm we were able to make way against 40kt+ winds and 3-4m breaking seas at a speed of 2kts or less.

The idea that a larger engine could make up for deficiencies in propeller design is not correct. Sailing boats have props that sacrifice blade area and , consequently motoring performance, in favour of sail performance.

There are some advantages to be had from a larger engine with "throttled output" and a bigger prop:

- more available power at partial load.
- lower fuel consumption
- greater cruising radius
- less noise
- less wear and tear
- better maneuvering
- faster acceleration

Disadvantages are:
- more expensive
- more weight
- lower speed (with down throttled rpm)
 
Well summarised, particularly the emphasis on displacement. If we take the common 9-12m range of modern boats we see displacements ranging from roughly 3.5- 9 tonnnes and engines ranging from 15-40hp. Not every manufacturer has a wide range of HP in this size range and the increments are large. Volvo has 20,30,40, Yanmar 15,21,30,40. So there tends to be boats which fall between two sizes and with these you tend to see the smaller as standard but still capable of achieving hull speed and the next size up as options.

Broadly speaking a 20hp will be fine up to 4.5-5 tonnes which could be a heavy 33' or a light 36'. 30hp covers a very wide range of boats from 33-38' ( up to 7 tonnes) when the 40hp takes over as standard. Length is not a particularly good measure of size nowadays with short overhangs and wide beams. My 33 (10m hull length) is the same displacement as the 37 it replaced (hull length 11.7m) and has the same size (30hp) engine.

The original question about whether a new, more powerful engine is worth having is really not the right question - or rather does not have a simple answer. Older boats were often under-engined, either because it was not the "done thing", or cost, space or even because there was not a suitable engine available. Many older boats benefit from having a more powerful engine than the original and also gain from more refinement, greater reliability, freshwater cooling allowing hot water etc. However this comes at a cost, often high in relation to the value of the boat, so the OPs choice of a recon engine of the same type is a viable alternative if it has adequate power. Downside is having a long obsolete engine with all the future worries about spares - and to be frank older style engines particularly saltwater cooled do not last as well as modern engines. Don't think to will find many who have replaced Bukh 20s with a Beta 25 or similar regretting their decision!
 
A bigger diameter prop will be to the detriment of the sailing performance, particularly light airs. You could go for a folder or a featherer, but these don't have the thrust in forward compared with a fixed (probably better astern).

It is all a compromise.
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Not necessarily. A good three or even four blade folder can have more thrust than a three blade fixed prop. Our Brunton three blade folder has move thrust than the one it replaced. In reverse it's less efficient than a fixed prop. It's about careful prop selection.
 
geem said:
Not necessarily. A good three or even four blade folder can have more thrust than a three blade fixed prop. Our Brunton three blade folder has move thrust than the one it replaced. In reverse it's less efficient than a fixed prop. It's about careful prop selection.
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Agreed. The Bruntons is probably an exception due to it's ability to change pitch. Never tried, just looked at it, so no direct experience. The surface area for the diameter looks less than a fixed - how does it perform in a sea? Does the variable pitch make up for the loss in area? Or is it that say a 14" fixed is replaced with a 15" Bruntons - ie is it a direct comparison, or is it better only if you have the space for a bigger one?
 
Pye_End said:
Agreed. The Bruntons is probably an exception due to it's ability to change pitch. Never tried, just looked at it, so no direct experience. The surface area for the diameter looks less than a fixed - how does it perform in a sea? Does the variable pitch make up for the loss in area? Or is it that say a 14" fixed is replaced with a 15" Bruntons - ie is it a direct comparison, or is it better only if you have the space for a bigger one?
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Mine isn't a variable pitch. That would be the Brunton Auoprop. Mine is a Varifoil. It's a three blade folder. The blades on my folder look like Mickey Mouse's ears. Lots of area?
 
Iliade said:
Who ever complained that their boat's engine was too powerful? Too heavy yes, but modern engines are nothing like as heavy as the traditional old lumps, unless you had a Dolphin that is!

I replaced my old Centaur's MD2B with a variety of engines, but the last one, a VP2003 at 27hp-ish, was far and away the best choice. I now have 50hp in a 34 footer, but would quite happily have something 50% more powerful as it gives the power to punch into anything without fretting about over stressing the motor.

Modern multi cylinder engines are definitely the way ahead too - Far less stress on every element of the boat, though I do miss the comforting old thudding.

The worry about under-stressing the engine is, IMHO, grossly exaggerated. But then maybe I have a leaden left hand. :)
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Me. My engine destroyed itself because it was no being driven hard enough. I re-powered with an engine rated at half the hp, and I can't tell the difference (other than it being quieter and cheaper on fuel). The rated hp is not the whole story though. My old engine was rated at 105hp, but that was at 4,300rpm. Nobody drives a boat engine at 4,300rpm. I used to cruise at about 2,000-2,500rpm (at 2,000rpm the engine was only producing about 30hp).

I replaced it with a 50hp unit, which even though rated at half the hp, had a cylinder capacity 10% bigger than the old one (so 10% more torque), and the max revs are 2,800rpm. I now cruise at exactly the same speed at between 1,500-1,900rpm and it feels no different to the old engine, but actually has more "snap" and grunt in close-quarters because of the extra torque.

So when choosing an engine, don't just look at the hp figure. Look at the power curve and the hp at the revs you will cruise at, and look at the cylinder capacity, as there's no substitute for cubes.
 
My first boat with an inboard was a Cobra 850 twin keeler. It came to us with a Yanmar YSE12 which turned out to be completely unreliable and seemingly well underpowered. After a couple of years it was decided to replace the engine. I looked at lots of different ones, looking for around 15 hp,however after many years of Nortons and BSAs I didn't want a vertical twin so it came down to either a Beta20hp or Volvo 20 hp. We settled on the Volvo as we got a great deal at the London boat show. It did turn out to be a bit overpowered as we never had to give it much throttle but it started on the button everytime and was totally reliable.

Next boat was a Hanse 301 which came with a Volvo 10 hp twin cylinder.This was fine as long as we kept her clean underneath and pushed us along at hull speed ok. It was also nice and smooth which dispelled my earlier misgivings about vertical twins.

Next and current boat is a Bavaria 350 (which is probably twice the weight of the Hanse) which has a Yanmar 3gm30f which again is probably a bit overpowered as we don't need to use anything like the full performance. We also ditched the std two bladed saildrive prop for a Kiwi which gave us a bit more speed under sail and a lot more initial bite under power for not a large expense.
 
Bobc said:
Me. My engine destroyed itself because it was no being driven hard enough. I re-powered with an engine rated at half the hp, and I can't tell the difference (other than it being quieter and cheaper on fuel). The rated hp is not the whole story though. My old engine was rated at 105hp, but that was at 4,300rpm. Nobody drives a boat engine at 4,300rpm. I used to cruise at about 2,000-2,500rpm (at 2,000rpm the engine was only producing about 30hp).

I replaced it with a 50hp unit, which even though rated at half the hp, had a cylinder capacity 10% bigger than the old one (so 10% more torque), and the max revs are 2,800rpm. I now cruise at exactly the same speed at between 1,500-1,900rpm and it feels no different to the old engine, but actually has more "snap" and grunt in close-quarters because of the extra torque.

So when choosing an engine, don't just look at the hp figure. Look at the power curve and the hp at the revs you will cruise at, and look at the cylinder capacity, as there's no substitute for cubes.
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Was your first engine turbo charged?
 
I was always told by salty old engineers that diesels like to be run hard. Unless there's reason to believe your boat is significantly underpowered, then I'd leave well alone. As others have pointed out, there's a lot more to consider than horses alone, and the interrelationship between all the different parts of the propulsion system is quite complex.
 
Bobc said:
Me. My engine destroyed itself because it was no being driven hard enough. I re-powered with an engine rated at half the hp, and I can't tell the difference (other than it being quieter and cheaper on fuel). The rated hp is not the whole story though. My old engine was rated at 105hp, but that was at 4,300rpm. Nobody drives a boat engine at 4,300rpm. I used to cruise at about 2,000-2,500rpm (at 2,000rpm the engine was only producing about 30hp).

I replaced it with a 50hp unit, which even though rated at half the hp, had a cylinder capacity 10% bigger than the old one (so 10% more torque), and the max revs are 2,800rpm. I now cruise at exactly the same speed at between 1,500-1,900rpm and it feels no different to the old engine, but actually has more "snap" and grunt in close-quarters because of the extra torque.

So when choosing an engine, don't just look at the hp figure. Look at the power curve and the hp at the revs you will cruise at, and look at the cylinder capacity, as there's no substitute for cubes.
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Yep, my engine is 86hp. Maximum torque at 1200rpm. 4.4litre spinning a 22" prop. We cruise at 6kts at 1200rpm in flat water@3.3l/hr. Maximum revs are 2000rpm. We are over propped. Theoretical max revs being 2400rpm.
 
BigAlbatrossBird said:
I was always told by salty old engineers that diesels like to be run hard. Unless there's reason to believe your boat is significantly underpowered, then I'd leave well alone. As others have pointed out, there's a lot more to consider than horses alone, and the interrelationship between all the different parts of the propulsion system is quite complex.
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Diesel van and car engines seem to have very long lives, spending most of those hours running at a small fraction of their max power.

There might be more truth in what you say for a seawater cooled engine which doesn't get properly warmed up at low power?
Does it apply to a modern freshwater cooled Kubota engine?
Surely those digger and generator engines run all day, most of it at light load?
 
Tranona said:
Broadly speaking a 20hp will be fine up to 4.5-5 tonnes which could be a heavy 33' or a light 36'. 30hp covers a very wide range of boats from 33-38' ( up to 7 tonnes) when the 40hp takes over as standard. Length is not a particularly good measure of size nowadays with short overhangs and wide beams. My 33 (10m hull length) is the same displacement as the 37 it replaced (hull length 11.7m) and has the same size (30hp) engine.

The original question about whether a new, more powerful engine is worth having is really not the right question - or rather does not have a simple answer. Older boats were often under-engined, either because it was not the "done thing", or cost, space or even because there was not a suitable engine available. Many older boats benefit from having a more powerful engine than the original and also gain from more refinement, greater reliability, freshwater cooling allowing hot water etc. However this comes at a cost, often high in relation to the value of the boat, so the OPs choice of a recon engine of the same type is a viable alternative if it has adequate power. Downside is having a long obsolete engine with all the future worries about spares - and to be frank older style engines particularly saltwater cooled do not last as well as modern engines. Don't think to will find many who have replaced Bukh 20s with a Beta 25 or similar regretting their decision!
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Hi Tranona,
The original question is due to finding myself in the situation where:
1st - I have to repair my old Bukh DV20 ( my boatyard quote around £3.5K),
2nd - Or get a recondiitioned replacement (in between £1.5-£2K) and keeo current engine for spares
3rd - since I have to repair or replace old engine, would it be worth it to "upgrade"?

The most tempting option looks the second, replacing like by like and keeping spares. However I would like to know if I am mising an opportunity to upgrade.

For what can I read, my 28" boat is about 3Tn displacement, it looks like the DV20 should be more than sufficient.

Thank you all.
 
Wing Mark said:
Diesel van and car engines seem to have very long lives, spending most of those hours running at a small fraction of their max power.

There might be more truth in what you say for a seawater cooled engine which doesn't get properly warmed up at low power?
Does it apply to a modern freshwater cooled Kubota engine?
Surely those digger and generator engines run all day, most of it at light load?
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Whilst not being an engineer, contrary to popular opinion, I think it’s a myth that light loads cause engine problems.

If we think about what causes engine wear…they don’t wear out when not running, what causes wear is moving parts, and we have oil to protect against that. So as long as the oil and filters are changed regularly that should prevent wear.
 
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