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

[Spanjaard]

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

 

[[70521]]

No...

20 HP on a 28 foot, 8.50 meters, sailboat boat might be seen as overpowered.

My last boat was 8,50 meters and had a 10 HP engine, my current 10 meter boat has an 18HP and we get pushed along at 5.5 knots STW with ease. I sail faster than propelled by engine.

 

[LadyInBed]

A bigger engine would just increase weight.
If you can maintain hull speed into a bit of a sea, no problem.
If you can't, first look at the prop, then the gearing.

 

[Spanjaard]

Thank you Sandy.

My engine is a Bukh DV20. The current boatyard services are too high to fully service the engine. The boatyard offered to install a Beta 20 but installation requires new gear box, new control panels and additional wok, possibly on the engine bay and shaft. Final quote went to a whopping £10K.

So I decided to get a reconditioned Bukh DV20 for under 2K and replace it for the current one which I could use for future spares.

I now the DV24, DV29 and DV36 would fit perfectly on the bay. Pointless if it does not bring any benefit when not
counterproductive.

Thanks.

 

[Spanjaard]

A bigger engine would just increase weight.
If you can maintain hull speed into a bit of a sea, no problem.
If you can't, first look at the prop, then the gearing.

What should I look on the prop? Higher efficiency?
What on the gearing?

Thanks

 

[coopec]

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

What is the design of your yacht?
What are the relative weights of both motor/gearboxes?

Will the increase in weight result in dragging the transom in the water?

 

[penfold]

I'd be surprised if a DV36 will fit where a DV20 has sat unless the engine compartment is cavernous; the extra cylinder adds a lot of length and some of the auxiliaries and plumbing are arranged differently. It's also a lot heavier than the already weighty DV20. The main benefit of upgrading to a DV24 or 29 is they are current models, but the DV20 is still well supported for spares.

 

[neilf39]

The Dv24 etc are very heavy. I replaced my dv20 with a beta 25. It is 90kg lighter than a dv20. I went 25 hp so I had power in the solent chop to power through it. I went for a bigger finer pitched prop for the same reason. At 2500 revs I do 6.5kn in a flat sea. It's worth talking to people like beta and darglow.

 

[fredrussell]

There is one advantage to bigger engines, or at least ones with more cylinders, and that’s the smoother operation an extra cylinder or two adds. I sailed on a mate’s 1GM-engined Folkboat the other day; that engine could not be said to purr like a kitten!

 

[scottie]

Does the current engine drive the boat at hull speed with a reserve of power to cope with head wind and chop if so as you say why go bigger except that you can however if you are struggling with the existing power then yes consider increasing hip but remember that changing the power will require a new propeller to absorb it or you are back where you started

A lot of boats were fitted with excess HP because that’s what the engine builders sizes were

Bukh engines are probably the last of specifically designed relatively low rev marine engines rather than marinised automotive or industrial units and are different in having heavy front located flywheels raw water cooling options and perhaps hand starting possibly and are expected to last as long as diesel is available and are rebuildable all of which comes at a price in size weight and cost whilst yanmar volvo beta etc offer light weight smaller size lower cost etc and the boating industry has chosen the latter
When faced with engine out expenses it is the time to look at all the options including what power you require .

Neilf39 went with smaller lighter more HP and it sounds the right choice

 

[Stemar]

There ain't no substitute for horses - up to a point. To my mind, that point comes when you can maintain hull speed against as much wind and wave as you're likely to want to go that fast in, with a little in reserve, because running an engine flat out too much isn't good for it.

In my Snapdragon 24, I replaced a dead 8hp engine, in reality, perhaps half that, with 28HP, because I was offered a free VP2003. It took some serious surgery of the companionway steps to get it in but, even adding a dedicated starter battery, it was lighter than the old MD1. It was WAY too much power - flat out was 7kt, a full knot over hull speed, but I never used it except for a few minutes every few months to deglaze the cylinders, but having that reserve did come in handy a few times when I missed the tide coming into Portsmouth Harbour. Would I do it again? For a freebie, absolutely. If I was paying, no. I reckon around 15hp would have been about right.

 

[Bobc]

Bad idea. You will add weight, but more importantly you will have an engine that you aren't running hard and at proper working temperature, due to it being too powerful for the boat. The result being that you shorten the life of the engine and cause problems with the engine coking-up.

 

[Pye_End]

To some extent you need to start at the other end - ie the prop. What is the biggest prop you can/want to swing? What is your hull speed etc. What gearbox will you use? If you cannot transmit extra horsepower and make the boat go faster then it is irrelevant.

If you have 2 engines with the same prop and gearbox ratios, one say a 10 hp the other a 30hp, the boat will go absolutely no faster.

You could pitch a prop up if you have reached your max prop diameter, eg with the intention of more power when the conditions are against you - ie relatively low boat speed with higher rpm, say into a big sea. However, under these conditions what you really need is a bigger diameter prop with a finer pitch to better use of the extra hp.

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.

 

[Bobc]

To some extent you need to start at the other end - ie the prop. What is the biggest prop you can/want to swing? What is your hull speed etc. What gearbox will you use? If you cannot transmit extra horsepower and make the boat go faster then it is irrelevant.

If you have 2 engines with the same prop and gearbox ratios, one say a 10 hp the other a 30hp, the boat will go absolutely no faster.

You could pitch a prop up if you have reached your max prop diameter, eg with the intention of more power when the conditions are against you - ie relatively low boat speed with higher rpm, say into a big sea. However, under these conditions what you really need is a bigger diameter prop with a finer pitch to better use of the extra hp.

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.

Spot on

 

[johnalison]

A lot depends on the type of sailing you do. If you are a weekend cruiser/racer you may well be content with the engine as an auxiliary to get you out of the marina to your sailing grounds, and this is how almost all boats were until the ‘80s or so. Given the choice I generally recommend people choose the larger engine option, such as my friend who chose 27hp rather then 18 for his Bav 34.

I had a friend who owned a Trapper 501 with a pathetic 10hp diesel. After a year or two cruising with us to Normandy and Holland he changed it to a 15hp Yanmar and found it a great improvement, though the engine was a tight squeeze and not ideal. Modern engines are light enough for weight not to be a major factor, so it is a matter of horses for courses.

 

[scottie]

As said earlier it all depends on performance with existing engine set up

 

[Wing Mark]

To some extent you need to start at the other end - ie the prop. What is the biggest prop you can/want to swing? What is your hull speed etc. What gearbox will you use? If you cannot transmit extra horsepower and make the boat go faster then it is irrelevant.

If you have 2 engines with the same prop and gearbox ratios, one say a 10 hp the other a 30hp, the boat will go absolutely no faster.

You could pitch a prop up if you have reached your max prop diameter, eg with the intention of more power when the conditions are against you - ie relatively low boat speed with higher rpm, say into a big sea. However, under these conditions what you really need is a bigger diameter prop with a finer pitch to better use of the extra hp.

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.

I've been looking at 26-28ft boats with engines of varying size.
The flip side of what you say is that a bigger engine can cope with a less efficient folding prop.
It has reserve torque so won't be sensitive to the prop not being the optimum pitch.
It has reserve power for driving into strong wind, chop, towing another boat, dragging the keel through the mud, having weed on the hull etc etc.
With the right prop, you can have a good cruising speed at a quiet low RPM.
The question is how small is 'small' and how big is plenty?

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?

I'm talking about a 80-90s shape of cruiser racer.
The OP may be talking about something more modern with a flatter stern which might be on a different scale.
The LWL could be the whole LOA for a start.
But the online calculators suggest not much real gain in going more than 15HP, so it's hard to see much point in going over 20HP.
Unless it's a hull shape where the online calculators are not using the right assumptions.

The other complication is, do 20 year old '8HP' engines still deliver the same as when they were new?

 

[neilf39]

When I got the bigger prop I went from 14 x 13 2 blade fixed to 15 x 8 3 blade feathering. All part of the engine upgrade. I did it getting boatshow deals a few years ago so much cheaper than otherwise.

 

[ashtead]

There is an interesting article in the recent PBO on reengining with a larger Cross rail diesel but for £20 it seemed a lot. That said our old 10.8 m Bav had a19hp Volvo and I always wished we had opted for a 29hp version -it seems to make most difference to smaller boats I notice when they try to enter Portsmouth harbour against the tide so I guess it rather depends on how important pushing against the tide is for you. Westerly centaurs always seemed to motor well with 25?hp at 26foot .

 

[johnalison]

I've been looking at 26-28ft boats with engines of varying size.
The flip side of what you say is that a bigger engine can cope with a less efficient folding prop.
It has reserve torque so won't be sensitive to the prop not being the optimum pitch.
It has reserve power for driving into strong wind, chop, towing another boat, dragging the keel through the mud, having weed on the hull etc etc.
With the right prop, you can have a good cruising speed at a quiet low RPM.
The question is how small is 'small' and how big is plenty?

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?

I'm talking about a 80-90s shape of cruiser racer.
The OP may be talking about something more modern with a flatter stern which might be on a different scale.
The LWL could be the whole LOA for a start.
But the online calculators suggest not much real gain in going more than 15HP, so it's hard to see much point in going over 20HP.
Unless it's a hull shape where the online calculators are not using the right assumptions.

The other complication is, do 20 year old '8HP' engines still deliver the same as when they were new?

Your 28ft is much the same as my friend with the Trapper, and also my one-time Sadler 29 with an 18hp Volvo. I would take the 7kn with a pinch of salt but you should have a comfortable cruising speed of around 6, while you would probably only be doing 5 and a bit with 8hp in flat water. Modern folding props are pretty good and if you enjoy sailing I would budget for that as well. You will lose a bit of snap when reversing but you soon get used to it, and 1/4-1/2 knot increase in speed when sailing is worth having. It is possible that an increase in engine size will increase fuel consumption, so it is as well to make sure the tank is adequate for your needs, though a change to a modern engine may actually be an improvement.