Propellor measurment

Jcorstorphine said:
Why don't you recalculate the prop based on (say) 1500 to 1750 engine rpm and the corresponding HP at that rpm. which will be a lot quieter. You might find however that the prop will be too big to fit in the aperture.

I had the same issue fitting a 1.6 litre Ford escort engine (cos I had one) into my 21 ft MS which was overpowered but I used a 1.5:1 reduction box and an over square prop. I run the engine at 1500 rpm with prop at 1000 and get 6.5 knots.
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See previous post.

I also worked backwards using the 17x11" result and this gives 20bhp at 2500rpm for 6.3kts - so a lot more revs, noise & probably fuel consumption.

It depends on what the op's aim is, but does serve to show how over-engining can be used to reduce engine rpm and noise as well as give a reserve for adverse conditions.
 
Thanks for the responses, I would like to be able to reach the hull speed but still have some rpm in reserve for adverse weather etc?
 
Just another point to think about. When a fellow berth holder changed his engine, the company doing the work married his new engine to an existing saildrive with the existing prop. Problem was that the new engine had a far higher tick over speed than the old clunker with the result that he could not keep the engine in gear coming into a berth and had to use a series of bursts and frequently banged into the main pontoon (the chap was in his nineties)

I initially had a similar problem with my engine as prop speed was a bit high due to the 1.5 : 1 reduction but fitting a 25 mm thick flywheel mass dropped the tick over to just over 500.

In your case, if you go for a bigger prop AND the reduction gear is different AND your tick over RPM is higher than your old engine, you might have a similar issue.
 
Jcorstorphine said:
In your case, if you go for a bigger prop AND the reduction gear is different AND your tick over RPM is higher than your old engine, you might have a similar issue.
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Thanks for this, I am hoping it won't be an issue as its matching to the same gearbox and engine with the exception of a Turbo.
 
aquaholic said:
Thanks for this, I am hoping it won't be an issue as its matching to the same gearbox and engine with the exception of a Turbo.
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Therein lies your problem. You have chosen the wrong engine for your boat. you will rarely be able to use enough power to get the engine up to using the turbo because you cannot fit a big enough propeller to absorb the power. Whatever you do -either fitting a steeper pitch prop to try and use some of the power, or running with your current prop and achieving the revs but not loading the engine will be unsatisfactory. The turbo engine was introduced so that the same basic engine could be used in boats 5' longer and displacing 25-30% more than your boat. Was not a success partly because it had some poor design features, particularly the oil cooler and partly because similar hp but naturally aspirateds engines came on the market which were far better in the low stress application in a yacht.

It would be good if you could simply remove the turbo, but not sure that is a straightforward job.
 
aquaholic said:
Thanks for this, I am hoping it won't be an issue as its matching to the same gearbox and engine with the exception of a Turbo.
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I think I'd go along with peteK's advice to try it with the existing prop first. I've a sneaking suspicion your boat may have been over-propped before, and you may find it all works satisfactorily.

Tranona had perhaps been a bit pessimistic about your choice of engine. I had a 2003T in a Hallberg-Rassy which was only a couple of feet longer than yours, and it was fine. It's very re-assuring to have a reserve of oomph when the going gets tough! I believe the prop on my boat was 15" x 16" pitch, 3-bladed. As for the engine not using the turbo, a glance at the power curves for the 2003 and 2003T will show that there's turbo boost even from low revs.

I note from your blog that you swapped the old gearbox on to the replacement engine. It's a bit late to say this, but did you check the splines carefully? If they're showing signs of wear, there's a cushioned drive plate kit which is available from Volvo Penta and is an easy DIY fit.

The two other things to watch for on the 2003T are corrosion on the oil cooler and the turbo feed oil pipe. The oil cooler has an alloy body and can corrode badly, leading to a leak which dumps all the oil in the bilge, followed by the engine stopping (for good). The turbo feed oil pipe is steel and can corrode, leading to pinhole leaks which dump all the oil into the bilge and, well, you know the rest. Just keep a close eye on them.
 
pvb said:
I think I'd go along with peteK's advice to try it with the existing prop first. I've a sneaking suspicion your boat may have been over-propped before, and you may find it all works satisfactorily.

Tranona had perhaps been a bit pessimistic about your choice of engine. I had a 2003T in a Hallberg-Rassy which was only a couple of feet longer than yours, and it was fine. It's very re-assuring to have a reserve of oomph when the going gets tough! I believe the prop on my boat was 15" x 16" pitch, 3-bladed. As for the engine not using the turbo, a glance at the power curves for the 2003 and 2003T will show that there's turbo boost even from low revs.

I note from your blog that you swapped the old gearbox on to the replacement engine. It's a bit late to say this, but did you check the splines carefully? If they're showing signs of wear, there's a cushioned drive plate kit which is available from Volvo Penta and is an easy DIY fit.

The two other things to watch for on the 2003T are corrosion on the oil cooler and the turbo feed oil pipe. The oil cooler has an alloy body and can corrode badly, leading to a leak which dumps all the oil in the bilge, followed by the engine stopping (for good). The turbo feed oil pipe is steel and can corrode, leading to pinhole leaks which dump all the oil into the bilge and, well, you know the rest. Just keep a close eye on them.
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Thanks very much for your advise, I spoke to the engineer who had been servicing the engine for a while and said the points you mentioned had been checked at the last service.....I also had someone look it over as well so fingers crossed. As for the prop, the boat came with a 16x10 as well as a 16x12, do you think it better to try the 16x12 first?
 
pvb said:
I think I'd go along with peteK's advice to try it with the existing prop first. I've a sneaking suspicion your boat may have been over-propped before, and you may find it all works satisfactorily.

Tranona had perhaps been a bit pessimistic about your choice of engine. I had a 2003T in a Hallberg-Rassy which was only a couple of feet longer than yours, and it was fine. It's very re-assuring to have a reserve of oomph when the going gets tough! I believe the prop on my boat was 15" x 16" pitch, 3-bladed. As for the engine not using the turbo, a glance at the power curves for the 2003 and 2003T will show that there's turbo boost even from low revs.
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Your HR displaced 6.7 tons compared with 5.2tons for the Contest and has a waterline nearly 2' longer. Big difference +(28%) - and your boat would have been underpowered with the smaller engine. It was boats like the 352 that prompted Volvo to turbocharge the engine. While the turbo does indeed come in at low revs, in the OPs boat it will simply not demand the power to use the turbo. The whole idea of the turbo was to increase maximum power without increasing the revs, or physical size of the engine.
 
Tranona said:
Your HR displaced 6.7 tons compared with 5.2tons for the Contest and has a waterline nearly 2' longer. Big difference +(28%) - and your boat would have been underpowered with the smaller engine. It was boats like the 352 that prompted Volvo to turbocharge the engine. While the turbo does indeed come in at low revs, in the OPs boat it will simply not demand the power to use the turbo. The whole idea of the turbo was to increase maximum power without increasing the revs, or physical size of the engine.
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Once a boat is moving, differences in displacement don't greatly affect the dynamics, although waterline length does. The 2003T's turbo operates at all revs, so you can't really say it won't demand the power to use the turbo.
 
pvb said:
Once a boat is moving, differences in displacement don't greatly affect the dynamics, although waterline length does. The 2003T's turbo operates at all revs, so you can't really say it won't demand the power to use the turbo.
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Displacement is really important for sizing the engine. While at low speed very little power is needed, once you get close to hull speed the combination of displacement, wave making and windage require significant increases in power to increase speed through the water. So once you get above , say 5 knots the power requirement for a shorter lighter, boat is substantially less. Your 352 has 25% more displacement and a 0.5 knot higher speed potential plus greater windage. So needs much more power to reach hull speed. The turbo does its work when the power requirement is higher than the power required for the shorter lighter boat. Even if he keeps the original prop the engine will never develop full power, and the correct size prop probably won't either as the hull speed will be achieved at only 2/3 maximum power. So trying to apply more revs will just result in the engine bogging down by being overfuelled.
 
Tranona said:
Therein lies your problem. You have chosen the wrong engine for your boat. you will rarely be able to use enough power to get the engine up to using the turbo because you cannot fit a big enough propeller to absorb the power. Whatever you do -either fitting a steeper pitch prop to try and use some of the power, or running with your current prop and achieving the revs but not loading the engine will be unsatisfactory. The turbo engine was introduced so that the same basic engine could be used in boats 5' longer and displacing 25-30% more than your boat. Was not a success partly because it had some poor design features, particularly the oil cooler and partly because similar hp but naturally aspirateds engines came on the market which were far better in the low stress application in a yacht.

It would be good if you could simply remove the turbo, but not sure that is a straightforward job.
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The turbo can be disabled by holding the wastegate open with piece of wire: there is no need to remove it.
In any case, if the prop is changed for a significantly larger one (dia & pitch) so the engine is being used to produce hull speed at much lower rpm - ie its working harder, the turbo will be needed and indeed, will give improved fuel efficiency.

The op has a spare 16x 12" prop and this may prove to be a good compromise.

In the meantime I recommend he contact a prop manufacturer such as T Norris for their advice, being sure to explain what is required from the new engine (presumably hull speed using far fewer revs than with the previous engine).
 
Tranona said:
Your HR displaced 6.7 tons compared with 5.2tons for the Contest and has a waterline nearly 2' longer. Big difference +(28%) - and your boat would have been underpowered with the smaller engine. It was boats like the 352 that prompted Volvo to turbocharge the engine. While the turbo does indeed come in at low revs, in the OPs boat it will simply not demand the power to use the turbo. The whole idea of the turbo was to increase maximum power without increasing the revs, or physical size of the engine.
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The turbo uses waste exhaust energy so will always produce some boost, the amount depending on engine load. If he props it 'by the book' - full rpm for hull speed using the existing prop, the load will be much less at cruising speed (about 16hp for 6kts) so the boost pressure will be less, but not zero. (Revving any turbo diesel out of gear will always produce some boost).

Modest over propping will increase the load at all rpms so the turbo will be producing more boost - this is good - it maximises fuel efficiency and minimises soot production. There is no requirement to use the maximum possible boost pressure - it is what it is for any given load just the same as on a motor vehicle.
 
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Tranona said:
Displacement is really important for sizing the engine. While at low speed very little power is needed, once you get close to hull speed the combination of displacement, wave making and windage require significant increases in power to increase speed through the water. So once you get above , say 5 knots the power requirement for a shorter lighter, boat is substantially less. Your 352 has 25% more displacement and a 0.5 knot higher speed potential plus greater windage. So needs much more power to reach hull speed. The turbo does its work when the power requirement is higher than the power required for the shorter lighter boat. Even if he keeps the original prop the engine will never develop full power, and the correct size prop probably won't either as the hull speed will be achieved at only 2/3 maximum power. So trying to apply more revs will just result in the engine bogging down by being overfuelled.
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Hull speed is not a brick wall.
Apply more power, the boat will go a bit faster and make bigger waves.
Many people are running engines and never reaching full power.
Much as I quite like having a car which only sees full power accelerating up steep hills, I'd be happy with a boat where the engine only saw peak power punchng into a gale.
YMMV.
 
lw395 said:
Hull speed is not a brick wall.
Apply more power, the boat will go a bit faster and make bigger waves.
Many people are running engines and never reaching full power.
Much as I quite like having a car which only sees full power accelerating up steep hills, I'd be happy with a boat where the engine only saw peak power punchng into a gale.
YMMV.
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That is correct, except that hull speed on the OPs boat is achieved with less than 2/3 of the available power, but the extra power is insufficient to overcome the extra resistance so trying to increase revs to use that power will not work, unless it has a smaller prop that will allow the revs - but not demand the extra power.

As I suggested earlier, he has a choice of a smaller prop so that the boat achieves hull speed at close to maximum revs, but not demanding full power, or fitting the right size prop and accepting that the engine will never exceed about 2/3 revs. Either choice will draw the same amount of power, but cruising speed will be a different RPM. The downside of the second is that a prop of the right diameter may not fit so the extra size will need to come from steeper pitch and the boat will be a nightmare at low speeds. Hence my suggestion of going with the small prop as the lesser of two evils.
 
pvb said:
In case this confuses the OP, there's no wastegate on a 2003T engine.
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Thanks, I didn't expect to cause such debate, although its great to see there is so much knowledge around!

Once in the water I will try both props and report back.
 
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