What does the team think? Replacing my current propeller.

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My boat is 9.1 metres in length and around 5000 kilograms displacement. It has a beta Marine sail drive of 20 horsepower, which drives a two blade folding propeller. The sail drive has a selva leg rather than a Volvo leg.

At 2000 rpm in flat water, the boat makes about 4.5 knots. Getting it much over five knots is difficult, and needs around 2700 rpm.

I think the engine is under propped. The current propeller is 15 inches diameter and 10 inches pitch. The people who I have spoken to supply these propellers say that should be sufficient. However, it clearly isn't.

The blades are quite easy to replace without having to take the propeller boss off.

What would be the effect of (1) using a larger diameter and (2) a larger pitch? It could hardly slow the boat down. What are the symptoms of being over propped? Could I get away with say a 16 inch diameter 12 inch pitch prop?
 
If the engine is working well you should be getting around 3400 - 3600rpm when flat out in gear. Anything less than this and you are over propped. Just be aware that you tacho may be under or over reading. You can buy an optical tacho for a tenner on ebay if you want to rule this out. You just fix a bit of reflective tape to the crankshaft pulley and point the meter at it. I'm sure someone will be along who can tell you the conversion between 2 and 3 bladed props but if you were looking at a 3 blade with a 2:1 gearbox Beta suggest a 13" x 9"
 
The gearbox gives a 2.2 to 1 reduction. The bigger problem is that I have a selva sail drive leg rather than a Volvo type. Not many companies make propellers with this fitting.
 
The prop is too big. You are losing at least 700 rpm which is the equivalent of 2" or so of pitch. In addition to not getting speed you will do long term damage to the engine because you are not loading it properly.

Having said that, the size you say you have should be about right, but non standard props can give different results from standard fixed props. For example my boat came as standard with a 16*13 fixed, but the folder is 16*11.

My first approach would be to fit the standard fixed prop that Beta recommend and see what that does. Changing folding blades is an expensive business as an experiment.
 
Do Beta have an explanation for the lack of revs and speed? I put your boat data in to Propcalc and it comes out with 14*11 and a speed of 6.8 knots. A 15*10 should achieve similar. What you are getting suggests strongly that it is not a 10" pitch, but more likely 12".
 
Do you know that the engine is in good condition, and its power is not restricted by e.g. choked exhaust elbow, fuel system, injectors, timing etc etc?
Is the hull clean and smooth?
Does the engine re freely when motor saling?
Is there any smoke?
 
There's no real lack in revs: it's just that at 2000 rpm the boat speed is about 4.5 knots, and that seems for too low. I really have to rev it to get 5 knots.

No smoke; hull is clean. it's been like this since new.
 
Tranona said:
The prop is too big. You are losing at least 700 rpm which is the equivalent of 2" or so of pitch. In addition to not getting speed you will do long term damage to the engine because you are not loading it properly.

/QUOTE]

I know the theory, diesels benefit from being driven hard. However I do not believe everyone runs their engine at high revs, I see lots of people pottering along. Many people take a pride in sailing everywhere and simply motoring gently in and out of marinas or from their moorings. Some even take pride in sailing onto their mooring, but I suspect they will have their engines running, slowly, in neutral. There are reports on these threads of lots of issues with diesels but I do not recall the diagnosis being the engine had been run at revs that were too low and as a result there has been 'long term damage'.

I would like to be educated :)

Jonathan
Click to expand...
 
Neeves said:
Tranona said:
The prop is too big. You are losing at least 700 rpm which is the equivalent of 2" or so of pitch. In addition to not getting speed you will do long term damage to the engine because you are not loading it properly.

/QUOTE]

I know the theory, diesels benefit from being driven hard. However I do not believe everyone runs their engine at high revs, I see lots of people pottering along. Many people take a pride in sailing everywhere and simply motoring gently in and out of marinas or from their moorings. Some even take pride in sailing onto their mooring, but I suspect they will have their engines running, slowly, in neutral. There are reports on these threads of lots of issues with diesels but I do not recall the diagnosis being the engine had been run at revs that were too low and as a result there has been 'long term damage'.

I would like to be educated :)

Jonathan
Click to expand...

I guess the thing to remember here is that the engine has a power rating based on using the full rev range. It may well achieve maximum torque at less than maximum revs but ultimately to get maximum power out you must be able to reach maximum revs. The engine will however have a throttle stop so you may be reaching maximum revs without fully loading the prop, if this is the case you are under proped. Without a full set of data showing boat speed V engine speed it is difficult to tell however I would suggest if you can motor into a strong headwind and achieve max RPM easily you are probably under proped. A good prop manufacturer would be well worth approaching for advice. You also need to make sure any trials are conducted with a clean bottom and prop - fouling makes a significant difference to results. A couple of other thoughts, adding diameter is generally better than adding pitch but you must maintain clearance of an inch or 2 between prop and hull for efficiency. Prop calculators may not be able to distinguish between different hull forms and a good propeller supplier will understand this and advise accordingly, I would approach 2 or 3 and listen carefully to what they say, compare notes and then make a decision on what to do. of course you need to ensure you ask the same question of each rather than be swayed by them selling a product so sticking to a simple 2 bladed folding prop.

Food for thought.

Yoda
Click to expand...
 
Tranona said:
Do Beta have an explanation for the lack of revs and speed? I put your boat data in to Propcalc and it comes out with 14*11 and a speed of 6.8 knots. A 15*10 should achieve similar. What you are getting suggests strongly that it is not a 10" pitch, but more likely 12".
Click to expand...

If you do have 15" x 10" and not a 15" x 12" it sounds like you have the right prop or at least very close to it. Before you go changing anything you need to establish that your tacho and log are accurate hence why it's a good idea to go down the optical tacho route. You also need to use a two way GPS run rather than the log.

http://r.ebay.com/lci5El

You should also check out your prop clearances http://www.yanmarhelp.com/i_propclr.htm

If these all check out and you are still not getting close to 3600rpm and 6.8 knots when flat out I suspect it's the engine rather than the prop that's at fault.

By way of reference I have a 4500kg 28 foot long keel and get a Max speed and rpm of 6.8 knots and 3400 rpm with a 3 blade 12" x 10" and 2:1 gearbox (I don't have sufficient tip clearance for anything much larger)
 
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There is a tendency to over dramatise (and getting a new prop is pretty dramatic, or it is for your wallet).

I wonder what maximum revs you get out of gear?

Jonathan
 
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Neeves said:
Tranona said:
The prop is too big. You are losing at least 700 rpm which is the equivalent of 2" or so of pitch. In addition to not getting speed you will do long term damage to the engine because you are not loading it properly.

/QUOTE]

I know the theory, diesels benefit from being driven hard. However I do not believe everyone runs their engine at high revs, I see lots of people pottering along. Many people take a pride in sailing everywhere and simply motoring gently in and out of marinas or from their moorings. Some even take pride in sailing onto their mooring, but I suspect they will have their engines running, slowly, in neutral. There are reports on these threads of lots of issues with diesels but I do not recall the diagnosis being the engine had been run at revs that were too low and as a result there has been 'long term damage'.

I would like to be educated :)

Jonathan
Click to expand...

You do not run the engine at high revs. The aim is to get a comfortable cruising speed at around 70% of maximum power. Engine manufacturers require the prop to permit the engine to achieve close to rated maximum before the installation can be signed off. They usually give a minimum, which in the case of your Volvo would be 3300 out of the maximum of 3600, and on my later Volvo it it 3000 out of 3200.
If the engine has enough power this will enable you to cruise at approx 2400 for your Volvo. at maximum revs, if the engine has the right power will achieve hull speed. On my old boat with a 2030 cruising speed was 5.4 knots at 2400 and maximum 6.9 knots at 3600. This gives a reserve to deal with adverse conditions.

I would expect the OPs boat to achieve similar results with the engine and prop that he has. As he says it has been like that from new it is unlikely that there is anything wrong with the engine. So the first thing to do is check whether it can achieve say 3400 at full throttle in flat water. If it cannot then it is the prop that is too big. If it can, but not achieve the speed (at least 6.5 knots) then the prop is inefficient. The latter might be the case as the speed quoted at 2000 is on the low side.

These tests need to be carried out in flat water, preferably with no tide running, doing runs of say half a mile and recording speed either against a measured distance or using GPS. This is exactly what we did when my new boat was commissioned to get the installation signed off by Volvo - particularly important because a non standard prop is fitted.
Click to expand...
 
Tranona,

Thanks for the education - and I'm impressed by the personal attention to detail :) edit Maybe that should be attention to personal detail!, close edt.

A suggestion is that if the yacht was signed off with the detail as provided - then it was incorrectly signed off?

Jonathan
 
Chaktoura said:
If you do have 15" x 10" and not a 15" x 12" it sounds like you have the right prop or at least very close to it. Before you go changing anything you need to establish that your tacho and log are accurate hence why it's a good idea to go down the optical tacho route. You also need to use a two way GPS run rather than the log.

http://r.ebay.com/lci5El

You should also check out your prop clearances http://www.yanmarhelp.com/i_propclr.htm

If these all check out and you are still not getting close to 3600rpm and 6.8 knots when flat out I suspect it's the engine rather than the prop that's at fault.

By way of reference I have a 4500kg 28 foot long keel and get a Max speed and rpm of 6.8 knots and 3400 rpm with a 3 blade 12" x 10" and 2:1 gearbox (I don't have sufficient tip clearance for anything much larger)
Click to expand...

Has the OP mentioned if the bottom is fouled or not? Obvious question. - sorry just noted - Hull Clean!
 
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