Volvo duoprop 290: how to tell what size?

skipjack27

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I have a 27 ft flybridge cabin cruiser weighing about 3 tons unloaded. Engine Volvo AQAD41A with duoprop 290 leg with an A2 props.

Had the engine rebuilt recently and the mechanic remarked that the turbocharger was barely cutting in WOT. He thought the boat was underpropped and suggested moving up two sizes. Subsequent cruising has confirmed that despite the "new" engine, we are not even getting up on the plane.

My question is this: it is not at all clear whether we should move up one or two sizes in prop pitch. If we don't move up far enough, obviously we will not get as much extra speed as we should. But if we move up too far: what are the consequences? In other words, if we move up two pitch sizes and that is more than enough to match the engine output, what is the downside? I presume we would overload the engine, leading to overheating, at WOT. So, is it simply a matter of not going to WOT, and keeping the revs just high enough to deliver the rated performance of the boat?

Can anyone explain the pluses and minuses and problems of installing a prop that is too highly pitched for a given diesel engine?

Jeff
 

Scott Danforth

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Re: Downside of props that are too highly pitched?

Re: Downside of props that are too highly pitched?

check with your local prop shop. many have loaner props to try. If you move up too far, the boat will struggle to get up on plane and you will over-load the engines vs the under-load situation you have now. I am guessing your hitting max RPM on the engine?
 

steelespike

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Re: Downside of props that are too highly pitched?

Re: Downside of props that are too highly pitched?

Volvo numbers have allways been a mystery for me you'll have to help me with at least the gear ratio and diameter and pitch of the props.and your prent wot rpm and speed.What rpm is the turbo supposed to cut in?
Is the mechanoc saying the motor doesn't rev enough to get the turtbo to cut in?If so then the motor is over propped,(Too much pitch)
Has this boat performed correctly before with this setup?Has the turbo been serviced?
An under propped boat would tend to over rev.Lowerr pitch numbers raise rpm higher pitch lower rpm.
 

Silvertip

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Re: Downside of props that are too highly pitched?

Re: Downside of props that are too highly pitched?

Something seems amiss here. Either you misunderstood what the mechanic was trying to describe, or he isn't up to speed on turbocharging. Turbo chargers do not have a cut-in rpm because they are always spinning due to the exhaust gas passing through them. The point at which they start creating "boost pressure" (spoling up) is part of the power plant design and perhaps that is what he was trying to describe. Turbochargers also have a relief valve that opens when maximum boost for the system is reached. At that point the excess pressure is bypassed. So -- the only way to detrmine what is or is not happening is to monitor boost pressure and engine rpm. If there is no boost pressure gauge I suggest you add one and learn what it is telling you. An exhaust gas temperature (EGT) gauge is also a valuable instrument on a turbocharged engine. Volvo documentation should have all of these specifications for WOT operation.
 

etracer68

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Re: Downside of props that are too highly pitched?

Re: Downside of props that are too highly pitched?

I 2nd that^^^^^
 

QC

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Re: Downside of props that are too highly pitched?

Re: Downside of props that are too highly pitched?

Not all turbocharged engine have waste gates. And many are on the exhaust side, not the boost side. EGT is not a typically valuable gauge anymore as EGT is relatively fixed if you are flowing the right amount of fuel. Boost pressure is helpful, but if there is a waste gate (cold side or hot side) then they become less valuable as boost is somewhat fixed at the high end.

If the tech was trying to say that boost was low AND you were at or above rated RPM then yes, that would indicate under propped. Boost is a result of fuel flow, if the engine isn't working hard (rated power RPM) then load is probably below available power. Do not confuse WOT RPM range with a gasoline powerplant. It's a little different with a diesel as the governor will limit RPM despite being under propped. This is why boost pressure (MAP) or fuel flow is very important to understand.
 

skipjack27

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Re: Downside of props that are too highly pitched?

Re: Downside of props that are too highly pitched?

Many thanks for the expert replies. Unfortunately, you have all gone far beyond my mechanical understanding. A few clarifications that might help sort things out:

1. There is no governor on the AQAD41A (re QC's comment)

2. Yes, the turbocharger has been very recently serviced at considerable cost (re Steelespike's comment)

3. Yes, even at max revs, there is no sign of the boat getting up on the plane or getting near it's rated max speed (supposed to be 21 knots - we were probably not getting more than 16 knots or so)

4. The AQAD41A is rated for max revs = 3800/3900rpm. On the sea-trial, we were getting in excess of 4000rpm. It's worth noting, though, that the hull and running gear were squeaky clean, only 3 people on board, and not much in the fuel or water tanks. Rather below what would be "normal" load conditions, in other words. The mechanic observed that the turbocharger (just serviced/reconditioned) was barely cutting in at max revs - which is what led him to speculate that the boat must be underpropped.

5. Further to Steelespike's comment: this boat is 27 years old and I strongly suspect that it has undergone progressive de-ratings of prop size to accommodate a progressively deteriorating engine. There have been serious problems with galvanic corrosion of props in the past, and I suspect that a relatively frequent turnover of props has encouraged previous owners to move down in pitch to match the lessening of engine power. Hence my suspicion that, with a newly re-built engine, I should return to a larger pitch size.

6. As with Steelespike, Volvo prop numbers are a mystery to me (and, apparently, to local businesses involved in selling props). Unlike most other props, there is no simple way of determining which size duoprop one has. Indeed, no way at all, according to the local "expert". This creates, to say the least, obvious problems in deciding how far up to move on the pitch ladder. I guess I don't believe that the only way to determine a Volvo duoprop size is to look for the model number stamped on it. I think this would be an interesting topic for a new thread.

I'm seriously grateful for the comments offered so far, and look forward to any further information that people can provide, in the light of what I've been able to say above.

Jeff
 

skipjack27

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Volvo duoprop 290: how to tell what size?

I have a Volvo Penta AQAD41A diesel running a Duoprop 290 leg. The props are the A series: 3 blades in front, and 4 behind. I suspect my boat is underpropped and would like to move up in pitch. But in order to buy a higher pitched set of props, I need to know what the current model number of my props is. The model numbers vary from A0-A10.

I've been told by local "experts" that the only way to determine the model number of my props is to look at the stamp on the props - apparently somewhere on the hub, and perhaps elsewhere. Given that my props are rather eroded and entirely covered in thick antifoul, finding the model number is likely to be a serious challenge.

Is there any way I can take measurements of the prop(s) in order to determine the model number? Surely there must be!

Jeff
 

Scott Danforth

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Re: Volvo duoprop 290: how to tell what size?

the part numbers are stamped on the hub on the mounting faces (not the outside). you have to remove the propellers to read them. based on you hitting 4000 RPM, I would say you are under propped, however your comment on the current props being erroded leads me to think you may just need new props.

either way, it sounds like you will be buying props.
 

Don S

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Re: Volvo duoprop 290: how to tell what size?

1. There is no governor on the AQAD41A (re QC's comment)

Your engine does have a governor, and should be set between 4100 and 4200. It's internal in the fuel injection pump.

You typically see 200 rpm per set size, so, if your A2 props get you 4000 rpm, an A3 set should bring it down to 3800 rpm. But if you are actually hitting the governor, then you have no way of knowing for sure, and it may take an A4 set to bring it into specs.

DO NOT mix props from (as an example) the front prop from an A2 and a rear prop from an A4. The prop pitches are variable on each prop and designed to work as sets. Mixing only confuses the water flow and slows you down.
 

steelespike

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Re: Volvo duoprop 290: how to tell what size?

You have spents thousands of dollars at with this dealer/marina getting your motor and turbo rebuilt it would seem
they would have a certain responsibility to get you setup with the right props.
I would think that Volvo could suggest a prop set for your setup with a description of your boat and typical load.
Also if the speed is supposed to be 21 knots then 21 +max rated rpm + gear ratio+ estimated slip should net approximate prop size.
Of course the duo prop complicates the issue.
 

QC

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Re: Volvo duoprop 290: how to tell what size?

Don is noting what I did, that if you are "hitting the governor", then you could be off by a lot. Hard to tell as you could ram the throttle in neutral and she'll stop at "high idle RPM". So how can RPM determine correct prop if you are above "rated RPM"? Which would also be your target WOT RPM. Be very careful about mixing up gasoline (Otto cycle) engine performance targets, rules of thumb, etc. with diesel.

And, ALL diesels have some type of governor. They have to ;)

Oh, and if you are getting 16 Knots in a 27 footer, you are on plane :)
 
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