Loss of Top End Revs

[richardbrennan]

My Volvo D1-30, which has done 550 hours, is about 200 rpm down on top end revs; I used to get about 3100, but it's now down to 2900. I suspect the exhaust elbow may be becoming blocked; does this sound a feasible place to start and what else might be the cause? There are no other symptoms and the engine starts immediately. Obviously I don't use full throttle very often, but it is useful on occasions through the entrance to Portsmouth Harbour against a big ebb tide.

 

[Tranona]

Unlikely at those hours to be exhaust elbow unless you habitually run for short periods at low load. My starting point would be fouled prop, fouled bottom or both. Does not take much to knock off a couple of hundred revs.

 

[Dipper]

That's what happened to my Volvo 2001. It would reach full revs in neutral but not under load. Otherwise everything seemed normal. I found the elbow had so much internal corrosion in it that I couldn't even poke my little finger through the hole. I replaced the elbow with a lovely shiny stainless steel one from the USA (via eBay).

Edit: Go for the cheap easy option in post #2 first though!

 

[jac]

unless you habitually run for short periods at low load

I suspect that this is a lot more common now. Engine sizes are larger now and so need less RPM to deliver cruising speed, and how many boats do you see under engine and main, motorsailing to windward with engine at maybe 1200 rpm or running to charge batteries.

 

[CCJ]

My Volvo D1-30, which has done 550 hours, is about 200 rpm down on top end revs; I used to get about 3100, but it's now down to 2900. I suspect the exhaust elbow may be becoming blocked; does this sound a feasible place to start and what else might be the cause? There are no other symptoms and the engine starts immediately. Obviously I don't use full throttle very often, but it is useful on occasions through the entrance to Portsmouth Harbour against a big ebb tide.

when did you last scrub your bottom & prop

 

[matthewriches]

Dirty bum? Can easily kill 6 knots off my mobos top speed!

 

[johnalison]

Dirty bum? Can easily kill 6 knots off my mobos top speed!

One of the many stories that my one-time engineer Steve used to tell in his favour was about a motorboat owner who complained that his mega-horsepower machine wouldn't plane. Steve told him that it was his dirty bottom but the owner was adamant that 500hp or whatever would be sufficient anyway. The point of the story was that the electronics of the engine had detected the additional load and cut the power to avoid causing pollution, so there is some virtue in having an older engine after all.

 

[richardbrennan]

Thanks for the replies.

I should have added that the boat has just been re-launched after antifouling and prop cleaning, this made about 50 rpm difference but I am still low.

 

[Keith 66]

How old is the rubber exhaust hose? It is not unknown for this to collapse internally when old.

 

[seaangler23]

recently serviced? Fuel or air restriction could cause it, any smoke? but probably injection elbow,

 

[Venus1]

How old is the rubber exhaust hose? It is not unknown for this to collapse internally when old.

Could you expand on this please? I also don't reach max revs (admittedly I am slightly over propped but not enough to account for 2800 max on my new D1-30).
There are no symptoms, performance is fine, no smoke etc, but now I'm wondering if my 25 year old exhaust hose might be restricting things and preventing more revs. How would I know if it had "collapsed internally"? Thanks

 

[duncan99210]

The exhaust hose is composed of a number of layers, stiffened by a spiral of steel wire. It's possible for one or more of the inner layers to become detached from the estate of the hose and to collapse into the space in the middle of the hose. Once water and exhaust get behind that collapsed layer, the it will form a partial blockage.

It's difficult to know from external examination if the hose has collapsed: if the exhaust elbow is clear, the prop and boat are clean and there's no fuel supply problem, then it'd be worth removing the hose to check for internal collapse by looking down the hose. Most likely location for any collapse is in the bit between the elbow and water trap/silencer as that's where the gasses are hottest and therefore most likely to cause damage.

 

[RichardS]

The exhaust hose is composed of a number of layers, stiffened by a spiral of steel wire. It's possible for one or more of the inner layers to become detached from the estate of the hose and to collapse into the space in the middle of the hose. Once water and exhaust get behind that collapsed layer, the it will form a partial blockage.

It's difficult to know from external examination if the hose has collapsed: if the exhaust elbow is clear, the prop and boat are clean and there's no fuel supply problem, then it'd be worth removing the hose to check for internal collapse by looking down the hose. Most likely location for any collapse is in the bit between the elbow and water trap/silencer as that's where the gasses are hottest and therefore most likely to cause damage.

I'm sure that this is possible and a couple of people have reported it as happening to them, I think, but I have to say that it seem extremely unlikely unless the outside wall of the hose is also mechanically and visibly crimped or distorted in some way.

My hose is 51mm and the wall thickness is say, 5mm. That leaves roughly 2.5mm of inner hose material to block an aperture which is 20 times that diameter. Unless you had a massive internal flap of rubber coming detached in such a way that it would not be simply blown aside by the exhaust/water flow, I'm struggling to see that such an occurrence is ever likely to happen. However, I do accept that lightning does strike a person sometimes. :ambivalence:

Richard

 

[Tranona]

Could you expand on this please? I also don't reach max revs (admittedly I am slightly over propped but not enough to account for 2800 max on my new D1-30).
There are no symptoms, performance is fine, no smoke etc, but now I'm wondering if my 25 year old exhaust hose might be restricting things and preventing more revs. How would I know if it had "collapsed internally"? Thanks

One inch of pitch or diameter is 250-300 rpm. So taking an inch off will get you back to the acceptable minimum WTO of 3050 rpm for that engine. You would not get your installation signed off for warranty with the current prop, although probably irrelevant now.

If the exhaust hose is collapsed (and it does happen) the effect is usually far more noticeable than losing a couple of hundred rpm.

 

[blan321]

Duncan 99210 is absolutely right. Took my engine and water trap and hose between the two out this winter for fuel tank replacement. Hose looked fine outside. Completely collapsed internally exactly as described.
No idea how common though. My hose likely boat age at 28 yrs old.

 

[malcp]

Although less likely than the above mentioned things, if the air filter hasn't been changed it can also restrict engine performance

 

[richardbrennan]

Just had a call from the engineer and he confirmed that the exhaust elbow is not only bunged up, but is also badly corroded and needs replacing.Unfortunately I don't have time to source a stainless one, so it's the same thing back on again at a cost of £250 for the part alone.

Is it just me, or is this borderline acceptable on an engine that's only done 550 hours?

 

[Tranona]

Just had a call from the engineer and he confirmed that the exhaust elbow is not only bunged up, but is also badly corroded and needs replacing.Unfortunately I don't have time to source a stainless one, so it's the same thing back on again at a cost of £250 for the part alone.

Is it just me, or is this borderline acceptable on an engine that's only done 550 hours?

More likely the way you use it rather than the hours. My last engine a 2030, also known for bunged up and corroded elbows ran 3500 hours and still had the original elbow. However was a charter boat in the med and most of those hours were under load at 2400 rpm.

I now have a D1 30 like yours and probably too powerful for the boat which comes as standard with a D1 20. So very conscious of the need to run it 2200 most of the time and minimise the low speed, low load running. Not easy to do in our mixed sailing conditions and generally low hours each year.

 

[Dipper]

Is it just me, or is this borderline acceptable on an engine that's only done 550 hours?

Thanks for the update.

I would have thought that the age of the elbow has more bearing on the corrosion than engine hours. I don't use my engine much (don't have an hour meter) but it was over 20 years old.

 

[Venus1]

Thank you tranona and others. I will follow up checking, and possible replacement, of my exhaust hose. (My prop is 17x11 on a 38ft 6t cruiser-racer, so I don't think I am grossly overpropped.)
Blan321, was there a difference in revs/performance when you replaced your collapsed one? Cheers.