Not Reaching Max RPM

[Fire99]

Hi gang, this may or may not be a simple problem but I think it's always best to ask..

My Trader runs twin Cat 3126B (Heui) engines rated at 450hp (yes the max for these) and a stated 2800 rpm at WOT.
The boat had new shafts and propellers before I purchased the boat and they were sourced from the Taiwan manufacturer.. Don't ask me why..

Anyway ever since I've had the boat, both engines have maxxed out at 2600 rpm with a clean bottom. Both nigh on identical. Once the engines were hot I ran the engines without load and they will exceed the max load rpm so they are not config restricted from what I can see.

So my guess, with both engines being nigh on identical, the propellers were over-size/ over-pitched? Would that be a fair assessment?

Thanks folks.

 

[Tranona]

Yes sounds like overpropping but that may be deliberate if the boat is overpowered. Perhaps get somebody like Clements Engineering or Hamble Propellers to run the numbers for you.

 

[Blue Seas]

Do a simple test with a digital tachometer first (cheap on amazon +/- £25) and a 5 min job. I did this last week (on a Trader coincidentally) and both engines were again reading 200rpm under - when tested they were actually reaching correct WOT.
While you're at it, do a double check (at sea) on the prop shafts and use the gearbox ratio to confirm.
Cheap and easy first.

 

[jrudge]

In essence yes. Assuming that the props are also clean ( you say th bottom is clean ) and also how clean is clean.

The other outside case could be weight if there was any chance of a large volume of hidden trapped water for example. Not that likely

Can you ask the manufacturer of their view ?

Finally electronics. Don't underestimate the extent to which the electronics control the engines and if one is low revving it can restrict the other.

 

[Fire99]

Thanks guys.. She ran 2600rpm when I sea trialed her and she literally was lifted in just before we ran the trial. She was clean..
Weight-wise she is as the book and the water tank was near empty so thats 1 tonne of water missing. I'm running Caterpillar's digital Marine Power Displays and checking ET etc all the figures seem exact. 688 idle rpm specified and exactly that displayed. The engines have a sync function where you can run both from a single throttle. I've tested the engines with it disengaged and then controlled by port and stbd respectively and I get pretty much the same results every time. The displays show the correct max (unloaded) rpm specified in Cat's Electronic Technician..

I was thinking contacting the likes of Clements Engineering and taking some measurements at the next lift. Since she had new props I was thinking that may be my first port of call (excuse the pun)

 

[MapisM]

The displays show the correct max (unloaded) rpm specified in Cat's Electronic Technician.

That's definitely good news, and supports your initial theory that the boat is overpropped.
BUT, that may well be deliberate, as Tranona suggested, and not uncommon at all in boats which typically cruise way under the max rated engine rpm.

Since you mention that you've got the MPDs, in one of their pages you can check the lifetime average load (as well as fuel burn, and other stats).
In a Trader, I bet you will find that's nowhere near the 80% that is considered optimal for those engines in the E rating version, which is what you've got.
Which per se is no big deal - I actually wouldn't want to run those puppies at high load for any longer than the few minutes it takes to test them!
But the point is, if you would shorten the props just as much as needed to reach 2800 at WOT, at your optimal cruise speed (which mostly depends on the hull, rather than the powerplant), you would run at a slightly higher RPM but with a slightly lower load.
And if your engines are already being running underloaded (which is just my assumption, but if you check that on your MPDs I'd be curious to hear your findings), that ain't a good thing.
In a boat like yours, I for one would privilege the optimal setup for cruising speed over the "religious" rule of propping for max rated RPM, any day of the week!

BTW, we are talking of 200rpm missing out of 2800, and in engines which are what, 30yo?
I'm not sure I'd bother shortening the props even if we were talking of a planning boat, where such rule makes more sense in order to avoid running overloaded, which is definitely worse than the other way round.

 

[MapisM]

Do a simple test with a digital tachometer first (cheap on amazon +/- £25) and a 5 min job.

Wise suggestion, on any mechanical engines with the analogue VDO tach of old.
But I the reading the OP can get on his digital displays is razor sharp.

 

[Fire99]

That's definitely good news, and supports your initial theory that the boat is overpropped.
BUT, that may well be deliberate, as Tranona suggested, and not uncommon at all in boats which typically cruise way under the max rated engine rpm.

Since you mention that you've got the MPDs, in one of their pages you can check the lifetime average load (as well as fuel burn, and other stats).
In a Trader, I bet you will find that's nowhere near the 80% that is considered optimal for those engines in the E rating version, which is what you've got.
Which per se is no big deal - I actually wouldn't want to run those puppies at high load for any longer than the few minutes it takes to test them!
But the point is, if you would shorten the props just as much as needed to reach 2800 at WOT, at your optimal cruise speed (which mostly depends on the hull, rather than the powerplant), you would run at a slightly higher RPM but with a slightly lower load.
And if your engines are already being running underloaded (which is just my assumption, but if you check that on your MPDs I'd be curious to hear your findings), that ain't a good thing.
In a boat like yours, I for one would privilege the optimal setup for cruising speed over the "religious" rule of propping for max rated RPM, any day of the week!

BTW, we are talking of 200rpm missing out of 2800, and in engines which are what, 30yo?
I'm not sure I'd bother shortening the props even if we were talking of a planning boat, where such rule makes more sense in order to avoid running overloaded, which is definitely worse than the other way round.

Thanks mate.. Thats really helpful. I've owned the boat for getting towards 5 years now, (As a matter of interest the boat was launched in 2002.. A press boat would you believe.. So externally she is on a 2002 Trader brochure which I have) and she has around 1400 hours on her, (I'm responsible for around 500hours). She had a bit of a chequered history for a few years before my ownership (including a new engine block due to a crack during my survey / purchase) so historical load figures wouldn't be too reliable.

My general cruising would be 10-14kts which has the boat settled and not too roly-poly.

The only part I would probably disagree with would be it being deliberate to over-prop the vessel. Judging by saga of when the props/shafts were replaced, I'm amazed they are shafts for a boat and not an egg whisk!

The E rating is interesting.. I'm a bit of a tech nerd so find these things interesting. Very interesting when you read the Caterpillar technical docs the difference in ratings with lower powered 3126B's being continuous rated and when you get up to the 450's expected use is just 250 to 1000 hours a year.

(Attached is the press pic!)

 

[Blue Seas]

Wise suggestion, on any mechanical engines with the analogue VDO tach of old.
But I the reading the OP can get on his digital displays is razor sharp.

100% - I (mistakenly) assumed mechanical controls due to age of boat.

 

[Fire99]

100% - I (mistakenly) assumed mechanical controls due to age of boat.

Ouch.. She's almost brand new at 24 years old

 

[Blue Seas]

Ouch.. She's almost brand new at 24 years old

As well matured as any fine wine

 

[Fire99]

As well matured as any fine wine

That's the way I'm going to look at it.. Or as Michael Sarrazin said in 'The Gumball Rally', "you know some things get meaner as they get older."