Crusing vs Max Speed

MonArk

MonArk

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Hi,

I have a pair of old Perkins T6354 engines. The T stands for Turbo. They max rev at about 2250 and the turbo pressure starts to climb at about 1000. I tend to cruise at about 1800 revs which brings in the turbo to about 0.25 (not sure what it is 0.25 of) but have often felt that she would be just as happy at 2000 with the turbo at just over 0.5. My temperature gauges are old (oil pipes raher than electronic) but consistent although one engine always displays as slightly hotter but that doesn't mean it is. Cruising at 2000 will generally lift the temp from a reading of low 80's to about 90 but I haven't cruised at 2000 for a long period.

Does anyone have a pet formula for decidig what is a sensible "cruising speed"? I am not talking about maximum fuel eficiency although that would be interesting but sensible speed for looking after the engines.

Many thanks
Nick
 
seem's like you already no the sensible crusing speed to me monark...
if you have a hot water clarifier fitted that would be why you have a temp difference.
look after the old girl and she will look after you..
 
75% max revs, so you are just a bit over, but if they seem happy thats OK. Fuel consumption will increase dramatically for little increase in speed if you go to 2000+
 
most engines have a fast cruise rating as well as whatever the engines will deliver at WOT!

it varies depending on a number of factors - use the manufactures rating.

obviously all these figures assume that the instalation and loadings are realistic ie propping.

fuel consumption (in terms of mpg) will vary dependent on a huge number of factors eg a marginal installation in a fully planing hull may end up with a linear mpg curve as the reduced drag form increased hull speeds offset the pure engine fuel consumption curve.
 
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I have the HT6354's and they are rated at 2400 WOT, 2250 continuous fast cruising.

Interestingly my manual states they are good for WOT for 60 minutes, then you must drop to 2250 for at least 15 minutes, before resuming WOT again if required.

I have run mine at 2400 for up to an hour and seen an increase in temperature from 70c to 85c but they held steady at that.

Looking at the fuel curves, and the speed at each throttle setting (SD cruiser) she is just as economical at WOT due to better planing speed.

The engines also burn a lot cleaner after a good run :-)

If you are only achieving 2250 you could have weed growth or be over propped.
 
Cruising Speeds

Thanks everyone. I am not sure wot WOT means - optimum something I guess. She hadn't better be over propped because they are only 3 or 4 years old and were spec'd by the prop engineers in collaboration with TT Boat Design (Arthur Mursell) who hold the boat drawings. Not sure about the pitch but we went from 3 blades at about 21" to 4 blades at about 20". She shouldn't be too overgrown being antifouled each year but I have no idea how much growth is significant.

Couldn't that lack of power just be down to the age of the engines or even "tuning"?

Incidentally pics at http://www.flickr.com/photos/redladyofark/4916954329/in/set-72157624645029049/
 
Max cruising

That makes more sense.

The temperatures hold steady once the rev's are up but not sure how valid the absolute reading is. Extrtapolating your manual would suggest I can max for a reasonable period as long as I come down to my notch down which could be as high as 2000 fairly regularly as well and watch the temperature. I probably haven't maxed for more than 5 or 10 minutes so far so a bit longer shouldn't do any harm.
 
That 0.25 may well be bar, where 1 bar = 15psi more or less, so 3.5lb of boost.

The 6354 series were designed as truck engines are are therefore not from the same school as more modern engines designed with lighter duty in fast planing craft in mind. As such they awill probably happily sit at 80% of max revs all day, and higher with some care, but remember: they are relatively old now and want some tlc.

I'm heading down to London over the BH weekend and plan to blow a bit of soot out during the process, its good for a diesel to be given some serious work to do but change revs sensibly, not a slam open of the throttles, the system does not spin up that fast. Likewise when slowing down let the turbos spin down properly.
 
She is a beautiful boat.
And yes, WOT = Wide Open Throttle.

Regarding the max rev's achieved, there could be a number of reasons for not getting as high as they should, but if both are the same, I would be surprised if it was anything other than the hull needing cleaning, or being over propped.
There are a number of 6-354 variants, with power outputs ranging from 120bhp to 180bhp - were your props matched to the correct power rating?
They all had a minimum of 2400rpm, with a couple rated for 2600 and 2800rpm

At the end of the day, unless it is a known hull, there is always room for error with matching props for the engine / boat combo - especially as you have changed the number of blades from standard.

If it was over propped you might expect to see black smoke at WOT.
You really need someone who can confirm the engines are running as designed, then perhaps get the props re-pitched slightly lower?
 
Smoke

Thanks Brian for the further insight. No the smoke is white - plenty of it - but we were asking that question ourselves in the context of what caused the smoke, white was the definite conclusion (well actually a pale grey). Does changing the pitch "slightly" mean changing the props or just grinding them a bit?

Nick
 
MonArk

I should add that she is stuck in Dartmouth at the moment waiting for a weather "window" when we can bring her home. Looks like we'll be there for the regatta but there is some sign of a respite on Sunday/Monday.
 
MonArk said:
Does changing the pitch "slightly" mean changing the props or just grinding them a bit?

Nick
Click to expand...

Hi Nick,
No grinding, just re-twisting the blade to a slightly shallower angle.

Given you do not appear to be overloading the engines (as there is no black smoke) then it may not be worthwhile the expense.

The only reason I could see to change them, was if the boat was able to plane more efficiently at max rpm and you were going to use it like that.
For me 2250rmp = 12 knots, 2400rpm = 16 knots, and for the same fuel consumption per mile. - More fuel per hour, but a proportionately faster speed.
 
Propellor Pitch

That is interesting. I get about 14 knots at 2000 and 17 to 18 at 2250 to 2300 (stbd doesn't quite get to 2300 but port does). I will give that some thought in the run up to the winter. I would like to get that bit extra out of her.

Having said that I am committing the cardinal sin by contemplating selling her with a view to something bigger - older aft cabin Aquastar 38's or Humber 35's. We would love a bit more weight and space as my children's legs now get entangles with everything now they are older. It would be a great wrench.

I greatly appreciate everyone's help.

Regards

Nick
 
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