Replacement engines

Latestarter1 said:
Rob, you are approching this with a sawn off shotgun..............Take a step backwards.

IF you want to consider using existing boxes with QSB at the prime 480 rating please digest the note I sent you. We have moved a million miles from my original suggestion.

MG 507 is 1.5:1, QSB 480 is 3,400 rpm rating, unlike the 425 which is 3,000 rpm, no clue as to your prop diameter but lets say it is 20 inches, to absorb this power will require hugely oversquare wheel with 30 inch pitch which is less than ideal. We have now moved vessel into 46 knot plus zone with cruise speed of around 43 knots. There is now a need to discuss hull strengh and bottom loading calculations. You have another issue, MG507 is great for absorbing torque, however something in my memory bank tells me that unlike sister boxes it has lower input speed capacity, I will look at Twin Disc capacity tables.

This exercise requires careful planning, put the shotgun away and lets start gathering some data.
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I have twin disc table at home, or contact lancing marine they have correct tables for twin disc.

I doubt this boat would do 43 knots safely, no point in over hp surely, make it a sweet project with just a bit more on tap, the Cummins must be lighter than the sabre surely.
 
Latestarter1 said:
Rob, you are approching this with a sawn off shotgun..............Take a step backwards.

IF you want to consider using existing boxes with QSB at the prime 480 rating please digest the note I sent you. We have moved a million miles from my original suggestion.

MG 507 is 1.5:1, QSB 480 is 3,400 rpm rating, unlike the 425 which is 3,000 rpm, no clue as to your prop diameter but lets say it is 20 inches, to absorb this power will require hugely oversquare wheel with 30 inch pitch which is less than ideal. We have now moved vessel into 46 knot plus zone with cruise speed of around 43 knots. There is now a need to discuss hull strengh and bottom loading calculations. You have another issue, MG507 is great for absorbing torque, however something in my memory bank tells me that unlike sister boxes it has lower input speed capacity, I will look at Twin Disc capacity tables.

This exercise requires careful planning, put the shotgun away and lets start gathering some data.
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:) I was actually curious, just to what the differences with gearboxes and the range of ratio does. I have no intention of surface drives or anything else other than whats fitted. just from an informed point of interest.
I looked at the plate on one of mine and it says 1:5something... looked like 1:51:1
now these mean nothing to me, except that I know they must be critical somewhere along the line with input speed/shaft speed. the effect on hull and drive i know nothing.
Sorry if my method of gathering info misconstrued...my sluggish brain cell in overdrive...
just trying to understand the relationship between engine rpm/power ratio, shaft speed etc...
 
Last edited:
I have dug out my Twin Disc capacity tables.

Remember when Volvo D6 quote came into picture I said refer to VolvoPaul...........Mg 507 has a maximum input speed of 3,200 rpm. You will have to stay within the envelope.

Using your existing gearboxes is critical to keeping re-power simple as you have accurate datum to work from, they are great boxes, and have done few hours.

IF you purchased the QSB 480's you would have to have them re-calibrated to 425's, no big deal as I think this power node would be a perfect match for the hull.

Your current Ford Sabre motors have pretty average torque rise with just over 1,050 Nm @ 1850 rpm. The QSB 425 will transform your over the hump performance, at the same rpm as the Ford peak torque you have 100 Nm more and at 2,000 rpm 425 maxes out at 1230 Nm, mid range urge will be transformed.

Moving on to gearbox ratio, think you currently have 2,600 rpm rated speed so with 1.5:1 boxes your shaft speed is 1,733 rpm at WOT, with a 3,000 rpm rated motor same box will give you a still realistic shaft speed of 2,000 rpm. Have not done the calcs as I do not know diameter of your wheels, however suspect prop pitch will be within graunching range, more cash saved.

Remember up to 30 knots prop diameter is better way to transmit power, above 30 knots increasing diameter holds you back and pitch and cupping become the key.
 
Hi latestarter,
I have a few more figures here for your calculations if you would be so kind.
I had been in contact with a chap ex sabre, who gave me the figure below.

"The engines are rated to achieve full power at 2600 revs/min.
Running up to 2700 revs/min at maximum load is not a problem. "

Still not sure on max rpm though. Although i have seen 2700 2800 on the tachos briefly. With what felt like more to come. I will be getting these checked for accuracy, but they have given me no reason not to trust them.

The props are also marked up 22x27.

Do these figures tally with what you had in mind.
Cheers
Rob
 
Wouldn't touch volvos with a barge pole. Too fussy,sensitive and expensive to maintain. Also you will never recover the cost of engine replacement unless you're doing many hundreds of hours a Year. Giving your existing engines a rebuild and maybe (if possible) fitting a turbo might be a better option.
 
Robg71 said:
This is purely theoretical....
It already has turbos, and apparently spares are like rocking horse shi£
I would go with cummins anyway :)
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Makes sense given the options. I have a similar problem with my 1980's Detroit Diesels 6-71 TI's, but at the end of the day they work like clockwork, have no common rail or ECU's to go wrong and I can do the servicing myself. So I live with the smoke, noise and vibration and keep reminding myself how much money I'm saving against a replacement.
 
Robg71 said:
And another point you missed about detroits, they sound hellish :)
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Nah ... they sing !! ( LOUD !!!) . :) .. actually the 6/71 in-line are actually very sweet running... and amongst the better 2 strokes they made. In respect to parts, I have never had any issues with anything from waterpumps to piston parts ... plenty of reconditioned and newly manufactured parts at very reasonable prices...
 
Divemaster1 said:
Nah ... they sing !! ( LOUD !!!) . :) .. actually the 6/71 in-line are actually very sweet running... and amongst the better 2 strokes they made. In respect to parts, I have never had any issues with anything from waterpumps to piston parts ... plenty of reconditioned and newly manufactured parts at very reasonable prices...
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One of the interesting side effects from two purring Detroits is that the harmonics from the exhaust lull you to sleep. Warm sun, clear skies, flat calm and no traffic make for a deadly combination. Last trip I was zonked out for half an hour and missed my waypoint and the harbour we were making for. Wife was no use. She was asleep on the fore deck lounger. Dangerous cruising in the Aegean!
 
Robg71 said:
Hi latestarter,
I have a few more figures here for your calculations if you would be so kind.
I had been in contact with a chap ex sabre, who gave me the figure below.

"The engines are rated to achieve full power at 2600 revs/min.
Running up to 2700 revs/min at maximum load is not a problem. "

Still not sure on max rpm though. Although i have seen 2700 2800 on the tachos briefly. With what felt like more to come. I will be getting these checked for accuracy, but they have given me no reason not to trust them.

The props are also marked up 22x27.

Do these figures tally with what you had in mind.
Cheers
Rob
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Rob,

320l motors had steel main bearing caps and were rated power at 2,650 rpm so guy is correct. Fuel pumps had a pretty tight governor droop so if correctly propped even so I would expect to see 2,800 clean on calibrated tach. Do a high idle test on each engine in turn, expect to see at least 2,850 rpm.

If you are getting around these #'s wait until CAC's cleand and run again, looks like you are in pretty good shape.

Give me accurate weight and I can run prop #'s for you, but life looks pretty good to me.
 
That sounds like good news to me. Thanks very much.
I shall get back with weight.
And let you know how bad air coolers are.
I have confirmation that they have never been cleaned...

Things looking rosey for the year...
 
Robg71 said:
And another point you missed about detroits, they sound hellish :)
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"Sound Hellish" I have 6V53s in my Sunseeker Travado 40 and whilst loud I think they sound terrific, bit like a Merlin!
They are loud, probably a bit thirsty but chep and easy to service, boat makes 32KN with a clean bottom too.
Classic Sunseekers are the way to go !
 
Robg71 said:
I shall clarify hellish.... Awesome :)
It wasnt an insult :)

I looked at travados, when i bought mine, as they have that bit more room...
Lovely boats...
There were a few for sale lastyear
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Yup mine was one of those for sale last year. One had Cats in with many hours, mine only has under 600 hours with Detroits. White, with dark blue and turquoise stripes. Really very pleased with the space, its the beam, she's massive!
 
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