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Old 27-05-2020, 19:13   #1
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Location: perth australia
Boat: chris craft 55' constelation
Posts: 80
New/different engines

Well the saga with my motors continues.
Refresh. Boat sunk, motors seized, motors removed, partially disassembled, mechanic a scammer, took me for $15k, now has thrown motors into a paddock, so rusted and most probably U/S.
I really don't know whether worthwhile pursuing to get motors back or look for different motors.
They were 320 HP Detroit 8V71's, driving through Allison 2:1 gearboxes with
1 3/4" prop shafts.
Do not wish to change anything under water so what is minimum and maximum HP I can go to get similar performance?
Chris Craft 55' Constellation at 32 ton.
Head height is minimal, with previous header tanks just hitting underside of floor, so can't get anything with a turbo sitting on top.
Plus - and most important - as no monies coming in and all just going out - I can't afford to just go and buy a matching set of motors and gearboxes and will have to get second handies.

Any sensible, constructive advice, much appreciated.



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Old 27-05-2020, 19:27   #2
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Re: New/different engines

Was the boat insured? Engine damage due to sinking boat should be covered.

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Old 27-05-2020, 20:13   #3
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Re: New/different engines

Well the core engines can be buggered but the marinisation equipment will all be ok, its mostly cast iron , inconel or bronze so grab all of that stuff and squirrel it away for when you can find a pair of used automotive 8-71s. The real problem is likely to be the reversing gears, extensive internal corrosion pretty much renders them unrepairable and they are likely to be expensive.
Keep up the search for a pair of used 8V-71s, that’s the key to success with the project
I can be sensible and constructive with the advice but at some point you are going to have to throw money at this project, the challenge is how best to deploy it without wasting time and effort.
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Old 27-05-2020, 20:54   #4
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Re: New/different engines

Two sensible questions and answer already. Thanks.
Boat was insured, but after fighting for 10 months, they gave me a 'good will gesture' of $20k and told me to bugger off!
We had blown a hole in a cast iron muffler and it pumped water into the bilge which when exhaust outlet got below water line - pumps couldn't hold.
Boat a classic 60 year old Chris Craft Constellation - so they said 'fair wear and tear'!
Second set of advice, yes, think what you say about retrieving all bits and pieces, may be the best way to go. I see 6V71's, 8V82's on the market and am wondering how horsepower, torque curve etc all equate?
I thought a pair of volvos, Cummings or Cats. may do the job, as there is more of them around - at better prices.
Saw two Fiats, with boxes, but they were 700Hp and had 3" shafts, so thought they a no go!
I have to try and do this right under water line(now that is a conundrum for me) exhaust prefab not a problem - but have to be careful with all 'extras'
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Old 28-05-2020, 12:43   #5
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Re: New/different engines

In the US, truck 8v71 are easy to come by. As Pete said, the marine stuff should be ok. 6v71s don't last as long as inline.The 4 cycle Detroits are unreliable.

Switching to a different engine means a number of changes in the engineroom. You need the same hp, gear ratio, and torque for the same results.

Cummings and Cats are good engines, but yacht versions less so.
If you're going to switch engine brands, get something common where you are, and make sure parts are available, and what they cost.
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Old 28-05-2020, 13:08   #6
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Re: New/different engines

I'd figure out what engines will physically fit in the space you have, what's not too different in weight, and then from there figure out if one of the engines out there will fit as a power / rpm / gearing combination that stands a chance of working with your current shafts and props.

Being able to keep the shafts is more likely than keeping the props in my mind, but both might be possible. Depending on your struts and cutlass bearings it might be possible up-size to 2" shafts if needed with just a stuffing box and cutlass bearing change (plus the new shafts).

As an example engine change, based on 2100 RPM for the Detroits and a guesstimated from a quick search top speed of 18 kts, a 2:1 trans recommends a 29x28 prop on the Michigan Wheel calculator (assuming 4 blade like a lot of the big Connies of that era). If we swap in a 405hp Cummins QSM11 (heavy duty rating) with a 1.8 ratio trans, the same prop would work (assuming top speed increases to 20 kts). With the 1800 RPM continuous rated 405hp QSM11, a 1.55 ratio trans will use the same 29x28 prop. The shaft loading should be similar enough, just a higher max shaft speed.
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Old 28-05-2020, 19:27   #7
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Re: New/different engines

Two more replies.
Lepke, I thought the old 4 stroke 8v71's an absolute war dog and would go for ever. Yes have heard 6V71's don't last as long - especially if turboed.
Rslifkin. You certainly know your stuff!
We got 16 knots out of the old girl with a clean bottom - but as she is now 60 odd years old, am not looking for any more speed.
Just need torque at low revs - say 1200 - to get her in and out of the pen (dock) without having to come in at full speed.
I had a bow thruster, but lost that when the dock yard dropped her in the water ( absolute pricks) after her being shipped from Seattle to Perth and sitting out of the water - in summer - for over 8 weeks.
I may be able to retrieve my gear boxes - Allison 2:1 so am looking for a motor that will take these.

Cheers and again, thanks for your constructive advice.


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