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Old 01-25-2015, 09:35 AM   #41
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Close reading of over propping threads I think pretty well explains why you don't overpropped a boat that has a much smaller engine repower and needs most of its hp to reach cruising speed....except in a few cases of out of the ordinary diesels.

Replies that discuss over propping engines being used to propel neat their upper limits only confuse the issue.

Over propping is usually only done to properly extract hp from an engine that has good torque lower in its rpm range and is being run way low in those ranges. Whether there is true success with a particular boat, engine, and cruising style is pretty specific and needs to be discussed with those in mind.
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Old 01-25-2015, 10:31 AM   #42
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I, for me do not subscribe to over propping, been there, ain't going there again. The proper gear, the proper prop all to put the engine at the most efficient rpm in its power curve while achieving factory reccomended RPM at wot is for me. Few if any armchair engineers can out do the professionals.
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Old 01-25-2015, 05:04 PM   #43
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Yes, Mule I do have the screw on Driftless set so the engine can reach full rated RPM.

I do understand, however that some folks prefer to "Overprop" so they can reach the necessary horsepower to cruise at a lower RPM than would be necessary if "correctly" propped.

The important thing to understand is that the power the propeller absorbs is proportional to the rpm SQUARED whereas the power developed by the engine is (roughly speaking) directly proportional to the rpm. This means that as we go down in rpm from rated speed, we reach the point where the propeller is using only a small fraction of the power the engine can produce. "Overpropping" means the power needed is delivered at lower rpm.

I suppose this might allow me to cruise at, say, 1300 or 1400 rpm than at 1850 as-now.

I chose not to, mainly because it would eliminate the possibility that I now have to increase speed to 8.5 - 9.0 knots if circumstances require, such as outrunning weather. There might be a small increase in fuel economy, but who cares? (I once came from Portland, Maine to Dighton, Massachusetts, including transiting the Cape Cod Canal and Buzzard's Bay, a distance of 195 miles on sixteen gallons of fuel.) It might reduce the sound somewhat also.

Overall, I'm quite pleased with the engine installation. The Isuzu engine has been quite up to it's reputation. Oddly, this one was manufactured by Yanmar, and private labeled by Isuzu. Go figure...

Carry on, keep calm and Cruise on.

JS
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Old 01-25-2015, 05:21 PM   #44
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Quote:
Originally Posted by Driftless View Post
Yes, Mule I do have the screw on Driftless set so the engine can reach full rated RPM.

I do understand, however that some folks prefer to "Overprop" so they can reach the necessary horsepower to cruise at a lower RPM than would be necessary if "correctly" propped.

The important thing to understand is that the power the propeller absorbs is proportional to the rpm SQUARED whereas the power developed by the engine is (roughly speaking) directly proportional to the rpm. This means that as we go down in rpm from rated speed, we reach the point where the propeller is using only a small fraction of the power the engine can produce. "Overpropping" means the power needed is delivered at lower rpm.

I suppose this might allow me to cruise at, say, 1300 or 1400 rpm than at 1850 as-now.

I chose not to, mainly because it would eliminate the possibility that I now have to increase speed to 8.5 - 9.0 knots if circumstances require, such as outrunning weather. There might be a small increase in fuel economy, but who cares? (I once came from Portland, Maine to Dighton, Massachusetts, including transiting the Cape Cod Canal and Buzzard's Bay, a distance of 195 miles on sixteen gallons of fuel.) It might reduce the sound somewhat also.

Overall, I'm quite pleased with the engine installation. The Isuzu engine has been quite up to it's reputation. Oddly, this one was manufactured by Yanmar, and private labeled by Isuzu. Go figure...

Carry on, keep calm and Cruise on.

JS
I totally get the idea regarding conversion of a industrial rated motor to a
marinized unit the expansion tank/heat exchanger and required oil coolers and raw rater pump is a no brainer.. you mentioned making up a water injection unit for the exhaust but there is no mention of a water cooled exhaust manifold?. What did you do regarding this?
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Old 01-25-2015, 06:33 PM   #45
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I, for me do not subscribe to over propping, been there, ain't going there again. The proper gear, the proper prop all to put the engine at the most efficient rpm in its power curve while achieving factory reccomended RPM at wot is for me. Few if any armchair engineers can out do the professionals.
Just remember many great advances in history were done by armchair engineers....

Orville and Wibur were bicycle guys before inventing controlled flight.
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Old 01-26-2015, 09:31 AM   #46
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IF the engine with a stock prop actually was anywhere near the most efficient RPM in the power curve for cruising there would never be a discussion of fitting a cruising prop.

As long as the engine is over sized for the boat (as most are) the use of a prop for WOT PM will have little in common with a prop for efficient cruising.

Oveproping (not my term, I prefer cruise prop)) and overloading are 2 different concepts.

Running at a proper load at lower RPM will extend the vessels range , lower the engine wear and the noise aboard.

Sadly most new boats are budget builds so a better compromise a CPP is never contemplated .

$$$ 10K to $ 15K to save 10% or even 25% on a 200 hour a year boat will never pay.
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Old 01-26-2015, 10:42 AM   #47
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Just remember many great advances in history were done by armchair engineers....

Orville and Wibur were bicycle guys before inventing controlled flight.
Yep, but propping has kindda been done just about every way possible, and documented. Flying by the Wright Brothers was an unplowed field.
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Old 01-26-2015, 10:49 AM   #48
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IF the engine with a stock prop actually was anywhere near the most efficient RPM in the power curve for cruising there would never be a discussion of fitting a cruising prop.

As long as the engine is over sized for the boat (as most are) the use of a prop for WOT PM will have little in common with a prop for efficient cruising.

Oveproping (not my term, I prefer cruise prop)) and overloading are 2 different concepts.

Running at a proper load at lower RPM will extend the vessels range , lower the engine wear and the noise aboard.

Sadly most new boats are budget builds so a better compromise a CPP is never contemplated .

$$$ 10K to $ 15K to save 10% or even 25% on a 200 hour a year boat will never pay.
Now FF, I defer to your superior knowledge on boats, not politics but boats. Why worry about the specific boat but instead about neutral WOT and WOT underway? My last boat had a 5.9 Cummins. Neutral WOT was 2800 RPM. In gear recommended was. 2600. With various props I went between 2250 and 2550, she was happiest at 2550.
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Old 01-26-2015, 11:03 AM   #49
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Yep, but propping has kindda been done just about every way possible, and documented. Flying by the Wright Brothers was an unplowed field.
I've done a fair few hours ploughing in my time, having grown up on a tillage farm. My favourite tractor was a Ford 6600 with 80hp and a 4 furrow plough.

How did we work out what gear to plough in given that each field had different type soil? Easy! Select the gear that gave you the fastest speed, but would still let you increase revs so that the engine wasn't overloaded. We poughed at about 2100 revs all day long , with wot at 2200.

Surely if you can increase revs on your boat engine to Wot, then by definition you are NOT over-propped? Conversely if you put on more throttle and nothing happens then you ARE definitely over-propped.

Us rednecks ain't as stupid as we look.
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Old 01-27-2015, 09:02 AM   #50
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Red face re-power

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Originally Posted by hollywood8118 View Post
We currently have a 225hp Volvo in our 40' Ocean Alexander.

It started life as a 275hp TAMD70C, have the injectors stepped down to the 225hp rating of the TAMD70 during a complete injector service.

We never used the upper 700 rpm of the range as it sucked fuel like a 66 Cadillac climbing a mountain pass.. we cruise the boat at 1600-1700 rpm making 8.3kts in flat water.. she would do 11kts fire walled.. no need to go that fast.

I have the opportunity to get a 10 year newer motor that is 110hp at a reasonable cost.. I would do all the re-power myself.. no big deal.

My TAMD70c has all new exchangers/coolers.. but has a lot of crap hung all over it in the form of pre heat start system etc.. the 70c motor is known as a bulletproof block.. hard to kill motor.. as in most marine diesels it is the stuff hung on the outside of the block that normally fails.. parts for the thing are very hard to get and wicked expensive.. the injection pump is almost the sive of the motor on my 7.5kw genset!

Its one really bad attribute is it smokes BAD when cold on start up.. but no smoke when warm.

So my long winded question is this...

Repower with the 110hp motor?

Strip off any unnecessary stuff ( aftercooler,cold start system that doesn't work anyway and simplify the tamd70 to a tmd70?

Is 110hp enough for the 40' hull to hit 8kts reasonably?

I am trying to keep this boat reasonable.. plan to take it to Mexico and possibly leave it for a few seasons and winter there..

The 70c motor is a monster.. a bit more room in the E.R. would be nice..

Personally I want to sell off everything and buy a Nordhavn and split .. but the Admiral isn't completely warmed up to full time cruising again just yet..But she loves the Mexico idea.

Thoughts and input from anyone ?

HOLLYWOOD
Hollywood.

Your straigth questings were: Shoud I go for derating my TAMD 70 engine.
OR buy a newer straith MD.70 110 Hp.?

I would take the newer engine 110 hp. have a red.ratio 3.1 Tvin Disc or ZF. marine gear, maybe already on the MD.70 now.?

Recommed the 3:1 reduction, - 600 800 rpm. on prop. for max. trust, OTTER is heavy to get tru Water.

Your engine smoke at start up, so do mine, at idle, 650/700 rpm. rev her up to a 1000/1200 rpm. or where she runs smooth, for better compression
and lesser smoke if any.
After a while when tept.start raising, let her run where she spins well, without smoke, - this unburned gasoil is a nuisance for all aboard.

I fully agree with you about all the exstra hang/up on a TAMD.
since "whats not there will not break" Simple and accessible.

Good luck.
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Old 01-27-2015, 11:33 AM   #51
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Hollywood:

On both of those repowers (with the Isuzu industrial diesels) we used the original "dry" exhaust manifolds, wrapping them with the insulation tape from Hamilton Marine. Works fine. They don't get particluarly hot, even at full power.

I wish the Isuzu 4BG1T were still available - I bet it would do a terrific job for you. When Isuzu first came to the states, a lot of them were sold to fishermen in Maine because they were (at that time) cheap. What they found was that they were also incredibly well made and durable.

My friend's Marine Trader 40 with that engine in it is incredibly quiet - it's as though we're being propelled by an electric motor.

Good Luck! Be calm and cruise on!

JS
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