What max cruising revs?

Our max revs are 3800rpm and we cruise at between 2800-3600rpm depending on the conditions. We happily cruise at 3600rpm in flat calm conditions without worrying about the engine. The boat seems to enjoy the extra revs and planes much happier.
 
CX54WEK said:
Our max revs are 3800rpm and we cruise at between 2800-3600rpm depending on the conditions. We happily cruise at 3600rpm in flat calm conditions without worrying about the engine. The boat seems to enjoy the extra revs and planes much happier.
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So you cruise at 95% of max revs if conditions allow? OK, I thought that would be an issue.
 
MikeJ42 said:
My engines rev to 3500 and I cruise at 3000. So thats 86% of their max revs.

Is that too high for extended runs (over 3hrs)?
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I think it's going to depend very much on the engines.
My old Perkins HT-6354's max out at 2400, and are rated for max rpm for 1 hour with a 15min drop to 22,500 rpm before running flat out again.

I can run continuously at 22,500
 
BGW said:
I think it's going to depend very much on the engines.
My old Perkins HT-6354's max out at 2400, and are rated for max rpm for 1 hour with a 15min drop to 22,500 rpm before running flat out again.

I can run continuously at 22,500
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er..!! i think you need to re-think this one.....
 
MikeJ42 said:
My engines rev to 3500 and I cruise at 3000. So thats 86% of their max revs.

Is that too high for extended runs (over 3hrs)?
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Difficult to generalise as condition and age of engines is a factor, as is hull form, prop size, and weight, but most diesel engines are designed to run happily at 75-80% rpm continuously. Indeed this is their optimum.

We top out at 4000rpm, and tend to cruise well up on the plane at 3000rpm (16kt@75%) for long passages, or 3400 (20kt@85%) if we are pressed for time. On our boat the "sweet" spot is 3000 (75%) and gives good fuel efficiency. For extended displacement cruising we run at 1600rpm (7.5kt@40%). We avoid extended low speed running to avoid glazing.
 
The first place to look is your engine manual which should give information on maximum cruising rpm. I'm not sure whether it's the case with your engines but most Volvo engines in boats I've owned had recommended cruising rpm at max rpm less 200rpm. Max rpm means whatever the boat achieves at the time not theoretical max rpm
 
BGW said:
I think it's going to depend very much on the engines.
My old Perkins HT-6354's max out at 2400, and are rated for max rpm for 1 hour with a 15min drop to 22,500 rpm before running flat out again.

I can run continuously at 22,500
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Christ 22500 revs
What have you got gas turbines!
 
Think you are well within the limits dropping off 500 RPM of max achieved. (think Volvo on the leisure engines state max obtained, less 200 RPM).

As a comparison, we have lower revving engines, derrived from industrial iron lumps (6.9 L @ 1.5 T each), which max out at 2300 RPM. Manufacturers advice is that max fast cruise is 2100, if 2300 is achieved (it is a % load issue)... however, for prolonged runs (that is classed as above 10 hours), they advise of max 1800 RPM.... Our planning runs is therefore spent at about 1850 as hull performs well and engines are running relatively light, which I am happy with..., but we still have the ability to run faster if we need to..
 
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MikeJ42 said:
My engines rev to 3500 and I cruise at 3000. So thats 86% of their max revs.

Is that too high for extended runs (over 3hrs)?
Click to expand...
There is quite a difference between flat out without any loading, and when the boat is in gear, and powering the boat along.

On our D3 160 I'd say not under load, our would rev to close to four thousand - though obviously never tried this. With the throttle right against the stops
and in gear, probably three and a half thousand.

I rarely set my speed above three thousand, but at this or perhaps a hundred or so less it will keep at this for seven or eight hours. Brixham to Poole for example.
 
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