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Old 04-29-2019, 10:38 PM   #1
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CHB 34 cruising speed?

With a 120 FL what is the cruising speed of a chb 34? Can it cruise at 8kts? Iíve heard 7kts up to 11kts.
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Old 04-29-2019, 10:46 PM   #2
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Greetings,
Welcome aboard. We had a MT 34' with a 120 Lehman. The two "sweet spots" were 1250 RPM and 1750 RPM. Can't remember actual speed (7 kn @ 1750 perhaps?). I figured as long as the engine sounded happy it really didn't matter much what the speed was. A knot or two either way doesn't matter to me.
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Old 04-29-2019, 11:04 PM   #3
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When a boat gets this close to FD the good cruising speed range is really quite small. A little under 7 to a bit over 7 is probably about right. However 6.5 wouldn’t be ridiculous and 7.5 would be fine for the trawler skipper only traveling short distances. 8 knots would be done only by those liking a big wake and not concerned about fuel burn.
So there’s not much “range” and as RT says it’s mostly a matter of the engine being at a speed (rpm) that is smoother than other speeds within the range.

Simple enough but higher speed SD hulls will have a wider range and make the choice of cruise speed more complicated. But the CHB is close to FD.
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Old 04-30-2019, 01:48 AM   #4
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Originally Posted by Grandbanks3642 View Post
With a 120 FL what is the cruising speed of a chb 34? Can it cruise at 8kts? Iíve heard 7kts up to 11kts.
Thanks
Depends on the engines. We had a
CHB 34 with twin Volvos, and cruised at 11 knots. With the single diesel, 7-8 sounds about right.
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Old 04-30-2019, 10:47 AM   #5
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I have a Marine Trader 34 which is a very similar hull (if not identical) and it is powered with the Ford Lehman 120. Like RT Firefly said, the sweet spot is right around 1800 rpm and that produces 5 - 9 knots depending on the wind, sea state, and current. on a calm day with no current the 1880 rpms will get you 6.5 - 7 knots. You can push it harder but it yields almost nothing except burning a lot more fuel. If you can live with the speeds I listed then you may be happy with the boat. If not, look for a different boat with a different hull design and power configuration. We love ours - but we aren't getting anywhere quickly.
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Old 04-30-2019, 11:14 AM   #6
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Our CHB 34 with a single 120 FL cruises best at around 1650 to 1700 RPM. If running late I'll push it to 1800 RPM. I have always used the philosophy that If I'm starting to lift the bow out of the water I'm burning more fuel then optimal. At 1650-1700 she runs at about 7 to 7.5 knots.
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Old 04-30-2019, 11:29 AM   #7
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What's the waterline length?

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Old 04-30-2019, 12:43 PM   #8
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Our CHB 34 with a single 120 FL cruises best at around 1650 to 1700 RPM. If running late I'll push it to 1800 RPM. I have always used the philosophy that If I'm starting to lift the bow out of the water I'm burning more fuel then optimal. At 1650-1700 she runs at about 7 to 7.5 knots.
Re the bow rising ..
I usta thing that was hull speed. Far from it. On my Willard the bow starts rising at about 5 to 5.5 knots. That would give excellent economy but my hull speed is 7 knots. I cruise at 6.15.
At 1800rpm I wouldn’t say you’re pushing it .... unless you’re overpropped.
But I agree fully once you start climbing your wave you will be burning more. But every time you increase throttle you burn more. Even w no load at all.
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Old 04-30-2019, 01:15 PM   #9
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I think the LWL on a CHB 34 is right at 30 feet, but don't quote me on that. If my numbers are right that would give a hull speed of 7.34 knots.

My probably misguided thought is that when I start lifting a 20,000 pound boat vertically instead of moving it forward I'm wasting energy that I could be using for moving forward. It is probably just a fluke that the nose on our boat starts climbing up at around that speed.
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Old 04-30-2019, 01:50 PM   #10
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Our CHB 34 with a single 120 FL cruises best at around 1650 to 1700 RPM. If running late I'll push it to 1800 RPM. I have always used the philosophy that If I'm starting to lift the bow out of the water I'm burning more fuel then optimal. At 1650-1700 she runs at about 7 to 7.5 knots.
i must be under propped i have a 38 and i get 6.5 at 1800 or so .
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Old 04-30-2019, 02:00 PM   #11
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I think the LWL on a CHB 34 is right at 30 feet, but don't quote me on that. If my numbers are right that would give a hull speed of 7.34 knots.

That calculation works, although I'd probably label that "maximum theoretical hull speed" versus just plain ol' "hull speed,"

Beebe suggests best fuel economy for that length at about 5.5-6.0 kts -- 1* or 1.1* SQRT(LWL).

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