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Old 09-23-2020, 11:43 AM   #1
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Thinking about prop size

We bought a new to us boat and I am thinking it is under propped. Here is why:
The boat is a Pelagic 36 trawler at about 30,000 gross. LOA is 36 beam is 11.6, 4.5 draft.
It has a good running Perkins NA 6.354 Range 4, with low hours. At 2200 RPM I am barely getting g7 knts. When we put the boat in gear or reverse the boat you cannot even feel it. We had another boat with the same engine at about the same weight with a 6.354 and the boat cruised at 7knts at 1850 RPM all day. Any thoughts?
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Old 09-23-2020, 11:46 AM   #2
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What is your rpm at wide open throttle? And it should be phototach rpms as dash tachs are often off. That will tell use how you are propped.

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Old 09-23-2020, 12:37 PM   #3
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Do you know what size prop is on the boat? Agree on WOT RPMs will tell a lot of the story.
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Old 09-23-2020, 12:59 PM   #4
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You also need to know the transmission ratio. 30,000 lbs is kind of heavy for a 36 foot trawler. That’s 4 ton more than my 36.
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Old 09-23-2020, 01:04 PM   #5
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The gear ratio is 1.5:1. The wot is 2800 the tach is verified. Yes it is a very heavy boat. It was built on the hull of a commercial fishing boat. the glass is 1.5" on the bottom and it carries 650 Gallons of fuel. When we had it hauled for survey last year I did not get the existing prop size.
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Old 09-23-2020, 01:10 PM   #6
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You also need to know the transmission ratio. 30,000 lbs is kind of heavy for a 36 foot trawler. That’s 4 ton more than my 36.
If it's 30k dry, I'd agree, it's heavy. If it's 30k loaded, I'd say that's not bad. A Grand Banks 36 is easily in that range and even my planing (not a trawler) 38 footer comes in around 26k loaded (and I'd expect a same size trawler to be heavier than my boat).
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Old 09-23-2020, 01:12 PM   #7
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2800 is over revving the 6.354, most iterations are 2400 WOT. So if your prop is as large as it can be (w/ .15D clearance to the hull) you need to add some pitch. However, I don't think you are going to gain much speed b/c assuming you are on a 33' watrline, you are already at hull speed.
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Old 09-23-2020, 01:17 PM   #8
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Opps you are correct it is the net weight. It came in at 28,000 on the lift with no water and 65% fuel on board. The Perkins is rated at 2800 in the owners manual and on the injection pump.
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Old 09-23-2020, 10:30 PM   #9
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If your boat was surveyed by a decent surveyor, the prop size should be on the survey.
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Old 09-23-2020, 10:33 PM   #10
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If your boat was surveyed by a decent surveyor, the prop size should be on the survey.
Absolutely.
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Old 09-24-2020, 05:53 AM   #11
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"The boat is a Pelagic 36 trawler at about 30,000 gross."

The use of "gross or net " tons scares me as the tonnage carved into the hull is a Volume measurement , not a weight measurement..

"Opps you are correct it is the net weight. It came in at 28,000 on the lift with no water and 65% fuel on board."

A marine ton is 2240 lbs , so your boat would be about 12 -13 tons and require about 35 to 45 hp. ay normal cruse.

Should run under 2 GPH if propped right.
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Old 09-24-2020, 07:24 AM   #12
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Unusual to use a 1.5:1 gear on such a boat. Double check that. I would expect 2:1 ranging up to 3:1. Should be a tag on the gear giving ratio. Can also check it with a phototach.

And read your survey. Should be some discussion of prop. Not all props are stamped with visible numbers, in that case the numbers will not be in the report.

Some surveys have photos of the prop, post them if you have them.

Not really enough info posted so far to do much analysis.
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