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Old 10-01-2023, 02:40 PM   #1
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Cruise speed and fuel consumption

Good afternoon everybody. Iím eyeing a 78 29í Prairie Trawler thatís fairly local to me. I love the small cozy layout. The seller said the 50 hp Perkins was recently rebuilt. What kind of cruising speed and fuel Milage do these pretty little trawlers get?
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Old 10-01-2023, 03:18 PM   #2
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1/2 gph @ 5 knots
1 gph @ 6 knots
2 gph @ 7 knots

Or close to that.
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Old 10-01-2023, 03:30 PM   #3
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That size makes 7 knots beyond displacement speed. I am skeptical that 50 hp is enough to push it faster than displacement speed.
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Old 10-01-2023, 04:19 PM   #4
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Quote:
Originally Posted by Jeff F View Post
1/2 gph @ 5 knots
1 gph @ 6 knots
2 gph @ 7 knots

Or close to that.
1/2 gph @ 5 knots
1 gph @ 6 knots
2 gph @ 6.7 knots

Better?

I owned a Willard 30 with a Perkins 4.108 50hp. I think Jeff F's numbers are pretty close. At displacement, a 29-footer will run 6 to 6-1/4 kts unless running WOT.

Peter
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Old 10-01-2023, 04:33 PM   #5
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It was fine before. I was really just trying to say don't expect to cruise at 7 knots without pushing the engine to its limits, and even then it may not be possible.
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Old 10-01-2023, 05:07 PM   #6
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Why would one expect that they will be able to run that boat more than hull speed? But below hull speed it should provide exceptional fuel economy.
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Old 10-01-2023, 06:20 PM   #7
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If you push the Perkins to 50hp you will burn 2.5 gallons an hour. As others have said your speed probably won’t reach 7kts.
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Old 10-01-2023, 07:49 PM   #8
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If semi displacement and very close to the planing end of that design... why would 7 knots be so inconceivable at near full throttle?

The pics I have seen of a Prarie 29 hull look like they have pretty flat aft sections.
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Old 10-01-2023, 08:24 PM   #9
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Quote:
Originally Posted by mvweebles View Post
1/2 gph @ 5 knots
1 gph @ 6 knots
2 gph @ 6.7 knots

Better?

I owned a Willard 30 with a Perkins 4.108 50hp. I think Jeff F's numbers are pretty close. At displacement, a 29-footer will run 6 to 6-1/4 kts unless running WOT.

Peter
Very close to my W30 experience.
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Old 10-01-2023, 08:41 PM   #10
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If semi displacement and very close to the planing end of that design... why would 7 knots be so inconceivable at near full throttle?

The pics I have seen of a Prarie 29 hull look like they have pretty flat aft sections.
May have a lot to do w their beam.
But Iíd guess the Prairie could be close to 10 tons.
Could be lower too.

Iím not even sure my W30 will reach 7 knots. Very close tho. But I was a bit shy of power at 37hp. And w the raked stem of the bow WLL is only 27.5í.
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Old 10-01-2023, 09:21 PM   #11
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If semi displacement and very close to the planing end of that design... why would 7 knots be so inconceivable at near full throttle?

The pics I have seen of a Prarie 29 hull look like they have pretty flat aft sections.
My intuition is that 50 hp (best case, engine may now make less) is not sufficient for that boat to get on plane. One of my first boats was underpowered but semi-displacement. If everything wasn't just right, it wouldn't be able to get on plane, but once it did, the engine could get a few extra knots. But, if the tanks were full, or I had too many people on board, or heading into wind, or chop, it would get stuck trying to get on plane (even though it was making full rpm.
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Old 10-02-2023, 01:00 AM   #12
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Originally Posted by Nomad Willy View Post
May have a lot to do w their beam.
But Iíd guess the Prairie could be close to 10 tons.
Could be lower too.

Iím not even sure my W30 will reach 7 knots. Very close tho. But I was a bit shy of power at 37hp. And w the raked stem of the bow WLL is only 27.5í.
My previous Fales 30 Boomarang (Willard 30 clone) was powered by a fresh 56hp Yanmar with a well-tuned and matched 3-blade prop. Lightly loaded with a clean bottom it was capable of 7.2 knots. This was observed over a timed, 2-way measured nautical mile and was repeatable. The bow rise was dramatic, and wake pronounced. My normal cruise was ~6.75 knots. All of my dock mates had much faster powerboats and really resented me when I told them how much diesel I used. One used to joke that I stopped every month at the fuel docks to drain my excess fuel. He said my boat 'made gas'!
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Old 10-02-2023, 10:19 AM   #13
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Quote:
Originally Posted by mvweebles View Post
1/2 gph @ 5 knots
1 gph @ 6 knots
2 gph @ 6.7 knots

Better?

I owned a Willard 30 with a Perkins 4.108 50hp. I think Jeff F's numbers are pretty close. At displacement, a 29-footer will run 6 to 6-1/4 kts unless running WOT.

Peter
5kts all day @ 1/2 gal/hr. Anything else is not reasonable. This was our Willard Vega ops @ 2200rpm. Anything more was really uncomfortable ride and noise wise.
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Old 10-02-2023, 11:06 AM   #14
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Tung Hwa 30. Similar hull with a Lehman 80 hp. Just finished a two month cruise of +700 miles, boat loaded to the gills, mostly at 6.4 knots. 1.4 gallons per hour including our tiny diesel heater. Top speed according to the Garmin was 11 knots, but that was current pushing in a passage. We tend to cruise between 1,600 and 1,800 (about 40 hp).
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