What's your cruising speed.

7.5 knots at 2400 rpm, depending on the current. About 2.5 GPH.

7.5 knots at 2000 RPM's.....Seems to be the happy spot with my 200hp Volvo/boat. Burns about 2.2 GPH at that number..

By "Happy Spot" I mean she's not digging a big hole in the water at the stern, "Feels" pretty efficient at that pace, and the Fuel usage is not killing me.

My typical cruise is 1060 rpm, 8.5 knots, 7 gph. 1200 rpm, 10 knots, 10 gph. Going faster is very inefficient: 2100 rpm 17 knot, about 50 gph. Going slower can greatly increase efficiency -- 600 rpm, 4 gph, 6.5 knots.

We are typically cruising between 7.0 and 7.5 knots with our AMS 40', running both FL120 between 1400 and 1550/1600 rpm, burning 1.3-1.6 l/nm of fuel.
Never had problems with the FL120s not to reach reasonable temperatures at 1400 rpm as reported by Marin (only at significant lower rpm as it is necessary for some inland channel passages in Europe).
On the high end we go up to 8.2 knots / 1800 rpm / 2.2 l/nm. Since fuel is in Europe still very expensive (up to 1.8 Euros/Liter) we don't like this, so only if it's really a must ...


best regards / med venlig hilsen
wadden

Just did a 300NM cruise to the Keys. 7.5 Knots @ 1750 RPM while getting down to Biscayne Bay. Then I pulled back to 6.5 Knots @ 1500 RPM and enjoyed the scenery......1.75 Gal. per hour overall total running time. I enjoy going slow. I am not in a hurry to get anywhere while cruising on my boat. It is the journey for me not the destination. If I am in a hurry to get somewhere I'll drive the 911.

1800 RPM.....7.0 knots

Since I first posted on the thread years ago, much has changed. I stated I cruised at 2300 RPM at 8Kts, which I thought to be the true RPM. Then a little later I decided to use a lower cruise RPM of 2000-2100 to reduce the cabin noise and got about 7 kts, give or take...probably closer to 7.5 kts. In May 2012, I had both props tuned by PropScan which improved my speed very slightly and reduced the vibrations significantly.

My Perkins 4.236 tachs receive alternator inputs and my port tach has always been jumpy, so I always relied on my steady stbd tach to set my power, then sync the port side to match. I replaced my stbd alternator last summer with a Balmar alt and found my tach reading changed substantially. Now I have verified the tach readings with a photo tach and found the stbd tach to be accurate within 40 RPM at all speeds. When the port side settles down (dependent on charging status), it matches the stbd tach very closely.

Baker, Edelweiss and Dougcole were right to question my numbers. As it turn out, they were all wrong! Although I thought I was cruising at 2300 RPM, I was really closer to 2100 RPM. My old 2000-2100 RPM cruise RPM is really 1800 RPM.

Here's what I know now...

WOT RPM on my engines is listed at 2800 RPM in the book, but both high pressure fuel pumps are placarded at 2600 RPM. My WOT tach reading is approx 2500 RPM, so I might be slightly overpropped. My cruise power sweet spot is now 1800 RPM (formerly 20-2100 RPM but the same throttle position and sound level) at about 7.5 kts. My average fuel consumption between top-offs is 3.2 GPH. This gives me 2.3 NMPG.

If I could tolerate a slower cruise speed, I could probably achieve a 3.0 NMPG cruise at 6-ish knots. To me, that's not something I'd be willing to try to save 0.5 gal / $2 per hour.

There's always a couple factors...budget for fuel per year and are you a once around the harbor type or the 3000+ NM per year type.

Some are forced into speed ranges based on their finances and cruising miles.

So stating facts are great....comments on what you can and can't tolerate is fluff...unless you ask a speedster to cruise with you...

Sure I'd like to outrun thunderstorms and leap tall buildings in a single bound....big deal if the only working package I cruise in can't do it.

Twin Detroit 8.2L's normal cruise @ 2000rpm = 8-9 kts 10gph. Goes up rom there. High cruise is 23-2400 rpm. 19-21kts; not worried about the fuel at that point

FlyWright said:

... My cruise power sweet spot is now 1800 RPM (formerly 20-2100 RPM but the same throttle position and sound level) at about 7.5 kts. ....

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My sweet spot too, but with only half as many engines, that gets me 6.3 knots.

chc said:

Twin Detroit 8.2L's normal cruise @ 2000rpm = 8-9 kts 10gph. Goes up rom there. High cruise is 23-2400 rpm. 19-21kts; not worried about the fuel at that point

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But it sure was fun to get your 34 LRC up to WOT!! Here's a view looking aft at the fastest I've gone on CHC's 34 Cal! I think it was 23 Kts WOT, right Chris?

Moonstruck said:

I cruise at 400-500 off the top. *-450 gives me 27 knots on flat, slack water.

It's really tough being in the minority again.

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Joining you in the minority here Don with similar speed. I'll worry about fuel consumption if we ever get a larger boat and go serious distance cruising. Until then the option of kicking it in the rear and getting where I want to go is nice.

- 4.5 to 5 knots on one engine = 2.75 to 3 nmpg (occasionally utilized in thin canal areas where docks / anchored / moored / small fishing boats abound)

- 6.5 to 7 knots on twins = 1.75 to 2 nmpg (often used in SF Delta - calced hull speed is 7.58 knots)

- 16 to 17 knots on full plane = 1 nmpg (whenever in relatively open water needing/wanting to get some distance covered)

- 21 to 22 knots at WOT = Way Too Much Fuel (used for short bursts when needed)

A single FL120 pushes our Taiwanese Tub at 7.5-8knots @ 1800-1900rpm. We have made it go up to 9- 9.5 knots but that is with a clean bottom and WOT. Also have a 4 blade prop if that makes any difference.

Unlike you other fellows, I have no idea what kind of fuel consumption we get any calculations ever done have been pretty rough.

Somewhere around 7.5 knots...we tend to run at the same rpm, so speed is sometimes faster or slower depending on currents and wind.

I cruise at or just below the 7kt hull speed which is about 2/3 throttle. The boat is overpowered by about 50% so this puts the engine power and speed where Yanmar recommends. Fuel burn is about 1.75gph or 4nmpg. When throttled back to 6.5 kts, the fuel burn is more like 1.25gph or about 5nmpg. My normal cruising grounds are about 25 nm away and I really wish now I had a somewhat faster boat. Having been a sailor most of my life, I thought I would be comfortable at those speeds, but I find that is less and less so.

In contrast, my prior boat was a beautifully restored Donzi 22 Classic which cruised very comfortably at about 45-50 kts, but was much faster. The fuel burn was horrible, but it was stable and fun. It was just a lark though. Maybe that's what wrong with me now, I was seduced by speed. Is there a rehab for this?

LarryM said:

I cruise at or just below the 7kt hull speed which is about 2/3 throttle. The boat is overpowered by about 50% so this puts the engine power and speed where Yanmar recommends. Fuel burn is about 1.75gph or 4nmpg. When throttled back to 6.5 kts, the fuel burn is more like 1.25gph or about 5nmpg. My normal cruising grounds are about 25 nm away and I really wish now I had a somewhat faster boat. Having been a sailor most of my life, I thought I would be comfortable at those speeds, but I find that is less and less so.

In contrast, my prior boat was a beautifully restored Donzi 22 Classic which cruised very comfortably at about 45-50 kts, but was much faster. The fuel burn was horrible, but it was stable and fun. It was just a lark though. Maybe that's what wrong with me now, I was seduced by speed. Is there a rehab for this?

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Get a Tolly 34' tri cabin; does it all - to a good extent. YRMV!

5.5knots at 1850 rpm's and .4 GPH. I think that figures out to about 14 mpg Little Yanmar 4JH3 in it with 150 gallons of fuel. Got to love that! 7 knots is 2650 rpm's and 1 gph, and that's faster than the hull really likes. Engine vibration sets my cruising speed and it doesn't like anything between 1850 and 2650 very much.

LarryM said:

I was seduced by speed. Is there a rehab for this?

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Wifey B: Speed...Maybe a fast boat to play around in or a fast woman to play around....with...I was careful there

High-speed (greater than hull-speed) is the antithesis of a trawler. Here, a knot below hull-speed:

BandB said:

Wifey B: Speed...Maybe a fast boat to play around in or a fast woman to play around....with...I was careful there

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Sounds like a great combination . . . I like the way your mind works

Just saw this in Sarasota. This is what we need for fuel economy. Wonder what the GPH is?




Two 9.9 yamaha 4 strokes. ?

I was once passed by a tow with 15 barges while we were both going upriver.

Twin Ford Lehman 120s, 7 knots, 1400 rpm. 1.8 gph total between both engines, though we round up to 2 for fuel consumption calculations...

River Cruiser said:

I was once passed by a tow with 15 barges while we were both going upriver.

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I got passed by tugs towing logs on my trip up the Inside Passage

AKDoug said:

I got passed by tugs towing logs on my trip up the Inside Passage

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Wifey B: I got passed by a snail and a turtle....and the turtle had a broken leg.

Tugs towing barges seem threatening to me.



(Carquinez Strait)


(northern San Francisco Bay)

Pushed barges seem to be under control.



(Petaluma River/slough)

Depending if

we can get diesel at "reasonable" price for refuelling !

If we navigate at economic speed :

1 engine @ 1500 rpm, speed 7 kts, 0,81lt/1 nm

2 engines @ 1780 rpm, speed 8kts, 1,14lt/ 1nm

And little quicker :

2 engines @ 1850 rpm, speed 9,8 kts, 1,78lt/nm (0,8 edited to 9,8 kts)


Maximum we do , just for this photo 11,6 lts @ 2300 rpm just for the photo

At 1000 RPM, 8.6 knots 4.7gph for 2.1mpg. I usually do 1200 RPM (10 knots) and 10gph for 1 nmpg. Fast cruise is 2200 rpm 21 knots 31.6 gph .77 mpg and WOT is 2625 for 27 knots, 46 gph .68 mpg.

For us, this is the best of both worlds. Most times we are at 10 knots or less.