The Case for Going Slow

I should also add that in my opinion if you're traveling at 1.2 SL and are so obsessed with range that you need calibrated monitoring the obvious solution is to reduce speed by 10% to increase range by something like 30%. Same with trying to optimize prop and stuff like that. It's all playing on the margins compared to a simple speed reduction.

This isn't directed at you, Peter. Just an observation that crosses my mind when discussions of hull speed come up.

RickyD said:

Well again the curved angle of the water line is touching the surface of the water. Do I count the curvature distance or not?

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Tied to a bulkhead or floating finger pier, with a right angle tool laid along the pier, mark the points on the pier where the bow and stern touch the water. Measure the distance between the two and you have waterline length.

Ted

Jeff F said:

I should also add that in my opinion if you're traveling at 1.2 SL and are so obsessed with range that you need calibrated monitoring the obvious solution is to reduce speed by 10% to increase range by something like 30%. Same with trying to optimize prop and stuff like that. It's all playing on the margins compared to a simple speed reduction.

This isn't directed at you, Peter. Just an observation that crosses my mind when discussions of hull speed come up.

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I agree with you Jeff. There's a pretty big fuel efficiency difference (e.g. MPG vs GPH) between S/L 1.32 andS/L 1.2. Circling back to the OP's comment on an article which seem to focus on cost savings of going slow, I think it's more interesting to discuss range and miles-per-day and time savings garnered through bypassing fuel stops that going fast would necessitate.

Peter

Greetings,
Wonderful exchange of banter. I get what everyone is saying. We "trawled" for a wonderful 20+ years (????). Great experience GREAT memories. Numbers and "economies" similar to what has been posted. 1979 Cheoy Lee 46' LRC. Sold (Bittersweet).

The memsaab and I did a 6 day shakedown cruise on our "new" cruising vessel this past spring. Not far (500 miles in total). Best I can remember optimum gas usage was 1.4 MPG @ 24 MPH. Cruising speed 24 to 38 MPH. I don't dare try to remember usage in the upper ranges. One of our best shakedowns with "lists" generated, changes planned and handling/maneuvering experience improved (I hope).

LOTS of gas and several gas stops. Do I care? Not a whit. A ton of fun. NOT a trawler but...

Sister ship:

Jeff F said:

I'd suggest a third option: 4nmpg at 6.5 knots.

A lot of people think that displacement speeds at 1.3 SL is the sweet spot. That's only in comparison to faster speeds.

I've built a chart for my boat (135 hp, 49' WLL) that illustrates the further efficiencies below hull speed. In my case MPG is doubled by reducing speed from 1.32 SL to 1.0 SL. I think that's pretty typical for a SD boat.

SL | l/hr
0.8 | 1.9
0.9 | 2.5
1.0 | 4.2
1.1 | 5.6
1.2 | 7.1
1.3 | 10.5
1.4 | 18.0
1.5 | 26.0

I'm usually happy cruising at < 7 knots (1.0 SL) getting 5+ nmpg.

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Our former boat ( lwl 60' beam 5m and 32t)
at 0.85 (6.6 kts) it was 6nm per gallon
the actual ( lwl 19.30m beam 5.90m weight 58t)
at 0.755 ( 7.05kts) it is 2.43nlm per gallon :-(

RT Firefly said:

Greetings,
Wonderful exchange of banter. I get what everyone is saying. We "trawled" for a wonderful 20+ years (????). Great experience GREAT memories. Numbers and "economies" similar to what has been posted. 1979 Cheoy Lee 46' LRC. Sold (Bittersweet).

The memsaab and I did a 6 day shakedown cruise on our "new" cruising vessel this past spring. Not far (500 miles in total). Best I can remember optimum gas usage was 1.4 MPG @ 24 MPH. Cruising speed 24 to 38 MPH. I don't dare try to remember usage in the upper ranges. One of our best shakedowns with "lists" generated, changes planned and handling/maneuvering experience improved (I hope).

LOTS of gas and several gas stops. Do I care? Not a whit. A ton of fun. NOT a trawler but...

Sister ship: View attachment 155899

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YOU REBEL. Next you will tell us you are replacing the Rocna with a danforth. Glad your plan is working to your satisfaction.

.... removed

I love all these best of conditions numbers. I don't have any of that. I don't even have fuel gauges or sight gages. I have a stick and a tape measure and a rough idea of how much fuel and inch is in each tank. I have four tanks.
I have gone about 3,130 nm and calculated my starting fuel at around 355 gallons. I have put 1,863 gallons in the tanks. The tanks are currently full. That gives about 2.1 nmpg. I ran the generator about 290 hours. This includes all running or idling time. I generally ran about 7 to 7.8 knots with an average of probably 7.2-7.3 knots, S/L ratio about 1.14 to1.25.
This is the bottom line of actual cruising for me with Stabilizers, AC, refrigeration, hot water, cooking, idling anchoring and battery charging.
Do I give a darn? NO, so I might increase speed in the future. The Detroit 4-53N's will thank me for it. My average diesel price paid was 3.38 per gallon US. Currently have 700 usable in the tanks ready for the 5 week trip this summer.

Jeff F said:

This is reported by the ECM, and I did two way tests in calm water.

I don't have a ton of confidence in the absolute value of each reading, but the numbers also align pretty well with the prop demand curve supplied by the manufacturer.

Obviously the values are going to vary by boat, displacement and engine, but I think the curve is probably typical.

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Thanks! I suspect the curve flattens out at a higher l/hr number for my 2x120hp 38'WLL vs your 1x135hp 49'WLL, but it'd be great to see real data. No Flo-scans on our ancient FL engines; I wonder about the max nmpg we could muster; suspect it might be around 6kts, not sure if I could get to 4nmpg tho.

Boatdiesel calcs show ~80hp required for me at 8.5kts vs 35hp at 6.5kts, so certainly possible we could double our nmpg assuming the engines are still able to operate efficiently at under 15% of rated output.

socalrider said:

Thanks! I suspect the curve flattens out at a higher l/hr number for my 2x120hp 38'WLL vs your 1x135hp 49'WLL, but it'd be great to see real data. No Flo-scans on our ancient FL engines; I wonder about the max nmpg we could muster; suspect it might be around 6kts, not sure if I could get to 4nmpg tho.

Boatdiesel calcs show ~80hp required for me at 8.5kts vs 35hp at 6.5kts, so certainly possible we could double our nmpg assuming the engines are still able to operate efficiently at under 15% of rated output.

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That sounds about right to me. I'd expect somewhere around 7 kts is the cliff for you where the speed gain vs drop in nmpg starts to get worse quickly.