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Fuel usage
How do I ballpark the fuel usage for a modified keel 37' ketch 18000# at 5 knots with a Westerbeke 4-108 with a fairly clean bottom? Gordon |
Fuel usage
"Gordon" wrote in message
m... How do I ballpark the fuel usage for a modified keel 37' ketch 18000# at 5 knots with a Westerbeke 4-108 with a fairly clean bottom? Gordon What is the waterline length? -- Roger Long |
Fuel usage
On Tue, 23 Sep 2008 08:19:36 -0700, Gordon wrote:
How do I ballpark the fuel usage for a modified keel 37' ketch 18000# at 5 knots with a Westerbeke 4-108 with a fairly clean bottom? Gordon With flat water, no wind, clean bottom and efficient prop it should be less than 1 gph, maybe as low as .6 or .7, all the way up to 1.5 or more with adverse conditions. |
Fuel usage
Roger Long wrote:
"Gordon" wrote in message m... How do I ballpark the fuel usage for a modified keel 37' ketch 18000# at 5 knots with a Westerbeke 4-108 with a fairly clean bottom? Gordon What is the waterline length? -- Roger Long 30'0" Gordon |
Fuel usage
OK. 30 foot waterline at 5 knots would be a speed length ratio of .91.
My boat should be fairly similar in characteristics, heavy but with a modified keel. Look at the graph towards the middle of this page: http://home.maine.rr.com/rlma/StriderFuelSystem.htm Waterline length of my boat is 26 feet so equivelent speed would be 4.64 knots. She reaches that speed at 1560 RPM with 4 POB. Horsepower at that RPM is only 4.2. Displacement is 13,500 pounds or 6.03 Long Tons so .70 HP/ton. Your displacement is 18,000 or 8.03 tons so you need 5.59 HP to go 5 knots. .056 gallons per HP per hour is typical for most diesels so you would burn .31 GPH. This doesn't sound like much but you are talking about a pretty slow speed. At an economical speed length ratio of 1.15, typical cruising speed for most sailboats, 6.3 knots for your boat, you would be burning a bit over twice that. At a speed length ratio of 1.29, flat out, you would be burning about 4 times that. -- Roger Long |
Fuel usage
Roger Long wrote:
OK. 30 foot waterline at 5 knots would be a speed length ratio of .91. My boat should be fairly similar in characteristics, heavy but with a modified keel. Look at the graph towards the middle of this page: http://home.maine.rr.com/rlma/StriderFuelSystem.htm Waterline length of my boat is 26 feet so equivelent speed would be 4.64 knots. She reaches that speed at 1560 RPM with 4 POB. Horsepower at that RPM is only 4.2. Displacement is 13,500 pounds or 6.03 Long Tons so .70 HP/ton. Your displacement is 18,000 or 8.03 tons so you need 5.59 HP to go 5 knots. .056 gallons per HP per hour is typical for most diesels so you would burn .31 GPH. This doesn't sound like much but you are talking about a pretty slow speed. At an economical speed length ratio of 1.15, typical cruising speed for most sailboats, 6.3 knots for your boat, you would be burning a bit over twice that. At a speed length ratio of 1.29, flat out, you would be burning about 4 times that. Appreciated! Gordon |
Fuel usage
On 2008-09-23 14:45:05 -0400, "Roger Long" said:
OK. 30 foot waterline at 5 knots would be a speed length ratio of .91. My boat should be fairly similar in characteristics, heavy but with a modified keel. Look at the graph towards the middle of this page: http://home.maine.rr.com/rlma/StriderFuelSystem.htm Waterline length of my boat is 26 feet so equivelent speed would be 4.64 knots. She reaches that speed at 1560 RPM with 4 POB. Horsepower at that RPM is only 4.2. Displacement is 13,500 pounds or 6.03 Long Tons so .70 HP/ton. Your displacement is 18,000 or 8.03 tons so you need 5.59 HP to go 5 knots. .056 gallons per HP per hour is typical for most diesels so you would burn .31 GPH. This doesn't sound like much but you are talking about a pretty slow speed. At an economical speed length ratio of 1.15, typical cruising speed for most sailboats, 6.3 knots for your boat, you would be burning a bit over twice that. At a speed length ratio of 1.29, flat out, you would be burning about 4 times that. Also depends upon the efficiency of the prop, hull & keel shape, wind, wave action, patience, etc. I suspect your estimate is not far off, as our 2GM20F has a minimum burn of about .26 gph, which I measured from about 3 knots (1200 rpm) through about 4.5 knots (1800). At our normal cruise of 5-5.5 (2200) it's about .33 gph. 3200 (6.5-7.4 knots), we burn a bit over 1 gph. [Yes, our waterline is only 22.5 and we shouldn't to better than 6.65, but those are multiply-checked speeds.] [and yes, the multi-day runs at 3 and 4 knots were excruciating.] -- Jere Lull Xan-à-Deux -- Tanzer 28 #4 out of Tolchester, MD Xan's pages: http://web.mac.com/jerelull/iWeb/Xan/ Our BVI trips & tips: http://homepage.mac.com/jerelull/BVI/ |
Fuel usage
On Tue, 23 Sep 2008 08:19:36 -0700, Gordon wrote: How do I ballpark the fuel usage for a modified keel 37' ketch 18000# at 5 knots with a Westerbeke 4-108 with a fairly clean bottom? Gordon My 37' Irwin with a 30' LWL, shoal keel and a Perkins 4-108 (the same engine) cruised at around 6-7 kts with a clean bottom at 2400-2600 rpm and used about 0.8 gph. Dirty bottom lowered that to 5kts and really dirty to less than 5kts. Steve |
Fuel usage
Gordon wrote:
How do I ballpark the fuel usage for a modified keel 37' ketch 18000# at 5 knots with a Westerbeke 4-108 with a fairly clean bottom? Had a fin keel that weighed around 8,500 and burned about 1.1 L/Hr at hull speed (18,000/8500)*(1.1 L/Hr) = 2.3-2.4 L/Hr Add 10% for funzies. Use 2.6 L/Hr for calculations till you prove it wrong. Lew |
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