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

"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

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.

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

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

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

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/

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

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