RESULTS OF FIRST FUEL EFFICIENCY TRIAL
[June 2022] The discovery that during her first season 38’ Cape Sable Stanley burned less than half a (US) gallon/hour (see below) has led to a commitment for careful readings at various rpms/speeds this season.
In a world where 2 US gallons/hr at 8 knots is considered excellent, the first test continued to show that the Cape Sable design is one of the most fuel-efficient live-aboard trawlers currently being built.
For a boat designed for cruising the ocean in comfort and safety, fuel-efficiency is a wonderful bonus.
The first trials were done with the boat cruising up the LaHave River between East LaHave and Dayspring on June 8 one hour before the top of the tide so with minimal current running against the boat with the following results:
- At 1600 rpm cruising at 6.5 knots Stanley’s Yanmar 4JH110 engine burned 0.45 liters in ten minutes. For a full hour the fuel consumption would be: 2.7 liters/hr or 0.71 gph or 0.59 Imp. gph. Or 9.15 nautical miles per US gallon or 10.7 miles/Imperial gallon.
- At 2000 rpm cruising at 8 knots the Cape Sable prototype burned 0.8 liters in ten minutes. The hour equivalents: 4.8 liters/hr or 1.26 gph or 1.055 Imp. gph. Or 6.36 nautical miles per US gallon or 7.58 nautical miles/Imperial gallon.
The 1.26 gph figure for the 8 knots trial is slightly better than what Yanmar expects for this engine, according to the graph in the left column of this web page.
For this test the engine drew fuel directly from a calibrated container and the fuel level was observed immediately before and after each run.
(Although it’s unlikely a long-range cruiser would run at 2400 rpm, a five-minute test was included at approx. 10 knots. Consumption: 0.95 liters. The hour equivalents: 11.4 liters/hr or 3.01 gph or 2.5 Imp. gph.)
The trial was witnessed by Canadian Coast Guard Captain Rodney Strowbridge who has confirmed this report as “accurate to what I witnessed.” (tstrowbridge@eastlink.ca). Capt. Strowbridge was on leave from his duties at the helm of the icebreaker Kopit Hopson 1752.
Further testing may include downriver cruising as well as moving against the current.
Contact Cape Sable co-designer John Steele to discuss these results: john@capeislandcruisers.com .
If you haven’t done so already, sign up for the Cape Island Cruisers newsletter to keep up to date on news and features.
THE CAPE SABLE DESIGN'S CLAIM TO SUPERIOR FUEL-EFFICIENCY
The Initial claim is based on fuel use divided by engine hours
In response to a PassageMaker “e-blast” circulated to their newsletter subscribers, a doctor was inspired to visit the Cape Island Cruisers website, where he noted a discrepancy in fuel efficiency numbers between separate web pages.
The ensuing email discussion with Cape Sable co-designer John Steele, inspired John to come up with figures based on direct evidence from 38" Cape Sable Stanley's first season last year (2021).
According to John, perhaps the single best approach is to divide the actual fuel used over a season’s use by the number of hours the engine was run. (See detailed discussion of issues related to gauging fuel consumption, below). This average fuel consumption takes into account all of the variables, plus time at idle, time at cruise, favourable and unfavourable conditions, etc.
Just under 0.5 gal/hr. average for the season
After our first Cape Sable was hauled for winter storage (fall, 2021) we did just that — carefully calculated the amount of fuel remaining in the tanks, subtracted this from the amount purchased using the actual invoice to give the exact amount of fuel burned, which we divided by the number of hours on the hour meter. The results are very impressive: average fuel consumption just under half a gallon / hour: 0.439 for a season that included a mix of offshore runs at 8-9 knots; bay and river cruising at 8 knots; some fishing at 0-9 knots; a few runs at full throttle to see how fast the boat would go and how she’d handle the sea at full speed; and of course idling and warm-up time. (See Cape Sable Stanley’s sea trial readings for speed at various rpm, below)
What does this mean for you as a boat buyer?
Yanmar engine data suggests that trawler cruising an average 8 knots will burn 1.25 gph. We expect our hull’s consumption will be close to 1.00 gph. What do our tests and research really say? Well, it gives a fairly accurate indication of two important questions to consider when buying a trawler: 1) How much fuel will the boat burn in a typical season?; and 2) What is the vessel’s range on a single fill- up?
Industry reports from brokers, surveyors, sale departments, etc., peg the average use of a pleasure boat between 150-200 hrs. Multiply those figures by the vessel’s average fuel consumption to calculate how much fuel you will use per season. The Cape Sable will burn less than a single 94 gallon fill-up per season. Not bad!
Turning to range: using 8 knots as the long-run cruising speed at 1.25 gph gives the standard Cape Sable a range of 600 nautical miles. With optional larger tanks and 200 gallons board this increases the range to 1280 nautical miles. A Nova Scotia owner of a Cape Sable can say, “Bermuda, here we come!”
Trawlers and Measuring Fuel Consumption: A Discussion
The issue of fuel consumption on a boat is complex. Unlike a car, where fuel consumption is standardized to fuel consumed / distance travelled typically for both city and highway driving, on a boat, such an approach to fuel consumption is much less meaningful and so seldom used.
This is because of several factors such as the type of hull (displacement, semi-displacement or planing ), the length of the boat, its total weight, the conditions at sea and the speed at which the boat is travelling. Each of these factors can greatly impact the amount of fuel consumed.
At Left: Yanmar's fuel curve for their 4JH110 engine. To see fuel specs, click on the image.
Often in this discussion, boats performance is given as nautical miles per gallon (NM/G) especially for trawlers where it is assumed that the speed will be about 8 knots and that this type of vessel will cover fairly long distances each year.
Why 8 knots? Because typically trawlers are displacement type hulls and their speed is limited not by the size of engine rather by their theoretical hull speed which is calculated as the square root of their waterline length x 1.34. If we apply this formula to displacement hulls from 36 feet through 46 feet the range of hull speeds is between a low of 8.1 to a high of 8.9 and so 8 knots becomes the logical “cruising speed” for trawlers in this size range.
Can these boats go faster than their calculated hull speed ? Yes, but not much and the impact will be significant on power needed / fuel consumed. The yardstick used is that for a 10% increase in speed it will require a 50% increase in fuel. Also displacement type hulls can only go a few knots faster, unlike semi-displacement and planing hulls which can go significantly faster, but again require much more power and fuel to do so.
The new Cape Sable is a displacement type hull. That said, due to the design’s lobster boat heritage-designed to carry a good load with a clean straight run aft plus the hull extension / swim platform - it makes very efficient use of its power and can reach 11 knots with the standard 110 h.p. engine wide open at 3200 rpm. Although, at this speed it will burn more fuel than the remarkable numbers seen at an 8 knot cruise.
Getting back to fuel consumption: how to determine how much fuel will a boat use? It is very difficult to say with accuracy what a given make/model will consume because of the many factors that impact the fuel consumed so significantly: weather and sea conditions, load, speed can each alter the performance drastically. Not to mention, whose behind the wheel!
Perhaps the single best approach is to divide the actual fuel used over a season’s use by the number of hours the engine was run to establish the boat’s average fuel consumption which will take into account all of the variables, plus time at idle, time at cruise, favourable and unfavourable conditions, etc.
After our first Cape Sable was hauled for winter storage we did just that - carefully calculated the amount of fuel remaining in the tanks, subtracted this from the amount purchased using the actual invoice to give the exact amount of fuel burned, which we divided by the number of hours on the hour meter. The results are very impressive: an average fuel consumption just under half a gallon / hour: 0.439 for a season that included a mix of offshore runs at 8-9 knots, bay and river cruising at 8 knots, some fishing at 0-9 knots and of course idling and warm-up time.
Our sea trial readings for speed at various rpm :
- Idle, 800 rpm - 3.5 knots
- 1800 rpm - 7.5 knots
- 2200 rpm - 8.5 knots
- 3200 rpm - 11 knots
If we strictly apply the figures from Yanmar’s fuel curve for the 4JH100 engine (see chart in left column) it would suggest that at 8 knots we will be burning just over 1.25 gph. Based on experience we expect our actual consumption will be slightly better - just 1 gph. We plan to accurately measure her consumption at different speeds once the boat is back in the water this spring.
Visit this web page occasionally to track this season’s fuel consumption readings. Or have occasional reports delivered to your email Inbox by signing up for our newsletter.
- John Steele
May 2022