Hull Speed of CD25 - Economy - Range - Kind of Rambling!

P

Pat Anderson

New member
The discussions in another thread about Mac's range in the Ranger 25, and how much range is needed in Alaska got me thinking about this topic. Pardon if this kind of bounces around a bit! I will echo the statement that there is NO problem finding fuel every 100 NM or so between Washington State and Ketchikan, AK - the key spot we were unsure of when we were planning was Hartly Bay, because it is a long way from Shearwater to Prince Rupert - this is a great modern facility now. We fueled on the trailer at a gas station in Blaine, WA; Gorge Harbor, BC; Sullivan Bay, BC; Shearwater, BC; Hartley Bay, BC; Prince Rupert, BC; and Ketchikan, basically running off the top half of the tank. But some day we might need to go farther between fuel stops, and it seems prudent to have a firm grasp on what you can do and how to do it.

With the re-propped Honda BF150 (15.5 x 15 x 3 AL) we are getting much better economy than before on our CD25...We now routinely turn in 2.5 NM per gallon (calculated, NM traveled per GPS trip computer divided by gallons used), of course that is a blend of fast transit and slow cruising for sightseeing. So for planning, I think I can very conservatively count on 2.0 NM gallon. With a 100 gallon tank, and a 25% reserve, I think I can count on a 150 NM range for whatever conditions I may encounter.

Maximum economy though I presume is attained at hull speed. I know the formula for hull speed is 1.34 times the square root of the water line length of the hull. I don't really know the water line length of the CD25 however. I think Bob Austin estimated it at 22 feet? If so, that would give a hull speed of about 6.2 knots. I am wondering if the water line length could be as short as 20 feet? That would give a hull speed of about 5.9 knots. The reason I wonder this is that I had a great chance to run Daydream on the kicker in Upper Priest Lake on an absolutely mirror-like surface, no apparent wind and of course no current. The kicker, a Honda BF15, gave us 5.7 knots (SOG per GPRS) at well below WOT (maybe 3/4 throttle, we do not have a tach on the kicker) and at WOT (and significantly more RPMs) it only crept up to 6.0 knots. I have also noticed that the BF150 gives its most economical performance per the Navman somewhere right around 5.7 knots - when I get up over 6.5 knots it really drops off. This makes me think hull speed is probably actually 5.7 knots, but that does not conform to the formula. No doubt there are other factors, but that seems to be what we are finding.

For figuring out range in a tight spot - a long ways to go between fuel stops - obviously 5.7 knots seems to be the best speed for our CD25. The fuel used by the BF150 at 5.7 knots does register on the Navman - it jumps all over the place by instantaneous economy. Actual computed economy is always less than instantaneous readings, but we see instantaneous readings around 8 - 10 MPG. Assuming it is actually somewhere around 5 NM per gallon at 5.7 knots (and I think it is higher, but have never run exclusively at this speed, so no way to really check), my range at 5.7 knots would still be at least 375 NM using the 25% reserve rule.

But the fuel flow to the BF15 at 5.7 knots does not even register (or register accurately) on the Navman - at 5.7 knots it jumps between 18 - 38 MPG, which I am pretty sure is an artifact of the inability to register really low flows. Now, this seems to indicate that the flow to the BF15 to attain the same 5.7 knot speed is less than the flow to the BF150. If the same amount of fuel was flowing to each engine at 5.7 knots, the Navman would register the same but it doesn't. This seems to fly in the face of what Bob was saying about the fuel required being based on the amount of "work" the engine is doing. Comments, Bob?

Anyway, it seems clear to me that in a tight spot, I would get the absolute maximum range running my BF15 kicker at 5.7 knots - does anybody have any idea of what kind of economy I might really be getting or range I would have doing this? I suppose I could put exactly a gallon (or maybe two) in the little day tank that came with the BF15 and see how far it takes me...
 
Hi Pat,

I've never run Wild Blue at less than 6 knots long enough to get a real accurate figure on mileage. After several days "around" that speed, we did calculate over 3.5 nmpg. Most of the time we see 2.2-2.5 nmpg per tank... and you've seen how loaded we travel.

Hull speed is based on more than just the waterline. We've seen small sailboats frequently exceed their theoretical hull speed, because they are not true displacement hulls. I would sure think this reasoning would apply to the C-Dory hulls, as well. Just going by the Navman (and our fuel bills), slower (OK, much slower certainly results in better mpg). I think it's great that we have that kind of versatility on our boats... if we have to stretch the range, we have that option.

Published figures on the Honda 135/150 state 5.7 nmpg at 1500 rpm, resulting in 5.1 knots. Step up to 2000 rpm, and you should see 5.43 nmpg at 6.5 knots. That could only be with a very lightly loaded boat in ideal conditions... no way can we duplicate those figures.

Here's a look at the numbers we found (not our calculations, but what we use for basic planning purposes)...

Performance_copy.jpg

HTH

Best wishes,
Jim B.
 
I would agree with most of your data and calculations Pat, except for the fuel tank capacity. Last fall I completely drained the fuel tank with a suction fuel pump tilting the boat bow down, bow up and level until no more fuel could be pumped. This spring I filled the boat with 97.158 gallons again tilting the boat for maximum fill. Unfortunately I forgot about expansion, and fuel would drain from the vents when the boat was warmed by the sun and had to drain out about 3 gallons. I would guess in the real world of cruising that our tanks have a usable capacity barely over 90 gallons. Deceptive advertising?
 
Pat,

I don't know if this helps but we have a BF 20 as an auxiliary on our boat and it burns about 1 gal/hr at WOT which is in line with what the Honda Rep. told us at the SBS this year.

We use our motor as an emergency unit and for Salmon trolling in the open ocean. Our main engine is a 245 hp Chevy V8 coupled to a Volvo Pents DP drive. I have kept extensive records on real world fuel consumption for our boat (much different hull than your 25 CD) and found the following: Main engine only at 17-20 Knot cruise = 1.7 Nm/gal; Main engine with BF 20 used for trolling (2/3 distance main engine and 1/3 for BF 20) = 3.2 Nm/gal; BF 20 at WOT = 5.3 Knots and 1Gal/hr.

Our hull has a 24ft WL and a bout 6.56 Nm/Hr calculated hull speed. However, I have not had the smooth water needed to verify this as all of the above tests were run in various sea conditions off of the Oregon coast in the open ocean.

Hope this helps .
 
Hank,
I have an '03 CD25, when new I carefully filled the tank with the boat level with the waterline. The dealer said he put about 3 gallons in it to start the engines when installed and to verify they started when I picked it up. I put 99.4 gallons into it, about the last 5 gallons were at or above the vent inlet because it didn't 'gurgle' for that part of the fill. From then on I only fill it till the vent just stops gurgling, want room for expansion.

I have a plastic tank, no venting or other problems whatsoever so far (now I've done it...!).
 
The water line length of the CD 25 is 21 feet 4 1/2" (just measured using the "scum line" foreward to waterline transom with the boat leveled.)--using 1.34 x sq root LWL this is very close to 6.2 knots. This 1.34 is the max hull speed, before the boat starts climbing over its bow wave. It is not the most effecient--that most effecient is closer to sq root LWL, or down about 5 knots. Actually even slower is even more effecient--but again there is getting there--against wind, waves, currents etc. 5.7 knots is not a bad figure to use--and is a reasonable speed. The flow meters don't read well at these low speeds--hard to know what the actual number is without using a small can and accurately measuring the amount used. Figure at one gallon an hour you can generate 14 hp with a gas engine. With a 9.9 if you are getting 6 knots, this would indicate that you are probably using about 3/4 of a gallon an hour--maybe slightly less. I would not be surprised at 10 miles a gallon at 5 knots. The smaller prop of the 9 hp is less effecient than the larger prop of the 150. There is also more internal friction in the bigger engine. Too many variables to be "super accurate". The "rules" are only generalizatons. I can relate my experience using 5 hp to drive a 26 foot sailboat, weighing more than a C Dory 25--but with relitatively similar hull lines--hard chine, about the same beam and LWL--a keel which causes more drag--and we would get at least 10 miles to a gallon at 5 knots. I believe that you can get 10 miles a gallon when you slow down to 5 knots--and with 100 gallons--you are going to be close to 1000 miles--if the water is calm...no head wind etc...Don't try a trip to Hawaii! I would go with the smaller engine for the best "mileage" at low speeds.

Rather than putting a gallon in a tank, it is best to fill a tank to full or some point, and then run for several hours, and refill, with an accurate guage. We used to do this with glass IV bottles, and plastic tubing. The bottles were calibrated very carefully and you could determine the consumption--under those conditions very accurately.

James, the BoatTest.com schedule which you posted, is subject to the problems with any flow meter at low speeds. I would have to run tests, but judging by what I find on the Centruy 18 CC it is very much in line with the 10 miles a gallon at 5 knots. I would be reluctant to use any of the BoatTest figures for range figuring. They give 390 miles at 13.8 knots or 3.5 nautical miles a gallon in a light boat. The Tom Cat 255 BoatTest.com gives 2.78 miles a gallon--I have a lot of miles on the TC and am below this even at my best runs--of course part is because the boat is carrying fuel and some gear etc.

Pat, Thanks for the information on Hartley Bay--when we wee there in the 90's it was a hit or miss propsition. However, I really appreciate the ability to go where I want and not have to worry if I have enoough fuel or not. We went up many out of the way inlets--where we rarely saw another boat. I think that a trip to Ocean Falls is a "must". Going around Queen Charlett is another plus, as is the outside of the Islands down from Icey Straits. The most charming places and most interesting people in these out of the way places...
 
pat , does scooter have a v-8? I could have had a v-8 :P , sorry I could not resist that one. I have never seen a dory with a i/o let alone a v-8. what speeds do you get? and how does it handle the chop?
 
My crew, Dave Olson and his son Alan, both old Alaska hands, thought we would probably be getting gas out of 50 gallon drums at Hartley Bay, if we could get gas there at all! Now they pump the gas and take VISA and Mastercard with a smile! The one thing I regret was having to get to Ketchikan on a schedule to meet Patty flying in...I missed Ocean Falls, and quite few other places I really wanted to visit. Next time (assuming Patty can be convinced to make this trip, probably it will have to after the Sea of Cortez!), we will take our time and see the sights! Never fear, we will NOT try to make it to Hawaii in Daydream no matter HOW good the mileage seems! Shipping her over would be a blast though, and I think David has got to be thinking seriously about that with his CD16...
thataway":r1i2e3c3 said:
Pat, Thanks for the information on Hartley Bay--when we wee there in the 90's it was a hit or miss propsition. However, I really appreciate the ability to go where I want and not have to worry if I have enoough fuel or not. We went up many out of the way inlets--where we rarely saw another boat. I think that a trip to Ocean Falls is a "must". Going around Queen Charlett is another plus, as is the outside of the Islands down from Icey Straits. The most charming places and most interesting people in these out of the way places...
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