Right props for CD-22 for full 5500-6000 RPM at WOT

[Sea Wolf]

C-Worthy-

To get a final answer to this question, you're going to have to have a fuel flow meter coupled to a GPS that together can give you instantaneous mileage figures as you change trim. Several C-Brats have this type of set up, and they should be able to give you figures they have derived from their experiences.

Triming the motors down under the boat to raise the stern and lower the bow increases the surface area exposed while softening the ride by breaking the chop with the sharpness of the bow. The extra drag should decrease your mileage figures, but the point may well be relatively moot if you value the comfort of the ride that way. Also contributing to the mileage loss is the inefficient thrust angle of the motor(s).

Shifting weight forward can have the same effect of lowering the bow and achieving a smoother ride with less mileage loss from the inefficient motor thrust angle created when trim is used to lower the bow. 100 ft of 1/4" chain (74 lbs) does this very efficiently. although there are other caveats of which to be aware. Is there any thing or anyone that can be moved forward?

The ultimate mileage figures in planing boats are usually achiueved with the bow trimed up as far as possible to minimize surface area with the boat planing only on the very back of the hull bottom, but th e mass and balance of the C-Dory doesn't permit this very easily and the resultant light bow is totally incapable of smoothly breaking choppy waves.

Let's get some hard numbers now from those who have the instrumentation to measure it. HTH. Joe.

[Casey]

I've played with the bow-up, bown-down (pitch attitude) quiet a lot in an effort to improve what I consider ... marginal gas mileage.

The set up: 2003 CD22 with Suzuki 90, heavily loaded (approx 4000#) on a long trip. Prop: 13" Propulse 4-blade adjusted to 18P.

We generally cruised at 4200rpm which yielded 15-16knots, and about 3statute mpg (s/mpg). The Navman fuel guage would read 5.x gph. (Fuel data was confirmed at fuel-ups.) The OB was generally trimmed almost all the way "down." While cruising along, I would tweek the trim tabs slightly and watch the gps speed and the NavMan settle as the instrumentation adjusted to the minor pitch changes. The changes were generally Very minor (ie. one second burst on the trim tabs) but you could see the effect on mph/gph - once things stabilized. (Rereading this paragraph it seems very awkward - but I hope you get the idea....)

After many days, and countless hours of doing this, my "sense" is that if you trim the bow down you will find a sweet spot that is the optimum for your boat/weight/speed configuration. I also think that a slightly faster cruise may put you higher on-step and actually improve mileage; but I haven't experimented with that idea (yet). The bad news is that without sufficient instrumentation, you might not be able to determine when you've reached that narrow point. With either gps or the NavMan gizmo you can get pretty close though. (1)Setup your cruise at a particular rpm. (2) Trim to what "feels" right, let things stabilize, then check your speed and make notes. (3) adjust the boat trim using either trim tabs or motor, and watch the effect on speed. When you're getting the most speed at a chosen rpm, you'll be real close to the best configuration. For what it's worth, motor trim seems to be the "gross" adjustment; trim tabs seem to be the "fine" adjustment.

My objective was to find the best cruising/economical configuration in an effort to maximize range (42 useable gallons x 3 s/mpg only yields about 120 statute miles max range. Carry extra fuel when necessary!) In my case, we learned that if we run at displacement speed (2000rpm @ 1.0 gph = 6.5 s/mph and (therefore) 6.5 s/mpg), it more than doubles your mileage; but, TAKE A LUNCH!!! (Thanks to Les for installing an Autopilot, which actually makes SLOW cruising Fun!)

Next summer's "trial" will be to experiment with the -5 degree shim recently installed beneath the motor, at the CD factory, and determine the impact on performance and economy. After that I may try a Doel-fin type plane on the OB. But, one step at a time. In addition, there is a good chance that a different prop will help. The list goes-on! (There are countless variables, and if too many are put-in-at-once I get confused!)

... I may have just beaten you over the head with obvious data; sorry.

Casey*
C-Dory Naknek

*the guy who always wanted to work for Consumer Reports! (come to think of it - Powerboat Reports would be even better!)

[Sea Wolf]

Casey- nice report! And not too detailed. I think we have to include full descriptions and data to be able to allow a through analysis, avoid misunderstandings, and make sure that the less technically inclined and interested can learn from these discussions.

This discussion give us some insights into the complex nature of trying to isolate variables and objectively evaluate performance factors. Nice job!!!


Two things seem to stick out and draw my attention from this report, aside from the discussion of how to evaluate and maximize fuel mileage.

1. Real observed fuel mileage calculated from fill-up and actual mileage covered figures are very different from the published figures on the C-Dory website. This has been discussed before, but the differential appears again and again, something on the order of about a little over 3 mpg vs almost 5 mpg from published data.

2. The real mileage calculated as above is also different from the instrument indicated mileage by about the same differential! How many hundreds of dollars does this combination instrument package cost that is off by 2 mpg on a actual figure of 3 mpg?

The instrumentation indicates 5 MPG, the real mileage is 3 mpg,

therefore the instrument indicated figure is approx. 167% of actual mileage, and

the error is 65-70% or thereabouts, with a lot of thereabouts for a lot of $$$!!!


Question: Do the magazine reports on these fuel mileage instrument packages indicate these levels of error after testing the products?


Some criticism is rightfully leveled at most of the boating magazines as simply being industry publications sponsored by boating industry manufacturers, with the conclusion being that all they do is test boats and equipment and give basically favorable evaluations to their advertising sponsors.

Power Boat Reports does it's testing independent of advertising sponsors, but the cost transferred to the subscriber is $78 per year for 12 issues or $6 per issue. Each issue has 5 articles, of which 2 are of real interest on the average, so the cost per article of interest becomes marginal. I'm a subscriber, but am evaluating the long-term value of this service. It would be great if they allowed their current subscribers full access to their archived reports, but they don't.

In some way, of course, it's actually a lot more fun to be able to participate first hand like we do here than read abut someone else's "research".

For those of you who don't know about it, The Hull Truth" website is another fun source of these discussions:

http://thehulltruth.net


Let me know if you find any articles about the accuracy of fuel mileage instrumentation systems.

Joe.

[Casey]

I don't know about the "big boys" or how they calibrate their instruments, etc. but when I'm doing this stuff, I get the data from the instruments (GPS, NavMan fuel gizmo, etc.) then compare the data with what it actually takes to fill-the-tank, and compare THAT with the GPS mileage. Interestingly enough, the NavMan fuel guage was surpringly accuratea right out of the box. As I recall, on a 93 mile calibration test, the guage said I would fill with 26.8 gallons, and the tank actually took 26.0 gallons. (Those may not have b een the exact figures, but I remember the guage was .8 gallons "off" over a 93 mile run!) Pretty impressive.

Even at best this stuff is a SWAG since there ARE so many variables (boat variables, wind, current, load, yada, yada, yada.)

...but it's fun.

(Lordy, I've GOT to get a better hobby!)

Casey
C-Dory Naknek

[C-Worthy]

Lots of good information. I'm really seeing the value of trimtabs. I sure appreciate all your replies.

[Sawdust]

Hey Casey,

Don't change hobbies, and get your lazy butt back up here and visit for a while. Miss you!

As I've said before (over and over to the point of yuk) I do this kind of thing a lot. Did it for Johnson on some of their big engines, and my predicted log racing days required precise (wow, how precise can you get) planning numbers.

The really practical numbers, like those El and Bill get from thousands of miles and countless fill-up checks, are very difficult to get, and are most important on long legs. So many variables are significant to our little guys. Total weight, hull condition, wind (both direction and velocity), topside drag, lateral and longitudinal trim, engine thrust angle, and the list goes on. Most of the published tests I've seen on boats I'm familiar with are very favorable to the boat/engine combination. They are usually done under ideal conditions, with a professional driver, and accurately measured with a radar gun. New boat, clean bottom, etc.These are not real world figures.

The GPS/ chart plotter systems have for the most part eliminated plotting errors. When I was teaching this stuff to professional seaman, I'd often find plotting errors of 2 or 3 tenths of mile on a leg. Steering errors add to that, so the actual miles run were generally more than the plot.

With the 5 22s I've had -- different main engines -- I have never been able to get 5.0 smpg. I'll get some numbers on the new 2-stroke, but with my Yamaha and Hondas I could get about 4 NMPG if, IF, conditions were ideal. Wind on either beam that requires rudder compensation, or bow-down trim to compensate for headwind chop, all decrease the NMPG significantly. I'm an old safety dude (coward a better word), so my planning figure is 3NMPG. I normal cruise long legs. Here to Bamfield is about 200 NM, and I like to have a decent reserve. With the limited range of the 22, that means I've got to stop enroute (Port Angeles, Sooke, or Neah Bay) and top off. Beats running out of fuel, and I agree with Casey -- better to slow to hull speed and go twice as far. Take along a good book - and have a good autopilot!

This doesn't help, but puts another oar in the water,

Dusty

[CatyMae n Steve]

This is making my hair fall out! well, not really...I'm pulling it out!

Reading here, reading Honda's website and I still don't have a clue! The boat's in the water, so I have no idea what prop is on there. What I DO know is we get 4500 RPM tops WOT and it should be between 5K and 6K. The spare they gave us is a 15 (pitch, I think) x 13 3/4" (diameter, right?) and I didn't open the box, but suspect it's 3 blade. The stock # isn't in the Honda catalog online (it's 58130-ZW1-W21ZA). I'm reading the website that there's a tach adjustment in the boat...we've asked the dealership about our concerns in the past, thinking the engine sounded like it was working way too hard and we've never achieved the speed others seem to get out of their boats...the dealership tells us it's just fine the way it is...I really don't want to break that engine in at improper speeds/RPMs, as I understand that can be a long range migraine, and we may already be too late to make corrections...Les...you out there? Can you give us some guidance?

[TyBoo]

CM n S - Googling your part number shows it to be a Honda or Solas (they make Honda's OEM props).

Now, here's my wild guess:

Since Sportcraft set up your boat, and they don't like Solas/Honda props because of the whine they often make at 2000 or so rpms, you have a black prop (Michigan Wheel) on your boat now. Right so far? My experience with both the CD22/75 Honda and the CD25/130 Honda is that the Michigan prop with the same size numbers as the Honda prop tops out at 200 to 300 rpm less. When I got my first CD (used) from Sportcraft, it had a Honda branded prop with 17" pitch. My WOT was 4800 rpm. I changed to a Honda branded prop with 15" pitch and went to 5400 rpm. Then I changed to a Solas 15" prop and made 5600 rpm. Plus it was a whole lot smoother feeling than any of the others. There was the whine, though, but the only time I noticed it was coming up the Skipanon channel at 2000 - 2500 rpm. It went away when trolling or running faster, so it was not a bother.

So I am guessing that you have a 17" pitch prop on there now, and it is likely a Michigan. If you change to your 15" Honda spare, you will be get over 5000 rpm.

If you find you have a 15" pitch on there now, then you better look real close at your throttle linkage at the motor and make sure you are getting full stroke and the throttle is really wide open when the controller is pushed all the way forward. (Actually, you should look at the linkage anyway, but it seems I remember you saying a while back that it had been checked.)

Let me know if any of my guesses are close, because B~C is offering a prize for this.

[Sea Wolf]

[CatyMae n Steve]

I'm printing this stuff and taking it to the boat with us this weekend...I don't know what color the prop is....maybe Steve can see the markings on it from the swim step...dunno....if we can't get a make on it from there, it'll have to be "ok" until we pull the boat the end of October...I'm saving what hair I have left on my head -- just noticed all the emoticons are bald! eeeeek!

[TyBoo]

[CatyMae n Steve]

[CatyMae n Steve]