cd-25 & mpg

[bcarli]

thought I'd pass this on. finally used my new fancy hond gauge. here's what I found out regarding mpg on my cd-25 with a 135 honda and a
s. 3x14 1/2 x 15 r prop.

rpm mpg knts
1500 3.9 4.5
2500 4 7
3000 3 8.5
3500 2.9 11
4000 2.7 15
4500 2.7 18
5000 2.5 22

seems once I'm up on a plane it doesn't really matter how fast I go.
If I wanted to loose some top end speed for better fuel economy at 4000 to 4500 rpm any thoughts on different props
bill

[Byrdman]

Interesting numbers. I think Bill and El on Halcyon have done a lot of prop work on their 22 with the way they like to travel. If I am not mistaking, they have found a nice prop mix as to allow them to get the best mpg for when they are "crusing" down the waterway. Have you checked the numbers in the middle of the ones you have posted to find that "loaded like she is" optimal cruising speed for your hull? Not sure if shifting weight forward and aft would effect your mileage or not either...? Will be interesting to track this.

Thanks for your information.

[Sea Wolf]

bill & chris carli-

(On Edit- Here's a link to a previous discussion on this topic:)

http://www.c-brats.com/viewtopic.php?t=2215

There's been some discussion of the prop choices on a CD-25 before, but some of it may be burried in other discussions and harder to find. here are some things I remember.

Generally, the consensus has been that motors in the 130-135 hp range are adequate for the boat, but at the lower limit of performance acceptability. Not that it probably matters to your or to most, but with a larger motor or pair of motors, some have achieved top speeds of up to around 35 mph with a CD-25. Just stating this for comparison purposes.

Your current top speed of 22 mph at 5000 rpm could possibly be improved with a slightly lower pitched prop which would also give you a better hole shot and general acceleration. Gas mileage could be better, but only actual measurement in the boat over a period of real usage would give you the final answer. Your fuel monitor system is great for instantaneous mpg, but actual mileage can vary somewhat based on your exact usage, especially if you spend a lot of time maneuvering at low speeds, trolling, etc.

The next lowest prop below the 15 inch pitch is usually a 13, which theoretically should yield about 5770 rpm, well within your range of 5000-6000 rpm max.

This 13 inch prop would seem about ideal, but I distinctly remember that someone (Tyboo?) tried a 13 on a 130 Honda, and went back to the 15. I also remember that Pat Anderson has tried several props on Daydream with a 150 hp Honda, and eventually settled on a 16 inch piitch. So you may already be about at the ideal point.

If you could find a 14 inch prop, or one with an adjustable pitch, such as the ProPulse, that could be adjusted to 14, you might have a winner. (The ProPulse, however, is a 4 blade prop. and with the extra blade and surface area, slips less, and may be just about equivalent to a 15.) Still, if it could be adjusted in 1 inch increments, it would be a good experimental tool.

A four-blade Solas 13 inch pitch would seem a real possibility, considering all the above. Maybe someone out there has tried one and can comment.

As with all prop discussions, a lot of what is said is speculation, and sometimes the reality check in the water yields surprising and unexpected results.

I'm hoping that this foray into the topic will bring out some of those who have "been there, done that" with the 25, and save you the trouble of experimentation.

I'll now go join DogGone Dan in the Swearing Off Prop Speculation Penalty Box for Violating the Vow of Silence. 5 minutes for Mumbling Around with Second Hand Information. Bye. (And Good Luck!) Joe.

[stevej]

Bill your numbers look to be about average for a 25 but you never know what a bit of tweaking might produce. I picked up about .5 mpg by dropping from a 14 x 17 to 14 x 16 and went from 4600 to 4800 at full throttle (4 blade propulse) with my 115hp Yamaha (2-stroke). The economy difference between my old carbed 2-stroke and the 4-strokes does not appear to be at cruising speed but in the low RPM range. My 115 gives a flat 2.6 mpg from about 1800 RPM up.

stevej