Optimum prop for 23' Cape Cruiser

[baensch-ak]

Greetings, Cape Cruiser owners! Karen and I joined your group this past summer and I have a question: What size prop are you running on that boat with 115HP Yamaha engines? Ours came with a SS Michigan Wheel of 13"x 12 3/8" pitch. Plotting RPM vs Speed in knots, we have found a steady gain from idle to 3400/just under 10 knots, a drop of almost 2 knots between 3400 to 3600 ( the "hole", boat transitioning to plane), then a somewhat steeper gain in speed per RPM from from 3600/9.4 to 4600/16, then a sudden, steep climb of 7 knots with 4700, after which the curve flattens to the previous angle, about 1.5 knots per 200 RPM, for a max at 5400 and 26.6 knots - on smooth water at 3000 altitude.
We are more interested in comfortable, economical cruising, say 12-14 knots, than in speed. Wondering if a flatter prop would be more efficient?
We have also noted that adding trim tabs boosts planing speed (very little below planing). We have seen 18/4200 with full tabs, vs 13 without. Above 18 knots the with-tabs curve tilts toward the without curce, crosses at 21, and flattens at 22.5, their drag canceling added power. Motor tilt was close to full down, 2-3 bars on the indicator, which seems best. Load 2 persons, modest gear, 1/2 fuel.
Would love to get your comments, especially about props you're using.
Thanks, Al

[Matt Gurnsey]

The Maximum RPM range on the Yamaha 115 is 5000-6000 RPM. You are right in that range. In theory if you dropped the pitch to the next size down you would pick up a few hunddred RPM at the top end, which would be okay.

It looks ,ike on your set up that a speed of 4000 rpm should give you the cruise speed you want, so I don't know if changing props will really gain you anything.

[C-Green]

Al, My guess is that you are at good fuel mileage. My experience on a CD-22 with Solas four blade props on twin Honda 40's, medium to heavy load on average. Averaging over a season's boating (700-1300 miles), my 12 inch pitch gets 4.5 mpg and the 11 inch pitch gets 4.0 mpg. The 12" has a WOT of 5400 rpm and the 11" 5900 rpm; top speed is about 1 mph less with the 11" and varies from 24 to 27 mph depending on all the variables. I originally had the 12's but on extended trips (with heavy loads) my WOT was 4700 and with the 11's that was upped to 5200, within the recommended WOT rpm range. So, if I plan a long lightly loaded trip I will remount the 12's and get 10+% better mileage.

Jay

[jennykatz]

We have the cc-23 with a f-115 yamaha with a13and 1/2 by 15pitch S/S 3 bladed wheel top[ speed is about 32 mph-33 mph at about 5500rpm

When we have full water and 50 gallons gas or so we can get about 26-27mph at 4700 rpm The boat will plane off at about 12mph 3500rpm or so .we burn about 6-6.4 gph at 4700 we are getting a little over 4mpg for cruising at sea level salt water When we were up in Lake Tellico we got about 3.8-4.0 give or take a little 900above sea level fresh water

I get the best mpg at about 4500rpm 23mph or so I'm away from the boat or else I could verify . You will lose a little performance being at 3000ft .

[Wefings]

You sound well propped to me as well . If you go sea level , you would still be good with your setup .
Marc

[baensch-ak]

WOW! Thanks so much for the quick come-back and your opinions. I took notes and appreciate your time. Looks like we are in the green range and I had best leave the thing alone.
Now, I still wonder about the recorded and repeatedly verified speed increase with tabs down (roughly 6" wide, 7"long, electric). In the range of 16 to 22 smph it is pretty significant (up to 5 knots). I found that with less fuel the best gain comes with 2/3 tabs down, with full fuel all the way down. Seems like flattening the hull in the water gains more than the drag of the tabs, in this range. (That may be the reason Ben T is moving the fuel weight to side tanks and the water to the bow in his new cruiser.)
Then one more thing. We spent a lot of time traveling thousands of mile at an average of just above 6 knots on Friendship. (8-11 knot sails happened only a handful of times over 3 years on the water, but they are memorable!) I assumed that theoretically one could at least double the normal milage of a planing boat by just going at displacement speed, let's say 7 knots with the C.C. (Thinking about looong trips.) My my thinking is that fuel consumption is fairly linear with RPM's, as air/fuel ratio must remain constant and air/fuel loading per piston stroke doesn't change much, either. True?
Anyway, looking at my preliminary curves I find that the speed gain per RPM in the displacement range is fairly constant up to the drop-off at 10-11 knots, but that gain is only .26 k/100 RPM in that range. On plane the boat gains 12 knots from 4000 to 5000, for 1.2 k/100 RPM. That tells me that the best miles per gallon likely falls into the middle to higher range of the planing speed on smooth water and blows my go slow for max milage theory right out the window. Any thoughts on that?
Finally, we found that the Wallas and the fridge (The Wallas had apparently never been used and smoked like a bandit on first heating) work just fine, but there is no extra ventilation in the cabinet. Does the fridge get enough air to do its job that way? Does the Wallas?
Thanks again, Al & Karen

[jennykatz]

Look into my photos WE bought a vent from WM for 7$8 The wallas and fridge need this also pull your fridge and check wiring I changed mine out to number 10 wire instead of 12 gauge .no problems once weput in vent and changed wiring.The only problem i have is the fridge smells like diesel talked to Dometric tundra people they said fridge is sealed? so smell could only come din through front door????

See you guys in Jan