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Will the lower pitch improve top end? It seems like the higher pitch would have a higher top end but a longer planning time while the lower pitch would have a shorter planning time but a slower top end speed?
Jim G.
Jim, it is counterintuitive, but yes, a lower pitch is likely to improve your top end. If you were still going to max out at 5000 RPM, a lower pitch prop would indeed result in a lower top speed. But the lower pitch will let your motor spin up higher, and the net is likely to be more speed.
The F70 is rated 70 horsepower at 5800 RPM. At the 5000 RPM you are currently hitting, I'm guessing the motor is producing between 50 and 55 horsepower (just a guess) so you are leaving a lot on the table. The rule of thumb is 1 inch pitch = 200 RPM, but like all rules of thumb, sometimes it works, sometimes it doesn't. You have a lot of RPM to play with, all the way to 6300, and in your situation it would be best to put your WOT RPM right up there - it will improve your performance in the rougher water when you can't go that fast anyway.
Of course, this assumes things are linear, which they aren't, but it gives you some idea why the 13-pitch won't necessarily slow you down.
Tough call between the 14 and the 13 - I guess, having thought about it a bit more, you might want to try the 14 first. With the usage model you describe, I would certainly keep trying props until you at least hit that 5800 RPM where your motor makes 70 horses.
Another possibility is that the SCD3 is just the wrong prop for your boat - I hope not, but it's not an exact science.
Tedious,
Thanks again for much good information. My last question would be about trim. I might of forgot to mention it up front, but the 5000 RPM was at between 3/4 down and 1/2 trim on the trim gauge. I didn't notice a significant rpm increase while trimming the motor up as on past boats I have owned if memory serves me right. How high do you trim to get max rpm's while not porpoising or robbing the motor of the ability to pump water? On older 2 strokes I have owned I have really never went too much higher than I did on my test run with the Yamaha.
Hello jgortva, let me add one additional design aspect regarding your prop search.
There are some performance differences between a "round ear" prop and a "semi-cleaver" design as are a few of the Yamaha (painted black) stainless props. I transitioned to the latter when replacing my Solas to Powertech and immediately notice that my "getting on plane" time was cut in half and with less rpm.
Semi-cleaver props as is my Powertech in some discussion circles are considered "stern-lifting" props while the round ear assist in "bow-lifting". I have experienced each condition with my props and for my 15 Dauntless I prefer to get the stern up quickly when getting on plane instead of looking at sky when the bow points up.
I always tuck my drive completely in when getting on plane and this did not help with my Solas prop. It took time and rpm to get up on plane, great top end (wot), but useless when cruising or entering "no-wake" areas as the Solas didn't like slower to idle speeds. Maybe a smaller pitch may have been better.
The Dauntless 15 is very sensitive to outboard trim input, porpoising will be the immediate effect if excessive. You will need to test trim positions and each will be dependent on the prop selection. Avoid cavitation and starving the water pump with trim input. Stainless Steel props are better suited for running near the surface (jack plates, drive trim, etc.). Outboard height on transom also has significant input on the performance and trim effects.
Hope this helps a bit and good luck with your prop search...
Angel
Edited by dauntless-n-miami on 06/21/16 - 6:47 AM
1996 Dauntless 15, Sportsman Pkg. - 2007 Yamaha 90 HP 2-Stroke
Jim, if you were not gaining more RPM when trimming out during your test run, then there's no additional speed gain to be had there. With the 16-pitch on my 15, I am a little underpropped (meaning too low a pitch) but I like it that way as it gives me the power to get good speed even when trimmed down in rough conditions. I don't know if you've spent time on a classic 15 but keeping it on the water can be a challenge! If I was looking for a full-on speed prop, I'd probably want to go up an inch or two, but the conditions when I could use it would be pretty rare.
Jgortva, which prop did you finally end up with? Can you post performance numbers? How does the f-70 Yamaha perform on your Dauntless 15 with a full capacity load?
I love the performance of my Mercury 75elpto but not the smell and fuel consumption.
jgortva wrote:
Phil,
The boat is used mostly on Lake Michigan loaded with me, (270 lbs), the wife, (160 lbs), wifes purse, (50lbs),good for nothing dog, (30 lbs), 12 gallons fuel, and not much else other than saftey equipment. I am looking for best all around performance as rarely does the boat go over 20-25 mph due to water conditions. While it is mostly a, "harbor toy", we do take cruises up to 15 miles to surrounding ports so mileage should be decent, however anything will be an improvement over my 1995 Evinrude 70 H.P 2 stroke that the Yamaha is replacing.
