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Hi everyone. New member. Just bought a 1997 15 ft dauntless with a 70 hp 4 stroke yamaha. Took it out in the glades on flat water. Motor is mounted in third hole up on transom. Running with a stock alum. Prop 13 1/2 x 15. Boat reaches 40mph but on the way there, it porpoising pretty bad. Question is, will raising the engine another notch help or getting a SS prop or both????
I have a 1998 Dauntless 15 with a Merc 60 2 stroke. The Dauntless 15 is pretty weight sensitive. Having weight forward helps with the porpoising. I'm running a stainless 12" x 11 pitch prop and that provides additional stern lift which helps, as does my Dole Fin. Still, my ability to trim the motor up is limited before the porpoising starts. My hole shot is great with the 11 pitch prop and WOT speed is 34 on GPS (fast enough for this old fart in a 15 foot boat).
Gary, thanks for the feedback. Im from key west and have always run and raced go fast sprint boats (allison, action marine...) Extremely over powered. Had a 13 ft whaler with a 55hp evinrude. Ran nice. Love this boat cause i live near the everglades and like to take it to the keys to run in the back country. Im turning 6k rpms with 2 adults getting 40mph on gps. Hard to get there though. Im gonna raise 1 notch on motor. Should i pitch up or down on the prop??
There are others on this forum that are more savy on prop selections than I am. Tom Clark is one resident Guru and may chime in.
That said, I went from a 10 1/2 X13 prop to the Stainless 12 X 11. I believe that lowering the pitch and increasing the prop diameter helped with the stern lift. This gave me a better hole shot and less porpoising. I did loose a few rpms in the process (WOT is now about 5100 - 5200 depending on sea conditions) Top speed stayed about the same.
My Merc is a small gear case motor, so the heavy 12" diameter stainless is all she can manage to spin, and I do get some minor prop rattle at low rpms. But, once she bites above 1200 rpms, its about 2-3 secs to plane with 2 aboard and gear (not much more with 4 adults). Also the lower pitch allows me to stay on plane at about 14 mph.
Hello Fire Dragon and glad to see your on W/C. Never mind which current mounting hole position is in use, how far above the keel center-line is your outboard's cavitation plate.
Make certain before measuring that the cavitation plate is as possibly level with the keel, a carpenter 4 foot level will be very helpful (as a straight edge and gage) during this process.
What is the weight of your F70? Current location of battery (for weight distribution)? Fuel capacity curiosity?
F.D. if you look at my signature you will note my Dauntless is over-powered but not by much. I feel these whalers where offered somewhat underpowered when available from the dealers. 4 adults fish or swim gear and they will struggle in a light to moderate bay chop with some of the small outboards I have viewed them with on the web.
Your F70 should be a torquey "son of a gun" and good on fuel, yes? Rule of thumb with "Prop Pitch" changes is, for each inch of increase 150-200 rpm decrease and just the opposite when decreasing pitch. These are simple guide lines allow for varibles with this calculation. I am currently going to test a "new to me" stainless prop on my Dauntless, from Power Tech 13" Dia. x 17" Pitch but a "Semi-Cleaver" type which should afford me better stern lift. This prop is modeled on Yamahas "OEM" Semi-Cleaver offering (typically painted in black).
Do you know what size is your lower-unit's gear case? 4.0 inch, 4.25 inch? Just currious, mine is a 4.25 inch I recall that being considered medium size.
I'm not to far North of you F.D., local Miami boater. Matheson Hammocks Marina is my desired launch area. Hopefully Tom will offer a suggestion to your inquiry.
