Lets talk props

[stinkynathan]

After my ignition timing via Seloc manual fiasco, I finally rented a timing light and spent some time out on a (very busy) small lake to get things fixed up. Runs MUCH smoother, starts easier, but I didn't gain any speed. I'm OK with that since it runs so much better. Also installed a TinyTach; cool little piece of kit that is.

Motor is an early 80s Mariner 15hp. I dinged my current aluminum prop (not too bad) early this season and fixed it up with a mallet and a file. I'm getting a GPS-verified 17.2+ mph on a small glassed out lake, or 16.5 mph on the choppier lake I timed it on. It has a 9.25x10P prop on it now that runs at 4800 RPM at WOT. Operating RPM range for this motor is 4500-5500.

I'm pretty satisfied with the holeshot right now; it pops up on plane pretty quickly then creeps up to top speed. What can I do to increase my top speed? It would be nice to get 20mph out of this thing. It seems that I could gain some speed by cruising at higher RPMs, but I don't know what I would need to order to get there.

Thanks for any help.

 

[cjldad]

What size boat? To get higher RPM's you would tend to drop prop pitch.. But you will lose some top end speed most likely.....

 

[stinkynathan]

It's a 14' V front.

 

[crazymanme2]

Is your motor as high as it can go?Your cavitation plate should be even or a little above the bottom of your boat.If you can trim your motor so it cares the front higher you might gain some MPH.Just make sure you don't go to high and loose water presure.

 

[stinkynathan]

I have the plate set at, or slightly above, the bottom of the boat. I can't go any higher on the transom because the mounts won't have enough to grab on to. Raising the motor got me to the point I'm at now.

 

[wasilvers]

I'm sure you tried this, but didn't read it on your post...

Did you change the tilt on your motor? My power tilt/trim is nice for this, I can swing my RPMs 800+(estimated) by playing with the trim and maintaining water for cooling and speed. I use it to reduce throttle (and gas usage) and maintain speed.

The idea is lifting the bow/boat further out of the water - removing additional resistance. It plays with your hole-shot and ride though.

If the ride gets too rough, I trim it back down to force the v into the waves more - cuts thru them nicely.

Good luck,

Will

 

[stinkynathan]

Yes. I've tried all angles and heights that my motor will sit on on that transom.