Prop loses grip...

[TBarCYa]

I've noticed the last two times I've been out that at certain times the prop seems to lose grip with the water for no good reason. Yesterday, I was cruising at about 25mph when it happened so I lowered the trim and it was fine. I then trimmed back up and it lost grip. This continued until I stopped the boat and tilted the drive full up/out to see if I had spun the hub. The engine was off and the throttle in forward and it ratched in one direction and didn't move in the other. Throttle in reverse did the same except that the directions were reversed. This is how it's supposed to work so it appeared to be normal. After that, she was fine the rest of the day. I was about 2/3 of the way up the trim portion of the gage (still using the first portion of the up switch) so I know the drive wasn't coming out of the water and when I tilted up I checked to make sure there was no debris on the drive. When I pulled out of the water, the hull looked clean and normal.

Any ideas on why this is happening? Thanks.

[seapuppy]

yeah..your cavitating.....what you do is to put the drive all the way down...then while up on plane...bump the drive up a little at a time...when you feel the boat take off and gain about 100 rpm or so...the boat is then trimmed properly......if your running the drive 3/4 of the way up ..your running it too hi.....your probably doing some damage to the coupler and the u-joint....be careful of running the drive too hi....

SP

[TBarCYa]

I'm not running 3/4 of the total range, I'm running about 2/3 of the trim range which is about 1/4 of the total range. I came to that position by setting the throttle and bumping the trim until I stop gaining rpm and mph. If you take a look at the pic in the link below, I drew a line about where I'm talking about. Hopefully you will agree that I'm safe running in that range and not damaging the driveline.

http://maxumownersclub.com/forum/galler ... temId=1578 (http://maxumownersclub.com/forum/gallery2.php?g2_itemId=1578)

[seapuppy]

well..I know each boat is different....and both boats' I've owned has only needed to bump the drive slightly higher to get the sweet spot...so..if your indicator can drop all the way down to the bottom..and your bumping up that high...then in my mind your way over trimmed......but if that is not the case ...and your slowly bumping the drive up..and it indicates what your showing...then your drive is pointed too hi and will be cavitating or you've spun the prop....my indicator is barely about 1/4 up off the bottom....it really doesn't take much.....
please understand..I'm not there ...I can't hear or feel or smell anything that is going on......so..I'm taking a wag.....

try putting the drive all the way...then just bump it up a little at a time.....you should never be as high as what your indicator is showing....set it as you have the indicator showing and take a pic of the drive angle....that will tell you how much your drive is up and if the indicator is out of adjustment or not...

SP

[TBarCYa]

I will definitely take some pics this week to get a better idea where the drive is pointing. It's a shame if that is running too high because it's really nice to pick up almost 10mph (over trimmed in) without touching the throttle.

[seapuppy]

if it's not a trim problem then it has to be a slipping prop or what is called a spun hub....

SP

[TBarCYa]

To diagnose a spun hub would involve putting the drive in gear and trying to turn the prop, right? If so, how much force should be required to turn the prop with a spun hub? I tried as hard as I could while leaning over the transom and could not turn it. I'll try again this afternoon.

[seapuppy]

well..I don't think you can do it by hand...pull the prop and pop the hub out...if the rubber is rounded out..then the prop has spun...and needs replacing the hub....shouldn't be that big of an effort...

when replacing the prop..make sure that you properly torque it ...something like 65 ft/lbs for a a1G2 drive..

SP

[TBarCYa]

Excellent, I will look at that this week also once it stops raining.

Thanks.

[racer2c]

For what it's worth, I can only trim my 18' out on glass conditions and even then I can only trim it up to the the middle of the hash marks before cavitation.
If I try to bump the drive up even a little in choppier water the boat will porpoise. That is why I was considering trim tabs to try to offset the porpoising but then I thought, why? Just leave it all the way down and roll with it.

[TBarCYa]

That's kinda good to hear. Good because it means I'm not alone with this 'issue'. Not good because you're probably losing performance and economy by not being able to trim your drive. I know that when I get to about 2/3 between the hash marks that on calm water I will have gained almost 10mph over fully drimmed in. If I understand it correctly, that's basically free speed since I haven't adjusted the throttle at all to gain it. I could be wrong about that and I'm sure there is some difference in fuel usage since I'm turning higher RPMs at that point, but it's got to be more efficient, right?

[racer2c]

That's kinda good to hear. Good because it means I'm not alone with this 'issue'. Not good because you're probably losing performance and economy by not being able to trim your drive. I know that when I get to about 2/3 between the hash marks that on calm water I will have gained almost 10mph over fully drimmed in. If I understand it correctly, that's basically free speed since I haven't adjusted the throttle at all to gain it. I could be wrong about that and I'm sure there is some difference in fuel usage since I'm turning higher RPMs at that point, but it's got to be more efficient, right?

Yep. The resistance of water on a deep hull boat is immense, thus the reason it takes 4000 rpm (and the gas to feed 4000rpms) in order to get and keep our boats at 40mph.
The more you can trim the boat so that it becomes more parallel with the water, the more efficient it is and the less resistance you need to overcome.
that's why some smart fella's invented hydroplanes, so that the hulls are up, out of the water and away from all that frictional resistance.
The problem with small boats like ours is that they are very susceptible to external forces i.e. chop. Chop throws trimming out the window on small boats.
That's why I don't even bother unless the water is glass. Now, I have heard from many that trim tabs help but it's just not that big of deal to me due to the water we boat on now. Now if I were on a smooth lake all the time it would be a different story.