hull vibration

Last Winter I restored my 1975 Mustang 17 with a new long block 307 RH, new stringers and floor. I took care in lining up the shaft/motor and installed new packings in shaft and rudder. I managed to get about 20 hours on the boat this year just enough to get the new block broken in. What a treat to have the girl start each time I went to use her. I did notice a hull vibration around 3k, but of course my wife did not. Since I live in the wild Northeast our season is nearing a close, so I yanked the boat last weekend. I pulled off the very tight prop, which appeared to be fine and measured the prop side of the shaft after the strut and got less than .001" variation.

Now I am a motorcycle junkee as well and got the idea to check the 12rh14 federal on my bike wheel balancer.
This may be a ridiculous idea, but I figured what the heck


It took nearly 3/4 oz of weight's on the hub to get it balanced. I figured it would require much less further out on a blade but no blade lined up where I needed the weight.






Is this enough to cause this vibration, or should I look further?

Every federal I've run vibrates pretty good. It becomes a lot more obvious if you put one on back to back with a decent hand finished prop (OJ) or a nice CNC wheel (acme). Once you feel the difference in performance an smoothness, that federal will go in the toolbox as a backup and (hopefully) never reemerge.

The only other thing worth checking is the strut bushing... Any slop?

There is a very slight movement, if I really yank it to the side (maybe a couple thousandths).

I have about an inch and a half between the prop and the strut. Is this excessive?

540 is the wrong prop on that hull, Zachy boy... 1210 is the 12.5" diameter version that fits on the smaller hulls.

If there is an allowable tolerance on strut bushing wear, I can't imagine .001" would be out. But wear is wear, and it might be a minor contributor.

1.5" is certainly excessive on the strut to prop clearance, but at the same time, it's not "abnormal" for cc to have set the boat up that way. I would not go out of my way to dial it in on an otherwise ok performing stock boat, but if the need for a new shaft arises, by all means get that clearance below an inch (ideally ~1/2").

My trig is rusty but with a 15* strut angle you'd lose .26" clearance.

I don't know how aggressive the rake is on an 1210 (not all Acmes are equal) but you'll gain some clearance back from it. My gut says you'd be fine.

I put the prop back on and measured 3/4" clearance to hull bottom and 1 1/2" to strut..

TRBenj wrote: The 1210 is raked just like the 540 as it is the same prop just cut down 1/2". M17 used the same 6a strut as the 71+ 16'5" boats (skier/Tique/m16). You will keep clearance the same with the 12.5" 1210 as a 12" federal or oj, possibly gain some clearance due to the aft rake. If you were to modify the shaft, which I would only encourage if you were replacing the shaft anyways OR were pushing the limits of the hull (speed wise) then HW is correct with his trig- you give up about 1/3 of the shortened length in hull clearance. 1" of clearance is plenty, I wouldn't sweat 3/4" much... Anything beyond that I'd have to consider carefully- but that should not be the case with the prop/hull/strut combo that you have.

Thank you... should the center of the shaft be dead-on the center of the rudder? I do notice more vibration while turning

should the center of the shaft be dead-on the center of the rudder?

On closer inspection I saw that the strut was slightly bent to starboard about 1/4"- 3/8". I left the shaft in and with a woodworkers pipe clamp, a big steel bar jammed into the trailer to pull against and my torches, I was able to straighten the strut perfectly centering the shaft into the log hole. I then quickly wrapped soaking wet rags around the strut to remove the heat and protect the hull. I then checked for a bent shaft and/or coupling at the motor and found nothing bent. I then removed the strut and the cutless bearing (ordered a new one from DB for $56 delivered. When the bearing arrives I will install it and remount the strut with 5200. Then I can check for alignment at the motor; hopefully I will be good to go. Anyone know where I can get a good deal on an ACME 1210?

If you intended on removing the shaft why the hell would you have messed with the strut with it still in? Have you disconnected the coupler and checked your alignment? Is the shaft centered in the log? Not really sure why you are so dead set on messing with all this. Sounds like I'm a day late but the shaft does not need to be centered on the rudder.

Surely you changed the angle of the shaft by moving the strut. Now you need to move the engine to align it with the new shaft location. Do your boat a favor and figure out what a drivetrain alignment is. Bolting the couplers together is not sufficient.

Once you unbolt the couplings the shaft will likely move! Chances are it probably won't be centered in the log anymore. This is assuming you had a proper alignment before the strut massage...

Maybe the strut didnt need to bend at all?! Alignment starts with the shaft centered in the log, then centered in the strut, and ends at the transmission couplings. You wanting it centered on the rudder is superficial. If strut, log and coupling alignment was fine before I wouldn't have bothered doing anything.

However, to get the shaft centered on the rudder I would have removed the rudder and shaft first. Next, loosen the strut bolts. Then shimmy the strut as necessary. There is some play in the bolt holes. Check by reinstalling shaft and rudder.

I think hammering on the strut with the shaft installed was a bad idea.

I don't have any 1210s. That's much smaller than I typically deal with.

Anytime the couplings are split or I bugger a prop the alignment gets checked. These struts are super soft and bend easy. ( my ACME is in rehab as I speak :( ) I will fess up though to taking a little twist out with a 15" crescent wrench and cheater bar. This I have done (twice) with the coupler split and my goal being the shaft centered in the shaft log. (pull the hose off for accuracy) Anything other than a slight bend and it goes to the strut press at the prop shop. In my experience if you hit bottom hard enough to fold a blade you have more than likely bent the strut.
I run fast and shallow over oyster shoals to where we ride... Ask Chris... he has seen it.
This also why I would never use 5200 to put one back on. I need a sealant, not an adhesive that will tear the gelcoat off next time I have to service the strut. The bolts should hold it on just fine :)

That reminds me, I should have bought Hollywoods prop... didnt think I needed two spares..Now Im gonna have to run the OJ for the weekend.

Spare ACME has a nice ring to it..

Interesting. I have ~1" on my Skier. It baffles me how 2 flat bottom boats with the same 6a strut could have such wildly different clearances (my prop to strut clearance is slightly higher than optimal, but not excessive... Around an inch iirc).