Prop Wash and Vibration: Wrong prop Sizing?

Bill1565 · Jul 22, 2013

coleman said:
I have a 3QM30H in my 1980 Hunter 37C.

Just did a haul out and had a local yard do a bunch of work:

[*]New stainless steel shaft with all new hardware
[*]New Duramax Cutless bearing.
[*] New PYI PSS Shaft Seal (dry in the bilge, now)
[*] Reputable prop shop that made the shaft also touched up the 3-blade prop that was on the boat, I couldn't determine the manufacturer and it's a 15X15 according to the only stamps I could find (front face of hub).
[*] New shaft zinc and Pettit Barnacle Barrier to try and protect the new hardware
[*]Engine alignment checked and was very good with no required adjustments.

I still get vibration which I can't reliably attribute to the engine (too low of frequency) and the prop wash in reverse has always been awful as long as I have been sailing on this boat.it is.

Check your motor mounts. Push,pull and pry on the engine to see how much it moves.

By prop wash, I guess you mean prop walk under power in reverse. Should not occur while sailing . Some people say that folding or feathering props will lessen this. Changing from one fixed 3 blade prop to another probably will not help.

My advice is learn to use it. You can turn your 37 end for end in38 feet. Yes, it is true you can only do this in one direction,and sometimes you must make a 270 degree turn, but you can do this easily in close quarters with practice. Also after you get in motion in reverse, the rudder will steer you, but it will also snap to full over unless you control it closely.

Don12364 · Jul 22, 2013

You are clearly serious about your vibration problem because of all the trouble and expense of all the work you have had done; otherwise I would not have been so bold as to offer an opinion.
But to my eye, your photo of the amount of overhang of the prop behind the cutless bearing is simply inviting vibration or whirling of the system. It may seem drastic but if the boat has always had this problem then a shaft with the prop as reasonably close behind the bearing as possible, e.g. 1/2", would provide much more rigid support and much less leverage of any out of balance loads, both dynamically and hydrostatically, back on the engine and its mountings.
The mounting of the strut looks to have sufficient rigidity and one presumes the bolts are tight; also presumably the engine mounts are okay - so it has to be something else.

A new shorter shaft need not be too expensive or you just need to borrow something to slip into place for a test and then have the new one shortened if the test is satisfactory. No need to go to the trouble of re-aligning to the last thou for this check. The prop shop or machine shop that made your new shaft may be in a position to help.

BTW I even worry about leaving sufficient gap for a line cutter.

kloudie1 · Jul 22, 2013

Good catch, Donalex.. I didn't see that it was that far aft of the strut.. MaineSail posted some info about his feathering prop having too much overhang and causing a lot of vibration.. The shaft can be easily shortened at the coupling end if you have room to pull the coupling .. ya just saw off as much as ya want then re-install the coupling.

Johnb · Jul 22, 2013

Per the surveyors recommendation we had the prop shaft shortened on our HC 37. This is what it looked like afterwards. The rudder is removed.

Later we went to a flex-O-fold prop and love it. The prop walk never goes away but you can either use it to your advantage or learn to briefly gun it and then steer as soon as some speed has built up.

rardiH36 · Jul 22, 2013

I've got a 1980 Hunter Cherubini 36. 2QM20 engine. Same basic construction as yours, but two instead of three cylinders. Mine also vibrates annoyingly. Not much difference when running it either out of gear or motoring. The vibration does cancel out somewhat at certain rpms, however.

I've read that 2QM20's are prone to shaking. Not sure about the 3QM series. I did change props from a 2-blade to a Campbell Sailor prop 3-blade (which is a different animal than the dumbo ears type of three blade). Didn't make much difference. However one item that improved the shaking was several years ago after I lost my engine power due to air in the line (I let the fuel tank level get too low). I put in all new fuel lines after discovering some minor cracking in the old ones. Afterwards, the vibration did moderate noticeably. Following, I did read that air in the lines can cause unbalanced detonation of the cylinders. Another item I've read about is the injector timing. If off on one of the cylinders vis-a-vis the others, then again the detonation of the various cylinders can be a few milliseconds off ideal. My engine runs very well, and motoring is generally limited to leaving the berth and 1.5 miles in the channel. So I'm not inclined to spend the time to go through the injector timing process.

As to prop-walk in reverse, one reason for buying the Campbell Sailor prop were reports that it could help with my also very pronounced prop-walk. (The other reason is that on-line and as tested by Maine Sail, the Campbell Sailor prop has significantly less resistance when under sail than standard props.) But actually the Campbell prop hasn't made much difference.

My 36 also has the prop mounted a bit far back from the cutless bearing -- about 4" or so. But if I were to shorten this distance to the 1/2" as mentioned by Donalex, then my prop blades would be too close to hull. As it stands, the clearance I have is less than is usual on boats. This also could be a reason for the pronounced prop walk, I have also read about.

Anyway for now, I just live with the walk and the vibration.

Blaise · Jul 23, 2013

The prop walk will make you look like a genius or a bafoon. I prefer genius. Two blade props walk less than three blade props. The shaft is definately too long. My local shop says no more than 1 inch from bearing to prop hub. When sizing props, 1 inch of diameter is generall worth 2 inches of pitch. In my friends 37c with a three blade fixed prop the best way to back it is to briefly go to full throttle with hard right rudder, then ease the throttle as the boat begins to back. Once you get the boat moving, the rudder will take over and you can back the boat anywhere. It takes a LOT longer to get Midnight Sun moving because of my two blade Martec folding prop, but once it does move the boat will back in any direction. While being bored waiting for starts I used to motor around in reverse standing in front of the wheel facing aft. Great view with the entire boat behind you. I stopped when a friend on another boat told me that I had just ripped one of my rudder fairing plates off.

Johnb · Jul 23, 2013

I would also inspect the mounts. The attached picture shows what a bad one looks like - it is bulging on one side.

The highly respected yard manager also said we had a mixture of mismatched mounts before we had ours replaced.

It actually made little difference and we did not have much vibration before anyway but I still felt good about having done it.

Prop Wash and Vibration: Wrong prop Sizing?

coleman

.

Attachments

Bill1565

.

Don12364

.

kloudie1

.

Johnb

.

Attachments

rardiH36

.

coleman

.

Blaise

.

Johnb

.

Attachments