Feathering Prop Problems

[Slowboat35]

I'be just fitted a Featherstream prop to my 35ft long-keeler and it seems to be producing a great deal of deep rumbling vibration. On the dock with no hull speed the propulaion system runs fine but underway there is a deep and quite heavy rumbling coming from under the cockpit floor well aft of the gearbox. It resembles the noise and vibes I had when the cutless bering had ⅛ inch of play in it - clearly something wrong.

The entire propulsion system is new (20hrs).

The prop has adequate tip clearance in its aperture but is fairly close to the deadwood of the keel which is 3-4 inches thick and more or less square-cut rather than chamfered.

Anyone with experience of this sort of thing or able to advise how to troubleshoot without buying a new fixed blade prop to compare it with?

 

[LONG_KEELER]

Just a thought. Is the propeller providing good thrust under power and has the sailing experience improved ?

 

[LittleSister]

Does it do the same at a similar speed in reverse?

I can imagine that a feathering, folding or variable pitch blade might potentially flutter as it passes through water flowing at different rates as a result of an obstruction such as a substantial width keel, but don't have any real knowledge of it.

I have a feathering prop behind a long keel, and I haven't noticed a problem, but the aft edge of my keel is relatively finely tapered in section and the prop is not as close to the keel as some.

 

[Ammonite]

I'be just fitted a Featherstream prop to my 35ft long-keeler and it seems to be producing a great deal of deep rumbling vibration. On the dock with no hull speed the propulaion system runs fine but underway there is a deep and quite heavy rumbling coming from under the cockpit floor well aft of the gearbox. It resembles the noise and vibes I had when the cutless bering had ⅛ inch of play in it - clearly something wrong.

The entire propulsion system is new (20hrs).

The prop has adequate tip clearance in its aperture but is fairly close to the deadwood of the keel which is 3-4 inches thick and more or less square-cut rather than chamfered.

Anyone with experience of this sort of thing or able to advise how to troubleshoot without buying a new fixed blade prop to compare it with?

I had something similar with my 28ft long keeler but resolved it while I had a three blade fixed prop before switching to a featherstream. In my case it was cavitation caused by inadequate clearance ahead and behind the prop, with clear evidence of this in the form of pitting / hammering of the prop antifoul on the side of the blades facing the deadwood. The later is circa 4 inches wide and square in section like yours. Dropping the prop back 20mm so there was 25mm of shaft aft of the cutless rather than the original 5mm and enlarging / fairing the cutout in the rudder completely resolved the problem. Im guessing that in your case the design of the featherstream means that the blades are not in the same position as the old prop and it doesnt take much to cause a problem if the clearances are on the tight side. Quite how the original owner had put up with it I have no idea as anything more than 2000rpm (2:1 gearbox) was unbearable whereas its now fine at 3600 max rpm with a cruising rpm of 2700/2800. Tip clearance remains tight but just within the recommended limits and this wasnt a problem in my case. An alternative would have been to narrow and fair the deadwood but that would have been a major undertaking compared with dropping the prop back and modifying the glass sheathed wooden rudder which I was able to do over winter in the warm at home, but if you have room a longer shaft (or dropping the engine back) might be a lot easier, but do check theres not something fundamentally wrong with the prop first. Just dont drop it back too far as there are recommendations for this based on the shaft diameter. From memory I think the unsupported length should not be more than the shaft diameter but would need to check. No more NOB gags please! See other thread