I recently bought a Cavalier 28, and I'm really happy with the way it sails. It came with the original Bukh 10 hp and an old-style folding prop. The photo below was taken in October 2019, when I got the pre-purchase survey. At the time, both prop blades opened and closed freely. The prop has been treated with PropSpeed, but it is worn and should be replaced at my next haul-out, whenever that will be. There is also some play in the P bracket, which I have to get fixed. The Bukh is quite rattly, but that's probably normal.

However, I am surprised at how much "judder" I'm getting. If I try to wind it up to more than 4 knots, it feels like its going to do some damage to something. The rudder shook so much, I had to disconnect the autopilot. If I sat down in the cockpit, it felt like my teeth were going to get shaken out of my head. Even if I can physically tolerate the vibration, I worry about the P bracket. The only way to make it bearable was to take it down to four knots or less, which made for slow progress (about 3 knots) when I was motoring out of Manly into the Moreton Bay Chop today. Coming back, with the wind behind me, I could only do about 4 knots.

I previously had a brief conversation with Austral props in Sydney about a stainless steel folding prop. The new style props are geared so that they open symmetrically and, presumably, don't cause asymmetric loading. But, they are not cheap.

Questions for the brains trust ...
* A Bukh 10 is only 10 HP, but should I expect more than 4 knots with an average prop on my Cav28?
* How much vibration should I expect from the Bukh?
* Would the growth on the prop significantly affect progress and cause the judder? I expect it would affect performance, but the question is how much.
* Would the old-style prop be causing the judder? It felt like it was not opening symmetrically, causing asymmetric loading, hence the wear on the P bracket? If this is the case, I should get the prop replaced sooner, rather than later.
* Recommendations for a prop specialist in Brisbane.

Thanks in advance.

Hi Achenar,
I have the same boat and engine as you. It is only 10hp so it's never going to fly, I get around 4 knots at around 1800 rpm which is where I usually run her. And she's not been bottom cleaned for around 18 months. I take her to 2000 rpm only when a bit more speed is needed such as going over the bar in strong current or swell.
I never get any bad vibration at any revs and I run a 3 blade fixed prop.

If it was me I would be getting her out again and replacing the prop and getting the play in the P bracket fixed before it damages something else.
Also check the shaft coupling for play and the whole shaft for correct alignment, either of which could cause excessive vibration. And maybe check the engine mounts?

I can't speak for your boat type or engine, but I did replace my engine mounts and was astonished at the difference it made when I didn't think it was that bad to begin with (I did it on a recommendation of a service). It also doesn't take a lot of growth to start cavitation and a lack of speed.

If it were me, P bracket rectified definitely, and everything else you mention could be one part of the whole problem. I would either start with a trial and error type fix starting with either what I am comfortable with trying to fix or the cheapest..

Hi Archernar
There are a few things that can cause excess vibration.
I will go through a few of them and maybe it will help you.
I hope someone with the same boat/engine will pop up and give you a better picture.

* A Bukh 10 is only 10 HP, but should I expect more than 4 knots with an average prop on my Cav28?
My guess is yes. Especially as you mention motoring with the wind behind you.

* How much vibration should I expect from the Bukh?
If the engine mounts are of reasonable quality and in reasonable condition, most of the engine vibrations should not be transferred to the boat
So from a small single cylinder diesel you can expect a reasonable amount of vibration, but not excessive as you have described.
(I hope that is a reasonable answer)

* Would the growth on the prop significantly affect progress and cause the judder? I expect it would affect performance, but the question is how much.
On a small prop even a few barnacles will effect flow and performance. Try to always have a clean prop. However, I have had so many barnacles on my (small) pop that the boat only just moved, but no excessive vibration. So I think the growth is not your cause.

* Would the old-style prop be causing the judder? It felt like it was not opening symmetrically, causing asymmetric loading, hence the wear on the P bracket? If this is the case, I should get the prop replaced sooner, rather than later.
An unbalanced or incorrectly opened folding/feathering prop WILL cause excessive vibration. I know you checked it on the hard, but it could have jammed or picked up a piece of rope. Time for a dive over the side. You say that you had a survey in October this year. What did the surveyor have to say about the cutlass bearing? On some boats/ engines you can get away with a reasonable (there is that word again) amount of wear in the cutlass bearing. In others , not so..

* Recommendations for a prop specialist in Brisbane.
Can't help you with that one , but there are a few things you can do before lashing out on a new (expensive) prop,

I previously had a brief conversation with Austral props in Sydney about a stainless steel folding prop
If I was working at the service department, I would recommend a new prop (to be reasonably sure), because that would be my job.

You need to do a bit of faultfinding.
Dive and check the prop.
Does it vibrate at idle or in neutral?
Disconnect the coupling and run it in gear. Does it vibrate with the prop/shaft/cutlass bearing disconnected?
Check engine mounts.
Engine alignment would be about the last thing to check as engines don't go out of alignment once set properly.



All the best

gary

I had a similar vibration on my Olympic 40. Turned out the guys in Mackay that services my prop reassembled it incorrectly causing vibration and wearing the cutlass bearing quickly. Wasn't too hard to fix.
Under power at 1800 rpm on both boats I've had 5 knots is all I aim for and that's with engines around 50 ho so 4 kn isn't bad for a 10 hp.

HI Achernar,
As Gary mentioned, normally barnacle growth makes your prop inefficient to the point it simply doesn't bite, so chances are slim it will cause juddering. If a leg of the prop is not opening properly that could do it, but I'd be looking for a loose mount. either engine or P bracket.
If you don't want to get in the water, a go- pro taped on the end of a broom handle is pretty quick and easy way of having a sticky beak.
You should be able to test by tying up to the dock nice and snug and loading it in forrard and reverse.
I'm heading down the boat later this morning, PM me if you like, I've got the go-pro on the boat if that helps.
Cheers,
SB

I have the same prop and mine is very worn. That amount of shell is enough to reduce thrust. I invariably shift in and out of gear every time I start to motor, sometimes three or four times to get the blades to fully open so the shaft rotates smoothly. One of these years I'm going to pull the prop apart and resurface the wear points even if I have to install washers. It's just one of the downsides to having a propeller that reduces drag when sailing.

All of the above plus your rudder gudgeons/pintles are probably worn exacerbating the shudder through the rudder.
Glenn at D H Porter propellers Grose Street Parramatta very knowledgeable.
I have the same vessel/engine combo but I have a 2 blade Gori folding prop. Clean bum clean prop should be good for 5 1/2 Knots.
Fouled prop can have a very big impact on performance.

Has the vibration been there since you purchased the boat or just started. If it has just started I would be looking at the prop. With that type of prop and the small horse power any fouling will effect performance. If there is anything stopping one blade from going back not allowing both blades to sit exactly the same it will create vibration. The alignment doesn't need to be out much to create vibration, on my 3 blade prop I cracked one of the nylon back stops which allowed one blade to move back only a couple of mil at the tip which was not ever noticeable by eye. This created a vibration between 1500 and 2400 rpm which just happens to be my cruising range. Unfortunately it was just before I went north for 3 months, so I had to put up with it. When we got back we not only had to repair the prop but we had to also replace the cutlass bearing which was damaged by the vibration.
A a rule single cylinder engines normally run smoother with more revs, but as others have said you still should check the mounts, coupling, cutlass bearing, P bracket and alignment but I would start by giving the prop a really good clean as at the moment with the harbour water temp being so warm, barnacles will just about grow overnight.

That looks a bit like a Streamline prop; a very simple unit. My 28'er used to have a 10hp and would do about 6 knots with a Streamline, although she's a bit lighter than a Cav 28 and due to clearance issues my prop was pitched out. The same boat did over 6 when switched to a 9.8 hp Tohatsu outboard so I'd say 4 knots is very low.

For some time when I was young and penniless, the 28'er used to run around with a broken P bracket, and many years ago some offshore racers were racing with no P bracket at all. While my boat did vibrate at speed when the P bracket was broken, it doesn't sound as bad as your experience. I'd therefore tend to think that one blade of the prop is not opening fully, or at all. I seem to recall that sometimes if it was quiet and I rotated the shaft, I could hear the blades drop open; perhaps you could try to listen for that and it may indicate that one of them is not opening. As Jode says, a single barnacle inside the folding area could be the issue.

I asked Barry Colson, who did the pre-purchase survey, and he suggested the following ...

"I think you will find the vibration is caused by the play in the folding propeller. It had shims fitted but still had play in them and any amount of play would wear the shims. This coupled with the 2mm of play in the cutlass bearing would have amplified the vibration. If you are not racing the boat the cheapest fix I would imagine would be to fit a fixed pitch two or three bladed propeller."

Barry also send me some phone numbers for prop specialists and marine engineers, which I will follow up.

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Cockpit said..
Hi Achenar,
I have the same boat and engine as you. It is only 10hp so it's never going to fly, I get around 4 knots at around 1800 rpm which is where I usually run her. And she's not been bottom cleaned for around 18 months. I take her to 2000 rpm only when a bit more speed is needed such as going over the bar in strong current or swell.
I never get any bad vibration at any revs and I run a 3 blade fixed prop.

If it was me I would be getting her out again and replacing the prop and getting the play in the P bracket fixed before it damages something else.
Also check the shaft coupling for play and the whole shaft for correct alignment, either of which could cause excessive vibration. And maybe check the engine mounts?

Just an update for you Achenar,
I finally got over the side and cleaned her bottom and there was a LOT of barnacles on the prop. Cleaned off and she will now do a good 5knots or so like Troubadour says his does.
Memo to self: clean the bottom much more often...

Get the P Bracket fixed ASAP. Very expensive to replace if it breaks. I had this happen to me when the prop hit an object snapping the blades off and breaking the P Bracket in half. Luckily insurance covered the 10K repair bill.

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Cockpit said..

Cockpit said..
Hi Achenar,
I have the same boat and engine as you. It is only 10hp so it's never going to fly, I get around 4 knots at around 1800 rpm which is where I usually run her. And she's not been bottom cleaned for around 18 months. I take her to 2000 rpm only when a bit more speed is needed such as going over the bar in strong current or swell.
I never get any bad vibration at any revs and I run a 3 blade fixed prop.

If it was me I would be getting her out again and replacing the prop and getting the play in the P bracket fixed before it damages something else.
Also check the shaft coupling for play and the whole shaft for correct alignment, either of which could cause excessive vibration. And maybe check the engine mounts?

Just an update for you Achenar,
I finally got over the side and cleaned her bottom and there was a LOT of barnacles on the prop. Cleaned off and she will now do a good 5knots or so like Troubadour says his does.
Memo to self: clean the bottom much more often...

I do a lot of hull cleaning for owners around Cronulla and if not done monthly at the least the growth will win. Pretty bad here.