Saildrive causing increase in revs

[asteven221]

Our boat has a D2-55 and a 120s saildrive, with the standard 3 blade fixed prop. When running the engine in fwd whilst tied to the pontoon at about 1500rpm all runs fine. However if I put the revs up to 2000 after a few minutes, the revs suddenly increase whilst at the same time the turbulence at the stern drops dramatically. There is turbulence there, but much less. If I play around with the throttle, I can get it back working again at 2000rpm for a short period and it does it again.

At 1500 rpm it runs normally and we did 32 hours motoring like that last year. I would really like to get it sorted going into the new season. Something to me appears to be slipping, but what? The prop or the saildrive? Has anyone else come across this problem? As always, any advice is appreciated.

 

[Tranona]

First thing to consider is the rubber bush bonded into the propeller. The bond can break down leading to the sort of symptoms you describe. If it is not that then it may be the clutch in the upper gearbox unit. BTW 1500 is a bit low for cruising - should be nearer 2000 to get between 5.5 and 6 knots. When it was running normally could you get over 3000dpm flat out?

 

[Capt Popeye]

Not familiar with your Saildrive system at all ; but ,. with outboards using a Flexi mounted prop , the friction required to enable top revs to be transmitted to the prop can fail , resulting in the revs not being able to take the load so the prop starts slipping on its connection (flexi prop bond) so the boat will just not increase its speed throigh the water ; Maybe its a similar failing on your boat ?

Opps Mr Tranona beat me to it

 

[Alan S]

Are you saying this only happens while tied to the pontoon.? If so it could be cavitation/ventilation. I've seen it happening testing my outboard when tied up.

 

[asteven221]

Thanks everyone for your help.

Would I be correct to say that if the problem is with the rubber bush, I will need to get a new prop i.e. the bush can't be replaced?

I have not tested it away from the dock, so I should do that. Nevertheless even if that works ok, I am concerned that the boat will not stop with a burst of high revs in reverse, as that in some ways is quite similar to testing the boat when tied to the dock.

We do normally cruise at 2000rpm, however the last time the issue happened, we were just heading off on our summer cruise. I didn't want to stress the saildrive and potentially ruin our holiday, hence our cruise at 1500 to 1700rpm.

Thanks again.

 

[johnalison]

I had a failure of the bush on my Volvo unit. What I suffered was loss of drive and we ended up limping into St Helier as a couple of knots. I don't remember if the revs increased. I imagine that I tried to get more power by revving but it was some years ago. It was a fairly simple job for an engineer to replace the bush, which cost something like £400, but I believe that reconditioned one’s are available. My normal cruising revs are around 2,500 + with a 2- blade folding prop.

 

[Capt Popeye]

Yep replacing the Flex bush is apparently a normal service job ; for equiped reapair shops ; my local OutBoard repair sold me another s/hand prop that he tested before selling it ; UTube has video of the procedures if interested

 

[Daydream believer]

Perhaps check the oil that you are using is compatible with the ceramic clutch in the saildrive.
Sounds like a slipping clutch varying with revs, load & heat in the drive
There is some discussion about Volvo issuing an update to change the oil between 15/40 & Automatic transmission fluid. From memory ( do not quote me) It is now supposed to be ATF.
I use 15W.40 in my 120S saildrive
A friend of mine developed trouble with his sail drive, selecting the gears, This was embarrassing when berthing. He even bought a second hand drive to swop the gear selector, but to no avail.
Eventually he spoke to a very clever technical guy at Volvo, who suggested a different oil. I do not recall the oil being a "run of the mill" grade. But anyhow he changed it & by magic the problem disappeared.

 

[oldharry]

Thanks everyone for your help.

Would I be correct to say that if the problem is with the rubber bush, I will need to get a new prop i.e. the bush can't be replaced?

I have not tested it away from the dock, so I should do that. Nevertheless even if that works ok, I am concerned that the boat will not stop with a burst of high revs in reverse, as that in some ways is quite similar to testing the boat when tied to the dock.

We do normally cruise at 2000rpm, however the last time the issue happened, we were just heading off on our summer cruise. I didn't want to stress the saildrive and potentially ruin our holiday, hence our cruise at 1500 to 1700rpm.

Thanks again.

It may simply be that the prop is cavitating: running up to full revs while tied up is a long way from normal operating conditions, and places a much heavier load on the engine at full throttle. The prop's pitch and diameter will have been calculated allowing for water flow into the prop at a given speed. Running up while tied up, the engine and prop have to work much harder, and as pointed out there is significant chance of cavitation. When that happens the prop loses grip on the water, allowing the engine to run much faster, and producing a lot of foaming water. It should never be able do this with the boat under way though.

 

[st599]

It may simply be that the prop is cavitating: running up to full revs while tied up is a long way from normal operating conditions, and places a much heavier load on the engine at full throttle. The prop's pitch and diameter will have been calculated allowing for water flow into the prop at a given speed. Running up while tied up, the engine and prop have to work much harder, and as pointed out there is significant chance of cavitation. When that happens the prop loses grip on the water, allowing the engine to run much faster, and producing a lot of foaming water. It should never be able do this with the boat under way though.

And causing vapour bubbles which trash your propeller.

 

[Tranona]

Thanks everyone for your help.

Would I be correct to say that if the problem is with the rubber bush, I will need to get a new prop i.e. the bush can't be replaced?

I have not tested it away from the dock, so I should do that. Nevertheless even if that works ok, I am concerned that the boat will not stop with a burst of high revs in reverse, as that in some ways is quite similar to testing the boat when tied to the dock.

We do normally cruise at 2000rpm, however the last time the issue happened, we were just heading off on our summer cruise. I didn't want to stress the saildrive and potentially ruin our holiday, hence our cruise at 1500 to 1700rpm.

Thanks again.

If it is the bush and you have a standard fixed blade Volvo prop probably not worth replacing. The hub is very thin and has a habit of splitting as the bush is pressed in and many prop specialists won't touch them. A new prop is not hugely expensive at around £400. Worth it for a bronze hub folding or feathering prop, firstly because it is easier to do and secondly a new hub is over twice the price of a bush replacement.

BTW not convinced about cavitation. I have had 2 saildrive boats and have often run them in the berth. Don't get full revs but up to 3000rpm in both forward and reverse. Have also spent many happy hours doing controlled bollard pull tests on development propellers without getting cavitation.