[Anonymous]

I have recently had a new Volvo folding prop fitted to my 1999 Bavaria 31 yacht and while it sails much better, I am unable to get good speed under engine only. Has anyone had the same problems?

Bryan Hunt

[Anonymous]

Is it the two or three blader?

[Anonymous]

It is a two bladed prop, great when sailing but under powered when motoring and against any tide – hopeless.

Bryan

[Anonymous]

@bryanhunt wrote:

It is a two bladed prop, great when sailing but under powered when motoring and against any tide – hopeless.

Bryan

Wow, mine seems OK might beworth checking you have the right size?

[Anonymous]

Bryan, Hi.
Looking at the spec for a 1999 31; The MD2020 has max revs of 3400RPM giving 19hp new. Your boat has a water line length of 27ft, the max hull speed is 6.23 knots. (1.2 x square root of water line length) If the engine has the power to start to climb over the bow wave a little you may in theory be able to get 6.5 knots water speed at full throttle, with a clean bottom on a good day.
The pitch of the prop you have is fixed. The pitch on fixed propellers is always a compromise between under pitched and over pitched to give the best overall performance. Think of it like a car gearbox, in 1st gear you have lots of pulling power but the engine revs high, speed is slow. In 4th the engine revs are low, there is more speed but no pulling power. The pitch of the prop is fixed somewhere between 1st and 4th to give you an overall “gear” that suits a number of conditions. If that makes sense?
I doubt that you are able to get the full rpm out of your engine under load, so you are probably looking at a best water speed of 5.5-5.8 knots at full throttle, allowing for the pitch compromise, prop drag, engine age and a max rpm under load of 3000?
If you add a headwind, tide and a bit of a swell, you could be down to 3-4 knots through the water, at best. If the prop has been supplied specificly for your boat I doubt that it is pitched wrong. But, it is a possibility.

However, I would suspect that you are barking up the wrong tree…………

I think you are not getting anywhere near 19hp out of your engine which is why it cannot drive the prop in 4th gear as it were. Before embarking on more time out the water or prop changing… Check that the exhaust manifold at the injector point is not partially blocked. This will restrict your engine power. You could be reving ok without a load on, but down to 7-8 HP under load.
It is very easy to check, 4 bolts, 2 hose clips. You don’t need to take the boat out the water. This is a very common problem. If the manifold has never been changed it will definitely need doing as they rarely go 5 years.
A very quick way to check is to remove the water injection hose at the elbow and see how big the hole in the casting is. It should be 12mm dia minimum and clear into the injection elbow. I bet it’s a lot less! And the exhaust manifold elbow will be down to a pencil width! Don’t bother trying to clean it up it will be rotten as a peach inside anyway 🙂 
New manifold is about £160.00 and takes half an hour to replace.

Hope this helps

Best regards Ant

[Anonymous]

Hi Ant,

Many thanks for the reply – as you suggest we may be barking up the wrong tree. It would be fantastic if we could solve it so simply. Will get Clipper to check it as I am in the Midlands (150 miles away)

Once again many thanks – put you a pint on the bar in Cowes!

Best wishes,

Bryan

[Anonymous]

Bryan, if you need a hand to look at it when your down next give me a shout (and save yourself a few quid) I am in Horndean near Portsmouth, so not far away.

Best regards.

Ant.

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