MF 635 Maximam Speed with Nanni 85hp ? Propeller Size?

[defiance]

Hoping for some advice and guidance from owners that have similar!
I have just bought a MF 635 with the Nanni 5.250tdi and would appreciate some advice on what speed I should expect and if it is ok to run the engine at full power all the time? She is governed to 2800rpm and doesn't rev beyond that, but is it ok to run near constantly at those revs? She seems to have plenty of power and lifts her bow a little, but stops at about 12 knots...... I read that she should do more like 16 knots?
I have always had boats with outboards before and I know she is not capable of planing, but should she semi - plane?  Did all the 635's with the 85hp Nanni have a standard prop or are there different pitches?
Thanks in advance!

[rene460]

Hi Defiance, welcome to the forum and congratulations on your purchase. You will find a treasure trove of useful and timeless information in the archives, well worth browsing in long evenings of lockdown and winter.

There are even threads on people who have had similar questions on other models, though I can’t recall whether there has been one on your particular combination of boat and engine.

The starting point for all these issues is similar. First make sure that the engine has been serviced and is running well, and that the propellor has not been nicked or damaged or even become home for a small barnacle, and that the hull is clean. Those things cannot be diagnosed from specifications or other people’s experience, but can make a huge difference to performance.

Then look out your engines specifications. In particular, you want to know the specified maximum continuous rpm. Also, is the propellor directly driven from the engine or is there a reduction gear between the engine and shaft. I would also look at the boat specifications to see what engine size or sizes are offered as an indication of the appropriate motor size.

Then on the water, in suitable conditions, try pushing the throttle towards wide open. Don’t push the rpm more than 10% over that specified maximum. If you are not at wide open throttle when that maximum rpm is reached, that tells you that the propellor is a bit small for the engine.

If on the other hand, at wide open throttle, the rpm does not reach that maximum specified rpm, then the propellor is probably a bit big.

These tests tell you whether your propellor is suitably matched to the engine to allow it to develop its maximum power without overload. Then you are in a position to talk to a propellor supplier. It is worth talking to two different ones as both price and performance indications. Some will even help with trialling more than one size, once you have decided on your preference.
As to whether you can run hard all day, industrial and marine diesels are intended to be worked hard and suffer more from running too lightly loaded. The general recommendation is at least 80% rpm at least 80% of the time. Loaded up with the correct propellor of course. This is very different from an automotive application, where the normal duty cycle involves a wide range of operating loads and speeds, with only short bursts of maximum power. And it’s not entirely intuitive, so many people like to limit the operating speed to something lower to “reduce wear” as well as for better economy, even though this generally goes against the engine manufacturers recommendation. Check your Nanni manual, it will probably have something to say on the matter.

And if your boat won’t pop up onto the plane while you are sorting out these issues, please consider others and minimise your wake. That half plane attitude with maximum wake damages the shore line and is very inconsiderate to other boats nearby due to the violent unnatural motion it causes as your wake passes through.  It also burns fuel out of proportion to any higher speed compared with displacement speed.

And please let us all know what you find, it will help others.

Rene460