MF805 lack of planing speed

[fredinireland]

I have a 2004 MF805 with Nanni 4.390 TD with 976 hours. After some issues with excess steam and smoking the boat has been completely serviced with new filters, pumps, jet washes, anti fouled etc. and the injectors have been cleaned/serviced professionally. She now has acceptable smoke and steam for this boat but the speed is still way down. Top speed is 14kts at 3200 revs. It will not go higher. This is way lower than spec. and hardly planing speed. We have 2 persons on board and little extra gear. I am out of ideas how to get a decent cruising speed of 17/18kts. Any ideas would be appreciated.

[rene460]

Hi Fredinireland,

It looks like none of the MF805 owners on the forum have seen your post yet, but there are a few things you can do to help understand what is going on in the mean time.  Certainly, not being able to get on the plane looks like a problem.  Hopefully someone will come in with their performance for comparison.

Less than 1000 hours after so many years does not indicate a lot of use, unless the Nani is a replacement of the original engine.  It might explain the excess steam and smoke, and you have attended to that by a major service, so it is reasonable to feel that there is nothing wrong with the engine.  You did not mention whether the boat used to go faster, so something has changed, or if it has never gone faster, at least not under your ownership.

I assume that the hull has been recently cleaned, and is not badly fouled with weed or barnacles, so next step is to look at the engine and propellor combination.

You quoted top engine speed of 3200 rpm.  I assume that this means wide open throttle.  I don’t know the Nani specs, but if you look up the specifications for your model, it should include a maximum rpm, and a maximum continuous rpm.  If the specified maximum rpm is higher than 3200, then the propellor is requiring more torque than the engine can provide.  On the other hand if the maximum rpm is less than 3200 rpm the engine has more torque available.  As an aside, Yanmar recommend that the wide open throttle rpm should be about 10% above maximum, and it is reasonable to assume that would also apply to the Nani.  It is also worth checking the rated engine power of the Nani with the specified power options in the boat specifications.  (Don’t proceed to wide open throttle if this means exceeding the specified maximum rpm by more than 10%.)

If the propellor is not fully loading the engine, it could be that it is too small.  Propellors diameters are usually selected as large as will fit without touching anything for best efficiency, but leaving about 10% of the diameter clear of the hull to avoid hull vibration from the blades passing.  It may well be a bit smaller to be within the range of available propellors, but will look out of place if it is a lot smaller.

On the other hand, if the propellor is requiring more torque than the engine can provide, there are at least two possible reasons.  Most commonly, especially if performance used to be better, is fouling of the propellor.  In my experience, even a single quite small barnacle on the propellor blade dramatically increases the blade drag, and hence the torque necessary to turn the propellor, so reducing the engine capacity to reach maximum speed and power.  Either go for a swim, or hire a diver to look and scrape any offender off.  Do not be surprised if only something tiny is found.  Obviously propellor damage can also increase the blade drag so increase the power required to turn the propellor.  Check the hull for any fouling or pick up of plastic bag or rope on the struts or rudder while you are there.

If the propellor is nice and clean and the blade area is smoothe, it is possible that you need a propellor with a bit more pitch.  Propellors are most efficient with a pitch about 10% higher than the diameter but a bit either side is pretty normal, as blade area and blade profile are also important factors.

By this stage, it should be possible to better understand what is going on, the first step to correcting the problem if cleaning the hull and propellor and engine servicing did not help.

Please come back and let us know how you go,

rene460

[fredinireland]

Thank you.

The hull was clean and prop polished.

The rated rpm loaded is 3600 and unloaded 4100/4300.

I recall doing 17+kts when we collected her. No point in thrashing a new purchase off the bat so was cruising home. Weather got bad and we reduced speed to 12/13kts or less. 7 hour trip. After 6 1/2 hours noticed steam from exhaust.

Next time I tried her I could only get 13kts max WOT.

Then we started the long saga to find out what went wrong.

Lack of Turbo power? Lack of fuel pressure? Lack of compression? All require special tools and skills except boost pressure.

Prop is as original I am sure of that.