Overpropped?

[jonathanhsm]

My Perkins Perama M20 18hp inboard engine (similar to Volvo MD2020) is rated to rev at 3600 at full throttle (WOT). But I can only get this in neutral. Best I can get in gear is 2600 or so - in fact there's barely any rev change in the top quarter of throttle travel. It seems to be severely underpowered when trying to run at top whack. NB this is the same when hull is clean, exhaust riser is clear - fuel filters have been changed and there's no noticeable black smoke.

I've lived with this for years as I don't lift out and antifoul on scrubbing grid between tides. and generally pootle about at half throttle.

But now for first time in aeons boat is ashore for a proper scrub check over etc...and I can do something about prop.

I have taken off the prop - first time for me- and it is caked in several layers of antifoul which I'm now removing.(See attached pic)
So my question (finally!) is: Could the historical antifouling cause a drop of 1000 rpm from rated WOT ?
Or am I overpropped?

I can just make out the prop is sized 15 X 11
Should I try another prop with a lower pitch ... say 15X10 or 15X9 ?
Or get my existing prop re-pitched?
Or stick old prop back on and live with it?
what does the panel think?
Many thanks

 

[penfold]

What boat is it and what ratio is the gearbox?

 

[saxonpirate]

One thing I'd like to add to @penfold 's question.. what's the wake like.. clean, turbulent, any signs of cavitation?

 

[jonathanhsm]

Its a Wauquiez Victorian 27 - pretty rare in Britain VICTORIAN 27 - sailboatdata
can't remember gearbox ratio off hand but it's an early Hurth - I'l check tomorrow

 

[jonathanhsm]

One thing I'd like to add to @penfold 's question.. what's the wake like.. clean, turbulent, any signs of cavitation?

Have to admit I've not really noticed. Looks pretty normal to me in my limited experience.

 

[MoodySabre]

I've recently had a new Beta 30 installed and I can't reach max revs. 2850 is it but I'm almost at hull speed. I'm overpropped with a Featherstream but at that speed more revs wouldn't achieve anything except wasting fuel.

 

[penfold]

The prop is manky and won't help but being short 1000 rpm is a lot. That engine should be shoving you along at hull speed, plenty of power for that. What speed are you achieving at 2600rpm? Is the speed control lever/fuel rack at the engine reaching maximum when the control lever is at full power?

 

[jonathanhsm]

Hull speed should be around 6 knots ... but can't achieve that without tide. And I'm afraid my impeller is another long term fail that needs fixing ! so usually rely on GPS which of course doesn't give me boat speed in water.
I'll check the fuel lever.
Random thought that has come to me while looking through my old maintenance logs. I had a fuel priming bulb which restricted flow so I replaced it. The 'new' on is several years old. Maybe it;s restricting flow too ?

 

[ithet]

I have a MD2030 driving a 16" Maxprop feathering prop. Until this year the max RPM was always 2800, well below the recommended band. Over winter I reset the the prop to next lower pitch, reducing by about 1". It now revs nicely to 3400 rpm under load.

 

[Tranona]

Yes, way overpropped A 13*8 or 9 would be much more appropriate if it has the common 2:1 reduction. Even if it has a 2.5:1 it would still be a 14*10 or thereabouts. I expect the prop is the original from the Volvo engine it had when new which has a much lower shaft speed.

So new propeller. check the the actual reduction then give the boat's data to a supplier such as Lake Engineering or T Norris who will do the sums and supply the correct size.

 

[neilf39]

I have a 15x8 3 blade on a beta 25 so a 15x11 on a 18hp seems a bit on the large side.

 

[ChromeDome]

If the prop is otherwise in good shape you may have the pitch changed to suit your need.

Less expensive than a new prop and can be bespoke as opposed to new prop's predefined pitch steps.

 

[jonathanhsm]

Yes, way overpropped A 13*8 or 9 would be much more appropriate if it has the common 2:1 reduction. Even if it has a 2.5:1 it would still be a 14*10 or thereabouts. I expect the prop is the original from the Volvo engine it had when new which has a much lower shaft speed.

So new propeller. check the the actual reduction then give the boat's data to a supplier such as Lake Engineering or T Norris who will do the sums and supply the correct size.

You may well be right about old engine. I've checked (on a French forum where I found previous owner) and apparently it was a Yanmar YSB8
if that's any help?
Meanwhile I'll get onto supplier as you suggest after I've identified gearbox.

 

[Boater Sam]

Is your tacho accurate? Most aren't.

 

[scottie]

How many times do people just use the old prop with a new engine to save a few bob?

 

[B27]

What is your goal in all this?
Traditionally, the object of the game was to use the maximum power available from an undersized engine, so it was crucial to load the engine to match its power curve to the torque required by the prop.

I think that starting point is open to question, with 18HP on a 19ft 4 waterline.
If you match the prop optimally for max power, all you will achieve is a lot of noise, a big wash and an extra fraction of a knot, pushing the hull a little further up the drag/speed curve in the region people lazily call hull speed. Using that extra power will cost you in fuel.
To get max power into the water may need a prop which is less effective at cruising speed and poor under sail.
With your boat, the prop will be restricted by the space around it.

The proper approach is to work out how much power you really want, and then make sure your engine can rev enough to give that power.
You want a reserve of thrust in case you need to bash into a gale or waves or plough a furrow in the riverbed.
So, I would be asking, how many RPM will it do in 'bollard pull'? Does the negine struggle to build revs when the boat is not moving?
Possibly your prop is on the coarse side, even from my point of view.

What I would suggest you don't need, is to be cruising above about 2200 RPM. It's much more pleasant to spend time on yacht which will cruise at lower RPM.
That is a big slice of the reason for putting a bigger motor in, to avoid spending time with a cement mixer engine doing max revs.

If that were my boat, I would be wanting a feathering prop for it though. Alas, they are pricey.


You need to find out the gearbox ratio to make any sense of it.

 

[parbuckle]

Non marine engines came with various camsshaft specs so could be marinised industrial unit say from a generator that ran all day at 2600rpm or may be a camshaft from a cannibalised unit the governor would be set at speed required

 

[Snowgoose-1]

Providing tip clearances OK, it would be nice to retain the current diameter prop with either reduced pitch or blade area. Think length of a gliders wing or luff length on a sail. As suggested, worth having a chat with professionals . Probably means a new prop.

 

[Tranona]

The boat is overpowered for its displacement. It only needs about 11hp to achieve hull speed. The correct prop will allow it to run up to maximum rpm and get 7 knots. Unlikely you will ever need this, but you also have to consider that the engine will be better running in the working rev range of 2-2500rpm which will allow a cruising speed of 5knots+. There is an argument for overpropping a bit to reduce the cruising rpm a bit more, but it has no effect on fuel consumption and that model engine is very smooth in the working range.

 

[scottie]

Non marine engines came with various camsshaft specs so could be marinised industrial unit say from a generator that ran all day at 2600rpm or may be a camshaft from a cannibalised unit the governor would be set at speed required

There are enough indicators that it the wrong prop and whilst your comments are valid they are a little exotic in this case perhaps