Roberts Spray 36 only does 3K with new 50hp volvo. Is this right?

[DennisRB]

Thanks, but according to my manual there are 2 versions. One that goes to 3000 and one that goes to 4000. Ours is the 3000 version.

[baeckmo]

Ok, that makes it a MD22L-A, produced 1995-2002, primarily sold together with the Penta saildrives. It has a lower compression ratio than its higher rated brother. Still, to come closer to a solution, we must know the gear ratio.

[DennisRB]

Yes the manual did list a few configurations one of which being the sail drive. This is just the shaft drive version. I will do my best to find out as much as I can about the ratios and prop when I next see the boat in 2 weeks.

[anthony goodson]

Check out your cylinder compressions if you can, these engines eat headgaskets ,especially if they are not used and the antifreeze solution is weak. Keep an eye on the exhaust manifold too ,another weak point,

[DennisRB]

This engine pumps seawater. It looks in great condition and sounds great.

[baeckmo]

Dennis, this engine has a closed fresh-water cooling cirquit that takes the heat from the cylinder block and heads. The heat is then transferred to the sea-water via a heat exchanger. So Anthony is correct (....he often is!) in pointing at the head gasket with referrence to anti-freeze/anti-corrosion fluid concentration.

This engine has not the best of reputations for endurance;your best chance to a long engine life is to be careful about service routines; clean fuel, clean lube oil filters, correct cooling. And, of course a basic understanding of its working principles!

But my personal approach is still to gather correct info on the transmission and prop, before digging into the engine "hardware".

[DennisRB]

I was fairly sure this was salt water cooled all the way, but I didn't really have a good look. I didn't notice a header tank or radiator cap anywhere for checking the coolant.

[anthony goodson]

Well Dennis if it is an md22 and it doesn't have closed circuit cooling ,then with a little judicious wiring between ally head and iron block you should be able to run a set of nav lights ,only joking. baeckmo is quite right of course it is a Perkins Prima painted green. The failures he rightly mentioned are, apart from head gaskets ,mainly marinisation parts problems, the basic engine is bombproof . On the plus side the engine was used in the Austin/ Rover Maestro/Montego and some LDV vans ,there is also a turbo version. Because of this and the fact that it is also a very popular engine transplant for "series " Landrovers there are a number of owners forums where you will find any information that you need .HAYNES also very unusually produced a separate manual for this engine.I know you are concentrating on the propeller at the moment ,but this posting may help you in the future.

[DennisRB]

This had an oyster farm on the prop. It does 6.5K at full power at around 2900 rpm and cruises at 5K at 2400rpm with it cleaned off. The prop still isn't 100% clean and the bottom is still a little dirty. The engine certainly is fresh water cooled too. We had only been on the boat the once and its a big drive away. But now we have it moving at a decent speed under power we will bring it up from Yamba to Brisbane. Still sails just as slow though.

[FAST FRED]

AS a mental concept Herrishoff sr. used to advise clients to envision the prop scaled to the size of the boat , growth and all, to get the idea of what the powering results would be like.

With a clean bottom and the correct prop you should see 6.5K or so at much lower rpm. Say 1800 -2200

FF

[baeckmo]

Originally Posted by FAST FRED

AS a mental concept Herreshoff sr. used to advise clients to envision the prop scaled to the size of the boat , growth and all, to get the idea of what the powering results would be like.FF

Hah, that's an excellent comparison!

[DennisRB]

Nice analogy.

We are going to get a wire brush on the prop before we go on our cruise. When we find a suitable place for the yacht we will do the bottom and really polish the prop too. I guess, only then it might be time to start looking at possible changes in prop etc.

Originally Posted by DennisRB

Nice analogy.

We are going to get a wire brush on the prop before we go on our cruise. When we find a suitable place for the yacht we will do the bottom and really polish the prop too. I guess, only then it might be time to start looking at possible changes in prop etc.

Right decision!

When only 5% of the prop surface is fouled, you´ll have already a 25% decrease of power (a broad rule of thumb).
With a clean body you might not have any issue left.

Regards
Richard

[FAST FRED]

only then it might be time to start looking at possible changes in prop etc.
Right decision!

First try to borrow a lazer Tach and reset your boat tach to be accurate at 2000rpm.

Then use the re-calibrated tach and a GPS to log a speed vs rpm graph .

With a clean bottom and clean prop you should be able to see how efficient the current prop is working.

Take your time , accurate counts.

FF