engine/propeller problem on HR42

[Marra]

Due to a breakdown of the original Volvo Penta MD31a engine earlier this year we replaced this engine with the Volkswagen 55 SDI.
After lengthy discussions on the options to replace the existing engine:
- recondition the original engine
- replace with a similar Volvo Penta MD31a, 3500 rev. 65 HP
- replace with a VW55 SDI or
- replace with a VW75 SDI

The premise also was that no replacement of the current propeller would be required.

The wharf advised that the VW55 SDI would do the job without any changes /
modifications needed to the propeller. This engine runs at 2500 revolutions and the
gearbox is a HURTH reduction 1:97.

After installation (june 2003) of the engine we suffer from severe cavitation and since then we receive conflicting information from the wharf, the VW-dealer as well as from propeller suppliers.

The current status is:
- the wharf states that the propeller fits this engine, only the current propeller
is worn out. Which is not the case.
- Volkswagen is looking into this but still no solution
- MAXPROP netherlands state that the current propeller 18` will never do the job
a 20` propeller is needed.
- other propeller suppliers mainly state that this type of engine requires a 20`
propeller. One supplier states a 18` propeller and another a 19` propeller.

We would like to learn wether there is some expertise around that reallly can help us out in establishing the true solution for this problem.

Additonal info:
- The engine can/does reach 2400 revolutions
- starting at 1800 - 1900 revolutions we encounter severe cavitation
fuel consumption then is quite high 9 - 10 liters/hour.

HR42: sailboat / sloop
- length over water 10.50 meters
- total length 12.93
- draft 1.85 meters
- beam 3.87 meters
- displacement 11.500 - 13.000
- hullspeed 8

Current propeller:
- MAXPROP 3-blade, type B70 or E70 depending on ctry list
- 18´
- 35 mm shaft
- 1:10 taper
- 80 mm hub
- 10 mm keyway
- 7/8 nut

Final:
Pse take into consideration that this engine can be upgraded to 75HP
by replacing the software at minor cost. Then the engine will run 3500 rev.

Looking forward to any replies.

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[Robin]

You need some expert advice plus some DIY research into propellor sizes. When we re-engined our last boat I agonised for hours between the various propeller sizes quoted by Volvo (the engine was a MD22L), Perkins (the engine was marinised by Perkins) and two different sterngear/propeller manufacturers. I did a lot of calculations myself from formulae in books like Nigel Calders, plus one I borrowed from the local library specifically on prop design - sorry I've forgotten the name. In the end I made the best informed guesss I could and luckily got it right first time.

As I understand it though cavitation has little to do with diameter or pitch, but more to do with insufficient blade area. I assume the prop is 3 bladed by the way.
The prop needs to be first matched to the engine in terms of the correct combination of diameter and pitch. The diameter is decided by the maximum available space and the need to maintain an adequate clearance between blade tip and hull. Having decided the diameter you can then calculate the pitch to go with it that will give utilise the available horsepower/gearbox/shaft revs/hull speed etc. After that combination is decided, you then need to do another calculation to see what blade area is needed to transmit the power without cavitation - you will probably find there is not enough on any standard prop! Since a 4 bladed prop on a sailboat is not an option, you can then redo the calculation and see what the max revs are that can be handled without cavitation, you may find this is above what you would normally wish to use.

As a last comment, it may seem daft but I believe I'm right in thinking that the prop size (for the same HP) is smaller if the shaft revs are higher, so a) your engine change reducing the revs from 3500 to 2500 required a bigger prop and b) increasing your engine revs (the software change you mentioned?) could possibly mean the existing prop might cope.



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[Chris_Stannard]

I would agree on the expert advice front. If you try to drive a large prop at higher speed the tips of the blades go too fast and cavitation is sure to follow. I have a Najad 373 and decided to replace the prop with a 3 Bladed Flex-o-fold from CJR at Southampton. The prop, as fitted by the builder, was an 18inch diameter 13 inch pitch three bladed unit. I got about 3400 revs, max out of the engine a Yanmar 53hp with a 1:2.63 reduction gearbox. Yanmar advised I fit a 17 inch diameter 13 inch pitch unit. The engine will now run flat out at 3700 rpm, as designed. This gives a maximum prop speed of about 1400 rpm. I have lost no speed and my fuel consumption is no different to slightly better. As with all propellers there is some slip.

With your current gearbox it would seem likely that if you increase the engine speed you will overspeed the propeller. Have you tried the HR owners association to see if anyone else has experience of this

<hr width=100% size=1>Chris Stannard

 

[ParaHandy]

Re: gbox ratio 1:97? you sure? nm

.

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