Engine Revs and Prop size/pitch

[Dyak]

Evening Gents.
I have recently changed boats and now have a Bavaria 36 that has a Volvo 2030 engine in it.
The survey has recommended that I change the prop when the boat is next lifted as it is a replacement 2 bladed folding prop that is suffering from de zinctification.
I am looking at putting either a 2 or 3 bladed prop on it but am unsure as to the size and pitch I need.
The engine will rev to 3600 when not in gear but only to 2700 when in gear with the current prop. Does this mean that the prop that is on it is undersized or has the incorrect pitch. The volvo engine manual says in gear it should run to 3300.
Any help or advice greatfully received. /forums/images/graemlins/confused.gif

 

[oldharry]

Underspeed at full throttle means - assuming everything else is OK- that the prop is oversize. You need to reduce either pitch or diameter.

Props are too expensive and too much hassle to change, to mess about trying different sizes, so you need either to speak to a specialist who knows the correct size for your engine/ boat, or you can download a programme such as 'Propcalc' which will give you some idea of the size. Trouble is different programmes give rather different results, because most of the 'freeware' programmes are too simple and do not take account of enough variables for real accuracy.

 

[Ships_Cat]

Here, a good prop maker will usually get the prop size right for a one off custom boat first go. With a production boat like the Bav they should be able to recommend the correct prop with 100% confidence. Maybe it doesn't work like that in the UK but I can't imagine so?

Just select a prop maker and buy the prop they recommend off them.

I am at a loss as to why in similar threads amateur peeps come up with "guidance" to others on prop size when professional advice is free of both cost and risk.

John

 

[Dyak]

John,
Thats what worries me as well because the prop that is on the boat at the moment was fitted by a volvo dealer but it is clearly the wrong size so really what I'm asking is do any Bav owners out there know what size prop they have. When i asked Opal Marine what the standard prop was for a 36 they could not tell me and they are the UK importers!!

 

[Ships_Cat]

But I'm suggesting that you go to a prop manufacturer not to boat dealers whose main business is pushing boats out the door.

This link http://www.henleyspropellers.com/form2.htm is a typical questionnaire from a manufacturer here. Obviously, any particular difficulty can be discussed with them and they will also input their experience to the selection. While they do not guarantee results they will pay 50% of a repitch should it prove necessary - have a look at the Q&A link on the same page.

Most reputable manufacturers will do similar (and Volvo too if they do not already know the exact fit to the boat).

John

 

[Budgie]

We changed our 2 bladed folder for a three bladed Volvo folder last year and I have been delighted with the result. The prop size was given to me by my Volvo dealer and has proved to be perfect.

The Volvo prop for our 40 ft Moody with a MD22L (50 hp) at around £800 was about half the cost of one of the fancy ones.

 

[Pye_End]

Presumably your engine is not achieving full revs because of the prop, and not some other reason (eg fuel starvation)?

Think this is worth considering before splashing out on a new prop.

My opinion of professional recommendations for props is that it is a starting point. They should be able to match up your engine/gearbox etc. and get that right, but sometime you have to consider your underwater profile (probably not a problem for you!) and also the conditions where you want the best drive from your prop - is it best top speed or best acceleration in a sea etc.

 

[brianhumber]

All good stuff from the other posters but make sure the engine is delivering the power in the first place.
In my exprience Volvos are not the longest lasting engine to maintain rated power output.

As for the comments re Opal I would refer you to the mag article about the backstay fitting which missed the backing plate completely. No more needs to be said.

 

[PCUK]

I fitted a 3 blade Volvo folder in place of a 2 blade flex-o-fold.
Speed increased from 4 to 6 knots at the same revs with real 'bite' in astern. Generally I hate anything Volvo but I have to admit they got it right with this prop'.