Confused - prop. calcs

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jackho

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Looking to re-prop my boat. Original prop (16x11 fixed 2blade) was intended for the md2030. My engine is D1 30 (different gear ratio 2.19 , max revs 3200 ). Boat 7 ton B37. Can't get max hull speed full revs nearly 7 kt.. LWL 9.82
Gori say I need 16.5 x 11 3 blade.
Volvo 17 x 12 (3 blade)P
Prop calc download says 16.5 x 11 3 blade ( Is thia what Gori use?)

Yachting monthly gear test not much help.dosn,t give prop sizes except for Gori. so max speed for the 323 is 7.65 (surely a lot more than the hull speed) but with what size prop. and no spec. on boat weight etc.? Again Gori say a 16.5 x 11 for a 323 - should I consider same for my boat (heavier , more beam etc??

Am I missing something here?

May ultmately go for the 3 blade Volvo but don't like the weight 6kg although no other maker seems to quote weight!!??

Your comments are welcome - thanks Jackho
 
Ariadne said:
Give these guys a ring and all will sorted out for you at no cost - well apart from a new prop!

http://www.sillette.co.uk/
Click to expand...

I'll second that. Manufacturer should sort it out. If it's wrong then it's his fault not yours!

Not all props of the same dia and pitch are the same anyway, blade width can vary for instance.

You don't say whether the new prop will be fixed, folding, feathering. If fixed, then it's possible that the 3 blade will give (even) more drag than a fixed 2 blade when sailing.

What's the problem with weight?
 
16*12 As you are not getting the speed but are getting the revs the pitch is too shallow.

I have an older 37 with a 2030 but I have just replaced the saildrive (don't ask!) with a 130 2.19 : 1 replacing the 2.42. Original was 16*12 and new prop is 16*11. Max speed identical at 7.4.

The shaft speed is virtually identical on your D1/2.19 at 1461rpm as my original 2030/2.42 at 1487.

I ignored the Volvo recommendation and based mine on Prop Calc.

If you are going for a fancy prop then fit whatever the supplier recommends as if it is wrong they are responsible.

Hope this helps
 
That is very good information, interesting. Thought it was implied but I intend to fit a folding prop (can't stand the whine of gearbox when sailing , also to many lobster pots get caught on fixed prop). I can get a 16x11 3 blade Volvo folding prop at a clearance price hense my problem vs. what is recommended by Bavaria ie 17x12.
The 16x11 3 blade folder _ one more blade ??? would that still be better than my 2 blade fixed?? Fully loaded my boat was weghed in at 8ton on the crane digital scales so perhaps this my account for the difference in the calculated values? Good topic for debate anyway. Thanks for your response.
 
Propcalc gives 15*12 3 blade, so a 16*11 would be close. 3 blade should in theory give better motoring performance than fixed two blade, and in general larger diameter is better. Don't think having folding blades makes any difference to catching ropes - sounds like a good idea, but in practice ropes etc are picked up when motoring. Only means of protection is a rope cutter. Make sure you have decided on the number of blades before you buy your cutter.
 
Just as a matter of interest - does propCalc assume a fixed or folding prop? As I guess there would be some difference in Dia. or Pitch !!!!
 
jackho said:
Just as a matter of interest - does propCalc assume a fixed or folding prop? As I guess there would be some difference in Dia. or Pitch !!!!
Click to expand...

Specifically says "standard"! which is not a lot of help. It is probably referring to the blade area ratio. As you probably know, there is always a bit of black art with sizing props, particularly when they are non standard designs like the Volvo folder. There is nothing like empirical evidence which is why best to take suppliers advice. That was the problem I faced with my now non-standard setup, but my fall back was that you can get the standard Volvo fixed prop repitched easily and cheaply.

If the price is right, the prop you have been offered is probably worth a punt - don't think it will be far out. You can always sell it on if it is way out.
 
Update

Removed my old prop today and to my surprise it is 16x13 and not 16x11 as stated. Bavaria have it listed for the 2006 bav 33 and 39 both with the d1 30 engine installations - dosn,t seem too logical to me - but I think I'm beginning to get some understanding of props but still have some way to go.
 
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