Duoprop propellers

[PCUK]

I'm used to calculating prop' sizes for shaft drives but how do you know which duoprop set to use. I seem unable to find any tables that give the correct set to use for a given speed range. (Not using a Volvo engine by the way).

 

[Momac]

Perhaps this will help ?
http://www.pentamarine.co.th/pdf/propellers.pdf




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[PCUK]

Thanks Martin,
Exactly what I was looking for.

 

[Scarron]

Volvo Penta Duo-Prop guides, wrong?

I personally wouldn't set too much store by those Volvo Penta prop guides - they make absolutely no logical sense to me..

How can you have; the same propset, on the same ratio leg and at the same revs produce markedly different boat speeds just with a with a different engine model?

To borrow a phrase from Latestarter 'props drive boats'

There are only three variables that produce boat speed - pitch x ratio x revs. And before anyone points out the missing variable, the huge differences they show surely can't be down to prop slip. Not for the first time I do wonder about Volvo penta's 'engineering' skills

Or perhaps I am being dim?

 

[PCUK]

I know what you mean Scarron, but I've got to start somewhere and calculating size for duo props using conventional means isn't going to work.

 

[[2068]]

Or perhaps I am being dim?

Ha, no, I spent some time looking at the guides before it clicked.

It should be read as "IF your boat can manage to do about 34kts flat out, and you have THIS engine with THAT gear ratio, then here is the propellor for you". Unfortunately, fitting an S28 with A10's will not turn it into a 50kt screamer.

That's also why a boat builder will experiment with different prop ratios so that the engine can hit rated RPM at max throttle - getting this right is a bit hit and miss.

The thing about different engines / different speeds for the same RPM and gear ratio is probably down to the power and torque curves being different.

 

[volvopaul]

Ha, no, I spent some time looking at the guides before it clicked.

It should be read as "IF your boat can manage to do about 34kts flat out, and you have THIS engine with THAT gear ratio, then here is the propellor for you". Unfortunately, fitting an S28 with A10's will not turn it into a 50kt screamer.

That's also why a boat builder will experiment with different prop ratios so that the engine can hit rated RPM at max throttle - getting this right is a bit hit and miss.

The thing about different engines / different speeds for the same RPM and gear ratio is probably down to the power and torque curves being different.

I know exactly what you mean, I've always been told by Volvo dealers that A series props are for diesels and B are for petrol engines and outright performance, so why do I see so many sea lines with diesels on B series props? Lanerboys included has B 5s on with kad 43s.

I have also seems mismatched props on the same boat, to discover the bottom drive ratio different giving the same rpm and speed, baffling I know it's all trial and error.

 

[PCUK]

I've got three sets of props, all 'B' s I believe but at least I've got a choice to start things off.

 

[Steve Duffell]

Duo props!!??

I have a sunseeker portofino with Volvo adaq40's, it has A3 duos fitted and does about 22 knots at 3500 revs! Volvo and sunseeker advise fitting A5 or 6's? But if you look at volvos chart an A7 would mean I could achieve nearly 40 knots! Now I ameven more confused!! ANY advise would be welcome??

 

[rlw]

Hi Steve,

I would check the revs with a photo tach or have this done.
These engines need to see 3800 rmp WOT. I had a Portofino 31 with AD41 on A6 which ran 31 knots.

Rob

 

[[2068]]

I have a sunseeker portofino with Volvo adaq40's, it has A3 duos fitted and does about 22 knots at 3500 revs! Volvo and sunseeker advise fitting A5 or 6's? But if you look at volvos chart an A7 would mean I could achieve nearly 40 knots! Now I ameven more confused!! ANY advise would be welcome??

no no no, that's the wrong way around.
IF your boat can do 40kts, you stick A7's on it.

IF your boat (with the perfect propellor on it) can only do 25kts, then sticking A10's on it will result in max rpm being 2500 and a strange burning smell

 

[dpb]

I have a sunseeker portofino with Volvo adaq40's, it has A3 duos fitted and does about 22 knots at 3500 revs! Volvo and sunseeker advise fitting A5 or 6's? But if you look at volvos chart an A7 would mean I could achieve nearly 40 knots! Now I ameven more confused!! ANY advise would be welcome??

If you are talking a Portofino 31 then mine has A6's and seems to perform properly with those.

 

[GrahamHR]

Isn't the number a clue as to the pitch? I read once that a F6 equates to roughly a 26" pitch, a F3 a 23" pitch etc

My last boat had 280 HP and F6s, my current boat ( same manufacturer, same hull shape) has 320 Hp and F5s, but is 2 foot longer and heavier by about 300 kg. It came with F6s but F5s seem to suit it better.

 

[PCUK]

That's interesting Graham. I'll measure the pitch on one of mine next week and see if it equates to what you are saying.