Urgent prop help needed.... Cant find the help i need

I

Ian MacAulay

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As a few of you may off seen with my recent posts I've had engines getting warm over 3k rpm . They set off the alarm but still sit below 90 degrees with no further climb

This weekend just gone i went past the 3k rpm and ignored the alarms (briefly) realised she still never climbed so continued to WOT

At WOT i had a poor 31mph and low 3500rpm suggesting the props are incorrectly pitched as the previous owner thought. Previous owner replaced the props before sale and ever since got warm. He was unable to see the markings on them and matched up best he could.

Now I'm lead to believe due to loosing nearly 500rpm I'm over propped causing my engines to work harder and produce more heat and the lack of rpm causing less water also to be pumped???? She should also be capable of around 40mph +


Now the questions I've been trying to ask dealers and prop companies is what props i need to buy to get the revs up to 3900 on my Ad41 engines 200hp each

The boats a Cranchi Aquamarine 31 and was loaded to medium to full load and currently has A7 props on her.
I have tried emailing Cranchi to find what she would of had from factory but no luck yet


Any help please before i spend out on new props and order the wrong ones


Thanks

Ian
 
Thanks for the reply.

I did message them but no real solid answer and very little help. I just first got a email reply saying A8 prop and prices then i said i need to increase rpm so i got a reply saying A6 prop with the prices...

When about to spend £900 i want to know I'm getting right ones


Thanks

Ian
 
http://www.volvopenta.com/SiteCollectionDocuments/Penta/Propeller guide/Propeller Guide.pdf

Have a look at the guide. I'd have thought you'd be better suited to a B4 or B5 sets; or C4 or C3's. When you get your answer and should it turn out that you are after C4 or C3's stainless steel sets, give us a pm. (They are 3k for two sets new, I have had mine refurbished, balanced and rehubbed at Castle Marine but are now surplus to requirement and potentially up for sale.)
 
I'm completely confused ... your previous post said:

-----------------------------

"Also at this point it was noticed the engines were turning 4100-4200rpm
Now I know the engines rpm is a bit high at WOT but just a few hundred rpm cause them to overheat??
Props are Volvo duo A7"

-----------------------------
 
Those figures where given by previous owner and when I then loaded the boat and finally got time to get in water that was flat enough to test I got 3500rpm at 31mph gps speed.

Ive never had the opertunity in the past to open her up and perform tests to see

Thanks

Ian
 
I was misinformed or miss heard, he may of said or ment 3100 for all I can remember.... But now I have real figures with a full service so may of got more out of her.


Thanks
 
Some logical thinking is needed here.

If 3500rpm =31mph,
then 4000rpm (assuming some magical extra power came from somewhere) = 35.5 mph, which is still not 40mph.

So, dropping a prop size might allow you to rev a bit higher, but you won't hit 40mph.
Going up a prop size will force the engine to rev even lower, and you definitely won't hit 40mph.

Have you checked the current props carefully?
i.e. check for damage, check that the part numbers for all 4 propellors really do match up with VP A7's.
Fuel filters ok?
etc.
 
FlowerPower said:
Some logical thinking is needed here.

If 3500rpm =31mph,
then 4000rpm (assuming some magical extra power came from somewhere) = 35.5 mph, which is still not 40mph.

So, dropping a prop size might allow you to rev a bit higher, but you won't hit 40mph.
Going up a prop size will force the engine to rev even lower, and you definitely won't hit 40mph.

Have you checked the current props carefully?
i.e. check for damage, check that the part numbers for all 4 propellors really do match up with VP A7's.
Fuel filters ok?
etc.
Click to expand...


TBH your probably a lot nearer the mark with your figure. Taking in to account the age of the engines, tide, wind, loading, trim...... the quoted 38knots by manufacture is not a real life figure.

i have replaced fuel and air filter BUT the only things i have not had a apart is the water separators.

The props are brand spanking new and all match up But i have since shown them a bit of sand and mud and taken a little of the pain off but thats about it.

The reason i believe she's over propped is the fact she is running at 5-10 degrees over and at 3500rpm and seams to suggest she is struggling


TBH its giving me a real head ache ..... neither Volvo or Cranchi have got back to me and nor can i find what she would of come out the factory with.

Thanks for the help


Ian
 
I think a rough guide was 200 RMP difference per prop size from memory. however i am not sure the overheating and prop size is linked. I have the same issue on KAD 44 and am considering dropping a prop size but there is no overheat.
In fact I send Bruce K a PM :)
 
Broshure top speed quotes is a bit of a fantastical figure. Stick with what your engine revs are telling you. If you have a clean hull, not disproportionately overloaded for your own personal average outing then you should be aiming to get WOT revs of 3900 and need to find propellors to match. If you can only achieve 3500 rpm, and you meet the above criteria, plus your engines have a clean bill of health, you are two sizes too big given that each increment equates to 200 rpm on average. Your overheating may or may not be related to being over propped, but does not substract from the fact that by all accounts given so far that you are.
 
Thanks very much. so i should get a set of A5 props or stainless equivalents... Bruce K pm me with what you have.

Wether the overheating issue is linked it all does ring true considering both overheat equal and I've cleaned everything with new stats and coolant too.. i have a clean Bottom :-) and full service and good water flow...

I belive i should get her propped correct before looking at anything else.


Thanks

IAN
 
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