Volvo Penta C Series Duoprops

BruceK

BruceK

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Can anybody shed some light on the pitch and diameter changes of the C series props C2-C7. Specifically diameter changes.

The question arises as a friends boat has twin AD40's at 160hp each on a 5 ton planing hull. Max speed is 33knts using a rather low diameter high pitch single prop 14x21. Mine (edit @8ton) has twin KAD42's at 220 hp and fitted with C4 duo props and where as I am not entirely sure of max speed certainly isn't anywhere near that. I'd say about 27knts. He cruises at ~27knts and I at about ~23knts both around 3.1k rpm.

His boat is not as quick on the plane. With C3's on mine will jump out of the hole and reach WOT at 19knts in seconds without the superchargers coming in outside of the predetermined range (post 2300 rpm). With C4's this is more sedate of about 20 seconds and the superchargers will stay in until about 2900 rpm but the speed is greatly increased. (far more than the usual 200 rpm gain per inch of pitch). However as the boat gets loaded and progressively fouled I can struggle to get on the plane in a short head sea and often have to follow it to get on the plane then double back. Once on the plane with both C4's and C3s WOT revs are easily reached indicating there is plenty power spare and I would readily try for C5's if it weren't for getting out of the hole onto the plane.
Interestingly the early Volvo propeller guides for the KAD42 on a gear ratio of 1.78 is ~35 knts for the C4 but on AD41 at 1.95 ratio ~ 25knts. Just to muddy the water I am pretty sure I am 1.68:1 ratio but seem to be achieving the speeds of a 1.95 ratio.

If a C5 is a smaller diameter coarser pitch it may be worth me stumping the 3k needed for a set to try out, otherwise I have reservations about reliably getting out the hole.
 
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to add. The boat was supplied with B5's straight out the factory which I am led to believe equates to a C4, however I understand that the rear prop on a B5 set is 3 inch coarser than the C4 equivalent.
 
There’s a lot of variables ^^^^ you could change as you suggest the pitch / dia of the prop set .But I think you will be dissatisfied clucking £££ at prop sets .

Here why ——

While you say your mates boat weighs 5 tons I take that ( you have not said ) that it’s about equal to yours for comparison sake .Yet his lower Hp boat is faster and I understand he has no obvious issues normally getting it on the plane .ie he does not have to go down wind in afollwing sea etc .

This tells me although both boats share the “5 tons “ they differ in hull form .
His will have more lift quicker so at what ever speed his boat has less drag therefore with out as much drag ,just goes faster .
In other words at 27 knots his % of his 160 Hp due to hull form —- sees less of the 5 tons than your % of 220 Hp at 27 knots
At 27 knots there’s so much drag from your 5 ton s hull form in the water your 220 Hp is all done .
Meanwhile your mate a got a bit more of his 160 Hp left to push on and importantly UP ,lifting the boat - reducing drag more so he see LESS of the 5 ton s trying to push the boat into the water - all the way up to his 33 knots

Your headroom diff in Hp is lost in drag and prop slip due to an inferior speedy hull form .
Assuming that - Both sets of engines reaching WOT - rpm and guages as they should be .

So the difference is the hull shape that carries/ lifts the same “5 tons “

So in a word the bit to change imho is the

BOAT
 
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sorry, yes I amended the original post. His is a 28 foot Sunseeker Offshore 28 at 5 ton mine is a 34 foot Formula 34PC at 8 ton.

His has a sharper Deep V at the bow (but both boats lift their bow clear when on the plane) and his deadrise measured is 19.5 degree to my 18. Where as I understand that drag is going to have a great impact I was under (perhaps misguided) impression that accounting for prop slippage the fact that the prop is spinning without excessive load at WOT (WOT easily reached once on the plane) the speed should be reasonably within the guide's allocated range.

It's not about whose boat is faster, I couldn't give a fig. It's just something I found intriguing as his prop is rather coarse for it's diameter and I was wondering if this lower diameter accounted for being able to climb the hole given his power to weight ratio is lower and if the C series followed a similar suit as the KAD 42 can take a C4 to C7 prop in the guide
 
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If his beam is norrower as well as smaller boat then dispite the grater deadrise , overall he will have a smaller surface area / contact .
Any how it’s not like for like , your boat was guessing set up for bigger Hp petrols ( USA market ?)

His is not a duoprop so the engine will be able to spin ONE higher pitched prop , but not two .
Anyhow different boat dims as you said now .

Back to your boat - if it struggles to get on the plane ( mid seasons fouled+ extra cruising Kg s ) then that’s gonna get worse with a bigger pitched set .Worse sooner in the season and possibly impossible layer on !
Even if - or - when it actually manages it , it then with trim etc eventually settles into a decent rpm , speed and guages are unremarkable- but a bit faster due to the new pitch .

It’s a pity there’s no facility to borrow / swap a set .
 
yes. The boat came with KADs from the factory but was designed with the 5.7 V8's in mind. With the big V8's yes they reach well into the 40 knt bracket and the latest incarnation with 7 litre monsters get low 50's.

But this is not about speed it's about getting the optimum prop. I find it starnge that with the current offering getting on the plane seems optimised with the C4 but once on the plane I literally have to snatch the throttles back to stop her over revving. (Subjective I know but a good 20 degree arc on the throttle levers). From 3krpm to 3900 rpm is quite literally about 4 seconds when the turbos kick in) yet from 1.8k rpm to about 2.8k rpm she is quite sluggish and nor can she hold those revs without the superchargers kicking in and out. It all seems rather imbalanced. The C3's could sit at any range and in fact the superchargers were decoupled at the time. But had a very low top end and the engines seemed be over reving unloaded. (like doing 50 in 2nd gear)
 
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.[
But the Q is are the engines powerful enough with the next pitch up to overcome the drag of your 8 tons pushing your hull form into water before any speed and lift is created reducing the 8 ton drag s to pop it out of the hole .

Maybe a 7.4 L petrol 350 Hp or what ever could and did in the design phase .
 
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But the Q is are the engines powerful enough with the next pitch up to overcome the drag of your 8 tons pushing your hull form into water before any speed and lift is created reducing the 8 ton drag s to pop it out of the hole .


No they aren't if the diameter remains the same. Which leads back to the original question. Is there a diameter change within the C series. Because it may well have the power if the diameter decreased with a coarser pitch.
 
BruceK said:
But the Q is are the engines powerful enough with the next pitch up to overcome the drag of your 8 tons pushing your hull form into water before any speed and lift is created reducing the 8 ton drag s to pop it out of the hole .


No they aren't if the diameter remains the same. Which leads back to the original question. Is there a diameter change within the C series. Because it may well have the power if the diameter decreased with a coarser pitch.
Click to expand...

Could be the same thrust more or less and back to sq1

Coarser pitch small dia = bigger dia finer pitch - —- thrust wise , there’s gotta be a x over point wherby one cancels t,other out .
Wait for someone to come along with the charts applicable for your drive .
 
I have no idea whether (water) props follow the same dynamics as (Air) props. But I can assure you that when I was an avid RC IMAC flyer that a lower diameter coarser pitch gave far more airspeed per hp without sacrificing thrust. The downside was an absence of a braking effect in a dive and less control for constant speed which was required when performing manoeuvres that incurred elevation changes. A larger diameter prop has more "bite" and so more sensitive to pitch increase. The reverse is true when decreasing diameter. I dont see why this should be all that different for water.
 
It sounds as if there is issue around the hull “hump speed” where the engine isn’t generating enough torque at 2000rpm+.
Have you checked your turbos recently for wear? (on the exhaust side, the housing can corrode and result in a gap between the housing and the turbine blades).
Are both engines doing exactly the same thing?

The other thing to bear in mind with these engines is the slightly odd torque curve. There is a peak at 2000rpm, then a drop to 2500rpm, and a more gentle decline further up the rev range. The peak at 2000rpm on an under powered boat will be at less than the speed required to actually get the thing planing, and you can get stuck. On a lighter boat with coarser props, that 2000rpm torque peak is closer to the speed to plane, and it goes steaming off.

.
 
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What you say I believe is exactly the case although at 2k rpm I am on the superchargers still. The turbos only kick in at 3.2k rpm, the boat only is properly on the plane at 2.9 krpm (18 knts) although can also sit on what I can only describe as semi-D plane at 14 kts. Quite unusual because there is nothing inbetween. it's 0-7, 14, 17 - and above knts. The 14knts speed has one other characteristic, she is revving at a speed just after supercharger drop out and at the same revs she would be doing at 17 knts if she were unloaded.
 
mmm I wonder now if the props are not too fine pitched now that you mention that. My KAD42's have this characteristics.

Normal aspiration to 1.8krpm - still at displacement speeds to 10kts
Fixed supercharger from 1.8krpm to 2.3krpm - up to 12knts
Variable supercharger (if engine senses its under load) from 2.3krpm to ~2.8krpm - 12- 17kts and entering planing phase
Normal aspiration from 2.8krp to 3.2krpm- 17-23knts but labours at 17 knts and liable to drop off the plane- super charger kicks in to hold it.
Turbo from 3.2krpm to 3.9krpm - 23 - ~27kts turbos totally unnecessary at this stage at the boat feels unloaded and turbos if anything give it too much power. As said above, transition from 3 to 3.9 krpm is measured in about 4-5 seconds

i.e. based on your point above the times that the "boosters" kick in do not really marry with when they are actually needed
 
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So that’s the thing about worn turbos ... the supercharger boost is dropping off beyond 2600rpm even when engaged, and the turbos should be really kicking in and taking over. Low boost around here will make a torque hole for you to fall into. Turbo should be fully spooled up way before 3200rpm.

Using a boost gauge and getting some stats will tell you one way or the other.
 
You need to eliminate as FP says turbo wear .
It comes on slow and intermittently as the the gap between the vane tips and body increases .
There diveded opinions on the cause
Two schools
1- corrosion inside the body roughens up the inside surface - reducing the seal ,so you loose boost
2- The tips burn away get red hot due to excessive EGT,s and no longer have tight fit
If you are struggle to get it up out of the holeshot then just reflect a moment of the unburnt fuel heating those vanes .

Classic sign is unusual kicking in of the S.C after it seems to spontaneously reduce the revs when cruising for no apparent reason - then its picks up and settles where it was .
This increases from once a trip to once every 1/2 hr or so -AND difficulties getting on the plane - which in a man math way you just put down to the usual - fouling / excess cruising stores / head wind - etc
 
Thanks guys. So first port of call is check turbos. If worn replace..... then look at coarser props?

I dont know when the turbos spool up, there is a "gate" mechanism after the air intake that divides the SC from the TC. I say 3.2krp for the turbos as that is about when you can hear the engine note pitch change as per the vid
 
Just had a chat with AET, new turbos are (relatively speaking) dirt cheap so may well be cheaper just to replace them than get an engineer in to do a diagnostic (boost test and inspection etc). £600 pound a pop. Less than it costs to fill the tanks. :encouragement:
 
For reference in case anybody else has the same question. I got these measurements from pattern props of the C series so may not be 100%

Diameter does not change from C3-C6 they are a constant 15" front 13" rear

Pitch changes
C3 17 19
C4 19 19
C5 19 21
C6 21 21
 
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