Volvo Penta Prop Guide - calculating estimated speed

[BruceK]

Up until 2006 Volvo Penta annual prop guides gave an estimated speed for a prop size and engine/ leg combination. For instance on KAD 42's with the 290 leg at a 1.78:1 reduction a C4 will give roughly 35 knts or 39 mph at 3800 - 3900 rpm

guide here pg 7 in this instance

If I can make 3900rpm quite easily but am making 25knts what is the issue? Or have Volvo stopped providing these figure because they are inaccurate?

i.e. I am wondering if I could be slipping on the bushes or if I should go up a prop size as the engines easily spins the props to max revs but I am not getting the expected performance.

 

[wongaero]

mmm, a Propeller-Library would be useful for this sort of thing. Like a Brick-Library!

 

[[2068]]

The way to read the prop guide is 'If my boat can reach 35kts, and the engines are turning at 3900rpm, then a C4 is the prop for me'.

Not "If if fit a C7 I could do 42kts".

 

[julians]

My boat is bang on the speed that prop library says it should be for my prop/drive/engine combination. So in my case its accurate

 

[[2068]]

My boat is bang on the speed that prop library says it should be for my prop/drive/engine combination. So in my case its accurate

It will be accurate for a boat that already has the correct prop fitted, moderate load, clean hull, and everything is working as it should.

 

[BruceK]

The way to read the prop guide is 'If my boat can reach 35kts, and the engines are turning at 3900rpm, then a C4 is the prop for me'.

Not "If if fit a C7 I could do 42kts".

Hi. That really doesn't make any sense to me. Even given a prop will slip if you spin it at x revs it's going to want to drive through the water at y speed surely? Not dissimilar to say a wheel over sand. What am I missing? What my boat should do with KADs I dont know. I do know with V8 petrols they exceed 50mph.

 

[darrenw]

It works for me too with my KAD32 at 3700rpm and A6 props delivering 31kts, but I see what you mean, it suggests a KAD42 turning at 3900rpm with C4 props should be pushing 35kts and there isn't a combo that equates to 25kts (I'm assuming you've got C4s and nothing lower which isn't supported).

But how does it take into account boat weight, dynamics, or number of engines ?

 

[BruceK]

Why should it. If you turn a wheel at 1k rpm and it does 50mph it will continue to do 50 mph regardless of it's carrying 100 kg or 1000kg. Likewise a prop surely, with some slippage yes but load would cause the revs to drop not the props to slip (at least not by the amount of 10mph) I'd have thought. But apparently not unless I have a problem or am wrong in my understanding which is quite likely, but if so, how?

 

[Momac]

Bruce Are the engines in your boat factory fitted from new? If not are you sure of the gear ratio in the legs?

 

[DavidJ]

Works for me I've got KAD43's but more or less the same as 42's for power. 290 drives, SS props "Type C4" and showing 32-34 knots which I can get with a clean bum on a flat day. Boat is a Sealine S37.
Interesting find, thanks Bruce.

 

[[2068]]

Hi. That really doesn't make any sense to me. Even given a prop will slip if you spin it at x revs it's going to want to drive through the water at y speed surely? Not dissimilar to say a wheel over sand. What am I missing? What my boat should do with KADs I dont know. I do know with V8 petrols they exceed 50mph.

Imagine fitting an otherwise standard Vauxhall Corsa 1.0 with a final drive unit that made the wheels turn at 2x the speed of the regular one.
Is that Corsa now going to hit 210mph?

It's the same with boats. You might be able to coax another couple of knots out of the top end by slightly overpropping, but this will be putting a lot more strain on the engine in normal or heavily loaded conditions. Running at max power, but forcing the engine to run more slowly at that max power setting is not a good thing.

 

[BruceK]

Thank you for the replies

 

[BruceK]

Imagine fitting an otherwise standard Vauxhall Corsa 1.0 with a final drive unit that made the wheels turn at 2x the speed of the regular one.
Is that Corsa now going to hit 210mph?

It's the same with boats. You might be able to coax another couple of knots out of the top end by slightly overpropping, but this will be putting a lot more strain on the engine in normal or heavily loaded conditions. Running at max power, but forcing the engine to run more slowly at that max power setting is not a good thing.

Exactly. Except the Corsa is hitting it's maximum revs without strain in this instance but doing 100. My boay makes mx revs within 20 seconds but gets to 25 and not 35 knts.

 

[BruceK]

Bruce Are the engines in your boat factory fitted from new? If not are you sure of the gear ratio in the legs?

I have replaced one of the lower gearbox housing. But your idea makes sense. However the gearboxes would have to be the maximum 2.30:1 as opposed to 1.78:1 to come close to the speed difference. That worries me some. I guess I will be manually spinning the motors and counting prop revs this next weekend