duo prop timing question

jon and michie

jon and michie

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Hi - is it true that the best way to ensure the props are positioned and timed correctly is to set the fwd prop stamp marks at 12 oclock and the rearward prop stamps at 6 oclock - if you get my drift.
or are set up another way ?

many thanks for any opinions

jon
 
I don't think it makes any difference how you fit the props (beyond them not being on backwards) , I couldn't find any volvo docs that state any need to install the props in a specific position.

I guess if you're really inquisitive you could install them like you have suggested then do a top speed test, then install them with the stamps lined up and try again.
 
Thank you Julians
Funny thing is that yesterday I aligned both props with the stamps next to each other and I am just questioning it.
Looking from past photos that I have kept it would appear that the stamps weren't side by side

Jon
 
As far as I know, but without any authority on the subject, no two blade should pass behind the leg together on B and C series 3 bladed props. i.e. perpendicular behind the leg or skeg (an I to the Y if that makes sense). Not sure how that works with A or J though or even if I am correct.
 
The Internet does seem to be full of tales about having to align the props in a certain way, but there's nothing about that in the volvo docs I have seen. But then I only have access to consumer volvo docs. Maybe one of the volvo engineers on here can comment?
 
Well we know they spin in opposite directions .

Do they spin at different speeds being different diameters ?
Vp have geared the dif rotation so it’s within there scope to have the two shafts geared at diff speeds .

I don,t know if they run at different speeds ( the inner and outer ) but if so then it does not matter lining up any marks as the blades will constantly pass each other in ever changing positions relative to each other and the body of the gear casings .
Netting out and precieved adverse effects .
 
You are correct, no reason why they couldn't but AFAIK they dont. Both follow the same reduction. However there is also the potential for the hub to gradually slip bar the J series which are fixed hub.
 
Thanks for the replies so far.
I am thinking that if they are supposed to be in a set position the forward prop flows the water onto the rear prop in a smoother screw turn and if the rear prop is a spline or two out then the flow of water is slowed down.
If this makes sense

Jon
 
I try and align mine so that the blades don’t line up underneath the exhaust plate.
I think this makes ... very little difference to anything.
I don’t walk underneath ladders, either.
 
From speaking to various people it does affect the top speed how the props are set up.
For example on the V24 Race boats they performed better with the 2 props not aligned at 12 o'clock. only 1 engine.

However speaking to another Hunton owner who has done a great deal of prop testing / tweaking,
On A Hunton 37 he has found that optimum top speed is achieved by having the props aligned, Twin Engines.

BUT each boat is totally different .
It really depends what you are actually trying to achieve,
To see what is best for your boat its record speeds at various rev's over a 2 way average, Then try props set differently and repeat .
This all needs to be done on the same day - = same conditions . Also need same fuel crew etc to obtain meaningful results.
 
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I suspect these are high speed effects, probably less relevant to a tubby SC35 that can only just about hit 30kts on a good day.
 
Yes I was only talking about ultimate top end speed. Do not know if it makes any difference to fuel consumption at lower speeds. I guess though you would need to still carry out 2 way average speed runs at various revs to see what is best for the boat
 
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