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Old 19 July 2007, 12:37   #1
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How will this affect my performance

I am currently running a quicksilver 13 X 18" Silverado prop on my Johnson 90.

It performs well giving me about 37 knots at 5,400 RPM

Just wondered what this 5 blader would do for me (if anything?)

http://cgi.ebay.co.uk/ws/eBayISAPI.d...ADME:B:SS:UK:1

Chris
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Old 19 July 2007, 13:10   #2
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I've read or been told before a general rule is that more blades = better acceleration but less topspeed.
Usually associate 5 bladers with heavy boats or ski boats and I don't think you'd get the same rpm with that prop so would probably lose some speed.
Harry
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Old 19 July 2007, 13:15   #3
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Chris,
As someone who has wasted a heap of money on trying to find the best prop,my advice is if your current prop works for you and it suggests by the figures it does leave well alone.Those elusive couple of knots more can turn very costly.
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Old 19 July 2007, 13:35   #4
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Quote:
Originally Posted by toby4594 View Post
Chris,
As someone who has wasted a heap of money on trying to find the best prop,my advice is if your current prop works for you and it suggests by the figures it does leave well alone.Those elusive couple of knots more can turn very costly.
I agree. Sounds like that boat is going fast enough.
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Old 19 July 2007, 18:41   #5
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Thanks Guys thought that might be the answer.
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Old 20 July 2007, 06:43   #6
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I disagree

On our 5.5m Tornado with a 115hp on the back with a 3 blade 19" ally prop our top speed was 31 kts.

We switched to a 21" 5 blade stainless and gained all round improvement
-> Higher top speed now 36kts
-> Better acceleration
-> better load capacity when carrying divers

OK faster / higher revs does mean more fuel used...

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Old 20 July 2007, 08:54   #7
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Five bladers are ski props really - it's an inch bigger prop and 1/4 inch bigger diameter, so you should lose some revs off the top end and acceleration will be good if your motor can pull it.

I think it's a fair bet that you'll lose top end!
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When a boat looks that good who needs tubes!!!
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Old 20 July 2007, 09:14   #8
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Are there any rules / calculations, based upon RPM, on how large a prop you can take and still hit maximum revs?

I currently have a a 13.5inch (I believe) standard alloy tohatsu prop on my 50hp tohatsu. But she'll go over the red line by about 250 revs. Is there anyway of caculating what the maximum revs would be if I moved to a 14 inch for instance?
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Old 20 July 2007, 09:51   #9
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I think you're getting pitch and diameter mixed up 13.5 will be the diameter, and as a rule it doesn't alter much with the same type of prop (ie Laser), you need to know the pitch.

If you have an aluminium prop, the change to stainless might be enough to bring the revs down and get more speed. Alternatively if you have say a 19" pitch prop, a 21" would bring you below the red line and almost certainly give you a better top speed - it would depend on how well the hull is working and if it is the same type of prop.

Without knowing a lot more detail, and it being a hull I am familiar with it is hard to say, but it would be worth borrowing a prop or two along those lines if you can - it's expensive to experiment!
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Old 20 July 2007, 10:07   #10
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Cheers Cookee - surely a change in diameter would have some bearing on performance though?
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