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Old 26 May 2016, 15:39   #1
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Propeller dilema

Some advice...

I'm running a Honda 75hp on a 5.5m Zodiac Rib. It has a Stainless steel 13.25 x 17 3 blade prop at present. I'm over revving slightly doing 5600rpm at wot. It should apperently be 4.5 -5.500rpm. I want to drop the revs a bit. I can hit around 34mph at max revs and light load.

Another member is selling a titan 4 blade stainless steel prop 13 x 17. Will the extra blade drop the revs?. The extra stern lift from a four blade is appealing as the Rib sits on its tube a bit at the back. But will it improve plane speed with lower revs?

Any thoughts appreciated
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Old 26 May 2016, 15:52   #2
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Not sure about a four blade but an inch drop in pitch will loose you 150-200. Rpm per inch.
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Old 26 May 2016, 16:56   #3
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Quote:
Originally Posted by cornishcoote View Post
Will the extra blade drop the revs?.
Yes by 150/200 rpm

Quote:
Originally Posted by cornishcoote View Post
The extra stern lift from a four blade is appealing as the Rib sits on its tube a bit at the back.
Correct it will give you notable stern lift

Quote:
Originally Posted by cornishcoote View Post
But will it improve plane speed with lower revs?
Yes

Here is one I recently sea trialled and re-proped
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Old 26 May 2016, 20:34   #4
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I had 4 blade stainless Solas prop which dropped my rpm by over 500, any chance you can try before you buy.

Jon
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Old 27 May 2016, 06:56   #5
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A 4 blade prop will give better acceleration & torque output but lower top speed.

Another 3 blade prop with a slightly higher pitch should slightly increase top speed but lower acceleration.
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Old 28 May 2016, 02:17   #6
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Thanks for the replies.

I don't think I can test it first and really don't want to loose any more top speed which is a concern with a 4 blade prop. It only hits 31 ish if I have a few friends on board. Trouble is these are a lot of money if you get it wrong.

Next thing I know I will have spend thousands on new props only to find out the current one is best....with some explaining to my good lady.

Maybe I should try a different 3 blade one. Was thinking of going two pitches up and have both props and swapping depending on what I am doing.
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Old 28 May 2016, 02:38   #7
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A 3 bladed cupped prop should do it nicely
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Old 28 May 2016, 02:39   #8
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Quote:
Originally Posted by lukewhiting View Post
A 4 blade prop will give better acceleration & torque output but lower top speed.

Another 3 blade prop with a slightly higher pitch should slightly increase top speed but lower acceleration.
Spot on
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Old 28 May 2016, 04:08   #9
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The only real way is to try different props on your boat if you can, as I found for us that the four blade Rev 4 was far better all around:

Mercury props: Al 19P / SS 19P / SS 21P / SS Rev4 19P
@ 3000 rpm: 24.7 / 23 / 26 / 26.6 kn
@ Wot: 36.1 / 40 / 39 / 39.7 kn

The basic 19 inch pitch aluminium 3 blade was surprisingly better than a 19P stainless version at cruising speed, but the 4 bladed was best. Nothing different at top end with the stainless props better than the stock aluminium. Acceleration and general response was noticeably best on our boat with the Mercury 4 bladed Rev 4.

The 21 pitch did reduce the top revs slightly.

The boat is an inboard mercruiser (190hp) regal 1900 bow rider with a stepped hull, which causes more cavitation challenges for the prop to deal with, but no issues at all at any speed with the Rev 4. One test with a 3 bladed stainless prop without the hub plugs in it wouldn't even get on the plane!

So we were really pleased with the chance to try these different combinations.
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Old 28 May 2016, 04:17   #10
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How do you find fuel consumption with the 4 blade compared to the 3?
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