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Old 26 May 2016, 19:39   #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, 19:52   #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, 20:56   #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 27 May 2016, 00:34   #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, 10:56   #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, 06:17   #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, 06:38   #7
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A 3 bladed cupped prop should do it nicely
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Old 28 May 2016, 06:39   #8
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Quote:
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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.
Spot on
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Old 28 May 2016, 08:08   #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, 08:17   #10
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How do you find fuel consumption with the 4 blade compared to the 3?
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Old 28 May 2016, 08:43   #11
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As we changed the prop shortly after buying the boat I don't really know. It must be better as we use less revs at cruising speed and it was just better all around - not very scientific I am afraid. Still uses lots of petrol
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Old 28 May 2016, 11:24   #12
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How do you find fuel consumption with the 4 blade compared to the 3?
If you have higher speed at the same (cruise) RPM you have less petrol consumption in this range.

Just look at post mentioned above by @"solent spence"
If he mostly uses 3000 RPM range the best petrol consumption he has on two last props
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Old 28 May 2016, 15:00   #13
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Useful replies guys😀

I'm going pay my local boat dealer a visit to see if he has any spare props to try.

I do think I need to go at least a pitch bigger to get the revs down. I'm cruising at over 4000 rpm which seems to high.

The four blade probably won't offer any more speed unless the extra stern lift gets the back out the water to reduce drag. Testing will only tell. A more economical cruising speed is my main priority.
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Old 02 June 2016, 21:21   #14
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Go do some tests using the mercury prop selector, great tool.. Aren't you perhaps cavitating? Is the motor trimmed correctly and at the right transom height?

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Old 18 June 2016, 19:04   #15
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Right. Managed to borrow at prop from a local supplier. It was a 13 1/4 x 19 which is the same diameter but next pitch up from what I got now, which was a result as it's the exact one I thought might work.
However, not great results today. Lowered revs by 300, but also lost top speed which surprised me. Light load it maxed out at 31 mph which was less than I had on the 17 pitch. It's still gave alright performance, but slower getting out the water. Thinking I will stick with what I have got unless any other thoughts?
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Old 18 June 2016, 20:09   #16
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Go back to your original prop.
You had better shot
Did you measure WOT on nominal load ?
You have mentioned in first post that you was a little over WOT (5600 - 100 over)
100 it can be even measure error and please remember that you have be in WOT range at nominal load.

So if you go always with other person and some bags - make test having such load.

Better shot, better high speed - why change prop

>>>>>>>>>>>>>>>>>>>>>

And you were in WOT range I suppose:

Full Throttle RPM Range 5,000-6,000 RPM HONDA BF75
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