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Old 23 May 2011, 11:58   #1
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Sr4 with yamie 50hp which prop?

Well I have a stainless 11.75" X 17 pitch prop on my SR4 & the Yamie 50hp will only rev to 4100 rpm & I believe it should be up to 5500 rpm! I can get 30 to 31 Knots max, what size prop should I be looking at?
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Old 23 May 2011, 14:17   #2
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10 3/8"x 14 on our BF50. Gives 28/29knts. Pops a skier out.
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Old 24 May 2011, 00:06   #3
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i will check on mine, I have a sr 4.0 with a 2002 mercury 50 hp 2 stroke, When I bought the motor it came off a hunting boat. it was propped for a heavy boat. I didnt even think about it and when I first tried it It jumped on plane but would start to sputter before I could get any real speed, didnt have a tach and thought I had a problem, found out I was hitting my rev limiter. I luckily have a nice shop that specializes in props and they actually let me try a couple first. I will check out what i ended up with but I tested a couple of props and went from 30 mps to almost 44 mph before the boat started to become a little squirrly. ill see if I can get the numbers and even speed for you,
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Old 24 May 2011, 09:49   #4
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The man to ask would be Gibbo on here-he's got the same setup.

I have a sneaking suspicion that a 14" or 15" ally prop of the same diameter will be about what you need though.

Ignore all the posts from people with different engines-unless the gearing,weight and recommended WOT RPM are the same (which they aren't), what they say is irrelevant to your specific needs.
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Old 31 May 2011, 06:55   #5
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same engine....

Currently running a 10 5/8 ali 12 on a yam 50 and getting 5100 rpm.
New prop enroute is a 11 ali 13 inch and will be interesting to see the difference WOT.
Currently running 32 - 33 WOT without all extra bits you need in a boat !
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Old 31 May 2011, 07:10   #6
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Quote:
Originally Posted by donegaldan View Post
Currently running a 10 5/8 ali 12 on a yam 50 and getting 5100 rpm.
That's thing's a prop? Damn, I thought you were spinning a serrated rope cutter on there! Hey, with a clean edge, there'll be no holding you back....
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Old 03 August 2011, 05:43   #7
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Quote:
Originally Posted by donegaldan View Post
Currently running a 10 5/8 ali 12 on a yam 50 and getting 5100 rpm.
New prop enroute is a 11 ali 13 inch and will be interesting to see the difference WOT.
Currently running 32 - 33 WOT without all extra bits you need in a boat !
Have you tried out the new prop yet?
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Old 03 August 2011, 10:46   #8
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I have a sneaking suspicion that a 14" or 15" ally prop of the same diameter will be about what you need though.
1400 rpm short? I'd think a larger drop in pitch is necessary. Even if you use 250rpm as the guideline for an inch of pitch (my downloaded rule of thumb says 100-200), you're looking at closer to an 11" pitch.

I've found that the "rule of thumb" works OK for smallish changes, but not so much for boats that are grossly mis-propped.

Here's the text from that tip, in case you're interested:

Quote:
Ok here is what some general rules are.

General rules for props:

switch from aluminum to SS (all dimensions are the same) should drop WOT RPM by ~100
an increase in diameter of 1/4 inch should drop WOT RPM by ~100
an increase in pitch of 1 inch should drop WOT RPM by ~150 (range is 100 to 200)

3-blade vs 4-blade
4 blade props have some advanteges over 3 blade. If you have like material, then 4 blades promote stern lift, give better mid range fuel econ, slower top speed, better holeshot, are more balanced and dont vibrate as much and allow you to mount your engine higher due to more blade surface area in the water.

Generally, the heavier the boat the more blade surface area you want, not more pitch. So, rather than going up in pitch, go up in diameter or number of blades or a combination of the 2. This promotes less slippage, but costs you top end. Make the boat more efficient in cruise rpm range.

These are REAL general. You must consider that manufacturer A's prop might have identical dimensions as manufacturer B's prop, but there will be a large difference in WOT RPM's. This is usually due to the variation of blade width, cup, hub flare... There are many things that can cause a difference.

The other thing that could cause such a difference is the current at the time of your test. Maybe if you were able to run the other direction with the props in question, you would see a speed decrease of 5 mph. This would make your average lower and affect your calculations. With out the second pass, you shouldn't count the data. unless you can compare the one way pass with these props to the same direction one way passes with all the other props. Then the comparisons would be level.

Edited by physicistkev 4/26/2005 9:33 AM


jky
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Old 03 August 2011, 12:28   #9
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Quote:
Originally Posted by jyasaki View Post
1400 rpm short? I'd think a larger drop in pitch is necessary. Even if you use 250rpm as the guideline for an inch of pitch (my downloaded rule of thumb says 100-200), you're looking at closer to an 11" pitch.

I've found that the "rule of thumb" works OK for smallish changes, but not so much for boats that are grossly mis-propped.

Here's the text from that tip, in case you're interested:





jky
I think he's running so short of 50hp at those RPM that he'll find a few more revs on top. Maybe 15" is a bit steep, but 14" isn't.
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Old 04 August 2011, 04:23   #10
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Julie at Steel Developments has suggest 11.3x13 aluminum prop so I have ordered one & will fit it & try Sunday!
Will let you know how I get on!
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