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Old 16 July 2014, 04:44   #311
keg
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Looks great

It looks like you had a great time Chris.

I'm a shade over 12st...... quite a big shadow actually and my SR4 seems well balanced.

I only have a single seat console and i have no idea what size or pitch the prop is.

As a relative newbie, how can i tell size and pitch please? is it stamped or engraved somewhere?
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Old 16 July 2014, 06:04   #312
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They I say my SR4 is tail heavy this is down to load/people in the boat wanting to sit at the back, it is fine when well loaded or just me!

Marking should be stamped on the prop!
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Old 16 July 2014, 11:20   #313
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Quote:
Originally Posted by keg View Post
It looks like you had a great time Chris.

I'm a shade over 12st...... quite a big shadow actually and my SR4 seems well balanced.

I only have a single seat console and i have no idea what size or pitch the prop is.

As a relative newbie, how can i tell size and pitch please? is it stamped or engraved somewhere?
I'm a shade over 12st myself, just

as Nick says, prop size should be stamped on the prop, or you may find it by taking the split pin, nut and washer off and looking on the end of the splined hub

Quote:
Originally Posted by Nick Hearne View Post
They I say my SR4 is tail heavy this is down to load/people in the boat wanting to sit at the back, it is fine when well loaded or just me!

Marking should be stamped on the prop!
is that a 11.75 x 17" prop you're using on your SR4 Nick?
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Old 16 July 2014, 11:53   #314
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is that a 11.75 x 17" prop you're using on your SR4 Nick?
I do still use this one (it is on it at the moment) but it is way over propped and will only hit 4100rpm, I bought from steel developments a aluminum 11.3 X 13 on there recommendation which is spot on and will hit max rpm now and even managing to pull me skiing
Top end speed is about the same.
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Old 16 July 2014, 12:26   #315
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Originally Posted by Nick Hearne View Post
I do still use this one (it is on it at the moment) but it is way over propped and will only hit 4100rpm, I bought from steel developments a aluminum 11.3 X 13 on there recommendation which is spot on and will hit max rpm now and even managing to pull me skiing
Top end speed is about the same.
I tried a 11 x 13 and it was hitting the rev limiter but only giving me around 30mph, it's definitely black magic this prop lark
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Old 16 July 2014, 19:10   #316
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started off with a 11" dia. X 13" pitch which induced revving out at around 30mph, currently running a 11" dia. X 14" which is now giving around 35mph, but still hitting 6200rpm, hopefully got a 11" dia. X 15" pitch on the way to try next time, but maybe need to go even bigger, I'd like to get up to around 40mph but reduce the revs to around 5500 to 5750'ish, but it's all a guessing game!
Hi Chris,

my congratulations to that one, really niiice!

RPM and speed shouldīnt only be a "guessing game", no black magic!

You know your gear ratio 2,08, you know the pitch of the prop, you know the rpm, and you know the slippage of the prop on that boat type
approx 10 %

You say 6200 rpm / approx 35 mph / 14 pitch

Let me calculate in centimetres, metres and kilometres per hour, itīs easyer for a german one.
35 mph ( x 1,6) = 56 kilometres per hour

One turn around of a 14 pitch prop (x 2,54 cm) = 35,56 cm forwards (1 inch = 2,54 cm)
minus 10% approx prop slippage = 32 cm forwards

6200 rpm : 2,08 gear ratio = 2980,77 prop turn arounds per minute
2980,77 x 32 cm forwards = 95384, 62 cm
95384,62 cm : 100 = 953,85 meters per minute
953,85 metres x 60 minutes = 57231 metres per hour
57231 metres per hour : 1000 metres = 57,23 kilometres per hour
57,23 kilometres per hour : 1,6 = 35,76 mph

perfect match to your data, no magic, no guessing.

You can calculate it out, not yust a guessing game.

Your plan is 40 mph with approx 5625 rpm. What pitch you need?
I would say 17 pitch. Lets calculate......

17 pitch x 2,54 cm = 43,18 cm, minus 10% slippage = 38,86 cm
(one prop turn around)
5625 rpm : 2,08 gear ratio = 2704,33 prop turn arounds per minute
2704,33 prop turn arounds x 38,86 cm = 105090,26 cm per minute
105090,26 cm : 100 = 1050,90 metres per minute
1050,90 metres per minute x 60 = 63054 metres per hour
63054 : 1000 = 63,05 Kilometres per hour
63,05 kilometres per hour : 1,6 = 39,41 mph

does your engine have enough power to match to this theoretic stuff?

I would better go up to 5850 rpm, and take a 16 pitch one, cause with a
17 pitch may be acceleration is lost too much....and anyway,
there is some chine dance waiting on you. Speed isnīt the whole game,
yust check out 15, 16, 17 pitch if you have the chance to do, to figure out comfortable mix of quick ride and E-Nuff acceleration.

Best regards

Heino
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Old 17 July 2014, 01:58   #317
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Heino, top to you for the maths

I was "guessing" that I may end up with a 15 or 16" prop and that a 17" may give me top end speed but the effort required to get the rib moving would not justify the 17, although I hadn't figured in any chine walking, but your calculations take the guessing out of the game I'm thinking I may end up somewhere between a 15 & 16" ally prop depending on how the SR4 behaves at the speeds generated by the prop, I'm not looking for high top end speed, but more comfortable top speed with good pickup especially when there's two people on board, maybe I'll end up with a 15" prop when were both on board and a 16" when it's just me............

but either way, I'm really enjoying the SR4 so far, it's great fun
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Old 17 July 2014, 03:01   #318
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Quote:
Originally Posted by palsteg View Post
Hi Chris,

my congratulations to that one, really niiice!

RPM and speed shouldīnt only be a "guessing game", no black magic!

You know your gear ratio 2,08, you know the pitch of the prop, you know the rpm, and you know the slippage of the prop on that boat type
approx 10 %

You say 6200 rpm / approx 35 mph / 14 pitch

Let me calculate in centimetres, metres and kilometres per hour, itīs easyer for a german one.
35 mph ( x 1,6) = 56 kilometres per hour

One turn around of a 14 pitch prop (x 2,54 cm) = 35,56 cm forwards (1 inch = 2,54 cm)
minus 10% approx prop slippage = 32 cm forwards

6200 rpm : 2,08 gear ratio = 2980,77 prop turn arounds per minute
2980,77 x 32 cm forwards = 95384, 62 cm
95384,62 cm : 100 = 953,85 meters per minute
953,85 metres x 60 minutes = 57231 metres per hour
57231 metres per hour : 1000 metres = 57,23 kilometres per hour
57,23 kilometres per hour : 1,6 = 35,76 mph

perfect match to your data, no magic, no guessing.

You can calculate it out, not yust a guessing game.

Your plan is 40 mph with approx 5625 rpm. What pitch you need?
I would say 17 pitch. Lets calculate......

17 pitch x 2,54 cm = 43,18 cm, minus 10% slippage = 38,86 cm
(one prop turn around)
5625 rpm : 2,08 gear ratio = 2704,33 prop turn arounds per minute
2704,33 prop turn arounds x 38,86 cm = 105090,26 cm per minute
105090,26 cm : 100 = 1050,90 metres per minute
1050,90 metres per minute x 60 = 63054 metres per hour
63054 : 1000 = 63,05 Kilometres per hour
63,05 kilometres per hour : 1,6 = 39,41 mph

does your engine have enough power to match to this theoretic stuff?

I would better go up to 5850 rpm, and take a 16 pitch one, cause with a
17 pitch may be acceleration is lost too much....and anyway,
there is some chine dance waiting on you. Speed isnīt the whole game,
yust check out 15, 16, 17 pitch if you have the chance to do, to figure out comfortable mix of quick ride and E-Nuff acceleration.

Best regards

Heino
On the face of it that's very impressive, particularly as it seems the figures were calculated in the early morning hours of the morning (probably after a few schnapps to celebrate winning some football tournament, or other) but they're totally flawed I'm afraid.....

You've completely forgotten to factor in the 12 stone of the person at the helm. So where you've factored in a 10% figure for 'slippage', I suggest you rework the figures and include (say 20%) for 'tonnage'.

It was a good try though.

My money's on a 16" by the way.
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Old 17 July 2014, 04:39   #319
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On the face of it that's very impressive, particularly as it seems the figures were calculated in the early morning hours of the morning (probably after a few schnapps to celebrate winning some football tournament, or other) but they're totally flawed I'm afraid.....

You've completely forgotten to factor in the 12 stone of the person at the helm. So where you've factored in a 10% figure for 'slippage', I suggest you rework the figures and include (say 20%) for 'tonnage'.

It was a good try though.

My money's on a 16" by the way.
I wasnīt drunk, (at that moment!) and itīs not flawed. Itīs always approx about the value of propeller slipping, boat type....influenced by many many many many values.
In that Rib range aproxx 10% is a good value taken over the thum.

Please give me your exact Yamaha outboard model, and iīll figure out the gear ratio, or yust give me your gear ratio. Give me your top speed rpm, your actual prop pitch, and donīt tell me how fast you go, cause i will calculate this with 10% prop slipping value, would this be fine to you?

Letīs see what happens, but please be honest in the end about your "matching" or not matching top speed.
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Old 17 July 2014, 05:32   #320
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I don't weight tons, just a few stones so less of the tonnage and more of the stoneage please!
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