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Old 30 January 2003, 14:19   #1
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Propeller Info

Hi everyone,

I currently have a Humber Attaque 5.3m with a Mariner 90Hp 2 Stroke with a stainless Mercury Marine Laser II fited.

The serial No. on the reverse is 48-16990-20 My questions are:-

1. Can enyone identify the size & pitch of this prop or tell me a way of measuring the pitch/diameter?

2. Who manufactures/supply these props.

I have reviewed the various comments made on propellers especially the formula for calculating prop selection, (is dimitri a mathmatician?) I do not know what what my starting point is.

Out on the water on sunday and with 50lts fuel and two adults recorded 38knots on gps @ 6000 rpm (Rev counter). My manual recomends to not exceed 5700rpm.

So am I over reving but engine? engine wanted to rev freely, could have gone a bit quicker sea conditions prevalent at the time.

However not worried to much about flat out top speed much rather have crusing speed at 3000 rpm 25-30 knots to maximise fuel consuption on cruises.

Any thoughts or comments are always welcomed.

cheers
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Old 30 January 2003, 15:26   #2
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Your part no 48-16990 is for an early 13 1/4" dia X 20"pitch Laser.

The details can usually be found on the hub. The prop is made by Mercury, and can be purchased from any Mercury/Mercruiser/Mariner dealer. If your motor is over revving, you ought to increase pitch by a couple of inches.

To calculate max speed you need to take max revs and divide by ratio, multiply this by Pitch, divide by 1056 and this will give theoretical max speed, deduct percentage of slip, around 10% is common, and you have top speed.
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Old 31 January 2003, 03:02   #3
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Propeller Info

Dirk,

Thanks for the reply, now I know what size my prop is I will give dimitri's formula a try & of course yours.

Can you tell me what you mean by ratio is it the gearbox ratio?

Cheers
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Old 31 January 2003, 03:39   #4
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Correct, from memory I believe the ratio of your gearbox is 2.33:1.

Your "theoretical" top speed at 6000rpm would be as follows;

6000/2.33= 2575 x 20 (pitch) / 1056 = 48.77 mph.

You then have to allow for prop slip. I don't know what this is on your boat, but 10% is quite common, so your max speed would be 44 mph
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Old 31 January 2003, 08:30   #5
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Prop Info

Dirk,

Many thanks for the tech info, this will be a great help.

So for now I will consider this thread closed.

Happy Ribbing
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Old 31 January 2003, 16:20   #6
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Dirk

excellent little formula

anybody wants it in an Excell spreadsheet format which includes a different range of pitches , mail me and I'll forward it (as only 17k attachment.) Thats bytes not price.

Tells me if I want my rib to do 80 mph, somehow I'll need to get my engine to rev to 12,000 rpm and change to a 21 inch prop !!! LOL
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Old 31 January 2003, 17:26   #7
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The formula seems spot on for my boat

I have made a webpage that does the calculations for you

http://ribs.loughlin.tv/propeller/default.asp

Daniel
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Old 31 January 2003, 17:32   #8
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Maybe Im asking a silly question but does the weight of the boat make any difference and the water line length and the waterline beam are needed to work out the theoretical speed. of the boat?

Cant see any of that on the formula thingy,its brill though,just not sure whether its accurate?
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Old 31 January 2003, 17:41   #9
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I'm sure weight and a hundred other things affect the top speed of the boat. But like most formulee you have to leave out some variables.

I guess if you know WOT RPM, max speed, gear ratio and pitch for your current setup you could work out the % slip and then use that to work out max speed using different pitch props
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Old 31 January 2003, 17:58   #10
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Im interested to see what the Kad works out at,I will get the imfo and see what she says using your formula.

I know what the Speed worked out is by a Naval architect ,so it will be of interest to see what your formula says,and then nock off 10 % as Dirk says for slippige and see what is left.

They use a complicated formula that costs money to work out,so a comparison will be of interest?.
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