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#21 erice

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Posted 20 January 2017 - 11:23 AM

online prop calculators can be useful for checking if props are matched well to boat + engine

 

if you've got all the numbers 

 

https://www.vicprop....cement_size.php

 

a more techie spreadsheet calculator downloadable here

 

http://www.marineeng...prop-calculator


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#22 native

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Posted 20 January 2017 - 11:26 AM

Alignment is one of those jobs we all dislike but its well worth it. I remove the flexible coupling (you should have one!) and use a bit of 3/8" tool steel to check clearances between the flanges. take your time, fiddle and swear until you get it within a few thou. Recently I redid mine and an annoying vibration at 1/2 throttle I had just disappeared like magic.

 

Online prop calcs can also be very misleading, the best starting point is other boats like yours or ask that water rat guy.


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#23 Chrisc

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Posted 20 January 2017 - 12:51 PM

Fish, I think the following is going imply that some people just shouldn't have engines but...
The original prop is a three bladed fixed sailor type. I haven't looked at it properly in terms of size and pitch, but the paperwork that came with the boat refers to it as a 14 x 7.
Figuring I was under propped (no smoke, max revs, no go) I went to Briski, gave all the particulars relating to boat and machinery and asked him to build me a prop to suit. I don't know what he supplied and I can't find the paperwork.
With the new prop I was hopelessly over propped so took it back for modification. This has been done and it is now a 14 x 8.
I don't know until I put the two props side by side but the new 14 inch looks a bit bigger than the old 14 inch one. Will know on Monday.
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#24 erice

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Posted 20 January 2017 - 01:22 PM

so banging the numbers you've given for hp (29), rpm (3600) + desired top speed (6) into the prop calc spreadsheet above

 

plus the displacement of a loaded compass h28 (12000lb) and water line length (22.8')

 

spits out a recommended 3 bladed prop of 14 x 8

 

with an alternative of a 13 x 10 (for if there are space issues etc with a 14' diameter prop)

 

interesting the calcs on the prop pitch page

 

6 - Speed in knots required from Power Reqd page

1800 - Max prop shaft rpm from Torque & SHP page    

1440 - 80% of max prop shaft rpm                

608 - desired speed expressed as feet per minute.                

0.42 - desired speed divided by max prop shaft rpm to give prop feet per minute.                 5.07 - Theoretical required prop pitch in inches.                

50.42% - Estimated prop slip at required top speed.                

8 - Required prop pitch for top speed.      


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#25 Chrisc

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Posted 20 January 2017 - 02:00 PM

Thanks Erice,
14 x 8 three bladed is what I now have to install. Hopefully my motive problems will soon be a thing of the past.
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#26 Muzza

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Posted 21 January 2017 - 01:07 AM

Chris Just make sure you have adequate tip clearance between propeller blade tips and the hull or rudder. An old rule of thumb was clearance minimum 12 % of the propeller diameter. 14 inch blade that equates to approximately 1 3/4" or just over 46 mm. 


Edited by Muzza, 21 January 2017 - 01:10 AM.

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#27 Chrisc

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Posted 21 January 2017 - 09:55 AM

Yep. All good there.
Although I was told that adequate tip clearance was important on boats with flat aft sections, on yachts with very acute aft sections, not so much.
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#28 Muzza

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Posted 21 January 2017 - 11:47 AM

Yep. All good there.
Although I was told that adequate tip clearance was important on boats with flat aft sections, on yachts with very acute aft sections, not so much.

Chris no mater what shape the hull is tip clearance is still important we did a displacement yacht once where the two bladed prop clearance  was slightly under  10% with this boat only the rudder was causing the clearance issue. we were getting bad cavitation  at anything over  2500rpm engine revolutions (3200rpm max rpm, 2:1 deduction) so effectively the boat was only able to use about  60% of the engines power. What does enable you to decrease clearance is decreasing shaft revs but certainly still giving yourself at least 10% clearance. The owner of this boat eventually went to the expense of a three blade feathering prop to increase bite but  with a diameter more suitable for the aperture size. The three blade prop was smoother and the boat lost much of the harmonic vibrations but most of all the boat could cruise at 7.5 knot at 2800rpm where before 2,500 gave just under 6 knots


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#29 Chrisc

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Posted 21 January 2017 - 03:35 PM

Thanks for the info Muzza. Still OK, I have 52mm clearance minimum in the aperture.
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#30 Fish

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Posted 21 January 2017 - 07:43 PM

Chris no mater what shape the hull is tip clearance is still important we did a displacement yacht once where the two bladed prop clearance  was slightly under  10% with this boat only the rudder was causing the clearance issue. we were getting bad cavitation  at anything over  2500rpm engine revolutions (3200rpm max rpm, 2:1 deduction) so effectively the boat was only able to use about  60% of the engines power. What does enable you to decrease clearance is decreasing shaft revs but certainly still giving yourself at least 10% clearance. The owner of this boat eventually went to the expense of a three blade feathering prop to increase bite but  with a diameter more suitable for the aperture size. The three blade prop was smoother and the boat lost much of the harmonic vibrations but most of all the boat could cruise at 7.5 knot at 2800rpm where before 2,500 gave just under 6 knots


So how do you tell if you have cavitation, esp above a certain rpm, does if have a particular sound or symptoms?
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