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Old 11-02-2013, 13:04   #16
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Re: Props for Dummies...

A 3 blade 16x9 pushes my boat at about hull speed at the hieght of the torque curve (2100rpm = 6.75knt). I was wondering about a two bladed prop that would accomplish the same thing (that could be lined up with the keel under sail).
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Old 11-02-2013, 17:53   #17
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Re: Props for Dummies...

Wolfenzee,
With a full keel heavy displacement boat like yours, I doubt that you'd see any difference under sail by going to a two bladed prop and would be very disappointed when you couldn't stop approaching the dock. My 27' 12,000# boat can be stopped with a goose on the throttle with the three blade, not so with the two. When I went to three I didn't notice a reduction of sailing speed.

Gelfling,
Rule of thumb; swing as big a diameter as fits the situation. Consider a 14" if you can. Talk the the Prop shop in Mukilteo.
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Old 11-02-2013, 19:09   #18
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Re: Props for Dummies...

I have a 16x9 now, I could probably fit a 17, it was suggested that I reduce the dia and increase the pitch. Looks like I shouldn't mess with something that works
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Old 12-02-2013, 02:17   #19
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Re: Props for Dummies...

Maybe similar discussion about my propeller will be helpful:
Dufour 3800 - what engine originally?
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Old 12-02-2013, 02:50   #20
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Re: Props for Dummies...

We sold the Campbell Sailor for many years and always recommended the three blade props for all sailing boats. The two blades will slip while going through the slipstream from the keel causing a possible vibration which is usually blamed on the engine. Only one blade of a three blade propeller is going through that slip stream at a time.
Email West By North dot com and have them size the propeller for you. You can fill in their answer sheet on line and there is no obligation. They usually send the prop out in less than a week. Shorter if you want to expedite delivery. Each propeller is built for your vessel, engine, transmission, shaft, etc.
We have one our our boat and are out cruising with it.


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Old 12-02-2013, 03:47   #21
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Re: Props for Dummies...

Quote:
Originally Posted by Gelfling View Post
I have searched the forum and have not found a situation like mine. It does appear that once a boat has had a certain number of owners for a certain number of years....no matter how well maintained the boat....there are parts that just don't match up!

My question has to do with the prop. My boat came stocked with a Universal M-15 (5411, 11hp), but has been upgraded to a M-18 (14hp). The previous owner told me that he did not change the prop when he upgraded the engine and after research it appears the manufacturer calls for the same size prop for both engines. With that said, I need a new prop and would like to go to a 3 blade as it now has a 2 blade. As we do a bit of gunk-holing I have found that having that extra under power is more beneficial than what I loose under sail.

The 'kicker' is, the current blade is stamped: 2X12RH (maybe a '1' at the beginning has been worn off??) and the manufacturer suggests a 13X10. I am not a 'prop guy' by any means....so what am I to make of this?

Now, I could use the MFG specs and calculate from there, but there seems to be so many 'generics' out there about how to size from a 2 blade to a 3 blade. How would I make the conversion so that I can maximize my speed under power?

You know my engine. Here is the boat: ERICSON 28+ sailboat specifications and details on sailboatdata.com

Any help would be greatly appreciated!

Why not try an online propeller calculator? Castle Marine of the UK do one but unfortunately if you have had Windows 7 64 bit foisted upon you after being burgled, the Castle Marine programme will not work as it runs 16 bit code. It used to run perfectly with Windows XP 32 bit but Windows 7 Home Premium is no good unless one pays a large fee for the XP downgrade.

One programme that does work is by Vicprop of Canada. Excellent!

One problem you may find is that your boat might be MUCH heavier than what is posted on the web. Examine the hull very carefully for signs of the original bootop hidden under the paint and check to see whether extra ballast has been added. I did have the misfortune to get sweet-talked into moving a boat for a friend a while ago and in spite of THREE surveys in the boats log book no one had noticed the two tons of extra ballast. Later it was noticed that the original bootop was hidden under the paint seven inches lower down. Luckily the additional weight was no problem at all for my Land Rover.

Vicprop - Propeller Calculator

It is important not to be over-propped as this will cause the engine to make black smoke. Eventually the cylinder head will get choked with carbon if things are left like this. Yanmar will not guarantee their engines unless a sea-trial is done which shows that the engine can reach its maximum revs.

Note that marine growths on the prop such as barnacles will also cause black smoke so it is advisable to be slightly under-propped to allow for the times when the propeller will not have a mirror finish.
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Old 12-02-2013, 08:00   #22
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Re: Props for Dummies...

You could check your findings with the 'Surfbaud Freeware Propeller Calculator for Excel' which is easily found by Googling 'propcalc.xls'.

Also try searching the CF with the keyword 'propcalc'.
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Old 12-02-2013, 11:49   #23
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Re: Props for Dummies...

My engine is rated at 33hp at 3000 rpm (max rpm, 33hp) according to the calculator would require a 15.2x8.3 to useing 30hp to move the boat at 6.75kt.
The calculator says at 2100rpm (top of torque curve and 22.9hp) would require 17.6x10.7 prop to move the boat at 6.02kt
At 2100rpm a 16x9 moves the boat at 6.75kt
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Old 12-02-2013, 15:43   #24
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Re: Props for Dummies...

Why top of the torque curve? Torque doesn't move a boat, power does. I don't recall maximum torque being a factor in prop sizing.

And 17.6x10.7 sounds like too much for a 33hp engine...
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Old 13-02-2013, 05:04   #25
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Quote:
Originally Posted by daddle View Post
Why top of the torque curve? Torque doesn't move a boat, power does. I don't recall maximum torque being a factor in prop sizing.

And 17.6x10.7 sounds like too much for a 33hp engine...
Not sure if he is considering it for the same reason...... but on a semi truck this is used for fuel consumption. But torque becomes a bigger factor on an 80,000lb `displacement` I would imagine.
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