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S&S Swan General - Propeller for swan 44
06 May 2013 - 21:26
#1
Join Date: 12 March 2013
Posts: 24

Propeller for swan 44


Dear The professor and all,

Regarding the last 2 months question about the propeler for my swan 44, which one will best for crusing and racing, here is the information about ratio and engine :

engine yanmar : 4jh3g-te1 reduction  2.7 N°E157557

Tk u and fairwinds,

César

09 May 2013 - 10:47
#2
Join Date: 02 January 2008
Posts: 1547

                     

Dear Cesar
Thank you for the engine type information and reduction ratio.
Your powerful engine would benefit from a bigger propeller, but the diameter is probably restricted to about 17" on this model. It would be worthwhile checking the maximum diameter that can be fitted, but some clearance is also needed.
The choice of propeller depends on what is important for you - maximum thrust in a strong headwind, good cruising performance, or small resistance under sail? As pointed out recently on the Forum there is no single propeller providing the best of all these, compromising is necessary.
If maximum thrust is important the suggestion with restricted diameter is to choose a 4-bladed propeller for maximum blade area, for cruising speeds a 3-bladed, and for small resistance a 2-bladed.
In order to give you an idea of the differences, a 4-bladed would provide about 16% more thrust than the 3-bladed, and a 2-bladed 20% less.
The Forum post dated 8 May 2010 discusses various aspects of propellers, and it is recommended that you read it in this connection. 
If you could check the maximum diameter, and indicate the preferred number of blades and type of propeller a proposal can be worked out.
Kind regards
Lars

09 May 2013 - 16:44
#3
Join Date: 12 March 2013
Posts: 24

tk u lars, i will have a look on the forum, probably going to chose a flexofold 3 blades.

Fair winds,

César

14 May 2013 - 14:34
#4
Join Date: 15 April 2011
Posts: 393

Dear Professor,

I just read the thread dated may 8, 2010 which describes in clear and thoughtful detail propellers, propeller tests and comparisons of propellers by use; e.g., flat blades vs. curved blades.

Thank you.  I continue to learn everytime I read something you write!

One question:  would the diameter of a 4-blade prop necessarily be smaller than a 3-bladed prop?

Chris

15 May 2013 - 10:13
#5
Join Date: 02 January 2008
Posts: 1547

Dear Chris
As I understand it your question refers to the relation between diameter and number of blades for sailing yachts.
Generally it can be said that fewer blades give better efficiency provided there is no limitation on diameter.
If the 3-blade propeller can use the optimum size, then an optimum 4-bladed propeller would be slightly smaller and slightly less efficient.
Often, however, the diameter is restricted for various reasons, and then blade area becomes an important parameter particularly with higher shaft powers.
When more blade area is needed and diameter is limited, increasing the number of blades is a way to get more area.
If the propeller in your fictive case is highly loaded at full power a 4-bladed with maximum allowed diameter would provide more thrust than a 3-bladed with the same diameter, but when you throttle back to less power all the area is not needed, a 3-bladed would perform better, and with low power a 2-blade would be best.
The choice of propeller is related to the condition when you want it to perform well, and it will then not perform as well in other conditions. Usually a compromise is chosen so the performance is decent at higher powers, otherwise you could as well have an engine with less power.
Did this answer your question?
Kind regards
Lars

15 May 2013 - 14:06
#6
Join Date: 15 April 2011
Posts: 393

Dear Lars,

Yes, that answers my question very clearly and, as usual, there is more to the answer than I anticipated. 

Thank you.

Chris

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