Gerry Douglas Response

[Tim April IV]

(Sent to the C350 mail list May 12th, 2006)

Dear Skip,

You are doing a great job as the MAINSHEET Tech. Editor, and it's nice to see the comments of appreciation from 350 owners.

The following address the prop/ cavitation issue on the 350. Would you please make this available on the web and in the MAINSHEET? Please contact me should you have questions.



There have been some owners concerned about noise and or vibration from the prop. The degree of the noise/vibrating seems to vary greatly from boat to boat and owner to owner because this is a very subjective issue and not measurable in any practical way.

It is my experience that every boat will experience cavitation at certain times. Simply stated this occurs when the propeller blade speed through the water exceeds the speed of the water flow over the blade surface leaving negative space behind the blade that is filled with oxygen pulled out of the water, when the flow "catches up" with blade speed the cavitaion stops. (This is an over simplification of a technical explanation, but basically correct ).

The conditions that can create cavitation are typically when the boats speed through the water is temporarily slowed by a head sea or when rapidly accelerating and continues until the boat speed through the water catches up with the prop speed.

There are ways to minimize the cavitation noise/vibration, one is to use a smaller diameter prop ( this would be much less efficient ) another is to slow the prop speed, ( this would require a much larger prop to achieve hull speed ) . The most practical is to decrease diameter to reduce tip speed and increase blade area to compensate for the diameter reduction and improve flow characteristics over the blade .
The props originally fitted to the 350 were 15" diameter 9" pitch 3 blades , a good compromise as is every sailboat prop ,where minimum drag under sail and efficiency under power are balanced.

In order to address the concerns of 350 owners about momentary cavitation we solicited the help of several 350 owners, on both coasts ,to do parallel testing of alternate prop designs. Three different designs were tested , during the test period Michigan Wheel, Catalina's prop supplier, introduced a new style prop designated the "M" series . Tests proved that the "M" series prop combined the best performance under power with minimum cavitation . The "M" series has a wider blade , therefore more frontal area than a traditional sailor prop and will produce slightly more drag under sail ,( all sailboat props are a compromise ), so racers or light air sailors may choose to stay with the original factory supplied prop.

Catalina will make the new "M" series props available to 350 owners , who wish to change props at our cost of $195.00 plus shipping. There are a limited number of props available at any one time , so we may not be able to fill all orders at once . The props will be shipped from the Largo Florida Plant and can be ordered Customer Service in Largo or though the Parts Department in Woodland Hills Ca.

Catalina has started installing the new Michigan Wheel "M" series Sailor props on hull number 400 and higher as standard.

Thank you , to the 350 owners on both coast who participated in the prop testing program.

Good Sailing, Gerry Douglas