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  Cavitation, chine, porpoise--What are they exactly?

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Author Topic:   Cavitation, chine, porpoise--What are they exactly?
band posted 05-24-2003 07:24 PM ET (US)   Profile for band   Send Email to band  
I'm trying to study up stabilizer fins and it's really tough to understand with unfamilar lingo. [Implied question: What is the meaning of cavitation, chine, and porpoise?]

Also, anyone know where I can find detailed specs on Stingray, Doel Fin, and SE Sport hydro-foils?


Lars Simonsen posted 05-24-2003 09:26 PM ET (US)     Profile for Lars Simonsen  Send Email to Lars Simonsen     
Chine is where the angle of the bottom of the hull meets the side of the hull. The "Smirk" of the whalers is referred to as a "reverse chine" because of the "fender" formed by the smirk (I'm not sure that made any sense).

Porpoising is when the bow of your boat rythmically rises and falls while underway as a result of having your motor trimmed too high. It feels like you're going over gentle waves, but it's actually caused by the motor being improperly trimmed.

Cavitation is when your propeller pulls air as it spins, often when your motor is trimmed to high, or the motor is mounted too high on the transom. The air bubbles can damage your prop because they explode with such force that they cause pitting (at least on an aluminum prop). You can usually hear it when you raise your motor too high.

There have been a lot of threads on doel fins on this website. Try using the search function on the main page of the site. I don't know any specs on these stabilizer fins myself.

band posted 05-24-2003 09:38 PM ET (US)     Profile for band  Send Email to band     
Thanks a mil Lars!

Off to search threads i go :)

JBCornwell posted 05-24-2003 10:12 PM ET (US)     Profile for JBCornwell  Send Email to JBCornwell     
Ahoy, Band.

What Lars describes as cavitation is more correctly called ventilation. It seems like nit picking, but the difference is important.

Cavitation occurs when the pressure on the trailing surface of the prop blades gets so low that the water vaporizes. It can be caused by too much pitch, but most often it is from a damaged prop. It will damage the prop surfaces and endanger the engine from over-revving.

Ventilation is what Lars has explained so well.

Red sky at night. . .

Sal DiMercurio posted 05-25-2003 12:41 AM ET (US)     Profile for Sal DiMercurio  Send Email to Sal DiMercurio     
JB is right on the ventilation.
Also, I think he means chine walking, not chine.
Chine walking is when the boat rides on one chine then bounces to the other real fast while at high speeds, kinda like porpoising from side to side.
jimh posted 05-25-2003 08:48 AM ET (US)     Profile for jimh  Send Email to jimh     
"Stabilizer fins" are seldom used on boats the size of a Boston Whaler. They are popular on larger boats, say 40-feet and bigger. Their purpose is to supress roll.

The term "roll" as applied to ships is a rotation about a fore-and-aft axis. This motion is particularly uncomfortable for the occupants of the vessel.

Stabilizer fins along the underwater portion of the hull can be used to suppress this rolling motion. The fins are often made moveable, and their angle of attack can be varied to counter the roll motion. Typically a gyroscope is used as a reference, and the roll motion detected with respect to the gyroscope. Actuators adjust the stablizer fins to fight against the ship's natural roll motion. The net effect is that the amplitude of the roll motion is reduced or in some cases eliminated.

Vessels that have stabilizer fins must be careful when mooring alongside quays, as the stabilizer fins are prone to damage. This is one reason you often see larger luxury boats using very large fendering to hold their hulls several feet away from the quay.

A good place to "study up" "unfamilliar lingo" is the dictionary. If you don't have a printed one you can use the Merrimam-Webster online dictionary. I look up many words every day. In the case of your inquiry, the standard dictionary explains "chine" and "cavitation" quite well. "Porpoise" is not explained in terms of hull movement in the dictionary. "Porpoise" is used metaphorically by boaters to describe the uncomfortable motion of the hull which looks and feels much like a propoise's swimming motion in and out of the water.

jimh posted 05-25-2003 09:11 AM ET (US)     Profile for jimh  Send Email to jimh     
Here is a hyperlink to the Merriman-Webster online dictionary. I have this link in my browser's toolbar so I am never more than a click away from it.
jimh posted 05-25-2003 06:30 PM ET (US)     Profile for jimh  Send Email to jimh     
[Administrative post]
hooter posted 05-25-2003 09:50 PM ET (US)     Profile for hooter    
Got a feelin' what you mean t'be studyin' is somethin' like the Doel Fins or similar "fins" to be mounted on your engine's lower unit. If that's your aim, Ah say just go buy a set and put them t'woik. Have put a set on ever boat Ah owned that size that Ah can remember over the last 30 years or so. Will get you up on plane quicker and at lower rpm, will keep you on plane at lower rpm, will reduce the tendency of most boats under 20' o'length to chine walk and porpoise, too. Hope that helps.
JBCornwell posted 05-25-2003 11:11 PM ET (US)     Profile for JBCornwell  Send Email to JBCornwell     
What Hooter said.

The correct term for what you are intrerested in is "Hydrofoil". Stabilizer fins are a different animal.

Red sky at night.. .

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