What Is Gearbox Blowout And How Is It Prevented

[jp560_0]

Everyone I Speak To Has A Different Opinion On What Blowout Means
Anyone Experienced It Or Know Excactly What It Is And Why It Happens
And Does A Nosecone Prevent It
Any Advice Would Be Greatly Appreciated
Cheers

[Corona Mike]

Welcome to Scream And Fly! Blow-out happens when you run faster than your stock gearcase allows you to, and, the water flowing around the case gets desturbed to the point that the hull falls off--usually to the left--it's plaining serface--pad. Adding a nose cone will allow you to run past the point of blow-out. The low water pick-up cases also allow you to run your motor higher thus more efficent--to a point!--You will experince blow-out more frequently if you run a "V" bottom boat than your air entrapment designes, and some "V" bottom boats are more sencitive than others--Allisons--Also not all nose cones are created equal, so in other words just because someone offers a nose cone kit, Doesn't mean your performance will allways be the same. If you are having a blow-out problem it's best to send your case to a quality shop the 1st time. This allways saves both time and money!! Hope that kind-of answers your question. Be Safe. Mike T.

[Fish]

hey jp560, welcome to scream and fly! here is a quote from Jeff G on blowout from a past thread:


First you need to have a complete understanding of blowout to correct the problem.
Blowout occurs when the propeller loses bite with the water due to the introduction of aeriated water. When this happens the rpm's go up, the boat loses stability, the bow drops and depending on the rotation of the prop makes a hard turn.
Blowout is caused by a low pressure area forming on the side or bottom of a gearcase. This low pressure area is formed from the lower unit not going straight through the water or by the trim angle being too great. As this low pressure area forms it gradually moves back until it meets with the exhaust gasses. The exhaust gasses then "flood" forward enveloping the gearcase in aeriated water, losing bite.
To stop this action a number of things have to be looked at.
Is the trailing edge of the gearcase squared off or is it rounded?
Did the gearcase originally have a blowout ring on the trailing edge of the gearcase that has been rounded or ground off?
A prop that has a through hub exhaust is resistant to blowout. What type prop?
Try a blow out ring of different lengths if you are using an over the hub prop.
Is the lower unit crabbing? Why if it is?
Is your trim angle excessive? Is your prop shaft at speed parallel to the boat's bottom? Try balancing the boat.
Is the gearcase smooth without low spots in the housing?
A torque tab can help but will also slow the speed. Try playing with the motor height and prop types.
A nose cone may help depending on it's design. Some nose cones actually do more harm than good. A cresent shape is generally prefered.

Hope this helps


if you want more info on gearcases, there is a fantastic thread started by Wayne Tripp that has probably more info than you ever wanted to know about gearcases.

This may be more than you wanted to know but I saw a fantastic thread on gearcases with a lot of great info from Wayne Tripp. I think pretty much everything you want to know about gearrcases is in there.

http://www.stvowners.com/forum/index.php?showtopic=694&hl=gearcase+mods

good luck and welcome to S&F,

fish

[jp560_0]

youve summed it all up for me you would laugh if you heard some of the things that other people thought it was
cheers:)

[Corona Mike]

Like I said some boats are much more critical of blow-out than others. With Allisons Depending on how fast you run, you need to run both a nose cone and a blow-out ring. They both do basicly the same thing--make the case longer. This gives you cleaner water for the prop. The one thing is in most cases there is a difference with case blow-out and prop blow-out. With your case causing a blow-out boat "usually" dips lefy--or in very bad cases turns left. The prop. loosing bite due to it being a POS--boat usually dips right, and very rairly makes a right turn.