please advise(one for bob??)

[JamesM]

Shame they banned the F1 cats from racing in bristol docks cus it was awesome, if a little dangerous. Any one got any photo's?

[michaelcaine]

dont say that i might just tow it down!!!!

[OCRDA]

You'd be most welcome !!

[Wacky Racer]

Re: Bristol Docks, got some on dvd, any idea how to upload it ?

[JamesM]

Nope, I'm currently looking through my old photo albums. If I find any I will then have to figure out how to use my scanner I've never used!!!

[michaelcaine]

[Jon Fuller]

Originally posted by OCRDA
No it is'nt but i won't argue about it !!

That IS arguing about it!

so explain your understanding of 'Blowout', don't keep it a secret.

[OCRDA]

Sorry Already have on previous page !! Assumedyou had seen it !!

[Jon Fuller]

no, but I have now, and it's not really an explanation of 'Blowout'.

I just found this site, which has a bit info on it.

in the blowout section, cause #4, near the bottom, = speed, have a look.

http://www.bassboatcentral.com/nosecones.htm


And if anyones interested, another good page on gearcase mods

http://www.hydrostream.org/Topic.htm

[michaelcaine]

jon,please have a look at good year thread.

[OCRDA]

Thanks mate but i think that is what i said !!

[Jon Fuller]

not really, you said that blowout is caused by the case tracking sideways, which obviously could induce, or encourage blowout, but it also occurs on twin counter rotating rigs, where this isn't an issue, but blowout still happens mainly for the classic 'too blunt, too fast' reason, as I don't think you'd ever find it (blowout) happening with a speedmaster type lower, unless maybe the drive was tracking way-off, but that would be another issue in itself. I have to admit, i've never experienced it myself.

but hey, ..

[Wayne Kerr]

Told ya Bob! ,,,,,,,

[John G]

Ok - this is going to sound boringly diplomatic but actually (IMHO) you are both right!

Blow out is caused when the water flow cannot follow the shape of the gear case (i.e. its too blunt / fat etc). However the seperation occurs down one side or the other.

This is down to the fact that we are not operating in (scientifically) perfect conditions. I think its known as chaos theory?? Same as the spiel Jeff Goldblum goes into at the start of the film Jurassic Park when he's chatting up the woman (that'll make sense if you remember/ have seen that!). Basically the gearbox is never perfectly straight, or, the sides are not of equal smoothness, or, the flow off the hull perfectly symetrical at that instance, or, there is a tiny amount of play in the steering, etc etc etc!

The effect of the flow seperating on one side only is that void it leaves behind creates a low pressure on that side. As the other side still has attached flow and therefore high pressure the gearbox is 'thumped' over to one side (high to low pressure) - it sort of feels like some huge b*stard with size 50's has kicked the stern of yr boat sideways! - like the effect OCRDA describes.

I think JBD could be on to something when he said perhaps its not so noticeable in big heavy boats.

The Void created by the flow seperation will also of course cause the prop to ventilate and 'lose grip' for an instance as JF says.

So this effect can happen to a boat travelling perfectly straight and can be brought on/worsened by changing trim etc (i.e. the shape of the gearbox relative to the oncoming flow). Nose cones etc help by creating an easier shape for the water to flow over - therefore it stays attached (on both sides) at higher speeds.

All MHO but I hope that all makes sense!

[Matt]

Yeh, but where does the butterfly flapping it's wings on the other side of the world come in?

[John G]

[Matt]

OK, but seriously I don't think it's chaos theory.
I also think that not all nose cones are an improvement - in fact I can imagine a sharp nosecone on a boat needing lots of trim to be dangerous, and pointed nosecones being more prone to letting go in the corners.
However, the "right" nosecone - completely agree.

[John G]

Yeh - don't know if it is chaos theory

Was trying to explain that the water will let go of one side or the other but not both - giving the sideways thump.

...but...

I've since learnt that in some extreme cases a massive blow-out can happen that doesn't give any feeling of sideways force and just results in the prop spinning out, like a bush has gone - just like Jon said

Some of the long nose cones can also produce too much lift if they are trimmed out to much. Like you say Matt - there ain't a best nose cone, its just the right nose cone

[Matt]

Too much lift if trimmed out?

[John G]

Originally posted by Matt
Too much lift if trimmed out?

DOH!!!! -

Trimmed in!

Where's the Hommer Simpson smilie?!?