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Old 10-02-2006, 07:27 PM   #1
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Question Extended Nose Cones

Fitted to cle's/sportmasters! Does anyone know the benefits and how they work? Speed? Stability?
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Old 10-02-2006, 07:44 PM   #2
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look in the lower unit for sale on scream and fly theres a titus case for sale .it explains it in the thread
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Old 11-02-2006, 10:53 AM   #3
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Can't speak for others, but ours gives us much better cornering ability and stabilty in a straight line, for boats going over 75 there are less prone to the gearbox "blowing out" with extended gearboxes.
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Old 11-02-2006, 02:11 PM   #4
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Quote:
Originally posted by Cookee
Can't speak for others, but ours gives us much better cornering ability and stabilty in a straight line, for boats going over 75 there are less prone to the gearbox "blowing out" with extended gearboxes.
Blowing out??
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Old 11-02-2006, 05:06 PM   #5
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Quote:
Originally posted by sinkunit0
look in the lower unit for sale on scream and fly theres a titus case for sale .it explains it in the thread
Thats not the same extension im thinkin of! It looks like a torpedo! It carries the width of the hub forward past the point where the water pick up normally is and moves it forward about 10 inches! Im borrowing one for tommorow and were going to see if it makes any difference! Hopefully it will be abit calm so we can see if it makes any difference to top end speed and stability!
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Old 11-02-2006, 05:28 PM   #6
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Nosecones

If your question is - what are the advantages of a nosecone...then here are a few...

A well-designed nose cone can eliminate blowout and improve propeller efficiency. Very few boats run into actual total blowout, but rather more often encounter propeller burn, a phenomenon that occurs prior to blowout. The addition of a nose cone can eliminate propeller burn/blowout, and can increase speed if the horsepower is available.

As we increase the length of the nose cone, the aspect ratio (Length/Diameter) increases, which provides superior water flow characteristics and gives the water flowing by the unit a greater chance to flow back on the surface of the lower unit before aerating the slip stream going by the propeller.

As water flows by the lower unit and begins to separate from the unit at higher speeds, a turbulent flow condition exists. This causes a much lower pressure behind the gear case. The addition of this lower pressure increases the drag of the lower unit. Additionally, the aeration of the water going through the turbulent flow and the water separation
causes propeller burning. This phenomenon leads to blowout, and can reduce top speed before blowout is obtained. (You may have noticed pitting on your propeller blades caused by this propeller burn).

A consideration when installing a nose cone can be an increased tail lift. The addition to gear case length (improved Aspect Ratio) also adds surface area to the gear case, which translates into increased tail lift. In many cases this is not desirable. When tail lift goes up, the bow goes down, and over-trimming to correct the problem wastes power. A solution in some nosecone designs is a parabolic design that creates a more efficient lift characteristic - a neutral gear case force that does not affect boat attitude.

Finally, the addition of a nosecone often gives the opportunity to provide a low water pickup. Outboard applications that run high enough to run conventional pickups out of the water will really benefit from a low-water pickup.
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Old 11-02-2006, 05:35 PM   #7
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Ah Thankyou! Explains it all!
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Old 11-02-2006, 06:22 PM   #8
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Hi!

You can also read a test of it here:

http://www.screamandfly.com/home/eva...ter_test_1.htm
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Old 11-02-2006, 06:45 PM   #9
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So is the blow out lip the raised edge just before the end of the gearbox behind the prop? And excuse my ignorance but what is blow out???
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Old 12-02-2006, 09:17 AM   #10
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Right i know what blow out is now! So am i right in thinking that the extended nose cones are done so that the bubbles and "buffiting" affect of them rolling over the front tip of the gearcase gives them a chance to settle down and dissapear as much as poss before hitting the prop? So if i get a nose cone made that just extends the shape of the hub forward its going to be the best shape? And i should leave the blowout ring on the rear of the box because if i remove this it will cause the blowout to occur at a lower speed?
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Old 12-02-2006, 09:42 AM   #11
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Yes and Yes.
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Old 12-02-2006, 11:27 AM   #12
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Quote:
Originally posted by scottyboy
Right i know what blow out is now! So am i right in thinking that the extended nose cones are done so that the bubbles and "buffiting" affect of them rolling over the front tip of the gearcase gives them a chance to settle down and dissapear as much as poss before hitting the prop? So if i get a nose cone made that just extends the shape of the hub forward its going to be the best shape? And i should leave the blowout ring on the rear of the box because if i remove this it will cause the blowout to occur at a lower speed?
And using a skeg with a raised lip on the right hand side like a few ive seen will also help stop the "crabbing" effect that im suffering from at high speed when i run the engine jacked? This is what im mostly trying to stop as it will be fine one minute and then all of a sudden whoops im crabbing to the left with light steering! And its not nice!
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Old 12-02-2006, 03:56 PM   #13
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Blow out is indeed not a nice thing to happen.I,ve had it with a ring 20 with a 2,4 EFI and 24 chopper with 1,87:1 ,at around 78 mph,it just spins a 180 degrees turn and tried to throw me and my nephew out of the boat.
The result was we both had a bruise on our head.
We were using a CLE lower with a torque tab.
Later we installed a blow out ring from Land& sea,but it did not made a big difference,it still blows out at around 78mph.
With a 4 blade our problem was gone,it did not pull so hard at the wheel anymore,and the crabbing was less than with a 3 blade
If you could turn the propshaft faster with less pitch just to achieve the same speed ,you should have also less prone to blow out,because the bigger the pitch of the prop ,the more it pulls the gearcase to the right (RH only) and you have less steering torque.
I have a Hydrostream Vector with a xr6 lower 1,78:1 (yes 1,78:1)
witha bobs big foot nose cone and welded torque tab,since a xr6 lowers have a small diameter housing ,the length vs diameter is better,this unit will not blow out at speeds up to 95 mph.

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Old 12-02-2006, 09:02 PM   #14
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Blowout

scottyboy - here is an explanation of Blowout and causes.

Blowout occurs when the ratio of air to water around the propeller gets so high that the propeller is no longer grabbing water, but is trying to propel itself through air (or a relative vacuum). This causes the propeller to lose "bite", and then a chain of events occurs that can range from merely a "loose" steering feeling, to a vicious turn to the right (typically). The speed at which this occurs varies with boat design, gear case design, and propeller design.

The four main contributors to blowout are:

1) Gear case inconsistencies:
If the gear case has been damaged (run up on the rocks a few too many times?); or has an improperly installed nosecone; or a damaged skeg; the gear case cannot provide the proper aerodynamic direction (steering) effect. The impact can be the need for the gear case to "crab" or slide sideways through the water, creating an area void of pure water, like a vacuum or air pocket in which the propeller tries to operate. This is bad for the propeller - it needs water to work properly. Cleaning up all nicks and gouges in the gear case so that it is very smooth will help.

2) Motor is too high:
If the motor is too high, the propeller will not be able to provide lift for the boat, causing the driver to apply excessive trim. This causes the propeller angle of attack to be angled downward (more than it needs to be) thus trying to force itself to go sideways through the water. This is bad for the propeller, and inefficient for the boat performance. Designing the hull with the engine at the optimum height will help overall performance.

3) Hull Design:
Some designs of boats are more susceptible to "blowout" than others are. Why do we think that is? Well, it's difficult to know whether you have a good one or a bad one, but the well-designed hull will have a dynamically balanced performance through all phases of performance (all operating speeds). The poorly designed or poorly dynamically balanced hull will need much more time and effort in "on the water" set-up. It is, of course, better to design the stability and performance characteristics into your hull ahead of time. This makes the set-up much easier, and the hull performance more predictable in all operating conditions.

4) Velocity:
When you go faster than a stock gear case is designed to perform, the water separates from the leading edge (front) of the blunt bullet and sort of "bounces" around the propeller. In engineering terms, we have a disturbed flow, and when this occurs near the propeller, it really impacts the propeller's performance. Smaller gear cases with smaller, aerodynamic bullets will always improve this situation, delaying "blowout" tendencies to a higher velocity. Adding a nosecone will also increase the velocity that a standard gear case can operate effectively.


The cause of blowout is typically a combination of all of these. Gear case modifications and propeller changes can reduce your chance of blowout. So can a properly designed and dynamically balanced hull. However, when you go fast, blowout becomes part of the business, so you will experience it eventually.

Typically, a blowout is immediately preceded by a "loose" steering feeling, an increase in RPM with no speed increase, a loss of lift, and a resulting drop of the nose of the boat. Hold on!


My explanation and more details can be seen in "Secrets of Propeller Design"
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Old 12-02-2006, 09:19 PM   #15
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Cheers for that info! exactly wot i needed to know!
Im getting the light steering and then all of a sudden going left but only slightly to the left! BUT im not jacking very high not even level and its doing it! It seems to be doing it worse than ever now and i have jacked higher previously so im wondering if i have a prop problem now???
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Old 12-02-2006, 09:29 PM   #16
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Or a bottem problem?
Maybe a few serieus chips fiberglass missing?
Or the prop ,as you said.
Could be worn out lower/upper mounts
Lower mounts are the most important,they must be able to keep your gearcase straith.
Maybe you trimmed a tad higher than before?
If your gearcase is running level with the bottem ,you should have reach the blowout point later than trimmed up more.

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Old 13-02-2006, 10:21 PM   #17
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Anyone got any pics???
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Old 14-02-2006, 12:02 AM   #18
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In my very, very limited experience, I found the backdraft (similar, but not the same I know) to dislike anything other 100% neutral trim.
At almost any kind of trim in you could feel the arse start prop riding, and at any kind of trim out, while the arse was more stable, it tended to chine walk more. Somewhere in the middle was the sweet zone. The propshaft is pretty much on a level with the bottom, no lifter.
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Old 14-02-2006, 08:47 AM   #19
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That's why (I'm told) the Batboats all run lifters - they run pretty much neutral trim and get the speed from raising the engine. Of course stepped hulls in general will never benefit from a great deal of trim as the angle of attack is set by the angle of the steps in relation to each other.
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Old 14-02-2006, 01:37 PM   #20
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Yeap agree with you both! Trimmed level and from then on any speed gains are obtained through jacking the engine not trimming! BUT i have a mark on jack gauge where the jack is level with the bottom of the boat and im not getting to this because i get "blowout" before! It never used to do it and has all of a sudden started doing it earlier but i can see any reason why?????
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