Author Topic: hyper grate for hamilton 212  (Read 5394 times)

Offline PM

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Re: hyper grate for hamilton 212
« Reply #15 on: October 26, 2012, 22:43:24 PM »
Isn't it the suction of the intake that allows a jet sprint boat to generate multiple G's in a corner. ???

Hull design.

Just try putting a pressure/vacuum gauge (like a boost gauge) at the front of the intake foot.  I would expect greatest suction to be when booting it from a standstill and greatest positive intake pressure would be when you close the throttle at high speed.

Done that

Offline PM

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Re: hyper grate for hamilton 212
« Reply #16 on: October 27, 2012, 12:27:02 PM »
Your example of the boat doing 50mph has some fatal flaws. Not only would the impeller create a restriction, but the size difference between the intake area and nozzle area would mean that pressures inside the pump would build up to limit volume flow,and remember, 15-20psi is at the front of the impeller, not at the nozzle after the drag of the impeller, stator, tunnel walls etc. Hence only a small volume of water would indeed exit the nozzle while under tow. (It is here that I wish to stress that a small volume is relative to when the motor is running, it would be considered a large volume relative to a garden hose!)

I agree that when you initally mash the gas you will see a tempory reduction of pressure, and when you get off the gas you'll get a spike. However, if you were to couple an HJ212 with 4.0kW turbo impeller to a 496cid HO big block and apply WOT while at rest the pump will simply cavitate. This is due to negative pressure infront of the impeller. Once the boat is making way, the hull and intake design start doing their job there is enough pressure infront of the impeller to ask for WOT and the pump respond accordingly.

To much pressure can be a problem as well. The amount of pressure becomming a problem is dependant on the hull and pump. When pressure tries to escape you'll get what some like to call reversion, this is when the excess pressure blows out of the intake of the pump causing the stern to jump leading to control instability. This is similar to what happens when you get off the gas in a hurry, or the engine seizes. This is why the NJBA (US National Jet Boat Assoc. look after 1/4mile drag boats) recommends the use of BOV's (Blow Off Valves, or pressure relief valves) and break away couplings in an effort to keep the impeller turning and relieve excess pressure out of the top of the pump so the 'pilot' can maintain some resemblance of control.

If the pump were to suck water from below the boat there would be no need for the all important 'spoon' on the bottom of the marathon boats, and the design of the intake would be nearly inconsequential. The many evolutions of design and money poured into this would suggest otherwise.

Waterjets simply don't like to suck, they require a small amount of pressure infront of the impeller to be at their most efficent. Thrust (acting on the boat) from the pump is measured by the pressure difference between inside and outside the thrust face (usually the nozzle) and not the difference between the front and back of the impellers. This is the fundamental difference in thrust behaviour between a propeller and a waterjet.

Offline Paul

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Re: hyper grate for hamilton 212
« Reply #17 on: October 27, 2012, 13:37:05 PM »
I can't be ar$ed bothereing to try and explain.

Out.

Offline JR

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Re: hyper grate for hamilton 212
« Reply #18 on: October 27, 2012, 13:44:29 PM »
I can't be ar$ed bothereing to try and explain.

Out.
Thats the sprit Paul  sl sl sl...  O)))) O))))...Cheers JR

Offline swifty

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Re: hyper grate for hamilton 212
« Reply #19 on: October 27, 2012, 14:06:01 PM »
I can't be ar$ed bothereing to try and explain.

Out.
To lift the water into the intake and flow it through the jet unit,its gotta suck it.I gonna ask hammys on here,they will know :).
If your feet are dry,maybe your boat wont go?

Offline mohawk

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Re: hyper grate for hamilton 212
« Reply #20 on: October 27, 2012, 14:21:05 PM »
To lift the water into the intake and flow it through the jet unit,its gotta suck it.I gonna ask hammys on here,they will know :).
Rubbish !!!! your wrong , PMs right  cou

IN ..

Offline swifty

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Re: hyper grate for hamilton 212
« Reply #21 on: October 27, 2012, 14:42:00 PM »
Well i spose it is always described as water being drawn into the jet unit,the question is ,is drawning water into a jet unit sucking. I think if you were describing a jet unit to someone without marine experience. You would tell them water is sucked in the grill and pumped out the nozzle at greater velocity. 
If your feet are dry,maybe your boat wont go?

Offline Redskin12

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Re: hyper grate for hamilton 212
« Reply #22 on: October 27, 2012, 17:37:28 PM »
Hull design.

Done that

Not quite sure how to phrase this question but here goes, does the intake side of the unit affect the static COG of the hull as a whole when in a dynamic state (ie a high speed tight turn ??) Obviously the Static one stays the same in relation to a fixed 3D coord within the confines of the boat, but are you saying the unit intake has no affect on the cornering ability of the boat ?

Given what looks like a pretty high polar moment of inertia (as a result of the bulk of the unit out of the back of the boat along way from the static COG), there could possibly be some improvements made here in terms of hull response??

Just thinking out loud LOL

Nice to see some innovation out there anyway

Offline zymes

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Re: hyper grate for hamilton 212
« Reply #23 on: October 27, 2012, 18:01:11 PM »
Good one Redskin.
You have simply put a question which many jetboaters have not asked themselves and it needs answering now. Everyone should retire to the dunny and not come out until they have a sensible solution.
Cheers

Offline Matthewf/Norbert/TuiDog

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Re: hyper grate for hamilton 212
« Reply #24 on: October 27, 2012, 19:28:45 PM »
Everyone should retire to the dunny and not come out until they have a sensible solution.

finally a real engineering solution that makes sense. love it   b>

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TUI - Clean for 369 days now! Hate the stuff

Offline FLM

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Re: hyper grate for hamilton 212
« Reply #25 on: October 27, 2012, 20:15:32 PM »
Well if the unit doesn't suck why do you have to turn it off to drop off stones etc. 2c 2c

Offline Hacky

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Re: hyper grate for hamilton 212
« Reply #26 on: October 27, 2012, 20:35:02 PM »
Didn't the old 750 hami's have an air bleed which sucked air into the intake, and I would have thought that to change the direction of water flow, especially at speed, to come up into the intake it would have to be sucked up ?   ??? ??? ??? ??? ??? ???

Offline mohawk

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Re: hyper grate for hamilton 212
« Reply #27 on: October 27, 2012, 21:13:18 PM »
Didn't the old 750 hami's have an air bleed which sucked air into the intake, and I would have thought that to change the direction of water flow, especially at speed, to come up into the intake it would have to be sucked up ?   ??? ??? ??? ??? ??? ???

All 7.5 inch hamiltons have an air bleed set up.
Re-read PMs post, the pump cant suck water that isnt there . The faster a boat goes the faster water has too be fed to the blade , so the front of the intake is curved to force feed water into the unit (hold the back of a teaspoon on the edge of your tap water and see how it bends ). 
The faster you go the more important the amount of water  becomes .

Offline zymes

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Re: hyper grate for hamilton 212
« Reply #28 on: October 27, 2012, 21:53:13 PM »
Hacky, The air bleed pipe is just that. It allows air to bleed out of the front top of the unit so water gets in and the unit is primed, when starting. No water comes out this pipe unless you let fall down to the the level of the unit, so it is not under pressure. It does not let air into the unit as most of the time the level of water up the pipe stays the same. Cheers

Offline mohawk

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Re: hyper grate for hamilton 212
« Reply #29 on: October 27, 2012, 21:59:23 PM »
Hacky, The air bleed pipe is just that. It allows air to bleed out of the front top of the unit so water gets in and the unit is primed, when starting. No water comes out this pipe unless you let fall down to the the level of the unit, so it is not under pressure. It does not let air into the unit as most of the time the level of water up the pipe stays the same. Cheers
Not quite , its there too allow a tiny stream of air bubbles into the unit too keep it quiet . Sometimes depending on blades and hull shape the effect can be quite surprising.  ;)