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cncer691
05-07-2002, 03:26 PM
So how much force is there on the transom of a jet boat. I know that a stern drive/outboard has a lot of twisting/pushing forces put on the transom, but does a jet? I ask because I found a soft spot at about a 10 o'clock position from pump that is approx. 4" x 6". I was going to fix it half-assed by injecting rot killer, then fiberglass repair. But... if this is a major stress point maybe I should take the extra time and money to do a "REAL" transom repair job. Any thoughts, because I got a poker run in 1 month that I am using for a repair rebuild deadline.

spectras only
05-07-2002, 03:33 PM
The JC, JG jet units are mounted to the bottom of the boat .The transom plate is there to just keep it watertight with the "O" ring.The forward trust forces are transfered mainly to the bottom of the hull,hence the lack of plywood reinforcements on better hulls ,ie; min 3/8" thickness hull sides.A good [preferably full lenght stringers suffice to support this force to keep the bottom straight.You'll find the stress points at the rear corner of the hulls where the hull meets the deck.
[This message has been edited by spectras only (edited May 07, 2002).]

gstark
05-08-2002, 05:31 AM
I've often wondered about force/load transfer in a jet boat. The force generated by the water jet is reacted along a thrust vector at the pump centerline. This thrust force is reacted thru the pump body into the intake at the bottom of the hull primarily as distributed shear forces, along with a local force-couple (bending) due to the eccentricity of the thrust line to the hull bottom. The transom will react part of this couple, so it does carry some load, but it is principally in the vertical up direction.
In aircraft, flat plates carrying loads and moment are ineffective structurally and exhibit deflection characteristics to be avoided. But, give them stiffness with ribs, doublers, etc. and they become decent structure. In a jet boat, this is achieved by adding contour to the inlet/plate so that it is not planar (flat). Where most intake grates couple to the hull (v- or mod-v hull), the profile is generally not flat, so stiffness is achieved. I don't know how this is done with tunnel hulls or near-flat bottom hulls.
In the quest for speed, the bottom hull profile, (hook elimination) is always discussed, but how much attention is paid to the stiffness and resulting deflection of this inlet region ? This is both a static and a dynamic problem. It would be interesting if dynamic measurements could be made. My guess is that this region is rather active, and in drag boats, may slightly alter geometry. Just a guess however.
In either case, "stress cracks" generally appear at areas of high loading with respect to the amount of material available to carry the load. Intersection points such as corners, also add load concentration factors, so when cracks are seen at these locations, one should not be surprised.
The term "stress crack", is a misnomer anyway. Stress is a calculated value to describe what is happening at a particular location. Strain is what is really happens when load is applied to a section of material. Without physical strain, there is no calculated stress. Stress and strain are related to one another by a material property (Youngs Modulus).

82SleekCraft
05-08-2002, 06:48 AM
And I'm sure you wrote all this off the top of your head. http://free.***boat.net/ubb/wink.gif

AGGRESSOR JETS
05-08-2002, 08:23 AM
Originally posted by gstark:
I've often wondered about force/load transfer in a jet boat. The force generated by the water jet is reacted along a thrust vector at the pump centerline. This thrust force is reacted thru the pump body into the intake at the bottom of the hull primarily as distributed shear forces, along with a local force-couple (bending) due to the eccentricity of the thrust line to the hull bottom.
In aircraft, flat plates carrying loads and moment are ineffective structurally and exhibit deflection characteristics to be avoided. But, give them stiffness with ribs, doublers, etc. and they become decent structure. In a jet boat, this is achieved by adding contour to the inlet/plate so that it is not planar (flat). Where most intake grates couple to the hull (v- or mod-v hull), the profile is generally not flat, so stiffness is achieved. I don't know how this is done with tunnel hulls or near-flat bottom hulls.
In the quest for speed, the bottom hull profile, (hook elimination) is always discussed, but how much attention is paid to the stiffness and resulting deflection of this inlet region ? This is both a static and a dynamic problem. It would be interesting if dynamic measurements could be made. My guess is that this region is rather active, and in drag boats, may slightly alter geometry. Just a guess however.
"Great Stuff" A Gentleman by the name of Jeff Bennett-Has a strong background & research in these areas. Jeff works for Boeing and involved in wind tunnel testing. I would like to see Jeff input here. Last I knew Jeff was involved in a Top Fuel Prop unit. Greg @ GS Marine & Jeff did some great study's on Lifts, Bottom Dimpling Etc.
The Intake load is the unknown portion of this industry. When I worked on Apollo, Rocketdyne our sister Co. did a great research on Jet Drives, this was released through the NASA dissemination centers. Have not been able to find anything on this.
I have talked to NASA Test Center and discussed Jet Pump Flows. We applied and received approval for Cray Test Time, using fluid dynamics as the basis. Our Problem is we must provide the model data in format.This would be maybe 50K or so.
What I would like to know, is can we not live with re-version on the intake by design.
Case & Point-One of our customers recently changed a Jacuzzi "W" Pump over to our Impeller Bowl Nozzle system. The boat has a Blown motor in it- (Do not know size or care)the boat made a sizable jump and is now running 98 MPH on a Jacuzzi Intake.
The shear size of intake tells me reversion is occuring at a high rate.
Is reversion controll-able. Compound this to 150-190 MPH. Hope this sparks some discussion. Dave

gstark
05-08-2002, 11:35 AM
Doing a CFD analysis of the intake and pump internals would be a real task. Assumptions like laminar/turbulent flow transition, angle of attack, etc. just make this a huge undertaking. The CRAY time would probably be well utilized, but the model prep would be huge. The results would also be specific only to that particular run and its geometry. I've long felt that a lot of pump optimization remains. Look at the LOX turbopumps for the Orbiter - these still see attention from the analysis folks. This is probably one reason why you got time allocated - anything pump/flow related may have an industry spin-off, as NASA is keen in pushing industry assist and technology dissemination.
I'm still focussed on the structural aspects of the hull/intake. You've gone several steps ahead of where I'm at.
If helpful, I have CATIA at my disposal. Perhaps this could assist in model prep ?
BTW, I believe Lock-Mart has also done a lot of black-world stuff on marine jet drives, although likely a lot larger that what we are thinking about.

Licketty Split
05-08-2002, 11:36 AM
One factor to not be left out is torque twist
My solution here bould be a sort of torque reactive device between pump and engine. The end resalt being lift with out thrust angle to CG deveation. But this would work on a hull that was fairly flimsy to start with

AGGRESSOR JETS
05-08-2002, 12:17 PM
Originally posted by gstark:
Doing a CFD analysis of the intake and pump internals would be a real task. Assumptions like laminar/turbulent flow transition, angle of attack, etc. just make this a huge undertaking. The CRAY time would probably be well utilized, but the model prep would be huge. The results would also be specific only to that particular run and its geometry. I've long felt that a lot of pump optimization remains. Look at the LOX turbopumps for the Orbiter - these still see attention from the analysis folks. This is probably one reason why you got time allocated - anything pump/flow related may have an industry spin-off, as NASA is keen in pushing industry assist and technology dissemination.
.
We made Patent Application on a self vacating suction for Aggressor. I know in our world of high speed will add a large amount of safety. My prime assumption of reversion is simple, can we not design a intake that maintains pressure while allowing excess pre3ssure to bypass. Some form of scoop etc.
Pre-load bubbles are a approach to this, except the bubble must vary in size based on speed demands. I think one can fit all but will be in shape design such as jets.
Well its a good thought. Love your question on stress,Some one told me the bottom of a Fuel Boat seen 3 Pounds per Sq. Inch of lift and that the transom had 5 pounds neg. force. sound right.Dave
[This message has been edited by AGGRESSOR JETS (edited May 09, 2002).]

SoCalPower
05-09-2002, 11:45 AM
SWANTECH (http://www.swantech.com)has experience on static pumps and can detect events such as cavitation and recirculation in a controlled environment. It would be interesting to develop a test routine for on-the-water testing to determine best efficiency based on stress wave energy levels under differing loads and hardware conditions. I work for SWANTECH and would be interested in supporting such a test with an OEM, Dave?
Call me and we can discuss 909/789-6256,
Dave Batista

RiverDave2
05-09-2002, 11:53 AM
Dave, I couldn't help but notice your post here..
We applied and received approval for Cray Test Time, using fluid dynamics as the basis. Our Problem is we must provide the model data in format.This would be maybe 50K or so.
Dave, you and I were talking about Scicon's services.. Transorming your product into 3-D computer models that can be used for FEA (stress analysis) and Flow testing is EXACTLY what I do here. If you would like to explore that avenue further we can talk about it when I come by. I can hand you a burned disc that you can present to NASA for ALOT less then 50 large. Pro Engineer will create a perfect platform becuase of the way it creates the data it's "watertight" in the models making it much easier to manipulated and interpreted by software such as FEA or Flow testing packages.
GStark, if you don't mind me asking what are you guys using CATIA for? (By that I mean what types of products are you designing?) A few of my customers use it, but primarilly for "styling" etc.. becuase the models are easily manipulated. Unfortunately that makes my job a little harder to build prototypes from them becuase sometimes the models are not water tight, but I've purchased an .IGES stitching software that has been working well for us to translate the data.
[This message has been edited by RiverDave2 (edited May 09, 2002).]

gstark
05-12-2002, 12:15 PM
RiverDave2:
We use Catia for general system integration work of hi temp (600 psig/1300F) pneumatic, cryo feedlines, and Environmental Control systems on aircraft. A very costly approach, but effective. We are driven to this solution by Lockhed Martin, Bombardier, Boeing, and virtually every airframe OEM and 1st tier manufacturers that I deal with. Of course, they all have Pro-E/Solid Works, and just about every other software seats installed, but the meat and potatoes (the master aircraft system and loft data) is Catia. This goes for B717-B777, Canadair, DeHav, S-Works, etc.
By the way, we never use Catia internal FE programs - this is an area best left for experienced Nastran analysts.
One of my biggest concerns is the marketing of FE software bundled into CAD s/w. Just because one can create, run and arrive at a FE solution does not make it correct or accurate. There is NO substitute for experience and education here.

searssears1
05-12-2002, 04:53 PM
Gstark,you sound like an engineer on the C-17 program at Boeing.If you are look me up in the engine dept.

gstark
05-12-2002, 07:49 PM
searssears1 - I don't work for Boeing but the company I work for does a lot of work with BLB, BHB, B-Wichita, and Commercial.