Performance Boats Forum banner

1 - 20 of 36 Posts

·
Registered
Joined
·
968 Posts
Discussion Starter #1 (Edited)
i have been having a discussion with someone about double bolting the droop...it occured to me to do some simple math...here is what i come up with:


7 inches ~=~ diameter at the droop/bowl interface

38 square inches ~=~ area of the droop/bowl interface

~300 pounds per square inch ~=~ bowl pressure (nice healthy big block like an unnamed sombody intend to run (an A impeller at 5900rpm)

~11400 pounds trying to separate the droop from the bowl

8 bolts puts this at ~1400 pounds on each bolt

each bolt is a 5/16" bolt...about 1/4 inch diameter of actual metal that the force is bearing upon...or ~ .05 (1/20) of a square inch.

~28000 psi is the force on each fastner by my dogfish90 calculation.

this is quite near ~70% the yield strength of some stainless steels that are commonly (the unnamed sombody) and many others use as studs in this application, but only 1/5 the ultimate strength.

Because i dont know I have not included the stress that the nut that holds the droop onto the bowl has on the stud...it has already preloaded the fastner to some extent, and in most cases the fastner is at an angle increasing the local stress further.

Because i cant quantify it I have not included the shock forces that unloading and reloading the pump has on these fasteners...it is going to happen...the waters and the way we run...it is going to happen.

i may be a little off on the bowl pressure, i guessed. I dont suppose wrong enough to change the story.

i have not included the fact that stainless (especially near aluminum and salt water) will do some unexpected corrosion stuff on you.

am i wrong that the bowl needs to be double bolted?

where have i gone wrong with the math?

does the reaction of the water leaving the nozzle offset these calculations and if it does, does that make the unloading/loading even that more hazardous?


i dont think double bolting can hurt but at what point is double bolting necessary?

Can i use this as a cheet:D:D;)

EDIT: i fix the 5/8 5/16 thing
 

·
Glendale Arizona
Joined
·
2,368 Posts
Well Mike...........

I think that with all the time it took to calculate the forces at work, the might be, and what if, etc. then post it up here, you could've already double bolted it. :D

After all what's it take? A drill press, a tap, some stainless allthread, nuts, washers and some time.

I'd think that if there's even a question it maybe should have double bolts then have at it.

How's your weather been? Getting out much?
 

·
Registered
Joined
·
2,234 Posts
I was always told the biggest reason for double drilling the droop is for lateral or shear strength, it does somewhat weaken the flange of the droop, so that needs to be taken into consideration.

I would think double drilling the bowl would be more important (my pump and bowl are both double drilled).

BTW, never seen a droop bolted on with 5/8" fasteners, only 5/16"...

GT :)hand
 

·
"The" masheenist
Joined
·
5,444 Posts
Yea.....exactly.

Doesn't hurt anything.

Well Mike...........

I think that with all the time it took to calculate the forces at work, the might be, and what if, etc. then post it up here, you could've already double bolted it. :D

After all what's it take? A drill press, a tap, some stainless allthread, nuts, washers and some time.

I'd think that if there's even a question it maybe should have double bolts then have at it.

How's your weather been? Getting out much?
 

·
Registered
Joined
·
968 Posts
Discussion Starter #5
Well Mike...........

I think that with all the time it took to calculate the forces at work, the might be, and what if, etc. then post it up here, you could've already double bolted it. :D
mine is double bolted...but i only run 580hp max. and it was done with someone with a drill and a beer..yes it works but it aint no machinist pride.



Well Mike...........

How's your weather been? Getting out much?
been really rainy...but that is not the real reason i havent been out...only person who can keep up is still putting it together...maybe this weekend (was supposed to be last weekend but it blew up on the lauch pad)


I was always told the biggest reason for double drilling the droop is for lateral or shear strength, it does somewhat weaken the flange of the droop, so that needs to be taken into consideration.

I would think double drilling the bowl would be more important (my pump and bowl are both double drilled).

BTW, never seen a droop bolted on with 5/8" fasteners, only 5/16"...

GT :)hand
yeah...Dogfish Head 90 had me there....5/16...but still .250 available meat for the stud.

the calc on the bowl/intake come out WAY good, if these simple calculations can be trusted.

I just wondered about the calculation. One can calculate till hell freeze over and bust and it dont reflect reality. On the other hand calculation can reveal something that reality and 'common sense' keeps hidden, to a wonderfull advantage.
 

·
21 Daytona
Joined
·
5,291 Posts
Call Jack @ MPD, he told me that mine needed double drilled at the bearing flange, bowl and droop.

I can't remember what HP he recommended it at.

maybe CS can tell us
 

·
Large Member
Joined
·
679 Posts
I am reading all this and wonder if the material that the bolt threads into is a strong as the bolt itself. Maybe I need to read it again to fully understand.
 

·
"The" masheenist
Joined
·
5,444 Posts
I am reading all this and wonder if the material that the bolt threads into is a strong as the bolt itself. Maybe I need to read it again to fully understand.
It definitely is not and are very suseptable to corrosion.

Brian
 

·
"The" masheenist
Joined
·
5,444 Posts
Everything is double drilled on mine. Just go for it so you do not have to worry about it, it wont hurt anything.

Travis

Agreed, don't be scared.
 

·
Thrust Whisperer
Joined
·
88 Posts
Your math is correct. Your thinking about cyclic loading and corrosion is also very correct. The weak point may not be the bolt material but rather the aluminum in the bowl and inlet flanges. As you probably know, the shear, yield and ultimate strength of the alum alloy is far below that of the stainless (or any other) fasteners. The web of the flange on the intake may or may not be in shear depending on how you consider the interplay between thrust force and bowl pressure force. But the studs coming out of the bowl are definitely putting the alum parent material in shear.

And then there's aluminum and the fatigue limit thingy. Depending on the make and vintage of your hardware, you could own some castings that are getting to be 30-40 years old and have had who knows how many cycles of loading?

Thanks for putting the word "math" in a post on this forum! :)devil
 

·
Registered
Joined
·
968 Posts
Discussion Starter #14
The weak point may not be the bolt material but rather the aluminum in the bowl and inlet flanges. As you probably know, the shear, yield and ultimate strength of the alum alloy is far below that of the stainless (or any other) fasteners.
here is a good place to work the math such that the fastner fails before the thread
http://www.engineersedge.com/thread_strength/thread_minimum_length_engagement.htm

it does not take into consideration the things you mentioned like distortion or weakening of the threads that happens by taking them in and out over the years....by the calculation it looks like the female threads are the week point for most...you got to have over an inch of engagement on a 5/16 bolt to even come close

And then there's aluminum and the fatigue limit thingy. Depending on the make and vintage of your hardware, you could own some castings that are getting to be 30-40 years old and have had who knows how many cycles of loading?
Yes, unseen is the actual conditon of the aluminum way up in the bowl. Also sometimes unseen are the things that happen to stainless, especially in salt water. Sometimes the failure appears like a stress rizer (like a rocker stud does)...other times the failure is associated with a black corrosion that has worked its way into the stainless. I have seen the liberal use of antisieze promote overtightning and cause the threads to pull too...i been using pipe dope as an alternative to antizieze too...sometimes both.

by the way, the jet handbook thingy you found is great... explains mathamaticly how massflow in the intake (as referenced from the source water) can be subtracted from masflow in the nozzle...
 

·
Junior Member
Joined
·
138 Posts
Aluminum Fatigue

I'm not a jet boater, but I am a structural engineer. Depending on the actual stresses in the system, the fatigue life could be infinite. Keep in mind that the stresses on the left side are stress ranges (final stress - initial stress) not actual stress which will be higher. You will need to divide your tensile stress by 145 to convert to MPa.

 

Attachments

·
Registered
Joined
·
738 Posts
no dyno sheets, but had a stock 454 about 450 hp and upgrades to a 496 650 hp. never had any probs with the 454, when I did the upgrade I kept blow'n out the gaskets between bowl and wedge

to make a long story short the bowl was the weak point as it pulled the threads out ( I use SS studs and nuts and washers flat & lock)

not real good with math,but I can say one thing..........................................I need to get mine double drilled :)sphss
 

·
I'm No Expert
Joined
·
3,139 Posts
Want to say i was told double drilling isnt really needed until around 700-800 HP. I may be wrong there. I double drilled mine after i blew my bowl seal and pulled a couple threads with a 500 HP deal. My problem had to do with the bolts not being long enough, Somthing i should have checked when i pulled it apart when i got the boat but figured hey, it's what came out, it's right.

My bearing cap was doubled, bowl was doubled, and drop was doubled. Glad i did it all at once because now with my new mill, i dont have to go and do extra work. Also now that i have a jetaway it bolts right up. Why not do it if you have the tools, time and the pumps out? I tend to get overboard through and try to plan too far into the future :)
 

·
"Try it Now!"
Joined
·
3,704 Posts
I'm not a jet boater, but I am a structural engineer. Depending on the actual stresses in the system, the fatigue life could be infinite. Keep in mind that the stresses on the left side are stress ranges (final stress - initial stress) not actual stress which will be higher. You will need to divide your tensile stress by 145 to convert to MPa.


UH OH....Where's Sleeper CP...we got a graph!
 
1 - 20 of 36 Posts
Top