Hydro-Physics?

[sws115]

i am having a balance issue maybe you guys can help me
a hydroplane is a tricky one to get set up
i got 400lbs on the back the hull is a converted dimarco inboard hydroplane
i built the transom so the outboard prop is right in the same location the inboard prop was
i get it prop riding for a second then i got to lift cause it starts rocking on top of the bubble
i have lowered the motor so the shaft is flush with the bottom of the sponsons and run a neagitive trim angle between 8 and 12* heres i video before i droped the motor a inch i have not tested it yet with the new set up
my 4" 4 blade high 5 seems to work the best
i got a yamaha 24 drag prop and a 14 28 clever either way all the prop start carring the whole boat as aproach mid rpm range

thanks for your support scott

http://www.youtube.com/watch?v=cnSek6B5bsc

[sam]

Sounds like a prop problem, Scott

[sws115]

yea thats what i was thinking i cant seem to find the right prop just yet
if i can get a balance going with the 4 blade maybe i can go back to the drag prop again
i got plans for a ssm and 15 inch mid to help with lowering the cg
im close i think ,it just start,s to get a rooster tail going and i got to lift

[Top Gun 3-Z]

LIFT IS MEASURED BY AND FIGURED BY THE MAD MAN THAT BUILDS THE BOAT. The more the lift the faster the boat is out the turns and the higher it flys ,unless the break is way forward.It all depends on what kind of ride u like .

[ZUL8TR]

[QUOTE=mdaspit;122007]

Is it too simple to think that the greater the sponson depth the more air can be compressed, therefore more lift created?

----------------------------

No not that simple. Just increasing sponson depth is not the answer. As sponson depth is increased the TML depth increased (assuming the afterplane length stays the same) and that results in a greater minimum angle of attack. If to great an angle there will be excessive aero and water drag. The idea is to keep the angle of attack as low as practical while still getting the needed aero and hydrodynamic lift and the least amount of aero and hydrodynamic drag.


Here are some of the specifics:

Sponson depth is part if it. The minimum compressed air amount under the boat (in the straight-a-way) is determined from the TML depth (described in my post above) and the width of the bottom between the sponsons which is same as the rear bottom width.

Thus the minimum air flow area (in straight-a-way) to feed air under the hull = TML x bottom width.

Note that when the sponsons are off the water (then lift depth is greater than the TML) there is more flow area to get air under the hull that will get compressed and also more air that will be released out along the sides of the air traps. Eventually a blow over!

Air flow over the deck provides: aero lift + aero lift induced drag + aero skin friction drag. The compression of air under the hull provides an upward force to help lift the hull by ground effects and some aero friction drag. The rear planing surface with its angle of attack to the water surface provides hydrodynamic lift + hydrodynamic lift induced drag + hydrodynamic skin friction drag.

Total Lift = aero lift force + compressed air force + hydrodynamic lift force

The engine prop thrust must overcome all the drag that includes those drags noted above and: lower unit water drag, prop water drag, turning fin water drag, driver aero drag, engine aero drag, wake drag, eddie aero drag, any other drag I missed.

[ZUL8TR]

Very good post. Don't forget that the bottom of the boat does not 100% represent the aerodynamic angle of attack unless the bottom and deck are exactly parallel (they usually aren't these days). The aerodynamic angle of attack will be closer to the average between the deck and bottom lines.

The old Swifts and Sidcrafts did have nearly parallel bottoms and decks. The old Swifts and Sids were something like 8 or 10 inches high at the back from the deck to bottom. Modern designs use an airfoil shaped deck that tapers down in the back to nearly nothing in relation. This changes the relationship of the the aerodynamic angle of attack (as the boat meets the air) and hydrodynamic angle of attack (as the boat meets the water).

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You are correct. I am aware of the different angles of attack for the air flow and the bottom planing surface but left those out of my short discussion.

I interpret the minimum air angle of attack as being made by the angle between the TML depth , the chord length (LC) of the hull from the rear of the after plane to the front leading edge between the pickels and the water surface. Math wise: SIN (air angle attack) = TML/LC

The average chord length you note between the bottom and the deck lines can be defined the same way as a wing airfoil shape called the mean chord line from rear trail edge to the front lead edge. Except for the wing airfoil the rear trail edge tapers to zero and the hydros do not.

The hydrodynamic angle of attack would be the angle the rear planing surface bottom makes with the water. On some bottom designs the minimum angle here is defined by the sponson depth (DS) and the afterplane length (AP). Math wise: SIN (minimum bottom angle of attack) = DS/AP.

[mercguy]

whew!!!!!!

-------------------------------

You are correct. I am aware of the different angles of attack for the air flow and the bottom planing surface but left those out of my short discussion.

I interpret the minimum air angle of attack as being made by the angle between the TML depth , the chord length (LC) of the hull from the rear of the after plane to the front leading edge between the pickels and the water surface. Math wise: SIN (air angle attack) = TML/LC

The average chord length you note between the bottom and the deck lines can be defined the same way as a wing airfoil shape called the mean chord line from rear trail edge to the front lead edge. Except for the wing airfoil the rear trail edge tapers to zero and the hydros do not.

The hydrodynamic angle of attack would be the angle the rear planing surface bottom makes with the water. On some bottom designs the minimum angle here is defined by the sponson depth (DS) and the afterplane length (AP). Math wise: SIN (minimum bottom angle of attack) = DS/AP.

now I know why I am a boat PAINTER and not a boat builder!!!!!!!

[bill van steenwyk]

clarify what you mean by rocking please

When you say "rocking" when you get it on the "bubble" what exactly do you mean. Does the boat actually rock back and forth in a side to side motion, with each sponson touching the water and then going back to the other one touching, or are you talking about the front lifting evenly and then settling again because you have to let off the power to keep from blowing over.

A side to side rocking motion can and usually is caused by the boat "spilling air" out either side just behind the sponson ends, or somewhere close to there in the length of the airtraps, and there are several factors that may cause that type of "rocking". You may have your motor (prop shaft in relation to the bottom of the boat) kicked out too far, trying to lift the front of the boat further that you have power for to hold it there without loosing lift. This is describing fore and aft motion. The spilling of air description is describing side to side motion. Another cause can be uneveness in the depth of either the air traps, one being slightly deeper than the other anywhere in the length, causing air to spill out from the "higher" side in relation to the surface of the water, that causing the boat to "rock" over that way until the air under the boat can then spill out the other side causing it to rock over then in that direction. One sponson being slightly deeper or shallower than the other can also cause the same thing. The reason I mention this is because you state that you have converted a boat orginally powered by an inboard to outboard power, and if air trap depth, sponson depth, etc., was changed and not exactly the same on both sides, that is a potential source of problems. It does not take much uneveness, in some cases a 1/4 inch or less one way or another, not in sync with the other side, to cause a very bad problem. I don't know very much about inboard hydro design, but all the ones I have seen are usually designed to run in a level attitude with the engine located in an entirely different location than where you now have yours. As you have probably gotten from the previous information on this thread, "balance" is very important in the running attitude of any raceboat, balance in this case meaning the location of the engine, driver, CG of the boat, etc., etc., etc.

Even if the bottom and air traps/sponsons are exactly straight and true, water conditions can sometimes cause the same problem, if running with a quartering wind and chop.

Good Luck

[sws115]

yes is its the forward and aft rocking motion as soon as the sponsons air out the whole boat stay air born then rocks on the bubble
when i say i lift i just really ment to say i just stop pouring the power on
once when i did have the motor back to depth it is now and the 24 drag wheel on i was able to hang the sponsons with the tail blown as well for a while then i just eased in the throttle and the right sponson lifted higher than the left and it hung there for a ways then i just backed off and dropped it back on the water still only in the mid rpm range

thanks for your support
i will re read your post over and over see if i can find somthing
i have only been doing this for a few years now and this is the second boat i built
my first design was a tail dragger
nothing has been changed on the dimarco other than is a recest outboard configuration

[johnsonm50]

i am having a balance issue maybe you guys can help me
a hydroplane is a tricky one to get set up
i got 400lbs on the back the hull is a converted dimarco inboard hydroplane
i built the transom so the outboard prop is right in the same location the inboard prop was
i get it prop riding for a second then i got to lift cause it starts rocking on top of the bubble
i have lowered the motor so the shaft is flush with the bottom of the sponsons and run a neagitive trim angle between 8 and 12* heres i video before i droped the motor a inch i have not tested it yet with the new set up
my 4" 4 blade high 5 seems to work the best
i got a yamaha 24 drag prop and a 14 28 clever either way all the prop start carring the whole boat as aproach mid rpm range

thanks for your support scott

http://www.youtube.com/watch?v=cnSek6B5bsc

I totally admit to guessing here.. Could it be that the powerplant location difference in/out board could mean a new prop location will help? Also have you tried moving the tilt angle out for less transome lift? Or might it be as simple as finding a prop that works ?

[MN1]

Almost sounds like porpoising. We need a better video with a high speed pass in front of the camera to see what the boat is doing.

Maybe more precise testing with one prop (at a time) at all of the different engine heights and trim angles written down in a note book.
This will be lots of testing but will be worth the time.
Include all speeds, RPMs and handling data. From this data you should be able to see a trend toward what your boat wants for a prop and engine height and trim.
Mark N

[sws115]

Almost sounds like porpoising. We need a better video with a high speed pass in front of the camera to see what the boat is doing.

Maybe more precise testing with one prop (at a time) at all of the different engine heights and trim angles written down in a note book.
This will be lots of testing but will be worth the time.
Include all speeds, RPMs and handling data. From this data you should be able to see a trend toward what your boat wants for a prop and engine height and trim.
Mark N

thanks mark i have been sticking to the 4 blade 25 now ,till i get that one right
i started first splash was shaft flush with the sponsons @ 4* negitive trim locked and threw all the changes both prop and angle variation plus up 1 inch the boat has never really taken a set for me yet
i was thinking maybe fuel slashing in the tank to now maybe the trim being un locked now and tucked in to 12 * has some movement allowing the rocking



maybe it could be a simiple as finding the right prop
i am really stumped on this one


thanks again for your support
scott

[mdaspit]

Jeez Louise

It is good to see America's Einsteins are alive and well. Thanks for sharing knowledge that must have taken years to gather.

[Kev43V]

I want to thank everyone that has shared some of their wealth of knowledge on this subject. You have really helped some of us have a better understanding. Of not only how deferent factors effect hydro performance, But also of how those factors effect each other.

[Dave M]

Sws115

I have not seen the video yet, has to wait until I can find some time at home. It looks like in the picture, that boat was designed to run an inboard, thus it is designed to run with the sponsons on the water and the tail in the air.
It just has that look about it in the pic I see. It could a number of things. I think your bottom break may be in the wrong location in relation to the amount of lift in the boat, and your driving location. You don't have enough power to hang it up and keep it up given the location of the bottom break. It creates the bubble effect you describe. Given this, it is not as simple as increasing power... you have to change the lift, etc. You add more power and she is likley to crash.

Basically, if you can't find a prop to tame it down, begin moving the bottom break forward a little at a time. Try it each time. You can add foam and glass, fairing it all in until you find that magic area. If it were a race boat, I would start over.