With my tail up it rides like a$$

[396_WAYS_TO_SPIT]

When I put the tail all the way up my boat wants to jump out of the water. I have a place diverter and a 5 degree wedge on right now. If I put cav plates on the back will this help my hull stay in the water? I love my tail but I hate the ride. If I keep it like this I will have to hire a chiropractor real soon. It makes me sick to hear guys talk about thier boats not getting out of the water when I cant keep mine in the water. What should I do to fix the problem? When I am running fast it doesnt porpoise that bad but when I run around 3200 it jumps real bad. I really want to keep my beautiful tail. :mad: :mad: :mad: :mad:

[lakesmodified]

396_WAYS_TO_SPIT:
When I put the tail all the way up my boat wants to jump out of the water. I have a place diverter and a 5 degree wedge on right now. If I put cav plates on the back will this help my hull stay in the water? I love my tail but I hate the ride. If I keep it like this I will have to hire a chiropractor real soon. It makes me sick to hear guys talk about thier boats not getting out of the water when I cant keep mine in the water. What should I do to fix the problem? When I am running fast it doesnt porpoise that bad but when I run around 3200 it jumps real bad. I really want to keep my beautiful tail. :mad: :mad: :mad: :mad: [/b]
Remove the wedge...

[Froggystyle]

Your boat is not "getting out of the water", it is porpoising.
Not to dwell too deeply on the subject, as we have covered it before but the principle of porpoising is what you need to remember. What the porpoise is, in short is this...
Your boat has a center of gravity. Usually it is toward the rear of the boat. Your motor provides thrust and forward motion, and this creates two kinds of lift. Mechanical (lever) lift coming from your thrust line, and hydrodynamic lift coming from the action of your hull against the water. (Tunnel hulls have aerodynamic lift as well, but this does not apply to you) Your thrust line with a jet is low against the hull, so imagine that your CG (Center of gravity) is a pivot point towards the rear of the boat, you are pushing against it with "X" power, and lifting the other side of it (your bow) with the same amount of force, divided by your lever arm. In addition, the wetted surface of the boat is creating hydrodynamic lift and lifting up on what you just were essentially pushing down, countering the action to a degree, but forcing the back of the boat out of the water more, and allowing your thrust to hold the nose up. When you push down on the rear too hard with the tail all the way up, you create a lot of drag with which the hyrodynamic force on the boat wants to overcome, so it fights you. The thrust and lift of the boat will only raise the bow so far though, because you will pass where the CG of the boat is, and it will come crashing down when the two combined lift's pass this point. Once the CG is overcome by both thrust and the lift of the boat, the boat will come down and start the process over again.
Having enough power to carry the nose for longer with the tail up is one solution. With my tail all the way up (Droop and diverter) I will porpoise like hell all the way through 50 mph. I don't even put it up until 50 now. Once the faster moving air, lifting strakes and thrust are holding me up higher, I can put the diverter all the way up without porpoise. This works up through 70 or so, when the 'tail cuts into the top speed of the boat. By diverting down at this point I get an immediate increase in speed, and the boat runs another 6-7 mph faster. Once we hit the higher speeds, this hull which creates an extraordinary amount of lift for a 21'er, is pushing the rear up, while the engine and thrust is carrying the now-dry nose. That is what they mean by "Getting the boat out of the water".
Short version... get more power if you want to keep the roost. Or, take out the wedge. It is too much diverter for the hull/engine combination you are running.

[djdtpr]

Will there be a test on this later??? :D

[Slick]

That was either a lot of bullshit or a pretty darned good explanation.

[jet496]

Very interesting. That was a nice explanation.

[Duke]

::fills in circle "b"::
good info!

[Jordy]

Slick:
That was either a lot of bullshit or a pretty darned good explanation.Possibly a combination of both? Just kidding Wes, sounded good to me. How's the intake project going?
Jordy :D

[PipesClean]

So basically he is tetor todering on his center of gravity!

[Froggystyle]

PipesClean:
So basically he is tetor todering on his center of gravity!Nearly exactly... He wouldn't be without the wedge/diverter combo, but with it he is exceeding the boats design parameters and resulting in the porpoise.
Don't ask me why I know all of this. It is a tragic tale of hanging out with naval architects...

[gotdamp]

Don't ask me why I know all of this. It is a tragic tale of hanging out with naval architects...[/QB][/QUOTE]
I have the same problem now that I've added a second battery and moved both batteries further back in the boat. I was told to adjust my ride plate more nuteral, it's 4 degree up now. The boat is a 23.5 ft Glastron/Carlson with a JE pump and diverter. Should I try the ride plate adjustment?

[396_WAYS_TO_SPIT]

What if I run cavatation plates and turn them down? Will that push the bow down? My friend has a 21ft baker tunnel hull with a 4 degree wedge and a droop = 12 degrees up and his rooster is high as hell and he never porpoises.Why is that?
Also, Froggy my boat does the same thing you explained about your boat. When i am running fast it doesnt porpoise too bad but when I slow down it sure kicks in fast. I want to keep the tail but if it porpoises I just might have to live with it. I am not trying to drag race anyone I am just having too much damn fun in a jet. Oh well if theres no solution then I will just have to live with it. "You gotta love them damn jets"
396_Ways_Not_To_Porpoise!

[jet496]

My kids LOVE the porpoise action I can get since I've added the droop. It's fairly controllable & goes away at high speed just like Froggy described.
That explanation might explain why it feels like I'm gaining speed as I trim up & down at WOT. Maybe it's helping to get the boat out of the water.

[Mohavekid]

I have the same action froggy described on my 21 Sanger. Depending on the load, anything above 3200 RPM's will cause porpoise unless the PD is almost all the way down. At high speed, I can raise the PD all the way with no problem and the boat flies nice and smooth. Max speed comes with the rooster 5-6 feet high. The boat is riding on the back 1/3 of the hull and is nicely aired out.
Does anybody have any ideas of how much if any speed I am losing by running the PD in a downward position as opposed to a more neutral position? Is it worth adjusting the ride plate or adding tabs to allow the PD to be more up at cruise speed?

[wsuwrhr]

Yea exactly what he said. Take out the wedge, or better yet more power. Same problem for a prop boat.
Brian

[Kwicherbichen]

Wes, did you stay at a Holiday Inn Express last night? j/k :D
BTW, great class on porpoise. I would have said it exactly the same way :D

[PipesClean]

I would have explained it a bit more in detail, that was kind of vague. :D :D :D
Froggy, I know what your talking about. I have to hang with Architects all day.
[ September 11, 2002, 08:37 AM: Message edited by: PipesClean ]

[Hallett19]

Big Gulps huh?
Cool!