Porpoise(ing) and trim tab info please

[RickyR]

In order to achieve my top speed, I need to trim my engine out pretty high. But once it goes past a certain point the boat begins porpoising. The prop is still in the water pushing the boat. I can trim back down, but how much help will trim tabs be here?
At present, I don't have a set but will these slow me down once they start pushing down on the transom? Or, will they allow me to stay level, and make more speed?
This is on a 23 center console.
I'm not as stupid as my question, but need to re-think a few things.
Thanks, Ricky

[vector mike]

How much setback are you running? What motor and prop are you running?

[RickyR]

How much setback are you running? What motor and prop are you running?


oops..
It is a 250 efi Merc that's bolted to the transom (no set back). I am running an 18" Mirage and can get just at 5k rpm's with a full load of fuel and me.
I normally run light with fuel I can get 5300 rpm's.

Thanks, Ricky

[vector mike]

If you add setback to it it will keep the prop in the water better and lift the bow with less trim using the leverage effect. You will also be able to lift it some depending on your water intake positioning and gain some top end.

[RickyR]

Thanks for the info. I prefer not to set the engine back on a bracket, and want to leave it as is.

Any idea of what I will gain, or loose if I go with tabs only?
Thanks, Ricky

[vector mike]

I have no idea. I don't know of anybody that uses them. Maybe somebody else will step in and give you an answer.

[SUPERBASH]

Trim tabs a.k.a. "water brakes" are designed to level out the boat side to side. Those of us that run in ocean swells with deep narrow vee's use them.
-They will stop porpoising, but at the price of extra drag. An O/B powered boat doesnt have the shear h.p. like a bigblock in the same boat to overcome that drag. The O/B relies on getting max. speed from "flying" it on the water surface where an I/O will push it thru.

Adding setback wont cure it....but may make for easier adjustments to height. If the propshaft is extremely deep try going up 1 bolt hole at a time. Since your 23' CC is of an unknown make I can only state the basics. Later.http://img482.imageshack.us/img482/9739/sasmokin9xf.gif (http://imageshack.us)
http://img482.imageshack.us/img482/4369/img05968oy.jpg (http://imageshack.us)

[SUPAJAY]

You need to be spinning more like 55-57, 5300 with a light load is too low.

[SUPERBASH]

Yeah. I just realized & calculated an 18P @ 5300rpm...we're only talking like mid-upper 40'smph speeds here :rolleyes:.
I guess you could just jump up & down to counteract the porpoise as well. http://img197.imageshack.us/img197/253/laugh1wx.gif (http://imageshack.us)

[RickyR]

Bash,
Thanks for the input, Your calculations are pretty close. I run about 51 mph (GPS) @ 5300 rpm's and 49 +/- @ 5050 rpm's.
The boat in question is a 1987 Donzi 23' CC.
I know the RPM's are low, but the way I am propped, it is for the cruising speed of 3600 RPM's, ( 36 mph by GPS) but would like ( as everyone else) to be able to open it up and fly the boat. But it is way to heavy for this, but I thought tabs may help some..:mad:
Thanks again, Ricky

edit: the engine is up as high as it can go without drilling new holes.

[Carter Powell]

Ricky:

You might want to have a good prop shop take a look at your prop and think about adding some more bow lift into the prop. If you dont want to think about adding some setback to the transom this might be the way to go. The extra bowlift will help so you dont have to overtrim the motor angle.
Make sure that you cav plate is NOT under the surface of the water. If it is get that motor raised up on the transom as this will really help your speed.

Best wishes, Carter Powell