my 19' donzi is very hard to keep on plane around 18-20 MPH when im pulling the tube and kneeboards. Any suggestions on what to use to get it to plane better at a slower speed? I alread have a hydrofoil and am running a 3 blade ss prop would going to a 4 blade help?

Mike,
Trim tabs would be the number one choice for additional lift to aid in planing.

Of course, expecting a boat with a 21*deadrise to plane at such slow speeds is not the easiest thing...

Any prop that would be transome lifting. Hydromotive's Intimidater is a 4 blade almost cleaver style. It will lift your transome a little more than a round ear 3 blade, Mirage style. If your boat doesn't need much transome lift however, the Hydromotive will slow you down on top end. Some boats like it, some don't. Find someone to swap with and give it atry.

i was going to put trim tabs on it but I only have 9" of clearance under my swim deck and no one makes tabs with that shor of actuators.

A set of Bennett trim tabs would do wonders.

Might require a little bit of ingenuity, could you possibly mount the cylinder on the swim platform??

I'm pretty sure that you could modify the bennetts to clear. I'm thinking that you'd mount the ram lower and have it attach further back on the tab. The tab is bent up at the end to receive the ram, but it wouldn't be hard to modify it.

My 16 Donzi had Bennetts (of course, it didn't have the swim platform) and they worked great. The four blade might help. For example, my Fountain is fine with a 4-blade but would hardly plane at all when I tried a Mirage 3-blade. (I hope that's not comparing apples to oranges.)

If you do a lot of skiing, knee boarding, etc, you might want to try a Merc High Five prop. Great hole shots and a lot of stern lift.

Griff is right on the money. The High Five is a fantastic for wake boarding or skiing. I had one on a 206 Rinker with Bennett trim tabs. I could stay on plain down to 18 mph.
Dan