Zip Help Needed - Cavitation, Turbulence, Transom Height?
Posted: Mon Sep 25, 2017 8:00 pm
Hello! First post, first time boat builder. I've been able to find answers to all of my questions during the build, so many many thanks to everybody who has shared your build experiences (gdcarpenter, vupilot, roberta, travis24, rbrandenstein, sawford, mooregard, Andy G, Ted G, ...)
Splashed! It's still a work in progress but I'm running out of summer and had to get it in the water.
Unfortunately, I am having problems with the prop losing traction. I need some help.
Glen-L Zip, 14'4" (stock). 1956 Mercury Mark 55E, regular one (16 1/2" transom). Transom built to 16 1/2" (measured vertically) per the 1961 Merc 400 plans I found on the web. With the pin in the 2nd hole, the motor AV plate is level-with and parallel-to the keel. In the first hole, the front of motor AV plate is about 1/4" below the keel and at the back of the motor it's lower than that. First test run was with 3 blade prop. Gas tank is under the bridge deck, with a 17 lb lawn mower battery in the back.
I started in the second hole. Holding my breath and wearing my life jacket ... the boat planed quickly and predictably, tracked straight, but as soon as I hit some waves/chop, I lost traction. It felt just like spinning the tires. So I quickly backed off the throttle. Same thing happened in turns.
So then I dropped the motor to the first hole (and thankfully not to the bottom of the lake ). It seemed to help some, but the problem was still there.
Skeg: The skeg starts between stations 4-5, slowly tapers to 1 7/8" tall at 3' from the transom, then tapers down to nothing and ends 2' from the transom. I followed the Glen-L recommendations to taper the rear end of it (webletter #41); on the squirt they said 18" from transom, I upped that to 24" for the Zip. The Mercury manual says nothing on the bottom for 48", but I was worried that there wouldn't be much left in the water when this boat is on plane!
So I'd like some opinions on what to do/try and where I went wrong. Here is my path forward, as I see it:
1. I have a 2 blade prop, I'm was going to try that first because it's easy, but I'm not very optimistic. Rationalizing that this is more of a coarse screw (like a drywall screw) than the 3 blade that would be more tolerant of turbulence.
2. Make the skeg shorter, and/or end farther from the transom. Could this be causing turbulence/cavitation?
3. Make the transom lower. I can only go about 5/8" lower without serious surgery because the clamp handles will hit the bottom of the motorwell.
4. Move weight farther to the back of the boat... anchor, tools, battery, gear? The boat didn't seem to be hose heavy or plowing at all, the balance seemed ok.
The boat is 1/4" okoume with 1/8" solid African mahogany on the hull sides. The deck is 1/4" okoume with 3/16" solid African/maple veneers. The transom is two 3/4" okoume plywood panels, with 1/4" African mahogany (two 1/8" layers perpendicular to each other) on the outside and 1/8" on the inside. All the frames/battens (T-batten reinforcements) is African mahogany. Keel is African Mahogany with 1/4" laminated okoume plywood on top.