baronbob

I have a 29' boat and have recently installed a stern jack and a procharger. the pocharger install has made the boat go faster and also porpoise. The tabs can take it out but at a cost of speed. I looked into my mercruiser install book and found a section on rocker vs hook bottoms. I have a 1/8' rocker over the last 5' of hull. I would like to start by straightening the bottom. Any power tools or hand tools particularly good for sanding out this rocker?

Cord

iTrader: (2)

Hmmm...you meant rocker, not a hook? A hook is when the bottom comes down. A rocker is when it goes up. Most boats have a hook which is detrimental to speed. If you do have a rocker, sanding will not take it out. In fact, I'm not sure if you'd want to take it out. Instead, I'd encourage you to examine your prop choices to find one that will help the boat carry it's bow at the higher speeds.

baronbob

I do mean rocker I took a 4' straight edge flush at the transom end and it had a 1/8" gap @ the other end away from the bottom. In other words the bottom is round not concaved . So I want to sand or grind out the round and make it flat. Any ideas?

Cord

iTrader: (2)

You don't grind out a rocker. You would add material. Frankly, I wouldn't mess with it. How fast was/is your boat running? What do you have for power? What were your props before and after the installation? Did you try any other props? What were the reslults? How much trim does your boat require to run fast? How well does it plane? Any slipping or blow out when planing? Do you have a standard Bravo drive or a shortie? Nose cone?

baronbob

Big cruiser application. Stern jack extends the bravo III drive 8". Cupped 26's. Boat tries to achieve 45MPH but porpoises back to 42. Tabs down, 1/4 travel, settles it down 3/8 travel in rougher water. 24's for high altitude not as bad. Procharger and 4' prop upgrade made it happen. planes good 500hp, trim out 2/3 travel

Cord

iTrader: (2)

What is happening with the porpoise is that the boat is trying to lift the bow, but isn't quite able to so it falls back into the water. With the added wetted surface the boat now makes more lift and lifts it's self back out of the water. The cycle just keeps on repeating. The large amount of trim that the boat requires reinforces the need for more bow lift. The rocker is actually helping to create the bow lift you so despiratly need. When the cup was added to the props did they carry the cup around the tip of the prop or is it only on the trailing edge?

baronbob

The cup seems to be in the leading edge as well. I talked to a friend about his application of a procharger with a drive extension. His boat is smaller lighter and goes much faster than mine; however he ground out his boats factory hook and ended up with a slight rocker bottom THE RESULT WAS A 6MPH SPEED LOSS. He put the hook back in and got his speed back and improved a cavitation problem he was having while getting on plane. The reason I put the drive extension on was because I was not able to trim the drive as it would cavitate very soon in the beginning of its travel. I put the extension on and saw a better trim up situation. Then I put the procharger on and new larger props. the boat is 20% faster up to a certain point where the increased speed in combination with the other up grades has resulted in porpoising. I think I am going to build up from the transom to about 18" forward with Marine Tex to take out the slight rocker and possibly create a slight hook. What do you think?

Cord

iTrader: (2)

I used marine tex for a situation like this once before and wasn't satisfied with the results. The stuff didn't set correctly and much of it released over a season. Usually they'll flip the boat and build the bottom with resin, which I don't see as being feasible with a cruiser.

baronbob

I just ordered a product called GOOP MARINE FAST FIX. Manufacture says it works well in bottom applications as it apparently does not sag. I am thinking of putting in a little hook between the ride pad and the first strake. I was thinking of starting with a 1/4" of hook. Any suggestions about trying this approach to eliminating the porpoising.

baronbob

Thanks for all the advice. I fixed it and here is how!! The product is Goop Marine fast fix. It works great on up side down applications. No sag or tear. 5 bottles no waste on the driveway or on me. I have a 8" ride pad, dead rise and then the first lifting strake about 18" up the rise. I built a dam out of aluminum and stuck it to the transom flush with the ride pad and then tapered it to fall 1/2" below the beginning or highest point of the strake. I started out with a 1/8" rocker and if you average the new material starting out zero @ the ride pad and sloping to the maximum of 1/2" @ the strake Thats 1/4" average over 18" each side. Factor out the hook and I ended up with a 1/8" hook over 36" of a 10' wide boat. Probably only 9' wide @ the chine. Therefore 33% changed from 1/8" rocker to 1/8" hook. Bingo the Porpoising is gone.