setback or wedge plate?

[zukiboy]

I HAVE A 16.5' V HULL ALUMACRAFT.WENT FROM A 75 MERC 2 STROKE (42MPH) TO A 70 SUZUKI 4 STROKE (33MPH).AFTER 4 DIFFERENT S/S PROP STYLES AND RAISED THE MOTOR UP TWICE.I CAN GET 36MPH.RPMS ARE 100 OVER MAX.NOW BOAT WILL START TO BOUNCE AT ANY RIPPLE AT WOT.HIT THE TRIM SWITCH DOWN TWO CLICKS AND ITS GONE,BUT SO IS ABOUT 2MPH.THINKING ABOUT A SET BACK OF 2" OR A WEDGE PLATE FOR MORE TUCK? ANY IDEAS?

 

[DJ]

Re: setback or wedge plate?

zukiboy,<br /><br />The 4-stroke is heavier-obviously.<br /><br />"Porpoising" is usually caused by too mauch weight aft (stern).<br /><br />Before I went to a wedge, I would try moving some weight around (batteries, fuel cans, etc) forward.<br /><br />Otherwise, the wedge will only allow you to run at the upper end of your trim range, however, your actual trim will remain the same.

 

[richg99]

Re: setback or wedge plate?

If, as you say, you can correct the porpoising by "two clicks"..then adding wedges will only give you the ability to go to 3 or 4 or 6 or whatever "clicks"...that doesn't sound like what you need. <br /><br />I cured porpoising asnd increased hole shot with a Bob's Stablization plate added. For $35.00 it gave me more stern lift. Many will say it will cut down on your mph even MORE, though it didn't in my case. <br /><br />You might want to consider Smart-Tabs. From what I have read, they will cut the porpoising down without losing mph???? I haven't tried them, but I have run into a boat mechanic who swears by them on his own boat. RichG

 

[Jdeagro]

Re: setback or wedge plate?

It is all about balance, and controlling the ride.<br /><br />The boat is now way out of balance, and the low end torq is less. The two stroke winds up quicker.<br /><br />The porpoising is a balance issue and will always restrict the top speed. When the bow comes up the stern drops, which changes the prop angle (even momentarily). "Gravity" pulls the bow down into the water "below the mean running position" which creates bow down drag or breaks. Then the "Water" squeezes the boat back out (like a inflatable toy in a pool)pushing the bow up inot the air, with the stern down again. And the cycle repeats. The breaking action of the re-entry keeps the boat from running at top speed. <br /><br />This action needs to be controlled. I would suspect that getting on plane is harder as well!<br /><br />Hydrofoils will likely keep the bow down but at 35 MPH +, they will push it too far down and the results will be less porpoising and still less speed.<br /><br />Try ST1290-60 Smart Tabs. Do a search and read the resposes by others.

 

[zukiboy]

Re: setback or wedge plate?

THANKS FOR YOUR HELP.AS FAR AS WEIGHT IS #38 POUNDS WAS THE DIFERENCE IN MOTORS,AND I HAVE TW0 LARGE DEEP CYCLES AND A PINPOINT UP FRONT AS BEFORE.NO CHANGE THERE THE MERC WOULD BE VERY HARD TO PORPOISE IF AT ALL.NAUTIJOHN DO YOU THINK 2" OF A SETBACK PLATE WOULD REBALANCE THE BOAT?AND HOLD UP THE BOW BETTER?GETTING ON PLANE IS FINE.I HAVE A BOBS PLATE BEFORE ON THE MERC AND IT DID RAISE THE STERAN AND SLOW DOWN THE BOAT,SO IT WAS REMOVED.

 

[Jdeagro]

Re: setback or wedge plate?

By the way - welcome to the iboats forums. First let me tell you that I am the owner on Nauticus and inventor of Smart Tabs, however my advise or opinions come from the heart, experience, and testing. <br /><br />Your previous experience with hydrofoils is typical. You also experienced very sensitive steering at cruising speeds. Adding any additional planing surface to the hull will infulence the boats handing and performance. Adding that additional surface directly to the motor and center of the boat is likely to cause handling issues and a loss of speed because the plate is rigid. The faster you go the more they lift, which is exactly what you do not want. At speeds you do not need to lift the stern any longer, it is up. Pushing it up more simply pushes the bow down and slows the boat.<br /><br />I doubt that moving the motor back two inches, or using a wedge is going to do too much. Here is why: The wedge only allows you to use the motor trim and the prop to put more pressure on the bow by lifting the stern. Keep in mind that the prop is for propelling not foe attitude adjustments. When trimmed in to lift the stern the prop is not at the correct angle (perpendicular to the water surface) to get the best forward thrust. It is however a popular remedy for and old problem, but it is a comprimise. The wedge simply allows you to comprimise more.<br /><br />If you move the motor back the Axis point moves back, which is likely to make the balance worse.<br /><br />Because trim tabs are adjustable you can get the early lift needed to keep the bow down on take off, and bring them up as required so they do not push the bow down when cruising. <br /><br />Smart Tabs do this automatically since they are regulated by the water pressure. The pressure adjusts to the conditions and speed, and the system has simple adjustment built in. You can fine tune it to your boat. Cost for your boat would be between $167. and $199. retail. Check this site for more information on the forums.