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View Full Version : Porpoising and chinewalk



raritan
04-16-2000, 10:18 AM
What causes this abberation and how can it be stopped? Thanks as usual for the help.

Donzi Dave
04-16-2000, 01:03 PM
Porpoising is usually a direct relationship between speed and trim angle. Generally the higher the speed, the higher the trim can be run. As you reduce speed, you must also reduce trim. At lower speeds with the trim up the prop is trying to raise the bow, but the speed is not high enough to hold it up so the bow falls. Then the prop tries to raise it again..and the cycle begins..up-down-up-down.

Chine-walk is the trade off for having a deep vee hull which gives us such great wave busting and smooth ride. At speed and high trim, the hull tends to roll off the vee from one side to the other looking for a flat surface to run on. It takes a little getting use to, but I love it! Great Yahoo factor when your really honkin'. This is why most new deep vees have a flat pad at the stern.

Varyak
04-16-2000, 09:43 PM
Donzi Dave states the truth very well but not the solution. An add-on "stabilizer" goes a long way to curing the Porpoising. The stabilizer is usually a plastic fin bolted to the lower unit. Look in any boating equipment catalog. Several models available. I use the Sting Ray Hydrofoil (34.99 plus shipping, part number 19154,from the Overtons Catalog 1-800-334-6541) and my Pal with identical Sweet 16 uses the SE Sport 300 ($39.99, same catalog, pn 37032). No discernable difference between the two in terms of top speed, and no degredation of top speed. Both substantially reduce porpoising, increase planning stability and control. Installation is simple.
Chine walking can be reduced by using trim planes, aka trim tabs. Same catalog, $339.00 - 369.00 depending on the size you choose. However, these may not be adjustable from the cockpit. Other sets are adjustable, and the Donzi factory can sell you an adjustable set for similar cost. Installation by a dealer will add $500 labor.

kingair
04-16-2000, 10:46 PM
One thing you should check is your steering linkage. If it has any play in it it could induce or aggrevate any chime walk. Adjustable trim tabs are infact great to have. They allow an infinite range of settings to counteract uneven load as well as wind and chop. I f you are looking at this option...make sure that they are set to atleast 5/8" in the negative or pointing up positionas to not add drag to the hull when not needed.
later
Philip

kingair
04-16-2000, 10:49 PM
Hey if you can...post the year, model and power of your boat it could be helpful
Philip

raritan
04-17-2000, 12:02 AM
1975 18' 3+2, 350/260 Volvo 270 no trim tabs

raritan
04-17-2000, 12:31 PM
Thats madpuppie, what kinda speed do you get with your 18 at WOT.

raritan
04-17-2000, 08:32 PM
57-58. Is this what you can expect from a 350/360. How much trouble would it be to put a 454 in an 18. Would the 280 drive take it

kingair
04-17-2000, 10:25 PM
The drive is very strong for smaller rats. keep in mind that the 18 lacks enough compartment space for the risers on most rats.
Later
Philip