Posted by JimM on July 31, 2002 at 08:09:24:
In Reply to: Porpoising problem posted by Eric on July 26, 2002 at 16:56:11:
Fiberglass boat?
That porpoising sounds like something more than engine or trim tab position problem, although you might contact Bennett or whoever made your trim tabs for advice. I assume you don't have some huge weight in back.
First, let's talk about engine height. The rule of thumb is the cavitation plate should be even with the keel. But that's affected by the shape of the hull and how far back the engine is off the transom (an aftermarket power trim unit might set it back 6"). A deep-V hull may need the engine higher, and if the engine is set back it needs to be higher. Weight distribution and the way the boat rides will affect ideal trim. The simple thing is to have someone drive in VERY CALM WATER, while you CAREFULLY lean over the transom and observe. Please don't fall in and blame me! The cavitation plate should be just barely underwater. I don't know if an engine too high can cause porpoising, but you might as well check it. Also, make sure you have full trim range, that the trim pin isn't set way out. Since you can push the bow way down, that doesn't sound like your problem.
Next potential problem is (shudder) hull distortion. If that's the problem, then HOPE it's this: The hull was popped out of the mold too soon after curing and it's a factory distortion. That can be fixed. BUT, it could also be rotted floor stringers (most use plywood which doesn't last many years in the bilges) which have made your hull too flexible. If your boat is old, that's very possible and it's a big job to fix.
Hoping it's the former, here's the drill. The aft 1/4 or so of the bottom should be described by straight lines (parallel to the keel) toward the transom. Climb underneath the trailer and lay a good straightedge along several lines on, and offset from, the keel. If the lines are curved either way, you have a problem. I've heard that even a 1/4" off can hurt performance. If the lines are "concave", then the hull is said to be "hogged": the bow & stern are higher than amidships. Your problem is probably opposite, "sagged". Hogging is common in wooden vessels.
To fix sagging, you'll have to apply filler aft of the low point, to form straight lines. Don't grind the high spots - I wouldn't want to sacrifice any precious gelcoat! Filling could be tricky if you have lift strakes, because the cross-section is complex. You might get away with only treating the bottom surfaces of the lift strakes. Afterall, that's where the lift is. Either way, it would make life a lot easier if you could turn the boat over, but it sounds like a 21 footer - might not be practical to do.
Now, the thought we dread, rotted floor stringers. They can be replaced! In short the technique is this: Support the hull with straight timbers to preserve/correct the shape. Carefully cut out the plywood floor to within an inch or two of the edge. Best to set a circ. saw to just the thickness of the ply, so you don't risk damaging the hull. Remove old stringer(s), leaving fiberglass "sockets" formed when they were glassed in. If they're well-rotted, it might not be too hard, but it's still a dirty nasty job. Get ALL of the wood out, and clean everything very thoroughly. I used detergent, big scrub brush and garden hose on full. Let dry for a few weeks, use heaters if necessary.
Make duplicate stringers, except TALLER THAN THE ORIGINALS by the thickness of the plywood floor. What to make them out of? Exterior plywood is great, until it rots out. I used cedar, which is very rot-resistant. My stringers were only 8" tall, so no problem. If yours are shorter, I'd probably use white oak. If you want to use plywood for ease and economy, then treat it real well. Probably the best material would be a sandwich panel of fiberglass with Nida-Core, but I'm not sure just how many plies of glass you'd have to use, and I'm not clear how'd you'd attach the floor to it. Try contacting Nida for advice. If you use solid wood or plywood, thoroughly soak it with CPES to preserve it. For this application I would use Smith's CPES, not some home-thinned epoxy. Install the new stringers into the sockets with lots of epoxy.
Finally, bed new floor plywood (soaked with CPES or similar product, and glassed both sides) ON TOP OF the old plywood edges. I think 5200 is the best product for mating with the stringers; I'd use lots of epoxy at the edges. Apparently, you don't use screws - you don't have enough thickness in the stringers to get screws into. Now you can glass in the edges and lay down carpet.
But guess what, there's still one more big job to do: true the bottom as above!
What's my experience? I've read a lot and got one boat 3/4 done, two more waiting. Then home remodeling got in the way! Wish I was 20 again.