Questions great Masters!!

Hello all, I've got a few questions and would appreciate any advise.
1) How tight should the prop be on the shaft? Should the prop be able to move at all on the shaft? If it does could it cause a vibration or "hull harmonics"?
2) I've been dealing with porpoising, and I suspect it's because the "hook" has been ground down. I think that's the proper term for it. I completely ground down the hull a couple of years ago and refinished it. I think in the process I didn't get it built back up. Does anybody know how much of a taper should be at the back - bottom of the hull on a 76 S-N? I read a post where somebody said to put a bead of silicone on the back and it will tell you how much needs to be built back up. The other option would be to install some type of trim tab. Feedback?
Thanks all.

Darren, If the prop has been in fact running loose, you must now definitely use some lapping compound and lap it or a new one in. You may have damaged the taper on the prop and or the shaft. Lapping compound can be purchased at any auto parts store. It is spread onto the shaft and then the prop is put on and twisted around. Then when you clean the lapping compound off, you will be able to tell if the taper is bad and still has high spots. The process may need to be repeated several times.

How bad is the porpoising?

Darren, The lapping compound is a abrasive so it will fix it to a certain point. Most of the wear will be on the prop because it is a softer material. If you are out more than I'd say .003" then I would send the prop in to the prop shop or the manufacturer to have the taper reamed.

You said you refinished the hull several years ago. What about the inside - stringers and wet foam? What I'm getting to is extra weight that may be causing the porpoising. You can have 100's of pounds of water in wet foam. Why do you think you removed the hook? Did you remove the complete gel?

My 77 will porpoise at higher speeds and forget about putting anyone in the back and do any high speed runs. The boats weren't designed for that anyway. They are ski boats!! Getting the hook back on is not going to be easy. I don't like trim tabs unless absolutly nessasary. Reid P, you may have noticed is one of our speed experts and has commented on the porpoising being prefered over "planting" the nose (stem) in the water for speed. It is a very fine line that is difficult to obtain. People have reported good results with prop changes. The props with more aft rake seem to direct more water out the back which may change the attitude of the boat enough to stop the porpoising. I did try a Acme CNC on my 77 with hopes of solving the porpoising as well as performance gains. I ran it for about 15 minutes and put the OJ back on. The Acme was a inch larger in dia. reducing the prop to hull clearance and the result was severe hull/prop vibration.

TRBenj wrote: Gangster wrote: I took it right down to the hull, new stringers, new foam, the works. Any chance you removed all the stringers before replacing any? If so, theres a chance the hull was not properly supported without any stringers and was rebuilt in a warped shape, which may be causing the poirposing. If not, then you may have just taken too much hook out. You can add a lip/hook back in. .

Sangster the Gangster, I understand your situation I believe. Several thoughts here. It's obviously porpoising to the point that it's annoying. Is the boat at least running as fast or faster than it did before in terms of top speed while its bouncing? I'm just curious as to whether the hook is just gone and allowing the bow to lift, and if so, the top speed should pick up. This may not seem like good news to you but if it's just the hook and not other deformation in the hull as Tim noted could be possible, then I would suggest going with a set of economical trim tabs. I just spoke to a very knowledgable guy with Smart Tabs, which is a simple non-fluid type which can be had for under $200. The reason I suggest this/these, is that you can get even more control over your ride characteristics with the ability to be set to where it can dictate even better bow-down positioning to compensate for heavy loading, provide better holeshots for pulling, etc.., as opposed to adding a bead of caulk. These tabs do not need a dash control, nor tubes or wires to be run. If interested check out this site and you can call and ask for John. I hope you don't mind but I explained your situation to him and he gave a part recommendation.     http://www.nauticusinc.com/

In summation, me personally, and with my known quest for speed, I would prefer a boat that had the potential of porpoising, (at full speed only), i.e., no hook at all, that was controllable with a tab or tabs, in the interest of higher speeds and better fuel economy.

"known quest for speed" This may be a understatement!!

I put Smart Tabs on an 88 17' no-name hull bowrider I had. It had a 3.0L GM 4-banger with a Cobra outdrive. It would do 44 mph on a good day before the tabs. 43 mph after installing the tabs.

The difference before and after was dramatic. That little POS boat had no bow lift and tracked better after putting the tabs on. Good product.

Mike

8122pbrainard wrote: My X55 Dunphy had 12" pieces of half oval aluminum trim screwed to the bottom at the rear. This created the "hook" on the boat bottom. I think the first owner had a marina do this to stop the porpoising. When I put a new bottom on it, I did not put them back on. I just remember not to put to many people in the rear seat!! You could try this and if it didn't work, you would at least be able to plug up the holes that would be in a spot that no one would see. The disadvantage to this method is that the "hook" is always "on" whereas the smart tabs do back off as the boat speed increases.

If the main goal is to eliminate the poirposing (and not gain top speed) then I would experiment with adding hook to the hull. Taping a rope or a bead of caulk would have the same effect as the 1/2" aluminum without any required drilling. Once you figure out the optimal amount of hook, you can have a fiberglass lip added in place of the makeshift apparatus. That would be the best looking solution.