Hermco/Suzuki question

[Bushwacker]

Originally Posted by hermco

I would not have any qualms with adding the extra bolts although I am not sure it is necessary, Denny/Bushwacker may have some input on the engineering aspects, I would say just be sure you have the engine height dialed in before doing so or u may be drilling more holes than u need.

Don's right, focus on getting the engine height correct first! Although the 23 is a different animal, my experience on a 20 is that AV plate will need to be almost 6" above keel to get AV plate above clean water, so you may need a jack plate to get it high enough.

Regarding bolt strength, 6 bolts is rather serious overkill! Although I would defer to the opinion of Capt. Terry, who was much more of a "hard metal" mechanical engineer than me (I specialized in the "hot air" heat transfer/thermodynamics end of the business!), my take on it is that 4 bolts are more than adequate. Per the Marx Mechanical Engineering Handbook, 4 Grade 5 1/2-13 UNC bolts are very conservatively good for a working stress of at least 85,000 PSI at a proof load and 120,000 PSI ultimate stress. This translates to a proof load of about 4800 lbs for 4 bolts; any higher than that and they'd start to stretch, but they'd go up to about 6800 lbs before they'd break. That means you could lift your entire boat with just those 4 bolts! If you wanted even more strength, Grade 8 bolts are good for 150,00 psi ultimate and high carbon Cro-moly steels are much stronger than that, but you may have trouble finding stainless Grade 8 bolts.

The main thing is to just make sure the bolts are torqued evenly and correctly with an accurate torque wrench. (I don't trust the fancy click-stop type because you have no way of knowing if they're out of calibration! With the simple direct reading beam-type, if it doesn't read 0 with no-load, it's real obvious if it's been over-torqued! Mine reads about 12 ft-lbs with no load on it, but it's real easy to compensate for that!)