repower?? - The Hull Truth

SLS233 · 09-21-2008, 11:33 PM

I have a 1976 formula 233. Originally it was powered w/ a 351 and omc. Around 1998 the original owner repowered with a 5.7L and cobra. When I got the boat a few months ago the engine block was cracked. I purchased a re-manufactured 5.7L, and began the install. I took it to the local shop to have the engine aligned. The mechanic told me he thought the outdrive was installed too low when the re-power was done. He explained that the "thingamajig" part of the outdrive, sorry I can't remember the name of the part (it is a horizontal fin just above the prop), should align on the same plane as the bottom of the boat when the outdrive is in its lowered position. Well, it doesn't. It's about 1.5" lower than the bottom of the boat. He explained a number of problems this might cause. The guy I got the boat from gave me an original OMC manual for installing this outdrive. In the manual there is a chart that shows install specs for installing the outdrive at +1", +.5", 0, -.5", and -1" (these measurements represent the difference between the aforementioned "thingamajig" and the bottom of the boat. Whoever did the outdrive install made some marks in the manual in the -1" sections, which makes me think he had a reason for installing it at that height. But he left no explanation. The manual gives no info on why to select which elevation. ANY IDEAS would be appreciated!

thomas70 · 09-22-2008, 04:45 AM

On an outboard the trim plate is usaully even with or up to one inch above the bottom of the boat. Outdrives are a little differant, they can be installed up to 2 inches below the bottom of the hull. I would not worry about it.

tkbsurf · 09-22-2008, 05:13 AM

Its referring to the "X" Dimension in the manual. That hull could probaly benifit from a slight raise in the "X" dimension because of the deadrise.

1bayouboy · 09-22-2008, 07:01 AM

The 'thingamajig" is the anti-cavitation plate. You'll lose some little top speed with it mounted too low because you have more drag from the sternleg below the bootom of the boat. Run it and see what kind of numbers you get for WOT RPMs, top speed, cruise speed and fuel burn. Changing the X dimension now isn't really an option.

SLS233 · 09-23-2008, 08:43 PM

Thanks for the info, guys! The mechanic I talked with had conserns that with the engine too low there may be a possibility of water surging into the engine through the exhaust. If it's just an issue of loosing a little top end speed, I can live with that. But, if there is a probability of engine damage, I'd really like to deal with any issues before that happens.