Transom height and lower cavitation plate

[MartyTn]

Hi All, I found a pretty solid little 14' vintage runabout to use with my vintage short shaft Merc Mk58A. This is a very simple boat which has no raised floor and no splash well and appears to be a pretty early fg boat. It seems very solid so maybe that's before they began to scrimp on materials ?? The 1 1/2" transom is solid with no evidence of water. Since there's no splash well, and the top of the transom was sealed it's all still dry. I'm puzzled however that the transom measures 17" instead of 16". It's been running for 45+ years, so I guess it's okay, but I thought it was ideal to have the transom at 16" so that the bottom cavitation plate would ride just at or below the hull to catch relatively undisturbed water?? If I use it as-is, the bottom cavitation plate on my motor will be a bit over 1" above the bottom of the hull. Since this boat has had a motor bouncing on that transom for over 40 years, there are a few stress cracks at the top of some of the transom laminates, so I want to beef up the transom a bit anyway which means cutting the cap to add resin/cloth, etc, and if I'm gonna do that anyway, I'm thinking I should cut the center portion of the transom down to the correct height so that when I add the new layers of fg cloth and resin, that it comes out to be right at 16". All opinions and help appreciated.<br />Many thanks,<br /> Marty

 

[Winger Ed.]

Re: Transom height and lower cavitation plate

If it runs OK, I'd leave it alone. Heck, maybe even raise it some more until it cavitates, then drop it back down an inch. <br /><br />As far as the height of the ventilation plate goes, look at some racing boats. They try and get the prop up as high as they can before it cavitates. <br /><br />Lots of the racers are running with the prop shaft even with the bottom of the hull. For the same application, lowering the motor a inch may take 5mph off the boat's top end at the same RPM from slippage due to the extra drag in the water.<br /><br /><br />Having that plate even with the bottom is a good, safe, and workable starting point. At the lower speeds, its fine. But its not a spec. that is written in stone. <br /><br />On most boats, you can get about 10-15% (some as much as 25%)of the top of the prop's arc above the bottom edge of the hull before it starts wanting to cavitate under acceleration or your water intakes get too high to pick up the engines water. Even then, a whale tail plate over the prop will often stop that.<br /><br />For the same rpm, raising the engine up like that may give you an extra 5-10mph by reducing the drag caused by the lower unit. <br /><br />Ed.

 

[JasonJ]

Re: Transom height and lower cavitation plate

Sounds like a good height to me. I raised my motor to 1 inch above keel and had an improvement. Just run it, if it runs good, has good speed and rpms, and does not cavitate, your in business. Good luck...

 

[MartyTn]

Re: Transom height and lower cavitation plate

Thanks for the great input guys. I did some careful measuring and ciphering today and I ended up cutting the center portion of the transom, but thanks to your input I cut it less than I had originally planned. This is a classic/antique boat & motor project and these old Mercs (before thru hub exhaust) have the water pickup in the bottom of the cavitation plate above the prop, so the plate has to be in (or right at the surface of) the water. With all my measuring I figured that plate would have been about 1/8" out of the water if I had left the transom as is. Since I'm going to add approx 3/16" - 1/4" of new fiberglass resin and cloth, I ended up cutting just a bit under 1/2" down on the center section. I cut the old fiberglass off the entire transom and the wood was dry and in great shape and when I cut that little bit of the center section the small laminate cracks in that area were not present 1/2" down, so that made me feel very good about the other small cracks. My plans now are to add two layers of fiberglass resin and cloth with possibly the first layer of cloth being roving woven for strength, and that should bring the transom back up the 3/16" - 1/4", and that should have the cavitation plate in the water approximately 1/8". Those are the best laid plans anyway...<br /><br />Many thanks for your help,<br /> Marty

 

[Winger Ed.]

Re: Transom height and lower cavitation plate

, so that made me feel very good about the other small cracks.[/QB]

Sounds like a great plan, I think you're on the right track and will have it come out just fine.<br /><br />Don't worry about small cracks in the fiberglass, these old boats are sort of like women:<br /><br />A little cracking with age is normal.<br /><br />Ed.