Cavitation Plate, Position?

[Duckbay]

We have a 17' C/C Aluminum Fishing Vessel.<br /><br />When mounting the engine, we have a small keel 3/4" in depth on the centre line. The transom is 15" and we mounted a Yamaha 50 Hp 4 stroke with a 20" leg. After having concerns, the dealer moved the engine up by 1 & 3/4", exposing 3 & 1/2" of clearway, above the mounting bracket and the transom.<br /><br />We now have the cavitation plate 1 & 1/2" below the transom chine. The concern from the dealer is, if we go even with the transom, the 3/4"<br />deep by 1 & 1/2" wide keel, may cause cavitation to the prop.<br /><br />I think it's do, to the fact that we have the costs associated to re-build the transom to 20".<br /><br />Any comments from the tweak meister's!<br /><br />Thx.

 

[FlyBoyMark]

Re: Cavitation Plate, Position?

I've been in the marine welding and fabricating buisness for about 20 years, so here's my input. Yes you can increase the transom height by TIG welding a sheet of aluminum of the same thickness to the top of the existing transom. Must be welded on BOTH sides. Of course the piece should be cut to astheticly blend with the rest of the transom. Some boats are "coped" out in the back below the gunwhales to accomidate a 15" motor, so adding in more height would look fine. Then you have to replace all the wood to make the inner support one piece. Then you you have to finish off the top of the transom with new trim as you will also the inner aluminum doubler.<br /> Another simple alternative is to get a 3"-4" thick jack plate and set it up high as possible. A lot cheaper and simpler unless you have a problem with water over the transom. The cavitation plate should be even to 1" above the bottom of the boat. 1" below if its some kind of tug or push boat. The keel, unless its some huge behemouth of junk hanging down under the boat should have generaly no effect.

 

[Duckbay]

Re: Cavitation Plate, Position?

I appreciate your thoughts.<br /><br />I have heard of jack plates on the forum, is this something that you fabricate yourself or are they available on the market?<br /><br />I'm thinking of modifying the transom to accomodate the 20" shaft and improve the structural integrity of current transom.<br /><br />Thank you.

 

[crab bait]

Re: Cavitation Plate, Position?

you can buy them at bass pro shops, cabela's & overtons .. all on line.. for around $400.00 .. the keel , actually it's called a skeg.. shouldn't run the ENTIRE lenght of boat.. should end way before reaching the stern,,( transom ) but hey,, you didn't design it...

 

[DJ]

Re: Cavitation Plate, Position?

Duckbay,<br /><br />Even if you do rebuild the ransom to 20" or go with a jack plate, you are still dealing with the boat keel, which I believe is a NON ISSUE. <br /><br />Jack plates were designed to accomplish two things: 1. Set the engine back into cleaner water. 2. Raise the height of the engine for max. performance and least L/U drag.<br />The jack plates you buy today were for the most part designed for high perf. boats with beefy transoms. A beefy transom is needed to handle the cantilever action of the jack plate on the transom under load.<br /><br />My solution would be to rebuild the transom to 20"-not that difficult on an aluminum boat.<br />If the wood is sandwiched between two sheets of aluminum, you will need to remove the inner sheet. This can be done by removing any bolts and drilling out a few rivets. Then you can laminate sheets of plywood together, using your old wood as a pattern-just 5" taller, seal the wood with West System epoxy and reinstall.<br />If the gunwales are in the way, you can just make the center section 5" taller. I just replaced the transom on my current project-cost about $40.00.<br /><br />Once you have the new transom in place, you should add bracing using aluminum "L" shpaed angle stock running down from the top of the transom and bolted to the knee brace that is riveted to the hull.<br /><br />It will be better than new.

 

[FlyBoyMark]

Re: Cavitation Plate, Position?

Crab..Manual jack plates like "Power Lift" are only $135.00 bucks prefabbed and pre drilled with all harware except the four bolts that hold the motor to it and it to the transom. Very cheap considering what a weld job and all the extra work will take and cost.

 

[Duckbay]

Re: Cavitation Plate, Position?

Thx gents, I appreciate the info!