Cavitation Plate, location

[Charger183]

I had a Force 85 replaced by an Evinrude 115 on a Chrysler Charger 183. The engine now cavitates before it comes up on plane. The cav plate is 1.5" above the stern bottom. I know it must be lowered but to where? Is there an insert for the lower unit to extend it instead of cutting the transum? I don't want to use a jack plate. Tnx.

 

[Joe Reeves]

Re: Cavitation Plate, location

The cavitation plate must be 3/4" to 1" below the bottom of the boat at the transom (not over 1"). There is no such extension kit that would enable one to extend an engine a inch or two.

 

[Forktail]

Re: Cavitation Plate, location

You must check the rigging specs for the Evenrude 115. Not all outboards have the same cavitation plat distance from bottom of hull.

 

[Spidybot]

Re: Cavitation Plate, location

Sorry Joe, but I cannot agree.<br /><br />The anti-cavitation plate as a rule should be level (and parallel) with the keel line on a planing hull. Depending on hull design (especially the rear part of the bottom) this may need to be adjusted.<br /><br />On fast boats, to get lower drag from the leg motors are often mounted higher - an inch or two or sometimes even on a plate that can be raised and lowered during run - in combination with a prop design that copes with surfacing (breaking the surface and running on an air/water mix) such as Cleavers, Choppers or 4- or 5-blade props.<br /><br />If you read the booklet 'Everything you need to know about propellers' you'll learn all about props, hulls, trim and fit. And be able to decide if your solution simply is to get another prop.<br /><br />The booklet can be downloaded (free) here http://www.iboats.com/cgi-bin/ubb/ultimatebb.cgi?ubb=get_topic;f=17;t=014920

 

[Charger183]

Re: Cavitation Plate, location

Thanks all. I will cut tyhe transum down to get the cav plate even with the bottom. This boat planes at about 17-18 MPH and I expect the top end with this engine and a 17"P prop to be in the 40 MPH zone. The Force 85 HP could push 33 MPH but never got above 3600 RPM with a 17P. Boat wet weight is 1300Lbs.

 

[Spidybot]

Re: Cavitation Plate, location

Charger, think again.<br /><br />Cutting the transom is a major alteration to your boat.<br /><br />I'd take any other solution before that and your problem CAN be solved by a prop. A good prop will increase your boat value - a cut transom will decrease it (make it worthless to an enthusiast).

 

[Joe Reeves]

Re: Cavitation Plate, location

With over 30 years of experience UU, I really don't need to read that book, but thanks anyway.

 

[HowieDean]

Re: Cavitation Plate, location

Tell me about your prop.... That's most likely your problem... Do Not cut your Transom... If you don't get it sealed write it will rot fast....<br />What is the length from the lowest part of the hull to top lip of the transom where the motor mounts ????<br />I run my cavation plate 12 1/2" above the bottom of the boat... I can pull a couple of skiers and run 105mph... Not that this means anything.. but Cavation plat hight is meaningless.... Where your prop shaft is in relation to the bottom of your boat is what you need to worry about... <br />

 

[Forktail]

Re: Cavitation Plate, location

UU wrote, "The anti-cavitation plate as a rule should be level (and parallel) with the keel line on a planing hull. Depending on hull design (especially the rear part of the bottom) this may need to be adjusted."<br /><br />No, as a rule of thumb the outboard should be mounted per the outboard and boat manufacturers rigging spec. This is rarely level with the keel line on a planing hull. It is usually slightly below the keel line (for the reasons stated below). I've seen specifications 2" or more below keel line. Joe's post is accurate for the most part.<br /><br />HowieDean wrote: "...but Cavation plat hight is meaningless.... Where your prop shaft is in relation to the bottom of your boat is what you need to worry about..."<br /><br />Nice outboard setup HD, but the cavitation plate is a vital instrument in maintaining bow lift, stability, proper rpm, proper cooling, and control (safety) of your boat. I don't think Charger 183 plans on doing 105 mph. I would not recommend a 12 1/2" lift.<br /><br />The best advice here is to work with your prop. Cupping will make a big difference in the cavitation problem. You may want a little less pitch and more cupping. I'd get a jack plate before I cut the transom. Your Evin dealer will have the rigging specs for your cavitation plate.

 

[Charger183]

Re: Cavitation Plate, location

Tnx for all the info. The dealer says level or 1/2" below the keel. The Charger was made for long shaft (25") engines. The Force cav plate sat level and worked fine. I am good enough with fiberglass and aluminum to change the transum without it leaking or being seen. As for the intrinsic worth of the boat, it was bought as a learning tool in the first place in very sad condition for a song. The prop is a 3 blade 13-3/4 x 17c Mich. Tried another on Sat. in case it was spun. No luck. What is 12-1/2" above the keel? If I tried that with the cav plat the prop would be looking at the white of the transum with the skeg half dry. Larry

 

[Spidybot]

Re: Cavitation Plate, location

No offence, Joe.<br /><br />I've been boating for 34 years and still think that this world can be a learning place.<br /><br />I take it you don't agree with that either.

 

[oldboat1]

Re: Cavitation Plate, location

There is a lot of good information on rebuilding transoms in the Boating forum. Cutting down the transom 1 1/2 inches to fit the motor is not a bad idea, in my view. Whether the motor well gives you enough room to do that, of course, depends on the boat.<br /><br />If you go this route, you might look into replacing/rebuilding the transom if you have any soft spots. I've done this on a couple of runabouts, and the results are excellent.<br /><br />Good luck -- interesting project.

 

[epresutti]

Re: Cavitation Plate, location

Folks,<br /><br />Lots of oppinions here. Some thoughts on ventilation VS. Cavitation.<br /><br />The plate above the prop is properly called the anti-ventilation plate, it is specifically used to prevent air entering the blade area from either above the water or from the through hub exhaust, air entering the prop area causes it to lose bite and also over-rev which can damage the motor, this is ventilation.<br /><br />Cavitation is the "boiling" of water as a result of a "very" low pressure condition behind or at the prop. There is normally low pressure behind the prop but not enough to boil the water. Bad prop design, damaged blades or the wrong propeller can cause this condition. The boiling of water is related to temperature and pressure. Water can boil at a very low temperature given that it is moving from an area of high pressure (in front of the prop) to an area of low pressure (behind the prop). When the air bubbles collapse they create energy which can erode the blades.<br /><br />OK, So how does this help? I don't know, but I thought you guys would like to know. It would seem to me that the whole issue is to keep water in below the ventialation plate and air out.<br /><br />On my boat and motor (18ft with 25" shaft 140) they are recommending even with the bottom of the hull (+- 1/2") for the ventilation plate.<br /><br />Safe boating to all.<br /><br />Cheers.<br /><br />emp.

 

[Beernutz]

Re: Cavitation Plate, location

Thanx, Ed. I'm pleased to see that someone has finally recognized the difference between cavitation and ventilation.<br /><br />I gotta go with the guys suspecting the prop. Is it worn to any degree? Have you checked to see if the hub is slipping? Check the thrust washer ahead of the prop for signs of wear around the outer circumference. I've seen Johnny-Rudes run like this for a long time. Once in awhile, til they wear completely thru the thrust washer. Not a pretty sight!<br /><br />I'd experiment with some props, before I started chopping up the boat.

 

[oldboat1]

Re: Cavitation Plate, location

good idea to make sure it's not the prop before taking on a rebuild of the transom, which is a pretty big job -- but rebuilding or small alterations of the transom is clearly within reach if you decide to go that route.<br /><br />I had similar symtoms with an 85 Evinrude on a 17 foot hull (incidentally, had reconstructed the transom on that hull because it was soft -- transom height not an issue). The problem was solved when I substituted a cupped prop -- which I strongly recommend if yours isn't cupped. Pitch changed operating characteristics (tried different props), but the key to cavitation/ventilation problems was the cupping, or lack of it.

 

[Forktail]

Re: Cavitation Plate, location

Good post on the ventilation vs. cavitation Ed.<br /><br />As an engineer, the cavitation terminology has always bothered me, but that seems to be the industry term and what the manufacturers like to call it. <br /><br />It gets confusing in that the bubbles that collapse behind the prop during cavitation are generally air bubbles. And an anti-ventilation plate actually helps to reduce the air entering this liquid pressure system (turning prop). Thus the plate, through eliminating air, actually reduces cavitation, and it adopts the term cavitation plate.<br /><br />Cupping of the prop changes the cavitation characteristics because it changes the pressure behind the prop. Cupping of a prop is always on the side of the blade where the water exits the pressure system.<br /><br />Good luck.

 

[epresutti]

Re: Cavitation Plate, location

Forktail, Beernutz,<br /><br />I suppose if we applied a current to the pressure system we could get some electrolysis going generate some hydrogen and oxygen add a spark and then we would have a rocket, or just a big explosion behind the boat. (couldn't help myself)<br /><br />I am a systems engineer but love physics as a hobbie. Always trying to learn new stuff. Cavitation vs. Ventilation is covered in my Clymer manual as well as many sources on the WEB.<br /><br />I like to know how things work as well as what is know to work. Sorry for the tangent on this thread. I am going to take a closer look at my prop and setup to make sure I am getting the most effeciency.<br /><br />Safe boating to all.<br /><br />Cheers.<br /><br />emp.

 

[Beernutz]

Re: Cavitation Plate, location

An afterburner!!!!

 

[Charger183]

Re: Cavitation Plate, location

Tnx guys: DavesWaves and I pulled boat out at 9:00AM, cut the transum only wide enough to fit the engine mount, lowered the engine 1.5", fixed a hole in the bow, re-fiberglassed, painted and ready to go back in the water at 3:30pm No other changes.... 35 MPH straight and level in a slight chop. Great for a 1500lb. 16 footer. Prop is like new Mich. 13-3/4 x 17 non-cupped. I will get a cupped 17 on it and keep the other as a spare. The only small problem we ran into was a lack of clearance for the steering bar at the end of its travel necessitating a new opening in the well wall and also I now must turn the engine full to port so there is clearance for the bar-to-steering rod to twist in order to use the TILT. There is no problem with TRIM. I think a change of prop design in this instance would have had only a minor value as there really is'nt enough speed to deal with. Good to see engineers on the line. Don't let your ring rust. Larry

 

[daveswaves]

Re: Cavitation Plate, location

Yeah, I only had to hang 3 feet off the stern to get it up to 35, LOL