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Jackplate Cavitation Issues
charkbait	#1  Posted on 04/01/09 - 11:24 AM
Member Posts: 82 Comments: 0 Joined: 08/01/08	Hey Folks, Got my 17' 95% of the way done and took her out and caught some stripers yesterday. I put a jackplate on so my motor is now offset 6" and raised 6" or so. My cavitation plate is around 3" above the keel. I found the Chesapeake dead flat for my maiden voyage and decided to let the old 88SPL unwind a bit, what a crushing blow when I continued to cavitate at anything above 1/2 throttle. I'd read 3" was about perfect for motor height on these 17's so that was my basis. It looks like I'll need to drop it about 4" to keep in the water but I don't know since I never reallly got on plane. I changed the layout of the boat a bit as well so this helps me get the bow down quite quickly but also makes my prop ride a bit higher as well. Between my casting platform, console, and fuel tanks I probably have 250# in the bow of the boat that many 17's don't have. Is there another level I should plan for with the cavitation plate or is this simply a trial and error venture? I'll post a link to some pics this weekend of my refreshed 1971 17'...looking so sweet, I'm really proud of it...shame it went so slow!
Phil T	#2  Posted on 04/01/09 - 11:55 AM
Administrator Personal Page Personal Album Project Albums Posts: 7043 Comments: 6 Joined: 03/26/05	I think you got some bad information.I think someone wrote inches instead of holes. Typical stock height is 1 1/2 to 2 inches at most. That can mean the top bolt has 1 to 3 empty holes above the bolt. Given your bow weight I would bring it down so the anti-cavitation plate is 3 inch above the keel, then test and adjust again. Edited by Phil T on 04/01/09 - 12:01 PM 1992 Outrage 17 I 2019 E-TEC 90, Viper 17 2+ 2018 Load Rite Elite 18280096VT
Derwd24	#3  Posted on 04/01/09 - 12:01 PM
Member Posts: 1607 Comments: 9 Joined: 05/09/07	And a lot will also depend on what prop you're running, especially as you start to push the limits with a jack plate. Stainless have much better holding power than aluminum, especially up high. As Phil says, it sounds like you need to lower the engine. I use the running rule, when you're on plane at speed, the anti-ventilation plate should be just at or slightly above the surface of the water. Start there and see what kind of performance you get, you can always go up incrementally from there. Dave - 1983 Outrage 22
charkbait	#4  Posted on 04/01/09 - 12:48 PM
Member Posts: 82 Comments: 0 Joined: 08/01/08	Phil T wrote: I think you got some bad information.I think someone wrote inches instead of holes. Typical stock height is 1 1/2 to 2 inches at most. That can mean the top bolt has 1 to 3 empty holes above the bolt. Given your bow weight I would bring it down so the anti-cavitation plate is 3 inch above the keel, then test and adjust again. my post was in error, the 3" I referred to is the height of the cav plate above the keel. That is where I'm at now. I'll lower it to even and try that this weekend.
Tom W Clark	#5  Posted on 04/01/09 - 1:29 PM
Member Personal Page Posts: 4280 Comments: 7 Joined: 09/30/05	What propeller are you using? Different props will tolerate different mounting heights. There is no one-size-fits-all in motor mounting height.
Mr T	#6  Posted on 04/01/09 - 5:41 PM
Member Personal Page Posts: 230 Comments: 0 Joined: 07/07/08	Let me know where you end up on yours, I'm holding off on raising mine right now as the word is to go up one hole, or 3/4 inch. Being as the bolt is 1/2 inch that only means 1/4 inch between holes. I'm not sure I can hold a drill steady enough to drill a hole that close without the bit walking itself into the old hole.
Joe Kriz	#7  Posted on 04/01/09 - 5:51 PM
Site Owner Personal Page Personal Album Photo Albums Project Albums Posts: 11447 Comments: 452 Joined: 03/18/05	T, You don't have to drill anymore holes. You just move the engine up in the holes that are already in the engine bracket. The holes in the engine bracket are spaced 3/4 of an inch apart.
Bake	#8  Posted on 04/01/09 - 9:33 PM
Member Personal Page Project Albums Posts: 902 Comments: 8 Joined: 01/30/06	I have never set a boat up with a set back but with the motor set on the boat I always thought the anti cavitation plate should be closer to the bottom of the boat like one to one and a half inch.I do understand the cleaner water with the set back is this the reason for being able to get three inches high.
Mr T	#9  Posted on 04/02/09 - 7:06 AM
Member Personal Page Posts: 230 Comments: 0 Joined: 07/07/08	Joe Kriz wrote: T, You don't have to drill anymore holes. You just move the engine up in the holes that are already in the engine bracket. The holes in the engine bracket are spaced 3/4 of an inch apart. Are you serious? I thought with an older motor and the blind hole pattern, I would have to redrill the lower holes.
Tom W Clark	#10  Posted on 04/02/09 - 7:32 AM
Member Personal Page Posts: 4280 Comments: 7 Joined: 09/30/05	Mr T, If your motor uses the blind holes, then you should fill those and drill new holes on the industry standard bolt layout so you can raise your motor, or any other motor in the future to whatever height you want. It is a simple chore. Edited by Tom W Clark on 04/02/09 - 7:33 AM
Mr T	#11  Posted on 04/02/09 - 7:47 AM
Member Personal Page Posts: 230 Comments: 0 Joined: 07/07/08	So, there will be more holes needing to be drilled? Sorry to sound dense about this but I was under the impression that there is only a single lower hole on each side. I've held off on dongin the motor rasie, because of a lack of lifting items to get the motor out of the way when doing the plugging and drilling for the new holes. Tom you state to go the new bolt pattern and that will allow me to get the adjustment I need. I know I've seen the pattern before but cannot recall where. Is it her eon the site? Edited by Mr T on 04/02/09 - 8:01 AM
Tom W Clark	#12  Posted on 04/02/09 - 8:02 AM
Member Personal Page Posts: 4280 Comments: 7 Joined: 09/30/05	Yes, of course. Fill two holes, drill two holes. No big deal. The bolt hole layout and dealing with the blind holes has been a very frequent FAQ. Here's thread where we talk about it: http://whalercentral.com/forum/viewth...post_22132 Edited by Tom W Clark on 04/02/09 - 8:13 AM
Phil T	#13  Posted on 04/02/09 - 8:13 AM
Administrator Personal Page Personal Album Project Albums Posts: 7043 Comments: 6 Joined: 03/26/05	Mr T - Have you seen this article by Joe on engine mounting? If you want to discuss this further, you may want to post some photos on a personal page and start a new thread. 1992 Outrage 17 I 2019 E-TEC 90, Viper 17 2+ 2018 Load Rite Elite 18280096VT
theo	#14  Posted on 04/02/09 - 11:04 AM
Member Personal Page Posts: 252 Comments: 0 Joined: 08/26/08	charkbait wrote:I'll lower it to even and try that this weekend. Hijack aside, your plan is good, but I'd say lower it to 1" above, just to tempt fate. Also, I agree that it's trial and error. Good luck. ps - When I was tweaking the height of my jack plate I just shut it down in mid-sea, crawled back there with a ratchet, and changed the height. Got caught though by a good Samaritan who thought I was stranded. And I rusted the heck out of my ratchet.
Tom W Clark	#15  Posted on 04/02/09 - 11:11 AM
Member Personal Page Posts: 4280 Comments: 7 Joined: 09/30/05	Again, I ask what propeller is being used? Three inches above the keel is not unheard of, but you have to have a good propeller to do this. I have personally operated a Montauk with only four inches of setback bracket and the cavitation plate on the Evinrude was 2-7/8" above the keel. I measured this myself. The propeller does not ventilate but it is a Stiletto Advantage which grips very well. You are not going to do this with an OMC SST and certainly not with a crappy aluminum prop.
charkbait	#16  Posted on 04/02/09 - 11:53 AM
Member Posts: 82 Comments: 0 Joined: 08/01/08	it is a stainless prop not sure the pitch. My cav plate at 1/2 throttle was WELL out of the water, not sure how any prop would help without lowering somewhat.
Tom W Clark	#17  Posted on 04/02/09 - 1:34 PM
Member Personal Page Posts: 4280 Comments: 7 Joined: 09/30/05	Look at the prop and get the part number, model name an/or diameter and pitch. That will tell us a lot. Yes, the cavitation plate can be well out of the water when the boat is on plane. This is not how it was done twenty or thirty years ago, but again the props used back then were not as good as they are today.