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Author Topic:   Optimum position of AV plate when on Plane
SpeedBump posted 08-27-2003 10:41 AM ET (US)   Profile for SpeedBump   Send Email to SpeedBump  
I have been assessing the position of my anti-cavitation plate (antivent Plate)when on a plane. I am running a 1998 70 HP Nissan W/ PT&T on a 1970 NAusett.

Currently when on plane the AV plate appears to be an inch to 1 1/2 inches below the water surface. I judge this by the location of the two smaller but simillarly shaped ribs above the AV plate on the lower unit which are clearly out of the water when on plane. The lower rib of the two rides a fraction of an inch above the flat stream of water off of the bottom of the boat.

The boat handles well in turns with out any spin out or Chine walk, and the prop never seems to break the water. I can get upto and cruise at 33MPH by my FF turning 5300RPM. I also get on plane and stay there at 3,000RPM.

The motor is mounted in accordance with the Nissan specs relative to hull bottom and location of AV plate. this seems different from what I have read here where Whaler recommendations locating the AV plate 1-2" above the bottom of the hull.

The motor is mounted at the top hole of the motor bracket which places the top edge of the bracket @ an inch above the top of transom.

I am planning on mounting a Turbo Lift to the AV plate in hte near future to help reduce porposing and smooth the ride.

Given the above informaiton does it appear that the AV plate is in the proper postion or should it be raised/lowered for better performance.

Bigshot posted 08-27-2003 11:16 AM ET (US)     Profile for Bigshot  Send Email to Bigshot     
Go as high as you can without sacrificing performance. You should do better than 33mph with a 70hp.
Marlin posted 08-27-2003 11:44 AM ET (US)     Profile for Marlin  Send Email to Marlin     
Don't put a foil on there until you've experimented with raising the engine. When I took my Merc 115-4S up by even one hole on my 160 Dauntless, the porpoising decreased dramatically. You'll know you're mounted too high when the prop ventilates in tight turns, or when you can't trim out sufficiently without ventilating.
SpeedBump posted 08-27-2003 12:14 PM ET (US)     Profile for SpeedBump  Send Email to SpeedBump     
I will try going higher, my only reservation is possibly getting the water intakes out of the water and causing cooling problems.

Again, when I go back to look at the postion of this stuff it doesn't apear that I can raise the AV plate more than perhaps an inch before the intakes will be at water surface. Also, I weigh in at @ 250 so when I go back to look things over I am sure I'm getting the stern a little lower than it would normally be when I am at the helm.

Next isssue will be with the mounting bolts. Currently the two top bolts are through transon and the motor mounting bracket, no problems here. The two lower bolts through the splash well are not through bolted on the motor bracket but appear to screw into a blind threaded area of the bracket. the pair of lower holes/slots of the bracket are not curently used. will I need to change our the bolts /hardware in raising the outboard?

prm1177 posted 08-27-2003 12:41 PM ET (US)     Profile for prm1177  Send Email to prm1177     
From your pilot position, take a look at your wake. Is it flat and without a rooster tail? If it is and you are getting minimal spray up from your AV plate, I'd say you are in the ball park. You don't mention what prop you have on your outboard, so I wouldn't hazard a guess on your top speed as a guide.
Bigshot posted 08-27-2003 12:55 PM ET (US)     Profile for Bigshot  Send Email to Bigshot     
You are screwed if using blind holes. no way to raise OR lower without filling in the lower holes and drilling new ones in the transom.
SpeedBump posted 08-27-2003 01:09 PM ET (US)     Profile for SpeedBump  Send Email to SpeedBump     
Wake is fairly flat, there is very little - no rooster tail from prop except when motor is not trimmed properly.

I do get a rooster spray from the FF transducer mounted on transom starboard side.

I closely monitor the trim using the trim meter which I find very accurate. When not properly trimmed its easy to do so by feel/overall performance and guage reference is always right on target.

The prop is a standard elephant eared, 3 blade alluminum Nissan prop w/a 17 pitch, standard issue with this motor.

Best top speed I have had measured by the paddle wheel Speedo on the new FF is 36MPH, 12 gal of gas, myself and one young kid, pretty flat water, minimal wind but was going with the breeze. that was early this season w/ fresh bottom paint.

SpeedBump posted 08-27-2003 01:20 PM ET (US)     Profile for SpeedBump  Send Email to SpeedBump     
Bigshot - It appears when motor was changed out they used the existing transom hole pattern for the new motor. I understand motor mounting brackets are very standardized these days. Shouldn't the lower blind mounting points on the outboard be just above above the adjustable mounting slots so if the motor was raised the bolts would still be in line with the mounting slots?

I seem to recall another post a while back about lower motor mounting holes and bolts in older whalers that present a problem relative to using the lower mounting slots on an outboard.

IF this is the case how about mounting a static or manual (low cost) jack plate. I should still get the height as well as benefit from a set back of 5-10".

Does this sound correct or am I missing something?

SpeedBump posted 08-27-2003 02:45 PM ET (US)     Profile for SpeedBump  Send Email to SpeedBump     
The 4 top holes are 327MM (12.9") apart on center and provide for 3" of verticle lift from current position in the top hole to the bottom hole.

The 2 lower blind holes are also 327MM apart on center.

The two bottom slots are 251 MM (9.8") apart on center and have a slot allowing 2 1/2" verticle. The lower slots are also 2-3 " lower than where the blind bolt hole is.

The lower transom holes are practically in the bottom of the splash well now at their higher setting. I don't think I can go lower and still be above the water line or bolt through the transom inside of the splash well.

Bolts are 12MM X 105 MM

JohnJ80 posted 08-27-2003 04:09 PM ET (US)     Profile for JohnJ80  Send Email to JohnJ80     
Several comments:

1. Using the Nissan mounting instructions is probably not useful. There is no way that Nissan can anticipate all the different hull forms and configurations. The one to use is Whaler's and experience. Whaler recommends that the AV plate be 0-3/4" ABOVE the bottom of the hull. Experience seems to show that this should be 1-1.5" above for optimal performance. Raising this up will most likely have a big impact on your ride, porpoising and top end. I have my motor on my 15 Dauntless mounted up 1.5" OVER the bottom of the boat. Everything improved every time I went up a hole.

2. Fishfinder speed measurements can often be suspect. Mine (Humminbird) tracks pretty well up to about 35mph (around 7% accuracy) but over that the error goes sky high (20% high or so). The best way to figure out your top end speed is with a GPS.

3. Ideally, your AV plate should be right AT the water surface in flat water. Each 3/4" that you are below this represents very substantial drag. Look at the surface area this represents and then imagine trying to hold that surface perpendicular to the water flow at max speed. you can see that this is a huge amount of drag that you are burning horsepower on overcoming. Fiddling with height is critical to so many things - ride quality, speed, hull attitude, porpoising. A few hours spent on experimenting with this will most likely pay large dividends. There is really no way to deal with this without experimentation.


ratherwhalering posted 08-28-2003 01:55 PM ET (US)     Profile for ratherwhalering  Send Email to ratherwhalering     
I am very interested in this thread, as I have been learning a great deal about mounting holes, jack plates, anti-cavitation plate position and the like. I have a 1987 90hp Johnson mounted to a 1987 17' Montauk, currently mounted using the upper through holes and lower blind holes, so my set up is essentially the same. Bigshot is right. Currently, I cannot adjust the height of my engine without removing the lower blind bolts, filling the holes, then drilling new lower through holes that allign with the bracket's adjustable lower mounting "slides". I don't want to do this, so I decided to use either a jack plate, or a . Ideally, I would use a black rite-hite, pictured here.
Thus, I could mount the transom side in the current upper through holes and lower blind holes, and the engine side in either the matching upper holes and lower blind holes, or the upper matching holes and the lower "slide" holes. Alas, I know that eventually I will be upgrading to a heavier engine, either a 4-stroke 70 or a 2-stroke 90 EFI, so I'm looking for a manuel jack plate that will work with both engines. I think the 6 inch set back of the rite-hite may be too much, as sugguested in this reference section:
To quote "By mounting the engine on a jack plate, the effective transom height can be adjusted to an optimum setting. This bracket (Cook Power Lift) affords a minimum of set back, which is appropriate for this installation as the modern engine weight is considerably more than was originally contemplated when the hull was designed. (Author Rex Bettis)

So it appears to me that there are a couple of alternatives:

1. A cheap, simple two peice, fixed set back bracket. Mounted using the existing upper through holes and lower blind holes on the transom side, and the upper holes and lower adjustable "slides" on the engine bracket, with a 4" set back. This is cheap, with the hardest and least amount of adjustment, essentially using the engine bracket's own adjustments.


ratherwhalering posted 08-28-2003 02:15 PM ET (US)     Profile for ratherwhalering  Send Email to ratherwhalering     
To continue ad nauseum:

2. A fancy jack plate like the rite-hite,
Detwiler Jack Plate , or a cheaper Hi-jacker
These all seem to have a 6" set back, but are easier to adjust.

3. My choice will probably be the one piece Power Lift . It has a 5" set back, and seems easy enough to adjust. The price of arount $200.00 also fits my budget.

I would appreciate any comments forum members have regarding these products, as my final decision won't be made for a while.

Lastly, For a great review of jack plates and their relative performance and cost, see this article

Bigshot posted 08-28-2003 02:39 PM ET (US)     Profile for Bigshot  Send Email to Bigshot     
The power lift(last one) is a good choice. I have a 5" setback Vance industries that is great. I got it for $139 on E-Bay and added a $15 1/4" thick alum plate to essentially make it a one piece lift. For $204, that plate is a great alternative and ready to mount.
JohnJ80 posted 08-28-2003 03:42 PM ET (US)     Profile for JohnJ80  Send Email to JohnJ80     
I guess I am not all that familar with the jack plates and i didn't look at all of your links, but i did look at jack plates a bit ago.

what i would do would be to go with a conservative setback - like 4-6" and get one of these things that has the adjustment threaded rod, so you can go after it with a wrench. i don't think that once you adjust this, you are going to be doing a whole lot of adjustment after that so, the power adjustable ones are probably overkill.

others are probably more competent in this than I, but i can tell you that raising the motor is going to probably do good things. It is an experiment and your current setup doesn't lend itself to that well. The adjustable jack plate would be the trick, but I wouldn't worry about the power adjustable ones.


Bigshot posted 08-29-2003 10:46 AM ET (US)     Profile for Bigshot  Send Email to Bigshot     
John you should have read the links. The power one I refer to is a manual plate with a "power" name. I agree with you and since I made a few tweeks, I have NEVER touched it again.
4whaler posted 08-29-2003 11:36 AM ET (US)     Profile for 4whaler  Send Email to 4whaler     
Cooks Mfg. has what they call a setback plate described as follows that Ive been looking at:

"The two piece adjustable manual has 5" of slot adjustment at the motor mounting bracket and 1-1/2" of adjustment at the transom bracket. After height adjustment has been made, tighten the four bolts on the sides to 100 foot pounds to hold the position in place. Available in 5-1/2 or 4 inch set-back."

Got to their web site and click on "Setback Plates" to view Pic.

Or click on" Manual Power Lift" to see thier ML-65 which thier tech has recommended to me. It has a 5 1/2 inch set back which I think is to much, but does allow the wrench adjustment. I think the 4 inch is better. Im ass heavy already and the further back you go the worse the problem relative to stern down as the CG shifts rearward. Im thinking about mounting my Pate forward under the front thwart seat to off set. Its removable and would have to be jacked up, plus something put under the tank to level it with the ramps that start under it.

Playing with this is kinda fun, and with E-bay you can sell off most of the rejected/didn't work out hardware at cost.

JohnJ80 posted 08-30-2003 11:26 AM ET (US)     Profile for JohnJ80  Send Email to JohnJ80     
Sorry Bigshot. I got lazy.

That is exactly how I would do it. Mount it, set it and forget it.


knothead posted 09-02-2003 12:23 PM ET (US)     Profile for knothead  Send Email to knothead     
Just a question,

How well do these jackplates stand up to salt water?
Does anyone here use one on a boat moored in saltwater?


SpeedBump posted 09-03-2003 10:16 AM ET (US)     Profile for SpeedBump  Send Email to SpeedBump     
I used a custom fixed jack plate on my classic 13 to fit a long shaft Yamaha 40 HP. A friend who welds made it up from 1/4" aluminum plate and it filled the well cut out. it was a really nice job with internal gussets. a perfect fit.. I have had this plate on for the past 7 years and it is in like new condition.. Gets a wipe down twice a season to keep it clean.

The boat is moored in salt water. No ill effects to the bracket or the motor as I believe the motor mounting brackets are also aluminum and I coated the SS engine bolts when installed to help eliminate galvanic action due to dis-similar metals.

I would assume the after martket jack plate and lifts will be the same. I think that many suppliers also ofer their lifts/plates in stainless steel for a premium price. If you are getting a static plate or manual adjusting plate I wouldn't worry much as once it is set properly there is really no need for further adjusting and if something does seize it wont really matter.

mhoyt01 posted 09-03-2003 12:27 PM ET (US)     Profile for mhoyt01  Send Email to mhoyt01     

If your AV plate is under the surface at full out and proper trim, would you be limiting your max rpm?

All the prop clalculator said I needed 19 to 21 pitch, so I went stainless 19's.

I'm running 5100 rpm at 41 mph wot.
22 outrage w/d twin honda 130's. The engines are rated for 5000-6000 wot. My engines are raised as far as they can go, which solved most of the porpoising, but it still will propoise some from about 36mph +

I think the boat should go more like 46-48mph and not porspoise. It went 47mph at 5250 with twin merc 125's with 19 pitch aluminum props(300 pounds less than the honda's)

JohnJ80 posted 09-03-2003 12:40 PM ET (US)     Profile for JohnJ80  Send Email to JohnJ80     
I don't think you would be limiting your revs at all, the prop wouldn't know or care as long as it was in the water. If your prop is breaking out (would be really noisy) then the revs would increase since it isn't in solid water.

The problem with being too low is that for each hole you are down, you are adding an enormous amount of drag. Picture a surface 3/4" thick and the width of your lower unit. Now try to hold that stable in a 40mph flow of water - takes an enormous amount of force. By raising the motor, you are removing that drag which will allow you to go faster for the same HP applied (props and all being equal). In your case, the problem is doubled since you have two motors.

Are you over the weight for the motors for your boat? If you are and the stern is squatting, then this makes for the same problem as your motors too low only now it is the stern instead. Honda's are heavy motors - probably among the heaviest. In addition, when I looked at them, they seem to carry their weight aft on the motor which has the effect of making them seem even heavier since the weight is out on a lever arm the distance they are back off the transom.

If you have done the fins - and I would recommend that you do (my favorite is Hydro Lift), raised the motors, added cupped high raked props (they also help to lift the stern) then the next step is trim tabs.

Trim tabs can do wonders. If you provide a little lift in the stern your can really reduce drag and get the boat going. They will also stop porpoising completely.

You can also raise your motor with jack plates, but I have no personal experience with these and there are others more competent than I to comment on those.


mhoyt01 posted 09-03-2003 01:35 PM ET (US)     Profile for mhoyt01  Send Email to mhoyt01     

Really appreciate the advice.

Yes, the motors are too heavy. They are not over the weight limit, as there was none provided by whaler for this model.

We did some flat water testing last friday, with and without the fins. The fins made not difference at speed, but helped the boat get on the plane tremendously, so the verdict was: FINS GOOD.

As for the engines height. I inititially put the them where the mercs where.(hole #2 from the bottom, 2nd lowest position) and the porpoising was out of control) I moved them all the way up to hole #4, highest engine position) and the porpoising was much better, and like I said, only starts at 36+ mph. As for the AV plate, I did not look at it, but I'm assuming it's under the surface.

There is a guy on this forum with my boat with twin honda 115's(exact same weight). He reports 41 mph with NO porpoising. He does not know what his prop pitch is however, and his engines are also all the way, but those two levels maybe different, due to hole location.

Trim tabs are not really a option with the whaler as i understand it, so i guess that leaves me with drilling new holes, getting jack plates, and cupping/raking props...


JohnJ80 posted 09-03-2003 11:20 PM ET (US)     Profile for JohnJ80  Send Email to JohnJ80     
Why are trim tabs not an option on a whaler? I just put them on my Dauntless 15 (Lenco Electrics) because of the very choppy waters I boat it and I wanted to control the bow attitude better.

They have made large difference in handling and ride in heavy chop. They might be a big help to you. Call Lenco and see what they say.

But first, do the prop thing if you havent all ready. A good prop with stern lift capability will help you (about 10-15% on a single engine, in my estimation). I would bet it would be a bigger deal on duals. Call Stilletto and have them walk you through it.

With the fins, did you still have the porpoising or are we now just looking for more top end?


mhoyt01 posted 09-04-2003 12:25 AM ET (US)     Profile for mhoyt01  Send Email to mhoyt01     
The problem with the trim tabs is hte whaler drive. There's is just really no where to put it.

I am still porpoising at above 35 mph. I do want the top end, but not because I like to go that fast regularly, but more because, I feel this setup should go 47 mph on the flat water with no porpoising, and when it does, then I will feel confident my boat is setup right. The dole fins help get the boat on the plane by a lot, but then once it's up the boat behaves identical with or without...

I will definetly look into the prop thing, but first I hav eto be positive that the 19 pitch is the right thing. All the book data points to 19 pitch, but when on the water, she only gets to 5100 with a light load. I'd like to be more in the middle of the 5-6000 WOT range, especiallly with the light load. Perhaps if the engines were yet still higher, the porpoising would go and the speed and rpm's would kick up that last little bit...

Thanks again for helping out,


Perry posted 09-04-2003 02:19 AM ET (US)     Profile for Perry  Send Email to Perry     
Matt, are your anti-ventilation plates on the surface at full plane? 5100 rpm at wot is too low. You need to bring it up near 6000 rpm. If you put on 16 pitch props you would gain 600 rpm and a better hole shot. 15 pitch props would get you to around 5900 assuming your motor is at the right height.

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