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Author Topic:   Anti-Ventilation Plate Position When on Hydroplane
montaukman posted 05-25-2005 12:28 PM ET (US)   Profile for montaukman   Send Email to montaukman  
Have other Montauk 170 owners installed [Doel-Fin Hydrofoil Stabilizer], Tigershark or Stingray fins on their 90-HP four-strokes outboards. If so, was the factory setting on engine height OK?

I know that the [anti-ventilation] plate is suppposed to be at the water surfae when on hydroplane, but since I am usually alone when I go out, I can't check to see where my [anti-ventilation] plate is when on plane. My engine height is at the factory setting, which I think is [one hole up].

Any feedback would be greatly appreciated. Thanks, Alan

bigjohn1 posted 05-25-2005 08:55 PM ET (US)     Profile for bigjohn1  Send Email to bigjohn1     
Alan, my Doel-fin is still submerged while on plane and the engine is mounted one set of holes up from the bottom. This is too low so naturally, I plan to raise my engine up probably one set of holes.
jimh posted 05-25-2005 11:08 PM ET (US)     Profile for jimh  Send Email to jimh     
[Administrative post]
kingfish posted 05-26-2005 09:11 AM ET (US)     Profile for kingfish  Send Email to kingfish     
With your boat on the trailer and your motor set to approximate normal running trim, put a straight-edge against the "V" bottom of your hull, oriented for-and-aft, and extend that line by sliding the straight edge back, keeping it alligned with the "V" bottom, until it reaches the leading edge of your lower unit. If your AV plate is below that point, it will be under water at planing speeds; if the AV plate is, say, 1/2" above that point or higher, it will be above the surface of the water at planing speeds. The water will "swell" up a little as it leaves the underside of the transom, related to the line discussed above, although that swell becomes less as speed is increased.
mikeyairtime posted 05-29-2005 08:58 PM ET (US)     Profile for mikeyairtime    
I went up two holes to the second from the highest and my perma trim now runs on top of the water and doesn't plow through it. No cavitation and noticably better mileage.
JohnJ80 posted 05-30-2005 04:26 PM ET (US)     Profile for JohnJ80  Send Email to JohnJ80     
I kind of get a big kick out of all this measuring of the AV plate to the hull adn all of this trading back and forth of settings on a boat.

It is really difficult, in the first place, to measure your AV plate to your hull. If you are trying to measure in a resolution of 3/4", you are going to have a very hard time of it because the angle of the AV plate is difficult to place if you have power trim and tilt. The tilt, on most motors swings through enough of a range to make this difficult. So, then do you do it when the plate is level? Referenced to what? Any way you cut it, the repeatability of this measurement from boat to boat is difficult if not impossible. Go ahead and try it - measure on your boat then launch your boat into the water, put it back on the trailer and see if you get the same measurement again - doubtful you will - at least within a 3/4" resolution.

On top of that, one set up as another even on THE SAME BOAT AND MOTOR is still problematic since the effects of the sea state, the manner in which the boat is operated and the way the boat is loaded all have very large impacts on this.

For example, if you operate in ocean chop, you may not be able to put your motor as high as if you are on inland lakes. Or if you have to do lots of dramtatic turns you may need it lower too. If you run for long periods in relatively straight lines, then higher is ok.

Whaler's recommendations where they mount motors is based on a huge average across all users and wide range of conditions - hardly the same as what most of use for our boats for.

No dealer can do this for you unless he is going to invest the time to be with you and see what happens. Most dealers mount them too low because they just have less issues with people having prop blowout etc... The default mount for a dealer is down.

In addition, dealers are going to only get anecdotal evidence of what some owner (who incidentally, have WIDELY varying skills) tells them the problem is. So, I doubt that they know the anwer - unless they are your best friend, boat with you often and see first hand.

The only sure thing in this is that you need to get your AV plate/fin at the surface when operating. If you don't the drag is very high and you can get squirrely performance as the lift will be varying between up and down as the boat hits waves.

All you can really say about this is that you need to experiment to get it right for the way you use your boat. Having been there and having done that, I can tell you it makes a big difference. Any other advice that purports to be the one best answer is probably best if seasoned with a healthy dose of skepticism first.


Perry posted 05-30-2005 05:17 PM ET (US)     Profile for Perry  Send Email to Perry     
Now that new Whalers are pre-rigged from the factory, it is interesting to see how high the factory rigs motors on new Whalers. I agree that dealers usually rig them too low but Boston Whaler knows what works best for their boats and I would think that they do some sort of testing before hand. I would assume that they would mount a motor in a position where the average user would have the best all around performance in a variety of conditions. My neighbor has a 2005 Nantucket with a 150 Optimax and the factory rigged the motor on the 3rd set of holes (up 2). I'm not sure about the other models that Boston Whaler makes but it would be interesting to know how high the factory rigs motors on their other hulls.
JohnJ80 posted 05-30-2005 07:22 PM ET (US)     Profile for JohnJ80  Send Email to JohnJ80     
That's exactly it - the AVERAGE. However, that band of 'average' is so broad that it becomes conservative. Whalers are used everywhere from little dinky inland lakes to miles offshore in the ocean. They are used anywhere from motly unloaded to loaded to max capacity most of the time. The performance in those parts of the envelope require different tuning. Tuning entail engine mounting, trim controls, and propeller choices.

What Whaler knows is reports of problems from dealers and from users.

I can tell you - having been there - that moving it around makes major differnences and it all depends on how you load the boat and use it. There is no way Whaler can predict that, so they go for the lowest common denominator that causes them the least problems. That does not necessarily, and probably doesn't, translate into optimum performance. Their primary worry is satisfying liability contraints, warranty repairs first and giving the best user experience under those conditions.


Perry posted 05-30-2005 11:09 PM ET (US)     Profile for Perry  Send Email to Perry     
I doubt Boston Whaler would mount a motor any higher than the 3rd set of holes because this may cause ventilation. I do think though that performance is a key factor in how high they mount a motor not what causes them the least problems.

Look at the Nantucket for example. With a 115 4 stroke, they use a Vengence prop which is basically a step up from aluminum. They mount this motor on the 2nd set of holes. The same boat with a 135 Optimax has a Mirage prop and is mounted one hole higher. They do this because the motor and prop provide better performance and will benefit from a higher engine mounting.

People buy boats for their intended use. I'm pretty sure a person would not buy a 210 Outrage for a dinky inland lake. He or she would probably use it in water that it was designed for. If someone loads a boat to the maximum on a regular basis, it would be in his or her best interest to use a prop for heavier loads. If a person uses an Outrage only on flat water, he or she can raise the motor to take advantage of its use. Those who don't fall in the "average" user category can always tune their boat to their own liking.

John, you may be right but I find it hard to believe that Boston Whaler's primary worry is satisfying liability constraints and warranty repairs.

bigjohn1 posted 05-31-2005 08:51 AM ET (US)     Profile for bigjohn1  Send Email to bigjohn1     
John, I see your point and have read all of your recent
informative responses on various threads discussing this. You have alot of experience in this area that I do not have. I was one of the guys who recently asked another forum member (Mikeyairtime) how he set up his engine since we have the same boat. I will not necesarily copy his exact setup but I will use it as a general baseline. I realize since I run in offshore water and currently still run an aluminum prop that I can't just replicate what a lake or river boater does with engine mounting as I will blow out easier. Being both time-challenged and lazy though, I like to use others experience as a baseline and go from there. I do see your point though...thanks.
JohnJ80 posted 05-31-2005 10:27 PM ET (US)     Profile for JohnJ80  Send Email to JohnJ80     
Hi bigjohn1 - that sounds like a good set of expectations.

Incidentally, I talked and talked with Whaler tech support about motor positioning. When I repowered, the dealer that mounted my motor put it down all the way. I was sick to death because the boat seemed completely unusable. That is when I had to figure out what and how all of this interacted or I would be out a big chunk of change.

What whaler said - virtually verbatim - was 'here is where we recommend you mount the motor (the 3/4" above bottom of hull thing). However, many people report better results when they move it up. It is a function of how you use and load the boat.'

So, getting close is good. each hole movement can have a big impact on performance - surprisingly so. So, even if you get it close you may be surprised at how poorly (or how well) it performs.


LHG posted 05-31-2005 10:58 PM ET (US)     Profile for LHG    
There has been tons of discussion here on engine "bolt hole" mounting height, which is why any adjustable jackplate is worth it's meager cost 10 times over. You instantly have the ability to fine tune your running height, and even change and adapt it for given conditions, or prop changes, in about 10 minutes of time. Now the newest designs have the adjusting nut on the top, so you can even do it while in the boat, if conditions change suddenly. The hydraulic ones are even better, for the $650 or so that they cost.
mikeyairtime posted 06-02-2005 09:11 PM ET (US)     Profile for mikeyairtime    
To further refine my findings: I use my 170 mostly for ocean fishing and runs to Catalina Island. The afternoon runs home are very rough for a 17 footer. The boat is usually heavily loaded with 2 adults, 2 kids 27 gal. Pate tank and 28 gal. bait tank. In the stock mounting position the lower unit plowed through the water and the trim tab was completely submerged. With the motor raised two holes (one hole below the highest position) the trim tab runs right on the surface, RPM's are lower at any given speed, tight turns can be performed with no blowout and holeshot is better. Way better all around performance than the stock mounting position. I think the reason the manufactures mount the motor low is that with the motor raised it is harder to discern cavitation caused by over trimming. In the stock holes the boat would porpoise and loose bite when overtrimmed. Now with the motor trimmed way up the boat will actually hold the bow higher without popoising and ever so slightly ventilate the prop. If someone didn't know what they were hearing and didn't adjust the trim back down a little they could cause damage if they allowed it to run this way for any amount of time.
JohnJ80 posted 06-03-2005 07:43 PM ET (US)     Profile for JohnJ80  Send Email to JohnJ80     
If the ventilation doesn't cause the motor to overspeed and if you still have cooling water to the motor (observe pee hole), there is no problem.


mikeyairtime posted 06-06-2005 03:25 PM ET (US)     Profile for mikeyairtime    
Actually the kind of cavitation we're talking does cause damage. It is detrimental to the prop itself. I've seen props with the edges worn down from cavitation. It's called propeller erosion.
kingfish posted 06-06-2005 03:43 PM ET (US)     Profile for kingfish  Send Email to kingfish     
I kind of get a big kick out of all the hot air that is generated when a simple question like, "How can I tell when I have my AV plate abovce the water line at planing speeds without going back there and looking while in the boat alone in the water at planing speed?" is asked.

If you can't "measure your AV plate to the hull" within a 3/4" resloution and with reasonable repeatability following the simple instructions I posted above, you either didn't read my post carefully or you didn't follow the instructions therein.

prm1177 posted 06-06-2005 10:06 PM ET (US)     Profile for prm1177  Send Email to prm1177     
Let's kick it up a notch (pun intended). How about twins? My Merc service manual indicates that, with twins, the outside edge of each anti-ventilation plate should be the measuring point. As you can see by these pictures I took when painting my Conquest, the AV plate is a little bit below the water line on the outside edges:

I can see confirmation when on plane because each engine throws up spray from the AV plate to the outside of the boat, but not inside.

When on plane and trimmed in, you should see clean water on each side of the engine. If your lower unit is throwing up spray, thats a reasonable indication that the AV plate is running low.

JohnJ80 posted 06-07-2005 09:51 PM ET (US)     Profile for JohnJ80  Send Email to JohnJ80     
kingfish, I'm sorry but your method does not work repeatably.

First of all, there is not an accurate scale to figure out what 'normal trim' is. Getting back to the same angle is not repeatable, period. None of the gauges are precise enough.

Secondly, "normal trim" - whatever that is - is going to vary from boat to boat significantly based on loading, seaway, wind etc... I don't think this is a reliable measure from boat to boat either.

Third, the distance that the AV plate is from the hull, is actually pretty fair distance. If you have the angle only a little differnent - by even an amount that you cannot eyeball, then you are going to have significant error in measuring - more than 3/4". If your error is more than 3/4", then you can't even get it back in the same hole reliably.

4th, In order to properly position your motor in relation to the boat, you are going to have to figure out where the neutral trim position is. Since the boat can be sitting at different attitudes on the trailer each time, you can't use a level.

The only way this can be done is by deteriming the angle from the transom to the motor and getting it precisely back to that each time. I would guess that this would take a pretty precise protractor and some precise measuring marks (of which there are none on the motor built in) to get this right. IIRC, most motors trim in (negative trim) a few degrees - like less than 5) - and out much more but probably a trim angle of 30 degrees is way too much.

So, if you cannot precisely locate this to some datum on the hull, then you can't make this measurement precisely. If you can't do this precisely since the motor movements in trim are just a few degrees, then this measurement is a waste of time. For example, if the separation is 12" from the hull, then just a 5 degree variance results in 1.04" variance on the hull. Well more than 3/4" precision. 5 degrees is awfully small to "eyeball" accurately. 1 degree results in about a qtr inch.

Try it and see - make a small trim adjustment and see how much the end of your straight edge moves on the hull. It is a lot!

All goes back to my original premise - raising and lowering your motor is best done experimentally. This measuring to get it exact is a total waste of time.


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