Anti ventilation plate height

[Skiffing]

FWIW

As soon as the water clears the stern it rises up.

The cav plate is submerged 1.5" - 2" in the best conditions.

That's a lot of unnecessary dag & resistance.

If you do use a jack plate as Bowhunter has done you will have to raise the engine even higher to gain performance.

The performance is not just speed. It is lower drag - rpm, better fuel efficiency and prop clearance, etc.....

 

[turbotodd]

My biggest boating pet peeves are:

1) Most boat dealers rig the OBs too deep

2) Most OBs are rigged with the wrong (stoo steep) a pitch prop

Ever wonder why dealers do that? Liability. And I have been down that road. Rigged a customer's brand new duck boat the exact same way that mine is, which is conservative but performs well (but not as well as it "could" ). Guy owns it about a week, calls me says it runs like poo after a few minutes of run time. He drops it off, I run it over to the lake, ran it about 15 minutes at full throttle 6000-6050 RPM and never missed a beat. Checked all fluids, checked impeller and key, etc. Found nothing. Told him I couldn't find anything wrong. It's duck season, guy runs it. Brings it back, low compression on top cylinder. Been hot/overheated. I start asking questions and find out that he's running 4 people in the boat, him in the back and 3 in the front. At full throttle it was pulling the foot up out of the water far enough to let the water pump ventilate but he keeps running it because it's duck season and the world will end if they don't have a boat to hunt out of. So in short, we ended up buying the guy a new powerhead. After installing, I dropped the motor back down on the transom (from a 3" lift), which put the AV plate at zero. Trying to get the guy the best performance possible ended up costing us about $1500.

Agree with most guys putting too much pitch in the prop on initial setup. Even Yamaha sometimes suggests more prop than that particular rig can handle. I see it listed on their performance bulletins and sometimes they'll list RPM tested at 5600 or something like that. An inch less pitch would help a ton, IMO, in cases like that. Those performance bulletins are real-world tests that dealers have supplied the info for. Other reason is that if a boat builder/rigger puts an inch more pitch than he knows is optimal, he more or less reduces the chances of over-rev, say, if Mr. Customer decides to ditch a battery or two, the trolling motor, etc...to see how fast it'll go. In the case of 99% of the boats that I've sold, nary a tach gets installed so guys can very easily twist the motor up well beyond what it's designed for. So lot of times we'll suggest using 12" pitch where an 11" would be optimal. Or if they choose 11" we'll tell them that it's optimal with a certain load. Less load=more pitch and more load=less pitch. One prop won't do it all but a lot of people will try. Props are, after all, kind of expensive. I have two. A 11" SS that I normally run, or if my GF and I go together, I'll put a stocker 10.5" pitch back on. Gets out of the hole faster, which is kind of important because she has to take everything (including the kitchen's sink) with her when we go fishin. It don't matter to me much...I'm just happy that she enjoys going.

 

[scoobeb]

Ty all for the info,helps allot. What I did was go back to look at pictures of were my other motors were mounted on the back of my other jon boats,what I found was interesting. All,I mean all my outboards were at the same level or even lower than mine is now and they all performed well like that. I always hit a great over all top end speed and hole shot was superb. Honestly if I'm losing a mph or two I'm not overly concerned about that,if I get near 30mph like I said that is extremely satisfying to me,any faster on a tiller motor is scary anyway. I may just be over thinking this as it should perform great as all my others did. If it doesn't than I will worry about it. I was going to move it up an inch till I saw I had little clamp movement left. If I must do a jack plate then I will. I don't need perfect,it's not a speed boat,lol. I would like it to perform good though and if it does as it sits I will leave it.

 

[scoobeb]

Here are some pics of one of my other boats i had,look closely at the anti vent plate here,it's near as low if not lower then the one i have now and that was a 15hp as all i needed to do was change out the reed valve from the 9.9 to the 15 reed valve to make it a 15.This boat with me and my ex wife and my child did over 22mph and the hole shot was good for a under powered boat rated for a 25.No problem with this one.I looked at all my boats i took pics of and they all hang below the bottom of the keel on all 5 of them at least an inch or 2 and all performed extremely well.I think it will be fine as is but again if it doesn't work as it should then i will at least know what the issue is.I'm putting a 12 pitch prop on it to.It came with a 11pitch prop but from all i have read this motor has mega power and will more then likely over rev with the standard prop so i went up a size.I will report back as soon as i take it out to test run it.My local lakes ramp is closed due to making it better so i'm going to have to wait for a nice windless day to test it out right.Here is the pics i'm talking about,thanks.

 

[scoobeb]

My biggest boating pet peeves are:

1) Most boat dealers rig the OBs too deep

2) Most OBs are rigged with the wrong (stoo steep) a pitch prop

Ever wonder why dealers do that? Liability. And I have been down that road. Rigged a customer's brand new duck boat the exact same way that mine is, which is conservative but performs well (but not as well as it "could" ). Guy owns it about a week, calls me says it runs like poo after a few minutes of run time. He drops it off, I run it over to the lake, ran it about 15 minutes at full throttle 6000-6050 RPM and never missed a beat. Checked all fluids, checked impeller and key, etc. Found nothing. Told him I couldn't find anything wrong. It's duck season, guy runs it. Brings it back, low compression on top cylinder. Been hot/overheated. I start asking questions and find out that he's running 4 people in the boat, him in the back and 3 in the front. At full throttle it was pulling the foot up out of the water far enough to let the water pump ventilate but he keeps running it because it's duck season and the world will end if they don't have a boat to hunt out of. So in short, we ended up buying the guy a new powerhead. After installing, I dropped the motor back down on the transom (from a 3" lift), which put the AV plate at zero. Trying to get the guy the best performance possible ended up costing us about $1500.

Agree with most guys putting too much pitch in the prop on initial setup. Even Yamaha sometimes suggests more prop than that particular rig can handle. I see it listed on their performance bulletins and sometimes they'll list RPM tested at 5600 or something like that. An inch less pitch would help a ton, IMO, in cases like that. Those performance bulletins are real-world tests that dealers have supplied the info for. Other reason is that if a boat builder/rigger puts an inch more pitch than he knows is optimal, he more or less reduces the chances of over-rev, say, if Mr. Customer decides to ditch a battery or two, the trolling motor, etc...to see how fast it'll go. In the case of 99% of the boats that I've sold, nary a tach gets installed so guys can very easily twist the motor up well beyond what it's designed for. So lot of times we'll suggest using 12" pitch where an 11" would be optimal. Or if they choose 11" we'll tell them that it's optimal with a certain load. Less load=more pitch and more load=less pitch. One prop won't do it all but a lot of people will try. Props are, after all, kind of expensive. I have two. A 11" SS that I normally run, or if my GF and I go together, I'll put a stocker 10.5" pitch back on. Gets out of the hole faster, which is kind of important because she has to take everything (including the kitchen's sink) with her when we go fishin. It don't matter to me much...I'm just happy that she enjoys going.

So with all that your telling me to leave it as be and enjoy my boat. If I get good performance were everything is I'm leaving it as be. From all I read it won't hurt anything like it is. I don't need to gain a mph or two if it performs well already. I will test it here soon and if It really suffers on performance then I will do what needs to be done to correct it. I realize it's not a speed boat,lol.

 

[Bowhunter1661]

You can make what ever end decision you like. My boat performs fantastic with my current engine height. It runs 32 MPH turning 5400 rpm WOT on a 1440 Lowe, 1988 Johnson 25. Also it sips fuel now with the anti vent plate completely out of the water.

 

[scoobeb]

Would you mind posting good pictures of the whole homemade Jack plate you made so I can get a idea how to put it together if I need to do so? Your boat is a hair over 200lbs,I was going to buy one a while back and had I known how crappy this alumacraft was built I would have bought a new lowe 1648. I've had awesome luck with all the alumacraft jon boats I have purchased until this one. My boat is I believe 290-310lbs so with this 25hp I should get near or close to 28-30mph with one or two people in the boat. If I hit that I'm completely satisfied,if I hit say like 22-25mph then I know there is a problem with the way it's set up and needs height adjustment.

 

[Bowhunter1661]

At the top of page two there are two pics. Unfortunately those are the only ones I have at the moment. I went to the local steel supply company. Bought 4 feet of 3"x3"X1/4" aluminum angle. I had them cut it into four sections 12" long. Though I did end up cutting them down to 10" each.

Here is where I got my idea. This guy did a great job showing the build process.
https://www.theonlinefisherman.com/forum/index.php/topic/13203-how-to-build-your-own-jackplate/

 

[scoobeb]

Your running your anti ventilation plate dead even with the bottom of the boat while getting your current speed? I can just use a wood shim and move it up darn close to where the anti ventilation plate is level with the keel. I am going to get a shim today to see how it sets. I can probably move the motor up at least 3/4 to 1" if needed. First is first,I need to test run it before I do anything because it may run awesome as is,you never know.

 

[Bowhunter1661]

I have mine set approximately 1.5-2" above the transom. But, I have the jack plate. With the the plate it gives your motor setback, aprox 4 1/2" on mine. This allows me to raise the motor even more that is it were raised on the transom because as the water flows under your boat it starts to rise immediately. Allowing higher motor placement. Since this pic I have actually raised the motor anoth 1/2".

 

[Bowhunter1661]

Never mind all the crap in the background. I live in a condo and my neighbors think their garage is a scrap yard.

 

[scoobeb]

Well,here is what I'm deciding to do,makes logical sense since I'm going to keep this boat for many yrs. I'm going to sell this motor since it's brand new abd buy the same outboard. This time though I'm getting the whole package,electric tilt and trim,electric start this way no need for a jack plate and not thinking how hard it will be to tilt a 136lb outboard is worth the upgrade. The price difference of $400 is well worth it to me. No a question,how much trim do these outboards really go? If I was say were I'm at now 1-2 inches below the keel would a power trim system move me up at least the couple inches I need? If it wouldn't then this would be worth the money. I guess what I'm asking is how many inches up or down does a power tilt and trim system give you?

 

[Bowhunter1661]

If you really want tilt trim I would say that would be a good route to go. However, IMO (and I'm sure others will vary) that is not going to be your saving grace when it comes to the motor height. If you simply trim up with incorrect motor height it will cause more drag as your anti vent plate is now angled down below the surface of the water. It is a combination of engine height and tilt. For example, my AV plate sits exactly where you want it when cruising (level and out of the water), however, when accelerating I get a slight bit of ventilation with the motor in the third pin from the transom. If if had tilt/trim I could trim the motor all the way down, get on plane, then trim it up to get the anti vent plate out of the water. That is the optimal situation.

With your current motor height I feel your anti vent plate will still be submerged even with your motor tilted with Hyd tilt/trim.

Now, say you go the route you are speaking and trade the motor in for a Hyd tilt trim model. I would still suggest you either buy or make a jack plate. The way I see it is your leaving chips on the table. You have invested a lot of money in a little Jon boat, why not make it perform as best it can. The marketed plates aren't all that expensive, I just love building things. That's why I made my own.

I do plan on building my own tilt trim unit so I can take of fully trimmed down to eliminate ventalation, then once on plane trim up to get the plate even with the water. If I run with my trim down, even with my current elevated motor height, the anti vent plate digs in. I have to be trimmed in the third hole so the anti vent plate is level when on plane.

 

[scoobeb]

So you don't think power trim would move my motor up far enough to at least be even with the keel,I'm only 1-2 inches from being even with the keel,I don't see how that wouldn't do it. What's the purpose of selling an outboard with power tilt and trim if it wouldn't trim to the right height,that would be useless.

 

[JL8Jeff]

Power trim only tilts the motor, it does not raise the motor height when running. You would need a jack plate to raise the motor higher while running.

 

[scoobeb]

Ok,now I'm really lost here. I thought trim was up and down for engine height and tilt was tilting the motor up and down out of the water. How can they advertise power trim and tilt if they are the same thing. That wouldn't make any sense,you know what I mean?

 

[Bowhunter1661]

Trim is what you use when on plane or taking off from a stop. It adjusts about 1-3" I would say depending on model and engine size.

Tilt is used when removing boat from water to get the motor up high, or when installing a transom saver. Tilt kicks in when trim is at the top of the trim ram stroke, so aproximatley 1-2" -6-8" high.

Here is a hydraulic tilt/trim for a larger outboard. The trim raises, once that is maxed out the tilt raises the rest of the way. On a smaller out board I believe they only have one cylinder, this would be a combination tilt/trim. Likely to raise the motor from 0"-6" depending on length of the cylinder rod.

 

[scoobeb]

You know what's funny,what's the point in a short shaft outboard on a short shaft transom if it doesn't work right. That is extremely pointless. Almost all the boats I have watched on YouTube bigger and smaller have most of the av plates riding under the water,no I don't doubt for a minute it does cause drag but how much is really is unknown especially ona jon boat. I watched a few ppeople who got the height exactly right on their boats and they posted it made only a slight difference in speed and fuel consumption but they did say it helped very little. I know now that it won't hurt it any,it may knock a mph or two of my top end which is not a big deal. But I'm starting to understand the whole drag issues. I can see how it would help on a boat with a larger outboard that takes allot of fuel to run through out the day but imo if my boat does well what's the point in modifying the transom for so little gain. Like I said if it's a big issue andIit performs like poop then I will fix the issue. Still to me spending the extra money is worth it just to save my back.

 

[Bowhunter1661]

This is a small outboard tilt/trim cylinder. Likely similar to what the outboard you are looking will come with.

 

[Skiffing]

Trim is going to change the angle of the prop THRUST in relation to the hull. It's not going to compensate for too long or too short a shaft.

You'll still have the same issues you have now. Wouldn't rather have the prop pushing the boat forward instead of lifting or burying the bow?

Power trim is a good thing, though. Among other things - it will allow you to trim up and find the sweet spot for cruising, trim down if you porpoise in a chop, get a better hole shot, etc, etc.