AV Plate > Water Flow

[Locozodiac]

Been doing some trial experimentation on a 400 Mt Rib rigged with a Tohatsu tiller engine to optimize best transom height for top performance. As you know the issue to have into account when sitting an engine on transom states per owners nanual, word of mouth, rule of thumb, read it at boating forums whatever, that AV Plate must be set level or slight under keel/boat's bottom.

Water trial was performed on a rowing track with these spot on conditions : flat calm, no wind sea conditions, rib perfecly inflated, ballanced & trimmed. I already know the answer from trial, just wanted to know your opinion about : At which height number do you think water flow is passing through lower leg at plane 1-2-3 ? Which number would be ideal for your setting and why ? Just short answer as we're not discovering gun powder again...

Happy Boating

[dave3235]

Originally Posted by Locozodiac

Been doing some trial experimentation on a 400 Mt Rib rigged with a Tohatsu tiller engine to optimize best transom height for top performance. As you know the issue to have into account when sitting an engine on transom states per owners nanual, word of mouth, rule of thumb, read it at boating forums whatever, that AV Plate must be set level or slight under keel/boat's bottom.

Water trial was performed on a rowing track with these spot on conditions : flat calm, no wind sea conditions, rib perfecly inflated, ballanced & trimmed. I already know the answer from trial, just wanted to know your opinion about : At which height number do you think water flow is passing through lower leg at plane 1-2-3 ? Which number would be ideal for your setting and why ? Just short answer as we're not discovering gun powder again...

Happy Boating

Two would be my first answer but ill go with one

[Locozodiac]

Dave,

Number 2 is right in the middle, valid answer. Why would you pick 1 as to understand your concept, perception about fixed objects hitting water flow at speed.

Happy Boating

[dave3235]

Originally Posted by Locozodiac

Dave,

Number 2 is right in the middle, valid answer. Why would you pick 1 as to understand your concept, perception about fixed objects hitting water flow at speed.

Happy Boating

sorry I ment to pick 3 instead of 1

[jyasaki]

I'd guess 2, as the water the hull displaces is returning to it's normal level, so should be a little higher than the bottom of the hull. Kind of depends on speed as well, as slower means more time for the water to rise as it comes out from under the hull before it hits the motor.

jky

[Locozodiac]

Jyasaki,

Let's say constant speed at 3/4 to full wot throttle, as to maintain a constant & state of the art perfect plane on those previosly mentioned ideal boat & water conditions.

Dave, 3 is what you assumed was my trial test result or it's your ideal personal choice you would pick for your own boat/engine combo ?

Happy Boating

[dave3235]

Originally Posted by Locozodiac

Jyasaki,

Let's say constant speed at 3/4 to full wot throttle, as to maintain a constant & state of the art perfect plane on those previosly mentioned ideal boat & water conditions.

Dave, 3 is what you assumed was my trial test result or it's your ideal personal choice you would pick for your own boat/engine combo ?

Happy Boating

I make that the chioce on an article I read that the when an engine is set up correctly you should be able to see the cave plate in most water states.

[jyasaki]

So... Did I miss the question? I assumed Loco was asking where the water would cross the leg in his picture; perhaps he's asking where it would cross in an ideal setup.

If the latter I choose at the AV plate. At speed, it's not needed, as hydraulic pressure from the water going past will prevent air getting sucked into the prop.

If the former, I say at the AV plate or slightly above (assuming the AV plate is even with the keel of the boat.) Doesn't say anything about correct setup, just what I think would happen in this setup.

jky

[Locozodiac]

There were 2 questions asked :

1-According to my posted pic, where do you think water flow at speed/plane is passing by 1-2-3 ? 2-Your personal 1-2-3 choice for your boat/engine combo ?

So Dave and Jyasaki chose 3, right through AV plate and with exposed AV plate when on plane, assume same on their personal choice, right ? We can't speak firmly about a "correct setup", as there are some tech parameters that must be fullfilled in order to achieve a correct set up, only done under trial and error which is not mentioned on Owners Manuals.

Find that 100% of posters goes from what owners manuals states, articles read, word of mouth, that's fully understandable untill a boating maniac, that's me, comes along to distort what's been written or said about AV plate heigt.

This is another prime example of read it on engine owners manual + some badly rigged inconsistencies. Question what odd cruising issues do you see on this picture. Boat parameters : 48 Ft long, 50 passenger off shore transport, 2-300 HP Yam engines with high thrust SS props.

Happy Boating

[jyasaki]

Originally Posted by Locozodiac

We can't speak firmly about a "correct setup", as there are some tech parameters that must be fullfilled in order to achieve a correct set up, only done under trial and error which is not mentioned on Owners Manuals.

Agree 100% on this.

Find that 100% of posters goes from what owners manuals states, articles read, word of mouth, that's fully understandable

Well, where else would they learn it?

I think the biggest shortcoming isn't that the manual says what it does, rather it is that the manual doesn't state that it is a starting point (or a generally acceptable solution for most boats), and that further refinement of the setup will likely yield better results.

In the second set of pics, the boat looks awfully stern heavy when loaded and underway.

jky

[Locozodiac]

Jyasaki,

Plainly agreed on you position that further refinements on the set up will render much better results. The issue is that most boaters take for granted that you must sit engine on transom, add fuel and voila or are extreme lazy to conduct water trials as recommended.

They preffer going per owners manual, word of mouth which is the only available fountain of information. Went through same info, but in my particular sib/rib cases didn't work for me a bit, so started my own exhaustive water trial and error experimentation and it worked spot on.

Buying a boat and engine is not same as buying a car, you have to deal with many hull shapes, transom heights, engine brands models, leg lenght which are not same although being same S-L most size used by us.

When the same question was asked to other boaters about the 48 footer issues, no one replied, not even with a decent answer. So we can assume that if these boaters would own that boat/engine combo will be most pleased with its current performance.

You hit the spot right on, assume you are a boater that can tell if a boat is adecuately tunned when passing in front of your eyes, right ? Btw, are you plainly satisfied with your current boat/engine transom height set up ?

Going back to the 48 footer picture, that combo has 3 main tech issues, could you tell which 2 are not being considered ?

Happy Boating

[jyasaki]

Originally Posted by Locozodiac

Btw, are you plainly satisfied with your current boat/engine transom height set up ?

My boat is a little tail heavy, but I had the console pushed back to free up space for dive gear (that was offset a bit by having the motor on a pod a couple of feet back from the transom, but it's still a little nose light.) Not a big deal, and a compromise I can live with.

As near as I can tell, the engine height seems fine. No idea where the AV plate sits while running, as I'd have to climb back onto the pod to take a peek (which I am definitely not going to do at speed.) I don't get too much ventilation or spray, though, so I assume it's fairly optimal.

Going back to the 48 footer picture, that combo has 3 main tech issues, could you tell which 2 are not being considered ?

I'm going to assume the tail heavy thing was one; Appears that the transom is below the waterline (as are the motor powerheads); Dunno what else.

Hard to tell from a static picture though; impossible to say if they're accelerating out of the hole or what.

jky

[dave3235]

Originally Posted by jyasaki

My boat is a little tail heavy, but I had the console pushed back to free up space for dive gear (that was offset a bit by having the motor on a pod a couple of feet back from the transom, but it's still a little nose light.) Not a big deal, and a compromise I can live with.

As near as I can tell, the engine height seems fine. No idea where the AV plate sits while running, as I'd have to climb back onto the pod to take a peek (which I am definitely not going to do at speed.) I don't get too much ventilation or spray, though, so I assume it's fairly optimal.




I'm going to assume the tail heavy thing was one; Appears that the transom is below the waterline (as are the motor powerheads); Dunno what else.

Hard to tell from a static picture though; impossible to say if they're accelerating out of the hole or what.

jky

Saying about stern heavy my recently upgraded setup from a 15 to a 20hp mariner merc was fine with the 15 on .When on the plane the engine now sits at number 1 on the picture, being both short shaft engines and measure the same from clamp to cave plate below this is more length on the 20 so need a cheap but safe way to raise the lump any ideas.Also getting prop slip or venilating,prop has normal wear and is straight 13 pitch ally.

[jyasaki]

Hi, Dave;

Hmmm. Somebody on here mentioned that the "short" designation was iffy years ago. Might be what's getting you (I always thought "short" meant 15" from top of clamp bracket to centerline of prop shaft. Apparently that wasn't always true.)

If you're talking a couple of inches, you can build up the top of the transom to raise the motor; wood blocks to get the actual amount worked out, then permanently epoxy/screw the spacer (which could be wood, fiberglass, whatever), followed by some kind of protection (surfboard or other resin, epoxy paint or the like.)

If you're that deep, I wouldn't think ventilation would be an issue. Are you sure your hub isn't spun? Do you get the slippage at throttle-up, running straight, only in turns, or what?

jky

[dave3235]

Originally Posted by jyasaki

Hi, Dave;

Hmmm. Somebody on here mentioned that the "short" designation was iffy years ago. Might be what's getting you (I always thought "short" meant 15" from top of clamp bracket to centerline of prop shaft. Apparently that wasn't always true.)

If you're talking a couple of inches, you can build up the top of the transom to raise the motor; wood blocks to get the actual amount worked out, then permanently epoxy/screw the spacer (which could be wood, fiberglass, whatever), followed by some kind of protection (surfboard or other resin, epoxy paint or the like.)

If you're that deep, I wouldn't think ventilation would be an issue. Are you sure your hub isn't spun? Do you get the slippage at throttle-up, running straight, only in turns, or what?

jky

Hi Jky
What do you mean by spun.It happens when turnning but I am not making sharp turns my 15 you could make more sharper turns before it happened but thats not the same bench mark I suppose.Also on the straight when powering up and sometimes when cruising its a bit strange to say the least.

Ah I know what you mean by spun.Thats what I call the bushing centre bush on prop

[Locozodiac]

Dave,

Have it mixed up, probably a senior moment, thunbs down. Where 15 and 20 same Mariner brand ? Was number 1 height achieved on flat calm no wind waters, with well sib inflated & ballanced, engine sitting at a 90° angle regardless of the trim hole position locked to.

The results of the 4.00 mt trials are as follows. All 3 positions propulses the rib ok, that's on flat no wind waters when going at a straight course, but..

Position 1 : gives the best prop thrust and best wot rpm, prop has more water flow height for prop to grip better achieving faster hole shot, nice boat performance throughout the whole throttle range, excellent close turns at any speed, works best on slight choppy, windy waters with no cavitation, aereation whatsoever.

Position 2 : loses sligh prop thrust and rpm, less water flow level height with respect to 1, slight prop ventilation, aereation when giving full throttle for a fast hole shot on windy & choppy waters, ventilates slightly at close turns at any speed.

Position 3 : loses the most prop thrust and rpm, slow hole shot, much less water flow height for prop to grip well. Ventilates at full throtttle or under slight choppy windy waters, much worst if water cond changes dramatically. Worst performer at close turns at any speed, prop will nearly stop pushing boat. Will need to correct tiller position near instantly for prop to grip water again and thrust to get back.

1 is best all around water performer, perfect setting for any outboard no matter which brand it is, will have best water flow cut at sharp edge, instant flat water deflection to wake including a nice flatten out undisturbed wake.

If you need to raise engine on transom accordingly to match position 1, JKY is giving good advice follow it. Will add that as soon engine sits at the sweet heigh transom position will need to raise both inner and outter engine plates to a new different position for the engine clamps to lock in middle of inner plate. The rear plate can be even with the new transom height. Both well middle transom centered.

Happy Boating

[dave3235]

Originally Posted by Locozodiac

Dave,

Have it mixed up, probably a senior moment, thunbs down. Where 15 and 20 same Mariner brand ? Was number 1 height achieved on flat calm no wind waters, with well sib inflated & ballanced, engine sitting at a 90° angle regardless of the trim hole position locked to.

The results of the 4.00 mt trials are as follows. All 3 positions propulses the rib ok, that's on flat no wind waters when going at a straight course, but..

Position 1 : gives the best prop thrust and best wot rpm, prop has more water flow height for prop to grip better achieving faster hole shot, nice boat performance throughout the whole throttle range, excellent close turns at any speed, works best on slight choppy, windy waters with no cavitation, aereation whatsoever.

Position 2 : loses sligh prop thrust and rpm, less water flow level height with respect to 1, slight prop ventilation, aereation when giving full throttle for a fast hole shot on windy & choppy waters, ventilates slightly at close turns at any speed.

Position 3 : loses the most prop thrust and rpm, slow hole shot, much less water flow height for prop to grip well. Ventilates at full throtttle or under slight choppy windy waters, much worst if water cond changes dramatically. Worst performer at close turns at any speed, prop will nearly stop pushing boat. Will need to correct tiller position near instantly for prop to grip water again and thrust to get back.

1 is best all around water performer, perfect setting for any outboard no matter which brand it is, will have best water flow cut at sharp edge, instant flat water deflection to wake including a nice flatten out undisturbed wake.

If you need to raise engine on transom accordingly to match position 1, JKY is giving good advice follow it. Will add that as soon engine sits at the sweet heigh transom position will need to raise both inner and outter engine plates to a new different position for the engine clamps to lock in middle of inner plate. The rear plate can be even with the new transom height. Both well middle transom centered.

Happy Boating

Hi Loco

Yes the 15c was a 1982 and 20 is a 1988 E9 I believe, first a yam mariner the second a merc mariner.Jky mentioned about the hub could be spun how would I test for this.With the engine at 90 degrees there is definatley less prop slip but makes for a bit of a hard ride with the bow steer.When tested at 1hole up on trim the water state was calm and little wind.I know its a compromise.wonder weather the prop needs to be looked at.Or just put a block of wood under just to see if different for a short time.

[Locozodiac]

Dave,

Both 15 & 20 should be Yam/Merc models. From around 2000 + are Tohatsu. If you have a spun hub issue all the trial testing will turn irrelevant, that's why a healthy engine and prop is mandatory.

There's a fast way to check spun hubs. Revove prop, at the rear of the hub draw a straight line border to border or 2 small dots front of each other, mark it with a white sharpie, fingernail paint. Let dry, put prop back.

Go for a short wot spin, remove prop and check marks. If have moved slightly or heavily from the position they were painted you have a light or severe spun prop. I dare say you have this cond as that's a 26 year oldie prop and props doesn't last that long unlesss kept in a closet.

If with spun prop, place a new one and go back to same water trial. The performing difference should be abysmal.

Btw, the 4.00 rib trail achieved position 3, so AV plate even with keel doesn't work as stated, there's a 2" water flow height difference with severely exposed AV plate seen when on plane. In this particular case a transom chop down is mandatory to match position 1, or live with poor boat/engine performance issues.

Happy Boating

[jyasaki]

Originally Posted by Locozodiac

If with spun prop, place a new one and go back to same water trial. The performing difference should be abysmal.

I think he meant amazing, rather than abysmal (unless you get the wrong prop.)



jky

[dave3235]

Originally Posted by Locozodiac

Dave,

Both 15 & 20 should be Yam/Merc models. From around 2000 + are Tohatsu. If you have a spun hub issue all the trial testing will turn irrelevant, that's why a healthy engine and prop is mandatory.

There's a fast way to check spun hubs. Revove prop, at the rear of the hub draw a straight line border to border or 2 small dots front of each other, mark it with a white sharpie, fingernail paint. Let dry, put prop back.

Go for a short wot spin, remove prop and check marks. If have moved slightly or heavily from the position they were painted you have a light or severe spun prop. I dare say you have this cond as that's a 26 year oldie prop and props doesn't last that long unlesss kept in a closet.

If with spun prop, place a new one and go back to same water trial. The performing difference should be abysmal.

Btw, the 4.00 rib trail achieved position 3, so AV plate even with keel doesn't work as stated, there's a 2" water flow height difference with severely exposed AV plate seen when on plane. In this particular case a transom chop down is mandatory to match position 1, or live with poor boat/engine performance issues.

Happy Boating

Hi Loco, jky

With regards yam or merc I was told that all mariners merc engines were yam, only the legs are of different makes ie merc or mariner , thanks for all advice will test next time out as for the engine height will mount a block of wood just to see if any benifits and will act on that acordingly.As for the 48ft I wonder if the seating is to far to stern, if nice weather most of the time won't have to worry to much about the stern.


THANKS FOR ALL YOUR HELP.