AV Plate > Water Flow

[SIBRider]

Position #3 (as pictured) or near-abouts is best. Correct propeller must be selected for best boat performance (top speed, fuel economy). Motor height is important but is only one of factors. Dropping motor lower improves handling in rough water and hole shot but creates excessive drag resulting in lower top speed and poorer fuel economy. Described symptoms at #2 and #3 indicate under-propped motor. Select prop with higher pitch and try again.

Big boat appears heavily trimmed out judging by rooster tail and angle of power heads to boat. That alone will causes stern to dig-in. There might be other factors. Is it accelerating out of hole?

[Locozodiac]

Sibrider, you've burried in deep sea all my data that's been tested on ideal lab like water trails and not under out of this world Mars theory.

If what you have posted corresponds to your personal Zodiac F 470 with speed tubes and underpowered 25 HP engine trials, sorry mate, will not work same on any standard sib/rib, it happens to be 2 different animals, you are mixing prunes and water mellons in same sack. Standard sib/ribs have their own flaws. Zodiac F470 has it's own too. To each his own...

If you slightly or fully understood what's been explained here with proven facts and details, at which 1-2-3 lower leg height will engine need be seated in order to achieve perfect close tight 360 turns ? would like to know your plain theory on this one..

Happy Boating

[SIBRider]

Making circles a displacement speed is not my way of having fun. Not that I have any problem doing that.

Drop you motor and you will get better control, poor economy, reduced top speed.

[jyasaki]

I am in SIBRider's camp on this one.

There likely won't be a "perfect" setup that works ideally in all water conditions. Too many fixed data points (hull shape, available power, engine efficiency) and too many variables (water flow, wave/swell action, prop deformities.)

What you're looking for is a compromise - a setup that works adequately in all (or nearly all) situations, while doing well in the areas that you operate in most.

jky

[Locozodiac]

If you don't believe than once engine is correctly height seated, where it should, boat will perform very nice on all water cond. This helps boats that carries underpowered engines min 70% of their max rated power to perform much better.

Jky, what are you talking about. Any boat engine brand could me maximized, what's maximized for A will not apply to B. For each it's own individual test with their own engines. Those trials must be made on flat calm no wind waters to start with. What has to do waves, swells, prop deformities, or even driver with diarrhea, pure BS, you're on different page.

Sibrider, those merrry go round pics were shot just for you, being so technically experienced assumed would provide a much better answer than plain "will needd to drop the engine down", wow big deal, congrats have re invented the wheel.

There are 2 kind of boaters, the standard one that's happy with current set up although could not be the best one and the more specialized who likes to take the most power out of their engine. Which one are you will depend entirely on yourself.

Happy Boating

[jyasaki]

[QUOTE=Locozodiac;624228]Jky, what are you talking about. Any boat engine brand could me maximized, what's maximized for A will not apply to B. For each it's own individual test with their own engines. Those trials must be made on flat calm no wind waters to start with. What has to do waves, swells, prop deformities, or even driver with diarrhea, pure BS, you're on different page.[quote]

So you're saying that waves and swells have no affect on prop performance? You're saying that in the course of using a boat things like prop dings don't occur? Engines don't go out of tune? Loads don't change? Really?

Of course to set things up as optimally as possible you have to test each boat/motor combination. That's because each separate hull and motor possesses the variables which you just dismissed. Add to that differing skippers with different operating practices, and different goals, and you'll find that a setup for one is not ideal for another.

jky

p.s. I think I'm done with this conversation, as it seems to be getting a little out of the discussion realm.

[SIBRider]

Originally Posted by Locozodiac

If you don't believe than once engine is correctly height seated, where it should, boat will perform very nice on all water cond. This helps boats that carries underpowered engines min 70% of their max rated power to perform much better.

Loco.
Nice is matter of opinion. I like to have certain criteria for optimum performance. For me it's not making circles, it's good speed and economy. Which is what most often other boaters are looking for. 25 hp is insufficient power for my boat but acceptable. Originally it performed reasonably well or so it felt. Topped out at 20. A/V plate was very low. I lifted my motor 3" above edge of transom (F470 commonly comes with 18" transom). As result A/V plate now is just below transom edge. I switched to higher pitch (13) prop. Original prop (10" pitch) performed notably worse at new location (cavitation, poor grip, over-revving motor etc) Just asking for higher pitch. With new prop It tops out now at about 25 mph, cruises at 20 with 5 people. 4 blade prob might make small difference not consequential I think. To go faster I would need bigger motor. I didn't trying to invent gun power or something more potent just followed common advice, books, manuals.
You like you setup I like my setup. We are entitled to our opinions. I've got my observations I can share.

[Projectile]

When setting up the motor on the transom, one has no idea at what height water will pass around the shaft at plane.

Isn't it better to speak of what height the motor should sit with respect to the transom, and in this regard, why not speak of the height of the AV plate with respect to the bottom of the transom. Then we all know what everybody is talking about in standardized language.

I'd say 3 if I had to pick between those three options you provided, but that's *assuming* that the level that water flows past the shaft will be the same as the bottom of the transom.

If I could pick my ideal answer, it'd be between 2 and 3.

The correct answer should be the one that provides the fastest speed (without ventilation in a straight line or on mild turns, porpoising, etc) under the exact same conditions, where fastest is clearly fastest based on statistical analysis: multiple runs with an average of 26 mph on Monday, but 28 mph on Wednesday may not really be "faster" on Wednesday if the 2 mph difference is within the range of measurement error and normal variation.

[Locozodiac]

Gee this conversation is really getting very tech interesting. To straighten things out before we all go flying over the branches. Sibrider and Proyectile, what you found ideal on your Zodiac F 470 with under powered engine for that huge military size sib is that those parameters will not work same on recreational standard sib/ribs. They don’t count with lower speed tubes under main ones and those speed tubes makes the whole water performance difference. To each his own.

Jky, you have not stated which boat type you have including engine HP to start with. If you happen to have a cruiser, speed boat, sailboat whatever using an inboard or outboards propelled engines, these settings will not apply, different animals, this post was intended for standard recreational sib/ribs, mainly tiller driven which happens to be the most common engines we all use.

You've said before that you're a happy boater with your current boat/engine set up performance and that you’ll never check what’s happening at back engine as to check if engine height could be optimized for better water performance, you are a boater that falls into the standard boater category that takes for granted that your boat/engine is well set up for your boating needs and expectations, a very valid answer if that pleases you and only you.

We 3 will agree that once engine is transom seated must match each boat type, transom height, hull shape accordingly to the engine brand of your particular choice to squeeze out top HP output that engine can deliver, right ?

The issue with 99.99% of sib boaters installing old or new engines is that all start from what have been written on owner manual, cast on stone, word of mouth, read it somewhere else premises that AV plate must be even or slightly under keel or boat’s rear bottom. Wrong the distance for most engines brands states a distance 10 to 50 mm height measured from AV plate to lower keel/boat’s rear bottom, and that’s a starting point which can be moved to match your best boat/engine requirements.

Everybody makes references to the same side of the coin, that’s AV plate height. But coins have 2 sides, apparently nobody wants to flip the coin to test the other side. the other side I’m talking about setting small upper water deflector height as opposed to sateity mentioned AV plate heights.

Being this issue not written cast on stone, stated on owner manual, word of mouth, read it somewhere else nobody knows a thing about it, it’s simply not mentioned nowhere, all owner manuals will be in need to be re written again as a Tohatsu engineer concluded after testing the outstanding water performance achieved with a rib with perfect seated lower leg and underpowered engine HP for that size rib.

Boaters that have gone through this info and had tried it, found superb water performance at all water conditions, night and day difference, specially the ones that have underpowered engines seated on larger sibs/ribs.

Perfect sweet engine transom height for a given boat type with specific selected engine brand (Yam, Suzuki, Tohatsu, Evinrude, etc) will need to be water tested under wot perfect plane, trial and error tests until the opt lower leg/transom height is dialed spot on, that's provided you follow specific boating parameters to go from there.

Regarding 360° turns posted pictures must say that you can throttle all you want to, the issue is that will get so dizzy pretty darn fast in few turns that will end puking all over the sib. A grotesque and smelly scenario indeed.

BTW, both Tohatsu & Suzuki lower legs as seen on pics were correctly water tested under trial and error. After tests completion, both boats were transom height modified to achieve perfect match for each hull to work top with current chosen engine brand. Both engines performs top best at position 1, in position 2 prop cavitates slightly at very close turns, at 3 so worst due to excessive prop ventilation that engines loses near all prop forward thrust, less water bed height for prop to grip in comparison to 1-2 positions which has higher water beds heights. All 3 mentioned positions with no issues whatsoever when riding on flat calm no wind water conditions, that’s at speed and perfect plane, provided of course you go on a straight water course.

Sibrider, Proyectile I’ve read all your posts regarding your current inflatables and engines big problems issues, did you happen to see the light at the end of the long tunnel or still struggling with your respectively Zodiac/speed tubes water performance tech issues.

Each of you have expressed your ideal lower leg heights configurations that works for you, but that’s your personal experience with those military heavy duty heavy sibs of yours, useless keep mixing different type of sibs in same sack.

The general discussion point is that we are not in agreement to which lower best tail height must water flow pass by at speed when hull at sea level is riding on a perfect straight plane. Let’s get back then to ABC kindergarten boating guidelines for better understanding, will post back a new very illustrative assay that should be ready in some more days.

Are we all 3 and anyone that wants to chime in provided that are recreational sib/ribs boaters in same opened page so far ? Stay tunned folks!!

Happy Boating

[Projectile]

I wasn't challenging what you've written, Loco. I was just responding to your original question and telling you what I would have thought would be the best setting.

I think that you are absolutely right to perform sea trials with a variety of motor heights and find what works best. Given your extensive experience with multiple seat trials, I'd be curious to know if there is a consistent motor height that seems to work best, or does it vary so much from boat to boat that no generalizations can be made and individual testing will always be necessary.

My only suggestion is to talk about the height of the AV plate with respect to the bottom of the transom because that's something everybody can readily measure and agree upon.

[Max...]

Seems to be some arguing here which I want to stay out of but I've always understood that it should be 1. - mainly through reading so many of Loco's very helpful and techhy threads/posts on this like below:

http://www.rib.net/forum/f50/abc-sib...tml#post572946

This pic:

http://cdn.rib.net/forum/attachment....6&d=1380658461

The shape of the leg and my tests confirm this at least for me is probably the optimum position.

If you check any Zodiac manual it states that trasom height and leg height (assuming the correct 'short' designation) are all nominal heights not industry standards and therfore riser blocks may or may not be needed.

Certainly for me with the common combo of a 3.4m Zodiac and 9.8 Tohatsu a 20mm block was needed to leave the def plate just skimming over the water - worked great.

On my Bombard 380 with Mercury a 30mm block is needed to get the right height. Any more would likely lead to some cavitation in turns or rough water possibly, I need to test this but it requires mods as the clamps are as high as they can go
on the metal plate.

This particular boat still splashes as owners know as the unique stern V shape seems to direct flow in a certain way.

[SIBRider]

Originally Posted by Max...

I've always understood that is should be 1. - mainly through reading so many of Loco's very helpful and techhy threads/posts on this like below:

http://www.rib.net/forum/f50/abc-sib...tml#post572946

This pic:

http://cdn.rib.net/forum/attachment....6&d=1380658461

The shape of the leg and my tests confirm this at least for me is the optimum position.

If you check any Zodiac manual it states that trasom height and leg height (assuming the correct 'short' designation) are all nominal heights not industry standards and therfore riser blocks may or may not be needed.

Certainly for me withe the common combo of a 3.4m Zodiac and 9.8 Tohatsu a 20mm block was needed to leave the AV plate just skimming over the water - worked great.

On my Bombard 380 with Mercury a 30mm block is needed to get the right height. Any more would likely lead to some cavitation in turns or rough water.

This particular boat still splashes as owners know as the unique stern V shape seems to direct flow in a certain way.

What is relative position of your A/V plate and transom?
Picture you are referring to (the one seem to be proposed as optimum mounting height) has A/V plate several inches below transom. Is that how yours is mounted? Is that A/V plate still just skimming over water or are you referring to deflector plate?

[Max...]

Re' my link to the pic in my post above (#31) water is flowing just under the upper def-plate.

As I say though the Aerotec is different to all other SIB's due to the sharp V at the back so maybe not the best 'standard' to judge by??

I want to try the engine higher but will need to mod the rear transom clamp plate to try this.

[SIBRider]

Originally Posted by Max...

Re' my link to the pic in my post above (#31) water is flowing just under the upper def-plate.

As I say though the Aerotec is different to all other SIB's due to the sharp V at the back so maybe not the best 'standard' to judge by??

Flowing water isn't telling much. Water certainly should not flow over deflector plate. Where does your A/V plate sit in relation to transom? If you place a ruler on plate and extend it to transom. How far below is the plane.
This is where mine is:


This is what it looked like originally. Not quite a nice picture but you can tell deflector late is just above edge of transom.

Performance notably improved after I raised the motor AND upgraded the prop to higher pitch. That's what confounds me since Loco seems to suggest opposite for the former and doesn't mention matching propeller to motor/boat combination. Which I believe is fundamental.

[Max...]

I'll have to mount it and check...

[C-NUMB]

Interesting subject but also extremely complex.....only changing the make of prop will make a big difference in optimal height with same boat and engine. If changing make of engine but same boat and prop can also makes a difference, the design of lower gear varies by brands.

Top speed makes a huge difference. What i find in my own testing is that smaller boats speed 15-25 knots, cavitation plate even with keel level works typically OK, but on faster boats both performance and handling improves with engine higher than that(some times much higher).

On a very light speed boat, running a standard laser II prop, the chine walking was less the higher the engine was mounted....propeller axle 25 mm below keel line was ok handling wise.
That is off course extreme and does not work without lower water pickup or for "family boating".

Sib's are little difficult as the bottom shape can be different than on hard boat so optimal engine height requires more testing.

[Locozodiac]

Max,

Thumbs Up, congrats, didn't know you finally dialed the Sweet Height Spot under upper def plate on your standard sib, but V deep inflatable hulls, tubes with speed small tubes under, dual tube type sibs as catamarans fall into a different category much tricky to dial their SHS as both are experiencing. It's not my theme, theme know about or have trial as standard sibs/ribs so will leave it there.

With respect to the black long tail pic Max uploaded, must say that doesn't apply, was just pointing out all the possibel tech issues, splashes out/in transom at different engine heights. That's a 20 shaft sitting on a 18" transom. It's a out of this world transom height size, was transom matched very cheap than going for a costly 2" transom SS mount so to match a 20" height transom to a 20" shaft engine.

About C-NUMB post must say that the posted guideliness was meant for any brand engine using standard factory delivered props. If going for a particular shape, size, SS, diam/pitch along other type of non standard sibs/ribs, that's a different page ,so if you feel like following this interesting tech conversation and not get out of the main central discussion will leave other props and other type of inflatables out.

I'm finishing writting a proven assay about middle leg tech issues at plane/speed, that being the central theme of discussion/arguing going on. Hope will open minds to see & understand things differently than what you're up to now. Stay Tunned Folks!!

Happy Boating

[Max...]

Here is mine mounted - pretty much as it should be I guess, the Anti Cav plate is 1" below the most rear point of the keel at the back. As noted above the Aerotec keel is unique - on any other SIB the rear keel is non-existant and the engine position would be very different.

I reckon I could raise it another inch but as per second pic that requires major modding and is probably not worth the effort. I would need to fix a new rear engine support plate properly screwed through the transom etc with a more secure raiser system.

I'm not sure what 1" more would gain me in top speed? At present lightly loaded it's 18 to 19 kts but we cruise at 15 ish so it's not that important. A little less splashing would be nice...

Goodness knows why Bombard don't change the transom shape with a shallower cut out on top so we dont't need to comprimise like this with raiser blocks, too costly for them I guess in tooling or whatever.

[SIBRider]

I think you set it up nicely. I wouldn't worry about mounting it higher. Any gains will not be appreciable at that point. You would have to bolt it down for sure to mount any higher. If motor reaches top rpm at WOT without trouble I would try to fit higher pitch prop if you want to go faster.

[SIBRider]

Well in fact. Just guesstimating but if you have stock 9.25 x 9" pitch prop and reach 18-19 knots considering stock 2.15 gear case then you running at top RPM for sure. 10" pitch prop would be very safe bet to gain couple knots. 11 - 4-5 but you may start dropping RPMs with corresponding lesser gains. Only experiment will tell.