AV Plate > Water Flow

[Max...]

I'm not taking *any* side!

Most SIBs have a reasonably 'standard' height transom and most short shaft engines are a certain reasonably 'standard' length. Most times you put them together and it all works to a reasonable compromise. I've always run my combos either as they are or with a 20mm or so wooden raiser block.

That's all really!

You've made good points and others here have made good points, it's not super hi-tech state of the art Formula One, it's just little rubber boats and some farty old smoky engines!

[Projectile]

Hey, Loco.

What do you think of the way that the water is flowing past the lower end of this outboard? What can you tell from this?

[Locozodiac]

Proyectile, really can't tell, vid is too fast and lower leg is not very clear. Seems is riding well. If possible once combo is out of the water post a new still pic and mark where water flow passed by to have a look. Check this new one :

To corroborate what have been saying for light years, had a nice talk with an old boater who was about to make his maiden voyage with a 4.0 Mt center console rib and a new Yam 25 L HP engine. Prior to meeting with owner had a talk with Yam mechanic who happens is working for Yam dealer for the past 20 years, he "seems" to be a experienced guy.

He was rigging the stated combo and converting a tiller driven to full remote steering. Engine was finally seated on rib’s delivered standard transom at 90º. When inquired the mech how did he knew engine made a perfect transom height match, he replaid : It’s a Yam 25 L engine, and this brand always matches perfectly. He was 101% shure about his dry installation that combo didn't need a water trial before owner took the con.

The height between lower keel and AV plate is less than one inch, so under the tech parameters that's constantly stated as per engine manual, word of mouth, etc. Went for a wot test under flat calm no wind open sea, just driver and spotter. At wich lower leg height do you think WF was passing at plane and wot when spotted ? Was it passing right under WDP, middlle leg or over AVP. Roll the dice and give your best wild guess, it's for U2 Max...

Happy Boating

[SIBRider]

Projectile. "Look ma, no splashes!" Doesn't tell much else about boat performance.

Loco. Yam seem to be set up ok although appears to have a hydrofoil. Why? Hopefully water will pass below deflector plate but again why focus on that? That fin could throw monkey wrench into performance and flow. If go for WOT testing measure speed, RMP and relate it to propeller pitch that would give reasonable info about boat performance.

[Locozodiac]

I focus on WDP becaus everybody else including local Yam tech focus too much on AV plate and lower keel. Those parameters on on a dry home installation will not say much.

Know already the answer, the combo was sea tested same day, will say owner is not satisfied with overall water performance. Why wouldn't he be not satisfied, engine runs top, it's brand new out of the box.

Just wanted more boaters to chime in and read their wild guess tech opinion on that installation example.

Happy Boating

[Projectile]

What's WDP?

Visually, the distance between AV plate and bottom of transom looks like > 2 cm to me. Looks more like 3cm, ie. looks like > 1 inch to me.

Whatever lower leg height the water was passing around the lower unit with best performance, there will be a corresponding relationship between the AV plate and the bottom of the transom. Whatever you decide offers best performance, I want to know what that relationship is.

If the height at which water passes around the lower unit is important, you should be able to tell me something about the video I posted. You can watch it in slow motion.

[Locozodiac]

WDP stands for Water Deflector Plate and there's a close relationship between best engine performace and overall boat/engine performance depending on the height at which water flow passes by.

On my Yam example, not an issue if being 2 or 3 cm in height, remember it's a "dry installation" that's take engine out of the box and seat it on transom. As per engine manual, initial installations could go from 10 to 30 mm (2 to 3 cm) and some even up to 50 mm.

Hydrofoils were recommended by Yam man, the issue is owner doesn't undertand what they are for but looks real cool at lower leg. He simply goes full wot to his fishing destination, fishes and returns same, like driving his car.

Your video doesn't say much, lacks clarity, sharpness, and details, at simple sight there's no over or out transom water splash, rear cav plate is not exposed, seems riding on a good water level. Have not stated boat type, engine brand nor size ? How does it performs art very close turns at speed on choppy waters ? Prop ventilates ?

Next monday will post about your "I want to know what that relationship is" issue, just hope you fully understand all tech facts that will be stated there. Fingers crossed!!

Happy Boating

[Projectile]

You ask: "How does it performs art very close turns at speed on choppy waters ? Prop ventilates ?"

If the way that the water passes around the lower end is somehow important ... if it's somehow a surrogate of good performance (as I think you seem to suggest), then why do I have to answer these questions?

If the way that the water passes over the lower end is just one way of indicating the height of the motor in relation to the boat, then I don't see why the rest of the world should adopt your particular way of describing this relationship, and I don't see anything wrong with the way that the rest of the world already describes this relationship by referring to the AV plate and the bottom of the transom, which is objective & easily measured in the place where these adjustments are made: on land.

So let me be perfectly clear in what I'm saying: on land, the relationship between motor height and transom can be easily described using the standard terms. In the water, what matters is how the boat performs. Notice that no comments about how the water passes by the lower end are required.

(This is me being stubborn!)

But I eagerly await your essay on the topic.

When are you going to let us know what happened with this boat with the hydrofoil?

[Projectile]

Loco: say that with the motor at a given height, there is slight cavitation in a straight line, moderate cavitation on gentle turns, and marked cavitation on sharp turns.

What would you do next?

[Locozodiac]

Proyectile,

Call me a complete out of this world stubborn boater, what will state on my next assay is a new theme related to more complex engine/water situations.

It's not posted nowhere not even on the WWW. I'll go ahead saying that the whole argument with both valid height positions are to be blamed to 2 gentlemen called Thrust & Resistance, both keeps fighting back endlessly at each other at back transom. Have managed for both to join forces and now working in complete harmony.

Once you realize how they interact between them will have a better understanding point of view on my : better water performance is achieved under (WDP) water delector plate.

Monday will post at which lower tail height is water flow passing on the Yam example, you'll be surprrised to know that what's commonly stated on a dry installation does not work "well/optimum" on a wet wot trial.

Will start a new extensive well documented easy to follow thread called "Lower Leg >versus< Water Flow"

Stay tunned, happy weeek end.

Happy Boating

[Locozodiac]

This is the water trial result on the Yam 25 L at wot under flat calm water conditions. Water flow passes slightly above AV plate, hydrofoil barely is doing it's homework as needs more water height for prop to grip better and plane faster that rib. Engine loses best prop thrust and ventilates badly on choppy waters and close turns at wot and speed.

On straight courses works aceptable, that rib is an old heavy model with a strange hull design, is transom rated for a 40 HP engine. Being way under powered with just a 25 HP, engine needs its full top prop thrust to work more efficiently.

In order for that rib/engine to work as owner expects to do so, transom must be chopped down at least 4 cm and test again to check if at Sweet Transom Height Spot. Owner understands the tech issue involved and clearly agree to chop transom down.

Happy Boating

[SIBRider]

Chopping transom is major surgery. Start with removing hydrofoil. Why have this on console boat. Go for sea trial. What's the RPM, speed, and prop pitch. Why not start with something less irreversible such as finding correct prop. What's the boat weight?

[Locozodiac]

Sibrider,

Gee, with all respect forum members deserves, what have you been smoking lately, is someone advising you or it's your own tech thoughts on the matter. What you have said has no theory or practice ryhme or reason at all.

As you are expecting an answer, must confirm that planes excatly at same lower unit level stated height with or without hydrofoil, hydros are useless. Will be removed to fan and freshen my staff menbers, it's still heated down here, el Niño issues.

We happen to count with a mayor heart surgeon, he can raise or chop down transoms on anything that floats, except bathtubs.

BTW, assay is ready, check once complete posting is over. I'm sure will enlighten your inflatable/engine setup and boating skills.

Happy Boating

[Projectile]

I gotta say that the idea of trying the boat with a different prop seems much less dramatic than chopping the transom.

Furthermore, the boat only ventilates in choppy seas or hard turns, so maybe a different prop would help under these limited conditions while it performs well otherwise. From what I've read, more cup, more rake, stainless steel, and 4 blades rather than 3 all may help to reduce ventilation.

From what I've read, a different pitch has not been mentioned to reduce ventilation.

[Locozodiac]

Simple answer : would you rather invest $ 150-200 on a new prop which will not work no matter what you've read or go for a 1.5 inch transom chop down that only costs no more than $ 80.00 and put you straight at the sweet tansom spot in 4 hours max.

That's the average time it will take to remove engine, remote cables, chop transom down, level and replenish with fiberglass what's been cut down, let dry, install back engine and steering, remote cables.

With the money you'll be left over by not buying a new prop, you can buy lots of beer cases and go boating all day long. We raise and chop transoms down on permanent basis.

Before you play with props, pitches you must know the wot rpm numbers achieved with that combo, and that's with an induction tach and lightly loaded rib.

Happy Boating

[SIBRider]

Originally Posted by Locozodiac

Simple answer : would you rather invest $ 150-200 on a new prop which will not work no matter what you've read or go for a 1.5 inch transom chop down that only costs no more than $ 80.00 and put you straight at the sweet tansom spot in 4 hours max.

That's the average time it will take to remove engine, remote cables, chop transom down, level and replenish with fiberglass what's been cut down, let dry, install back engine and steering, remote cables.

With the money you'll be left over by not buying a new prop, you can buy lots of beer cases and go boating all day long. We raise and chop transoms down on permanent basis.

Before you play with props, pitches you must know the wot rpm numbers achieved with that combo, and that's with an induction tach and lightly loaded rib.

Happy Boating

What makes you so certain that playing with pitch would not help?
I was suspecting budget reasons for reluctance with prop matching: expensive parts and cheap labor. I would suggest trying that none the less.

USA is very much reversed. Props are cheap at $60-70 for small alum. 4 hours in shop will set one back $300-400 easily.

[Max...]

Originally Posted by Max...

Yep, cheers, will probably leave it as it is with height as per pic and standard prop - it picks up really well and 18 kts is fine really.

Not had my tiny tach on it yet to check WOT RPM but will soon.

Finally got round to doing some runs with the tacho on.

Mariner 15 with standard factory prop.

Best trim position was 2 and achieved 5750 rpm max at WOT. Touched 19 kts lightly loaded with myself 12 stone and young son around 5 stone. 19 kts was brief with behind us, average WOT more like 18 ish.

[SIBRider]

Sounds great. Actually fact that you are able to reach practically max WOT RPM (5200-5800?) tells you got some spare capacity in you motor/boat combo. Especially if that's your typical load. I believe mariner 15 horsepower is rated at 5500 RMP. Horsepower starts dropping when engine goes past that. You can up a pitch by 1 inch or so to go faster if you want to then.

[Max...]

I thought max on that engine is 5500 to 6000 so spot on at 5750 and no more to give. AFAIK??

[SIBRider]

Guess we need to be more specific. I looked up this one http://www.mercury-marine.eu/cache/D...cumentFull.pdf