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Old 17 June 2014, 16:16   #61
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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!

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Old 18 June 2014, 00:17   #62
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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?
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Old 20 June 2014, 01:25   #63
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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 90. 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...

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Old 20 June 2014, 18:17   #64
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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.
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Old 20 June 2014, 21:02   #65
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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.

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Old 20 June 2014, 23:58   #66
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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.
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Old 21 June 2014, 10:50   #67
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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!!

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Old 21 June 2014, 11:38   #68
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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?
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Old 21 June 2014, 12:05   #69
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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?
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Old 21 June 2014, 12:33   #70
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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
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