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Old 10 June 2014, 02:43   #31
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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:

ABC Sib-Rib Installation Guidelines.-

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.
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Old 10 June 2014, 03:21   #32
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Quote:
Originally Posted by Max... View Post
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:

ABC Sib-Rib Installation Guidelines.-

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?
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Old 10 June 2014, 11:55   #33
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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.
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Old 10 June 2014, 12:32   #34
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Originally Posted by Max... View Post
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.
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Old 10 June 2014, 13:09   #35
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I'll have to mount it and check...
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Old 10 June 2014, 17:28   #36
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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.
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Old 10 June 2014, 21:46   #37
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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
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Old 12 June 2014, 06:37   #38
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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.
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Old 12 June 2014, 11:39   #39
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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.
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Old 12 June 2014, 11:44   #40
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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.
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