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Old 13 May 2013, 16:44   #1
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Town: Lima-Peru
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Matching a long shaht engine to a medium size transom.

After sibbing for years starting with a OMC 305 sib and ending with a Sea Rider 420 sib, decided it was time to pass onto a shallow deep V keel rib, a perfect hull to boat in the calm South Pacific.

2 years ago imported a 420 rib which had it’s transom chopped down to match my existing 2 strokes Tohatsu 18 HP (S) engine. After a precise transom height tuning to perfection, performed wonderfully in all water conditions. Went for a new 430 rib, tech info stated a long shaft use, so bought a new L size same brand HP to avoid chopping down transom again as with previous 420. As I much enjoy tiller driving at rear transom as a Venice gondolier singing along included, decided to keep the 430 sib completely open. Looks so huge spacious as a carrier deck.

Installed the new horse, engine height at sight apparently seemed well matched, using pier crane and slings rib was placed into sea. After 5 minutes warm up time gave juice to throttle and my water bummer issues began in terms of small water splashes out transom and excessive same splashes at to throttle. Mission impossible attempting full wot operation, a real splash exaggeration, you could literally take a shower there...

Gee, what’s going on, total disapointment, a deep instant investigation was mandatory, sat on middle deck next to transom and at 1/3 throttle pulled my head over. The hidden problem inmediately showed up, water flow at speed was hitting the middle leg round portion swirling it’s way up and hitting swivel’s lower mount bracket and going out transom and luckily not over transom. The more engine acceleration the worst the shower splash became.

The only way to a have achieved a perfect engine/transom heigh match without placing wooden shims would have been to make a proper height SS engine mount and placed it on middle transom. This was not the way to go as intendeed to use my short shaft 18 HP as well and not grounding it forever and ever.

To be continued...
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Old 13 May 2013, 17:26   #2
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After some quick mental evaluation of the tech problem and how to solve it real quick & cheap decided to make a exact shorter replica of the lower deflector plate mounted on top of anticav plate. With 2 part epoxic resin made same shaped deflector replica including a holding base, let it dry for 48 hours.

The small def plate was attached to middle leg with several wrapps of 3M lectrical tape which holds very well, what height ? rolled the dice and went for a 7 cm clearance with respect to lower def plate and then go under trial & error from there lowering or raising def plate to stop water swiling up tail or at least minimizing water splash out transom.

Went for a 1/2 throttle spin, surprise, no splash watsoever, same, same, full wot same. Bingo and the Lotto, the ideal leg height was achieved in the first attempt. Now water splash out transom is deflected on top of water flow. Engine has gained 300 + revs runing at 41 Km/H at 5,780 revs out of it’s max 5,800 factory stated for that engine. Not bad numbers at all as this size rib is intended for a 30 HP minimum horse.

As have said before in many ocassions engine/transom heights stated on different engine brands owners manuals are just a refference from where to start. But impecable engine/transom sib/rib performance can only be achieved under trial, error, bers & patience.

Happy Boating
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Old 13 May 2013, 17:33   #3
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Good work I've seen some other mods in the past, check out post 13 here:

Tohatsu 9.8 2 stroke long shaft
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Old 13 May 2013, 17:34   #4
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Quote:
Originally Posted by Locozodiac View Post
After some quick mental evaluation of the tech problem and how to solve it real quick & cheap decided to make a exact shorter replica of the lower deflector plate mounted on top of anticav plate. With 2 part epoxic resin made same shaped deflector replica including a holding base, let it dry for 48 hours.

The small def plate was attached to middle leg with several wrapps of 3M lectrical tape which holds very well, what height ? rolled the dice and went for a 7 cm clearance with respect to lower def plate and then go under trial & error from there lowering or raising def plate to stop water swiling up tail or at least minimizing water splash out transom.

Went for a 1/2 throttle spin, surprise, no splash watsoever, same, same, full wot same. Bingo and the Lotto, the ideal leg height was achieved in the first attempt. Now water splash out transom is deflected on top of water flow. Engine has gained 300 + revs runing at 41 Km/H at 5,780 revs out of its max 5,800 factory stated for that engine. Not bad numbers at all as this size rib is intended for a 30 HP minimum horse.

As have said before in many ocassions engine/transom heights stated on different engine brands owners manuals are just a refference from where to start. But impecable engine/transom sib/rib performance can only be achieved under trial, error, bers & patience.

Happy Boating
That engine is waaaay to low when you look at the pictures surely it affects the performance when compared to a short shaft...
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Old 13 May 2013, 17:37   #5
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That engine is waaaay to low when you look at the pictures surely it affects the performance when compared to a short shaft...
Yep, it does look way to low
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Old 13 May 2013, 17:38   #6
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Loco you bad boy posting photos showing no one holding the tiller and kill cord not attached!
Now give yourself a good talking to
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Old 13 May 2013, 17:50   #7
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Quote:
Originally Posted by whisper View Post
Good work I've seen some other mods in the past, check out post 13 here:

Tohatsu 9.8 2 stroke long shaft
Some clarifications not mentioned intentionally on posts.

If you look closely the 9.8 long shaft engine has a double deflector plate, my 18 long shaft has just one. Boater is only experimenting splashes at displacement speeds, on mine was constant at any speed. Same example won't work on mine. Tohatsu's are real natural water splashers if not transom height seated correctly.

When placing my short shaft engine on same 18" transom gives aireation at close turns and at choppy seas, although has best top speed on straight water courses due to anticav plate being nearly exposed. You could not use at all a SS15 on a 20" transom, prop will be exposed...

Don't confuse a 15 short engine sitting on 20" transom and vicerse. It's not the same sitting a SS engine on a 18" transom. Much worst and anoying is over transom splashes than out transom splashes.

Having days ago that horrible UK rib accident, I know kill cord use is mandatory, but pics doesn't show the extra cord extension to my wrist, besides the nut that controls the load on engine's swivel bracket is adjusted tight to give tiller a straight cousre on flat calm waters.

Happy Boating
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