Wash Over Transom Reality or Myth.-

Sea Rider

Supreme Mariner
Joined
Sep 20, 2008
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Was intrigued after reading Capt?n Caution post that Duras large 380-420 Mtr Sibs were being built with long shaft transoms and long shaft OB's to avoid transom?s over water splashes when max rated HP OB?s are employed, while combo runs at full throttle and comes to an immediate sudden stop.

Being a kind of Myth Buster myself and never having tested this situation before went to try that out as have the means and nice water cond to test so, this is what was found.

Short shaft transom 380 and 430 Sibs with max HP transom rated for and running at max wot rpm range with short shaft OB?s while lightly loaded on flat calm no wind water cond and suddenly stopped resulted in 0 splash. Same 430 Sib with long shaft transom running with max HP transom rated and a long shaft OB resulted in equal 0 splash.

Situations to have in mind to understand : Doesn?t matter much which transom height along which OB size is used in any of both cases as there will be 0 water splashing over transom, ?provided that OB?s lower leg sits at the sweet transom height?. Testing was performed with combos running parallel to water level with maximized props while water flow at speed was passing right under upper water deflector plate. The middle wake formed is so darn flat that doesn?t form any rear wave at back of lower tail. When at a sudden stop, wot revs will be decreased gradually towards idle rpm, consequently the prop spinning less will decrease its rpm accordingly neutralizing whatever water shape if any tends to form there.

OTOH, same Sibs with long transoms employing long shaft OB?s sitting lower than the sweet transom/OB height did experience water splashes over transom. Will include short transom Sibs with short shaft OB?s as well.

Badly Seated OB.JPG

This is due to water flow banging against OB?s non edge, square portion located aft transom producing a wave like (yellow line) at back lower tail as in pic. If coming to a sudden stop wave will inevitably travel towards transom banging against it and producing water splash over it. Beside this, combo will be be pushed forward by wave. Imagine the huge wave produced when at full throttle..

So in reality it?s all about how well an OB?s lower leg is correctly height matched to transom independently if it?s a short transom Sib with short OB or a long transom Sib with long OB. I would not dare telling the customer that a long transom will have more benefits over short ones, as proven it?s not completely true, will depend entirely on which lower leg height side you see it from.

Happy Boating
 
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