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Old 25 March 2016, 09:27   #1
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Transom Height

After a lot of research with conflicting stories I have a formed opinion but want to offer it as a "straw man"

So, the exam question is if a transom is 23" from bracket height to stern, what length shaft is correct?

I think: the transom is not vertical, the outboard bracket and then the shaft adds distance from the transom. The cav plate is ideally 1" above or below the keel line, or on it. So a 20" long shaft, trimmed right in will be, probably as much as 2" above the keel measured at the cav plate? As you trim out the radius means the cav plate gets lower, i.e it moves to 1" above keel, and then zero and so on before the arc means it starts to rise again?

Am I right to think this?

An xl shaft at 25" while it might feel right as trimmed in will be near perfect, will actually cause drag and maybe produce a rooster tail as you trim out?

After all my research the only think I'm sure about is if it has wheels or a transom there are many opinions and versions of what's right 😄

In an ideal world you would try both shaft lengths and learn from the practical. That's not going to happen this needs a leap of faith so please help

Thanks


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Old 26 March 2016, 12:59   #2
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Simple measurement
Dependant on craft the maker will assist In advising how high to raise The egg whisk and recommend offset best suited,,
after all they have built and fitted various hp lumps to there boats over the years
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Old 26 March 2016, 13:13   #3
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Cheers Ed At 23" but I think measured down the angle and not at 90 degrees to ground, (does that matter?), I'm hoping that a long shaft will work? .5 inch ain't much said the actress to the bishop 😄
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Old 26 March 2016, 13:14   #4
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I've mailed the maker and left messages so hopefully by Tuesday things may be clearer
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Old 26 March 2016, 14:01   #5
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I think I'd be pretty sure it's a long shaft as you often see engines set higher (to gain more speed by reducing drag) bur rarely seen them sat lower
As your only 1/2 inch above the upper length for long shaft fairly safe bet it's just long and not x long
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Old 26 March 2016, 14:52   #6
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Thanks pal I'm going over the edge lol I think the same 👍
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Old 27 March 2016, 06:25   #7
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If you don't have one fitted already a water pressure gauge will give you essential info on water pick up
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Old 27 March 2016, 07:54   #8
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Not heard of that before but sounds useful. Like this? http://m.ebay.com/itm/231874369369
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Old 27 March 2016, 08:28   #9
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Yes that's it, most marine chandlers will have them or can get them, we T- Join into the tell tale inside the cowling and then run the water tube to the gauge. Some modern engines will have digital dials that will have water pressure built in, my old optimax had block bar pressure on the smartcraft gauges.

When I had the optimax I would run with water pressure showing on the tachometer and engine temperature on the speedo + a manual water pressure gauge, I cooked the power head a few years ago running without the smartcraft gauges trawling through Pwllheli marina, I must have sucked up a bag etc, an expensive lesson learned !
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Old 28 March 2016, 03:30   #10
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I dunno if I fully understand the question.

This picture from evinrude shows how to measure transom(straight I believe) And the merc manual shows how to measure outboard (straight also)Outboard height pic requests - Boatmad.com
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Old 28 March 2016, 06:00   #11
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So the way to measure is clear but when the transom isn't in front of you to measure???

I think a lot of folk who do measure, measure the actual transom and not the vertical. But transoms are supposed to be pretty standard re lengths now. So if the vertical is 20" and therefore aimed at a long shaft how long would the actual be? The angle of the transom would have an impact here, but that seems to be fairly standard at 15 degrees I think
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Old 28 March 2016, 06:27   #12
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20 + 15% = 23 ?

The maths seem to add up to your actual transom length
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