# Transom measurement question

Discussion in 'Boat Design' started by Bigtalljv, Jun 1, 2022.

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### BigtalljvSenior Member

Hi,

I’m building my transom and had a few questions about measurements. I’m planning on twin 25”outboards and I have some measurements from another person who did the same thing. It looks like He had the transom 25-1/2” tall on the engine centerlines but is t specific about the angle. Is that measured along the surface of the angled transom (the hypotenuse) or in a vertical orientation? A poorly drawn picture is probably worth a dozen word with my abilities. So should it be measure A or B?
Thanks
Jason

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### fallguySenior Member

B=25
Angle between A&B = 12 degrees?
B is hypotenuse, A is adjacent
Cosine 12 degrees = 0.98 = B/A
A = B/0.98=25.51

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### BigtalljvSenior Member

Thanks fallguy!

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### IkeSenior Member

Just a quick question or two. Is this a powerboat? Is it less than 20 feet long measured on the centerline? Monohull outboard boat less than 20 feet have rules for hp, and one of those involves transom height, and for this purpose the transom height is measure vertically from the top of the transom cutout where the motor(s) are mounted to the bottom of the hull at the center line of the transom. If it's 20 feet or more , or a multihull, you can ignore this.

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### baeckmoHydrodynamics

The theoretical rig dimension is "B", but in practice when dealing with double installation you must consider the bottom rise angle as well. In some cases, the "inside" edge of the antiventilation plate must be located in line with the bottom to avoid propeller ventilation. The engine CL will then be slightly lower than the nominal 25".

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### BigtalljvSenior Member

It’s a monohull powerboat, 27’ long, and is planned on having twin outboards. Hopefully its correct because the core is now glued in. I’m sure there is some fudge factor there, no holes yet so I have a little adjustment there.

jason

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### gonzoSenior Member

Outboards have adjustment holes, so you can move them up and down. There is a limit to it. As was mentioned previously, the measurement is to the bottom of the boat at the location of the motor and not to the keel.

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### fallguySenior Member

OP did say engine centerline on his original post.

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### BigtalljvSenior Member

Thanks all. To confirm I am measuring at the centerlines of the OB locations.

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