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Old 27-01-2022, 11:24   #1
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Prop Strut Design

Hi all: first time poster, long-time lurker.

Question about the design of prop struts that support propeller shafts.

Why do most of them consist of a single vertical strut that runs straight down from the hull? It seems that this renders the prop shaft vulnerable to sideways impacts which could bend the strut. I've seen a couple of examples of this over the years (

Would it not make sense for them to be a double strut? (as per image below). The double strut has two angled pieces and would be much stiffer when resisting a sideways load.

What's the engineering reason for preferring a single strut vs double?

Thanks all!

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Old 27-01-2022, 11:42   #2
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Re: Prop Strut Design

Welcome to the forum, Bluejay,

Probably drag first and cost second. Lateral impacts don't come up on the forum; its usually the leading edge of the keel that takes the blow, often damaging the hull or the keel bolts.
Never let anything mechanical know that you are in a hurry.
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Old 28-01-2022, 05:18   #3
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Re: Prop Strut Design

Greetings and welcome aboard the CF, Bluejay.
What tkeithlu said.
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Old 28-01-2022, 08:43   #4
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Re: Prop Strut Design

How good is good?
On sail boats side impacts are almost impossible.
On power boats more likely from grounding.
Is stronger better? Somewhere there has be some give in a system to prevent catastrophic failure.
Most bent ones I have seen come from dangling docking lines or other lines getting entangled on the prop.
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Old 28-01-2022, 08:50   #5
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Re: Prop Strut Design

On power boats, you see both designs depending on the boat. The V struts are stronger, but especially at higher speeds, they add more drag and disturb the water flow to the prop more.
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Old 28-01-2022, 09:15   #6
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Re: Prop Strut Design

You have to look at the strut....the twin strut pictured above has flanges on each leg...undoubtably, hiding behind the flange will be some bolts.
The bolts go thru' the hull, the flange will bear on the hull bottom, and the bolts will be tightened with a nut inside the hull.

A single strut typically goes through the hull and is supported against sideways movements inside the hull by various methods depending on the manufacturer. When you see a single strut there is likely as much lenght inside the hull as what appears outside the hull.

A single strut is typically quite substantial, it would take some force to mover or bend it.

My guess that the V-strut above, is likely for a power boat. The legs are not very long.
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