Discussion in 'Boat Design' started by aaronhl, Jun 26, 2023.

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### aaronhlSenior Member

Is variable deadrise equal to "concave" design? Are they similar or different and why?

Seems to me lifting strakes are used during the "change" in deadrise. Usually the aft deadrise is the one to be determined variable...should the bow area with strakes and a concave shape in-between the strakes also be considered variable? Is aft variable deadrise also concave?

Is the reverse/flat chine also a form of concavity in the sense of a spoon pushing water away?

Am I making sense on a deep-V with high length to beam ratio

I think I will post some pictures

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### gonzoSenior Member

Deadrise and concavity are two independent aspects of the design. Variable deadrise means the angle changes along the length of the hull.
Lifting strakes, sometimes called spray rails, can be at whatever location they are needed. Sometimes there are full length, others are short.
The reverse or flat chine is a strake taken to the edge of the hull.

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### aaronhlSenior Member

You said along the length of the hull - Then wouldn't most deep-V hulls be variable deadrise because they have a high deadrise in the bow area and lower to over 20 degrees at the transom? Here is a picture showing what I thought of variable deadrise; and is this Concavity too?

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### baeckmoHydrodynamics

The local (as at a specific longitudinal position) bottom rise/shape can be straight, or with any kind of curvature; f.i. a concave (NOT "VARIABLE"!) shape as in your picture. It has nothing to do with the variation of bottom rise along the length of the hull. The local deadrise describes the transverse shape, and the longitudinal variation of deadrise is described in terms of "constant", "twist", "warp" etc. Generally, this is of interest for the main planing area; bow sections have their own "pecularities".

Last edited: Jun 26, 2023
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### aaronhlSenior Member

Is the picture convex of concave? You mention convex but I was thinking the transverse is concave in that picture....

I am trying to figure out a fair compromise in design for high top speed and stability / without concern for comfort in riding,

Aft half of the boat 22 degree dead rise, flat no curvature, would it be fair to say a concave here would slow the boat down in top speed? Should concave be in the bow section to "trap" air/ deflect water?

Can you talk about some of the "peculiarities" in the bow curvature/deadrise again with max top speed and handling as the main concerns...

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### baeckmoHydrodynamics

Darn.... you caught me, of course it is concave! Thanx, it is now corrected.
Generally, you try to avoid concave surfaces for several reasons, a moderate convexity is positive, both from a strength perspective and hydrodynamics and comfort. There are many reports on the accelerations of wedges, showing a pressure build-up as the wedge is slamming down. Actually, much of the knowledge was collected in the early 1930:ies when there was a lot of research on the hull shape for sea-planes. Take a look at the hull of a Saunders-Roe "Cutty Sark" from 1931; there you have an excellent high-speed hull even with todays standard.

I suggest you read a collection of papers by Daniel Savitsky, du Cane and Jo Koelbel for a start, before inventing the octagonal wheel.

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### aaronhlSenior Member

Interesting, I see the Cutty Sark design, looks similar to the offshore racing boat hulls I am trying to better understand
So if they have a 24 degree deadrise at the transom, being a variable deadrise hull, what do you estimate the approx deadrise is at the bow and mid ?

Heres a picture

Last edited: Jun 27, 2023
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### aaronhlSenior Member

My research says approx 50* bow deadrise?????

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### baeckmoHydrodynamics

Sounds about correct, if you refer to the fwd "tip" of the bottom panel; might be a couple more as well.

I suggest that you read the following papers, where you also get a list of useful references:

"Procedures for Hydrodynamic Evaluation of Planing Hulls in Smooth and Rough Water"; D. Savitsky + W. Brown; Marine Technology, vol13, no 4, oct 1976, pp 381-400.

"Re-Evaluation of the Planing Hull Form"; D. Savitsky + J. Gore; J. Hydronautics, vol 14, no 2, pp 34-47.

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### aaronhlSenior Member

Thanks for the information Wish I could find a site to "download" them

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