Dellenbaugh angle

[MikeJohns]

Originally Posted by Paul Kotzebue

Dellenbaugh angle is useful for comparing the sail carrying ability of similar boats. It has too many assumptions to accurately predict heel angle or to compare different types of boats. Comparing Dellenbaugh angles of a heavy displacement cruising boat and a light displacement racing boat would not be meaningful.

Paul

I was wondering why you think it's too compromised by assumptions to make it useful?

If the definition is actual heel from an equivalent 1lb/ft2 of defined sail area I would think it quite indicative. Surely the compromising assumptions are actually from the quick and dirty assesments of stability ( extrapolating low angle stability linearly ) then that's not being fair to Dellenbaugh

These days it's easy to match an indicative heeling moment with a righting moment very accurately .

It is a relative indicator of the power to carry the sail area the vessel boasts in it's SAD and as a quick rough and ready indicator I think it is hard to beat since SAD is close to useless on it's own.

[Paul Kotzebue]

Originally Posted by MikeJohns

Paul
These days it's easy to match an indicative heeling moment with a righting moment very accurately .

It is a relative indicator of the power to carry the sail area the vessel boasts in it's SAD and as a quick rough and ready indicator I think it is hard to beat since SAD is close to useless on it's own.

Mike,

You are right about being able to match an indicative heeling moment with a righting moment very accurately. I do think Dellenbaugh is a useful tool and I do use it in the preliminary design process. However, if you want to compare Dellenbaugh angle to older designs you need to use RM at 1 degree instead of the calculated righting moment at the Dellenbaugh angle. Dellenbaugh assumes sails are flat on centerline and wind pressure is directly from the side. Dellenbaugh makes no allowance for the reduction of heeling arm and projected sail area with heel angle. Because of these assumptions I would stick to comparing similar boats. Comparing the Dellenbaugh angles of a 30 ft sport boat and a 55 ft full keel heavy displacement cruiser would not be meaningful. Comparing a new design to a similar boat with known stability charicteristics is very useful.

I agree SAD is close to useless because published displacement figures are rarely accurate, and the load condition at the published displacement is never available. That will skew the SAD figures into unreliable territory.

[MikeJohns]

Originally Posted by Paul Kotzebue

.............. Dellenbaugh assumes sails are flat on centerline and wind pressure is directly from the side. ...

Does he really? I always considered this was just rudimentary way of considering the worst case heeling moment from the sails to windward, that it is derived by considering a flat plane doesn't necessarily mean that the approach is incorrect.

Originally Posted by Paul Kotzebue

............
Dellenbaugh makes no allowance for the reduction of heeling arm and projected sail area with heel angle. ...............

Although we read that into the dellenbaugh angle anyway. And it doesn't reduce that much, consider a delenbaugh angle of 10 degrees, this reduces the projected area by a factor of (1-cos(angle)) , so if it's 10 degrees you still have the same area for all practical purposes, If the DA is 30 degrees you don't need to do any correction to know its a flip flop.

I agree it is for comparing similar sized vessels. I think it can be a really good indicator of the power you (might) be able to get from the rig.

cheers
Mike

[Alik]

Dellenbaugh angle is just too simple to be correct. OK, it works as rough estimate.

In terms of RM is does not work either. It uses metacentirc stability formula that is not accurate at 20-30 deg heel, does not consider reduction of stability as function of boat speed.

If one needs to know real heel angle VPP is the way to go.

[Alik]

Originally Posted by MikeJohns

Paul
It is a relative indicator of the power to carry the sail area the vessel boasts in it's SAD and as a quick rough and ready indicator I think it is hard to beat since SAD is close to useless on it's own.

That is partly true because Dellenbaugh angle does not say anything about counterbalancing monments of crew and water ballast. Enclose a diagram defining types of sailboats depending on SA-DISPL and LWL-DISPL ratios, considering means of providing RM.

1 - heavy cruisers; 2 - racer-cruisers (crew on rail); 3 - water ballasted boats; 4 - dinghies (crew on wire); 5 - multihulls; 6 - ULDB's.

One can aslo note lines of achievable Froude numbers (displacement related) FnV.

[MikeJohns]

Originally Posted by Alik

Dellenbaugh angle is just too simple to be correct. OK, it works as rough estimate.

In terms of RM is does not work either. It uses metacentirc stability formula that is not accurate at 20-30 deg heel, does not consider reduction of stability as function of boat speed.

If one needs to know real heel angle VPP is the way to go.

But is the quick and dirty GM method actually Delebaughs intent or just a practical solution in pre-computer days that became a normal way of assessing it?

Surely now you we should always assess it from the static stability curve ?

But I'm curious does anyone have the original proposition by Dellenbaugh that this was all based on, I've no idea where or when this came into common usage.