The Dave Gerr Method

[Andrewc42]

Hi,

Does anyone know the factor of safety that Dave Gerr implemented into this " elements of boat strength" scantling determination book?

As I used his method but the dimensions I got were very large.

[Andrewc42]

The boat is a Mini transat made out of marine Plywood

[LyndonJ]

You should use a class society scantling rules. Elements is a pretty robust approach that'll be a bit heavy. Class rules will give you a bare minimum refined through observation and engineering. A lot of folks here seem to like GL. ISO slowly coming of age bit by bit.

[bbsboat]

if the boat less than 24m, the ISO starnd is best,because the the designe of boat is lighter

[Nojjan]

Hi, I suggest that you read the introduction in Gerr's book. He is not claiming to produce the lightest structures but rather healthy and strong dimensions. I do not have personal experience with any boat built with Gerr's dimensions but I do with many other hulls made with ISO or others and I get a feeling (no quantified values) that many boats of today are strong enough but maybe they would inspire more confidence if built with slightly heavier structures. Regards / N

[LyndonJ]

Yeah, racing boats are built to the mininums of course, and they really are the bre minimums refined in the class rules. I think Gerrs scantlings are on the heavy side for steel A quick look at the measurements he suggests are even on the heavy side for commercial boats.

[alanrockwood]

Another question about Gerr's scantling rules. Do they satisfy the scantling rules of one or more class societies? To put it another way, would they fail to satisfy any particular part of the rules of one or more class societies?

[terhohalme]

No, it is for boatbuilders who like to do their own scantlings. It can produce as strong boats as some rules of class societes, but is not equivalent.

[Nojjan]

I have seen some other strings debating this issue. If I remember correctly Gerr's rules produce stronger than required by ISO as an example. On the other hand you need to consider your purpose. I would do as follows: if the boat is to be sold then follow the ISO rules (or other officially recognized standard) if not I think Gerr's approach inspires confidence an solid construction. / N

[Willallison]

As others have noted, Gerr's rules are generally pretty conservative and the resulting scantlings will, in my experience, be heavier than those that would result from other rules - though not always by very much.
Some of the detail is not always as obvious as you might think it is, so an understanding of boat structures will be a big advantage. The biggest disadvantage of Gerr's scantlings IMHO is that they don't allow you to stray too far from the prescribed structural layout, whereas other rules can generally be satisfied by whatever arrangement you like as long as it satisfies the required "strength".

[MikeJohns]

Have a look at my comment here:

Massive Stem bars ..........Why

[alanrockwood]

I am no expert, but in reading Gerr's book and other experts it is my understanding that recommended scantlings for small steel boats are beefier than they need to be based on strength only. The reasons for beefier scantlings seem to be the following: 1) It's not easy to weld thin metal without distortion, and 2) thicker scantlings give some reserve against thinning due to rust.

Any thoughts?

[Alik]

Alan, corrosion allowance is often included into scantling methods directly, in rules of classification societies.

[alanrockwood]

Originally Posted by Alik

Alan, corrosion allowance is often included into scantling methods directly, in rules of classification societies.

Thanks for that comment. No contradiction from me on that point. It seems to support the idea that steel scantlings for small boats are beefier than required for strength alone in order to account for other effects, such as corrosion. (It might even apply to some extent to larger boats as well, but is probably more significant for smaller boats.)