42 foot steel round bilge sailboat scantlings

I am the new owner of a 1956 steel ketch I am starting the restoration on.
The hull suffered quite a bit of galvanic corrosion in the form of pit corrosion which have turned into localized holes in several dozen places. One of the zincs is basically brand new, even though it was replaced at the same time as the one on the other side of the hull. I believe this zinc is not making electrical contact with the hull, and so the hull was under-zinced for a period of time.

I am looking for steel scantling reference so I can determine when to replace plating, or when to weld up the holes in "good plate". I have a survey from 20 years ago that claims the hull plating was done in 3/16"(4.7mm). I have a racing survey document from the Koninklike Verbonden Nederlandsche Watersport Vereenigingen from around when the boat was built that claims the plating is 3mm.

Ribs are 40x40 T shaped 4mm thick at 600mm spacing.

I am going to ultrasound survey, as well as drillbit survey the hull and want to know what my replacement minimum thicknesses would be. These minimum numbers are obviously different depending on the original hull thicknesses. So I figured I would try and get some understandings from you all on typical scantlings for this size of vessel as I have seen a range.

I am not sure if the hull is corten. Later model from 1968 was advertised as being built in corten, but I don't know how to test this without sending off a sample to a lab (has anyone done this?).

 

Welcome to the Forum Asa.
It sounds like you did not have a survey carried out on the vessel before you bought her?
Can you supply some more information about your 42' ketch please, and ideally post some photos of her for reference?
Including some photos showing some typical corrosion pits / holes?
And some exterior / interior photos - including the bilges.
What do the bilges look like generally - are they very clean and well painted, or rusty and corroded, or somewhere in between?
Steel boats usually rust from the inside out, and you do need to pay particular attention to the bilges.
And what is the condition generally of the 40 x 40 x 4 'T' section bent frames? Are they rusty, or clean and well painted?
The 'several dozen' localized holes in the shell plating worries me - that is a lot. Can you roughly gauge what the steel thickness is in way of the holes, prior to drilling / ultrasound testing?
Re the KVNWV survey claiming a hull thickness when built of 3 mm, I would be a bit skeptical about this. Possibly for the deck (but thin plate there is a disadvantage re flexing when people walk on it), and I would have thought that the topsides would be at least 4 mm, and the bottom at least 4.7 or 5 or even 6 mm, and the keel itself a fair bit thicker, never mind the relatively close frame spacing of 600 mm.

 

Hello and thank you for the response Bajansailor.
I did not have a survey as this was a "nearly free" project boat that was floating when I got her.
Overall interior condition including the bilges is very clean without visible rust weeping etc. The interior was removed 8 years ago and inside was primed and painted in preparation for an interior build that didn't happen. I'll be inspecting this further this weekend and can take more photos of the pitting / holes as well as measure the thickness of the plates in the areas of the holes.

Here are a couple of initial photos.
more hull hole pics in a few days.

 

Unfortunately there are no "typical" scantlings. For such small boats plate thickness depends on the compromise between ease of construction and weight. Sometimes the customers fears also play a role, especially with amateur construction.
You have an all transverse framed boat, probably professionally buildt judging by the fairness and T section frames, it wouldn't surprise me if it was 3mm. This thickness also plays well with a racing survey and dutch build.

The ultrasound survey will give you a definitive answer, you take multiple readings and average the results.

 

Thanks for the info Rumars.
Yes, from what I understand the boat was professionally built in the J.H. Groos Koninklijke Vorbonden, Holland yard. The designer was Henk Tingen, who was the chief Naval Architect for Royal Huisman at the time.

I'll measure a bunch and get some averages. Thanks.

 

Excellent! We eagerly await updates.

 

"Koninklijke Vorbonden" refers to the "Koninlijke Verbonden Nederlandsche Watersportvereenigingen" wich was the dutch watersport federation and not a yard. Tingen was also "chief designer" at Scheepswerf Kok in Vinkeveen, where most of his designs of the time were built, many for export to the USA.
It would actually be helpful to share what info you have on your boat, for example if we identify the model this can possibly help you get original drawings.

Most of this boats were built with plywood or wooden decks, I think yours are steel?

 

Do you still have a link for this vessel please, so that we can compare the two?

And I will second Rumars' request above - please provide as much information about the boat as possible.

 

Rumars, I stand corrected on the particulars, I am trying to gather together as much info as possible from dutch and english sources. Here is a page with many Tingen designs: Damenvlet Amisia type CB - Zeiljachten https://www-damenvlet-nl.translate.goog/en/zeiljachten?_x_tr_sl=auto&_x_tr_tl=en&_x_tr_hl=en&_x_tr_pto=wapp

The decks are steel.

Attached here is the sales PDF for LUCY, which is a slightly longer yawl from 1968 which claims corten.

I do have quite a few original drawings the previous owner received from a Dutch Naval Museum.

The model is "Orca 12" or "Cruisemate" in the US market I believe.

 

Attached is a copy of the racing handicap / survey report.

 

Here are some drawings I have.

 

Well, the certificate does say the steel deck is original, so I am inclined to believe them when they say it's 3mm overall.
What are you planing for insulation?

 

I was thinking to do blown in polyiso insulation up to the thickness of the frames(above the waterline). Sikaflex wooden battens behind the frames first so I have something for the interior to screw to. (I don't want to put holes in the frames).

Thats based on some books and youtubers. I haven't dove deep on that yet though. All thoughts are welcome!

 

Re your photos, the hull is very fair - they did a nice job when they built her all those years ago.
Round bilge steel construction requires a lot more skill generally when compared to the more simple multi-chine method.

And it does sound like your hull could well be 3 mm thick - a round bilge hull will (all else being equal) be stiffer / stronger than a chined hull built with flat plates wrapped around the frames, as the curvature of the hull increases the stiffness.
However this does not leave much of a margin for corrosion over the years.
I think that everything will depend now on your ultrasound survey and general assessment of the areas where there are holes in the hull.
If you have to re-plate the whole hull (or even a lot of it), it will probably be cheaper / easier in the long run to simply build a new hull from scratch.

 

Well I am not going to be building a whole new hull from scratch any time soon. I am going to resurrect this one and sail the high seas with her.
Ultrasound generally says 3mm thickness!
There are quite a few localized bad areas that I’ll have to chop out, and quite a few holes I will need to fill, but generally the plates are still 3mm.

I will be removing the concrete / iron ballast as well and putting lead down in the keel for peace of mind. The previous owner was concerned about the ballast situation and I want to remove that worry. Attached are some photos, including the largest hole I have found.