steel st pierre dory

on 8/15 Ganjiro inquired about an aluminum st pierre dory. he got no replies. so i'm not too optamistic about any replies about the feasability of a steel st pierre dory for coastal cruising, maybe 1/8" sides and 3/16" bottom, flat bar stringers, small diesel inboard, some sail for stabilityand get home downwind safety. i know enough about boats to make me dangerous. help. any thoughts are greatly appreciated.

 

Precious little info on St Pierres for whatever reason, at least on the net.
the 27 footer is a light boat for its waterline length. at about 21 ft. Steel is, however, a reasonable choice (aluminum is even better, but costly).
Calculating the weight, 1/8" would be about (guessing) 4.5 lbs per square ft, wood (say, 3/4") at less than half that. It would be fair to say, then, that steel will double the hull weight.
Aluminum might come in about the same as wood.
The steel hull would only be about 600-700 lbs heavier than wood or aluminum, I think. This is based on just the hull being steel. Decks and so forth would be wood.
I doodled around with the idea of a steel bottomed dory--- a real thick (1/2") bottom, with about a foot of sides welded around the perimeter. The bottom would have to be machine rolled to get the rocker, of course. That's okay, because it makes for a nice self-righting design--- the bottom would weigh about 800 lbs (the light displacement would maybe be 3000 lbs).
On to the skirted edge... this might be 1/4", welded at the chine inside and out to form a pan that you could bolt plywood to the outside of.
No framing inside the bottom, just gussets to side-bolt oak frames to.
Where St Pierres are so low-ceilinged, the bottom interior could just be epoxied and have thin removable cleated sole sections.
There are advantages to building this way. The primary rock-bashing member is ungodly thick steel. No bolting is required to attach the keel or skeg. There are few crevices to corrode. The garboard overlap (plywood over steel) and frame-to-gusset areas would require special protection/attention, but is very repairable in any case.
All of the centerboard trunk could be steel.

Best of all, the steel floor-pan makes a perfect rock-stable building form. You'll never see the hull weeping rust, as all of it is below the water. Drive the boat up onto a rock beach, and only the paint gets scraped.

Alan

 

Many thanks for the reply. It just so happens that today I was thumbing through Tom Colvins Steel Boatbuilding. He shows a section of a 33' St Pierre dory with a 15.3mm bottom (I think that's about 1/2" plate). He mentions John Gardners modified St. Pierre. Gardners boat is 27'. Your boat with the steel pan and plywood sides scares me but it makes sense. Lot to think about. Hope you don't mind if i get back to you after I sort thtough some of these ideas. I'm going to order plate within 2 months. Regards,Dave

 

Keep me posted, Dave. I am also working on plans for a St. Pierre (28 ft).
The steel pan and ply sides shouldn't be too scary. At some point, you must attach steel to plywood. If not at the chine, at the deck. I am not convinced the idea is viable at this point. I just haven't found anything wrong with it. Since it takes a few hundred lbs off the boat's weight and does away with steel topside frames (which are harder to attach innerds to), while providing ballast and grounding protection, I see the idea as an improvement over an all-steel hull.
The skirt might be more like 6" high rather than a foot. No different than an iron keel attached to a wooden boat, except the wood hull is continuous. In terms of corrosion, there's no difference. In terms of watertightness, there is a 6" overlap (though it might be better to have a two inch overlap and a filler plank below that).