Stretching a Steel Hull

[Capt Nemo]

Hi Folks

This is a question for the engineers out there.

I am thrashing around the idea of stretching my 40 ft. steel hulled trawler by 6 to 8 feet at the stern, while it is layed up over the winter.

We own a welding/fabricating shop up here in northern Ontario, so the project would not be an issue as far as the build is concerned.

My boat has twin Perkins T6-354's turning 24 x 20 inch wheels.

My main concern is : How will this extension affect the steering of my boat and will it affect any other mannerizms in an adverse way?


The purpose of this addition would be mainly for more lazarette storage.

Please feel free to offer your thoughts and feelings. I'd love to read them all.

Thank you

Capt Nemo

[mrohr]

Yes! ,Of course. BTW:I'm not an engineer or a N. A..
Best to contact the original naval A. ,that way you can get the same answer from someone that has a few letters after their name.

[skipmac]

So you would add length on the stern of the boat but leave the engine(s), shaft(s), prop(s) and rudder(s) in the original location? I would think this might have a negative impact on docking and maneuverability.

Disclaimer, I'm not a naval architect either. I think this is a question that might be best addressed to a professional.

[Blue Stocking]

Some years ago, maybe 25, Derecktor-Gunnell, in Dania Fl, added 8ft to a 68 ft Burger,IIRC, to carry 2 Harleys.
The extension was built, floated to her stern, bow was depressed to reduce transom draft, and piece was welded on.
No changes to running gear.
Manager of the yard was one of my boatyard instructors.

[S/V Alchemy]

Aside from the trim and handling issues (you are merely adding stern overhang and not moving the shaft or the rudder, correct?), it's certainly possible.

It's going to affect a lot of things, however, and may not be worth the effort.

But I agree that adding 20% to the LOA should be passed by a naval architect. Adding that weight and length may compromise the existing structural integrity.

[Mark1977]

This is done all the time around here to fishing boats. Most are fiberglass. I ran one last year that was 45 foot with 5 feet added on. It made the boat better going head to the waves.(didn't pound like it use to) No other problems with the handling that i found.

[sabray]

My guess no effect except that the ass end will be more lift when powering. If you store real heavy stuff in the back you'll do we'll in a short following sea.

[ewsponberg]

A lot depends on the hull geometry as it is now, and I could not give any specific advice unless I had a set of lines available. And that's the first thing to consider--the new hull lines. Done properly, the new extension will be adding more weight to the boat, and you want to counter that with more buoyancy to support that weight. Add too much buoyancy, and the new configuration will cause the boat to trim more down by the bow. Add not enough buoyancy, and the hull will trim more down by the stern. If the boat trims adversely now, such as too much trim by the stern, then you can design the new extension to correct that. The weight of the structure and the buoyancy of its shape all have to be considered, designed, and engineered together so that the trim comes out right in the end.

The new extension is also going to affect the stability of the boat because of the new submerged volume and weight change. All of that has to be tracked as the changes are made. It is most prudent to measure the drafts of the boat at the bow, stern, and amidships before you take the boat out of the water so that you can better judge and decide about changes as you go along. Presumably, you do have hull lines. If you don't, that makes doing a smart and proper job all that much more difficult.

As for steering, if you are not moving the rudder, then you do run the risk that steering capability is going to diminish because the submerged hull behind the rudder is causing extra drag, and so steering authority is going to diminish. The correction for that might be a larger rudder, but that impacts the size of the rudder shaft, the rudder tube and bearings, and the size of the steering gear. All that has to be taken into account.

6' to 8' on a 40' hull is a 15% to 20% increase in length--that's a lot, and so the advice above about seeking professional help is well given. Do so, because it will be easy to make mistakes if not thought through properly.

Good luck.

Eric

[gjordan]

Years ago I worked on an 84 foot steel motor yacht that had onboard a set of planes to extend the stern by 12 feet. The job was never done, but the plans were drawn up by the yard that had originally built the boat. The rudder and shafts were not moved. Recently I was doing research on a 45 foot steel sailboat that I was considering, and discovered that many of that class of boat had been extended by 5 feet. The Question is not if it can be done, it is only if it can be done correctly on your boat. Good Luck.____Grant.

[Capt Nemo]

Hi Folks

Thank you all for your input and wisdom concerning my question on stretching a steel hull.

The engines, propellers, rudders, and shafts would stay in their current locations,
and the extension would be from the existing transum aftward.

Another question for the naval architects and anyone else who would like to chime in.

Would adding 6' or 8' x 12 feet in width to the stern of my 40 foot steel trawler create enough buoyancy to effectivley support an additional weight of say, around 1500 lbs in extra cargo in this new addition (lazarette)?

Thanks again for your input.
Capt Nemo