Nahant dory lumber to plywood conversion

[Vineet]

Hello all,
I have a question about the Nahant Dory from Gardner's dory book. If the Nahant were to be planked out of plywood instead of dim. lumber, would any changes in the stem curvature need to considered in order to account for the lack of compound curvature of the plywood or can the garboard and broad strake be "tortured" adequately to the curve of the stem as it is?

 

[wet feet]

I don't think you need have any concerns,after all if you tried to force compound curvature into a solid boar you would probably cause it to crack.History tells us that dories were built without the curvature causing problems.In fact you may build a boat that is actually tougher if you use plywood.

 

[Vineet]

I don't think you need have any concerns,after all if you tried to force compound curvature into a solid boar you would probably cause it to crack.History tells us that dories were built without the curvature causing problems.In fact you may build a boat that is actually tougher if you use plywood.

And as far as plank/bottom thickness I was thinking 9mm for lapstrake planking and 12mm for the bottom panel. What do you think would work best, permanent battens or temp. battens with filleted seems?

 

[DCockey]

What do you think would work best, permanent battens or temp. battens with filleted seems

Why would you need battens, either temporary or permanent?

 

[Vineet]

Why would you need battens, either temporary or permanent?

If the laps provide enough strength between planks then great! wouldn't battens at least help keep planks fair during the construction aid the planing? If experience says no battens are needed for hull strength or ease of planking then I would gladly ditch the idea. I just know that battens have helped plane the edge of planks with a block attached to a plane running along the batten beneath the plank being planed.

 

[gonzo]

Making that boat a lot lighter is not a good idea. The waterline beam will be significantly reduced and so will its initial stability.

 

[Vineet]

Making that boat a lot lighter is not a good idea. The waterline beam will be significantly reduced and so will its initial stability.

That's what I was thinking which is why I thought 9mm planking and 12mm bottom, glassed inside and out would be better than many newer dory designs which only have 6mm ply throughout (CLC Nor'easter Dory is an example) I would also consider live ballast when solo rowing, puttering about. I wanted a classic rower that I can load up with camping gear to explore my home waters around Whidbey Island in the Puget sound and also for casual rowing. I haven't calculated the weight difference yet but what does your experience tell you about my assumption on the planking/bottom panel?

 

[TANSL]

The waterline beam will be significantly reduced and so will its initial stability.

That is very true, a beginner can see it but, since the immersed volume also decreases, one cannot know if the metacenter increases or decreases its height or by how much.

 

[gonzo]

The original bottom was 22mm. It appears to have oak stem and I assume framing. These were relatively heavy boats for hard work. I am not saying the hull won't be structurally sound for your use, rather that it will be too light and tippy.

 

[Blueknarr]

And as far as plank/bottom thickness I was thinking 9mm for lapstrake planking and 12mm for the bottom panel. What do you think would work best, permanent battens or temp. battens with filleted seems?

Putting fg cloth on lapstrake is nearly impossible.

 

[wet feet]

I suspect that getting too involved in reasoning about reduction of hull weight and stability might be a bit of a red herring.No doubt the hull itself will float a bit higher.On the other hand,the average human in the days when the original was built was very likely both shorter and lighter than the present day counterpart.Said occupant may well raise the C of G a bit but his extra weight will have sunk the boat a bit further.

 

[gonzo]

Even if the total weight was the same, the center of gravity would be higher. Therefore, there will be less stability. Further, if their height was less, it would put the CG lower too.

 

[wet feet]

We seem to agree.

 

[TANSL]

What does "less stabilty" mean?. How do you quantify stability?.

 

[wet feet]

In practical small boat terms,if you go swimming too often,you haven't got enough of it.In this instance of material substitution,if the boat sits on it's lines correctly the stability won't have changed too drastically but the possible increased height of the C of G may alter the roll period.