The Cosine Wherry - A Problem Solved?

[OtakiriLad]

I am probably going to horrify the purists but so what.

I have been happily adding nicely tapered strips to my wherry and have begun to notice a problem. While I thought I had faired all the forms using strips stapled on, taken off, frames shaved here and there on a continuing theme for too many days I have now found that when a number of strips are added there seem to be bulges and/or hollows when looking at the hull from a low angle from fore and aft. The more strips I added the worse it seemd to look. I didn't know whether I would have to remove all the strips and adjust the offending frames or what.

So instead of making such a radical move I though bugger it, unscrewed the bits and pieces holding frames 1 and 13 (the offending ones) to the strongback and with a few lusty blows from a lump hammer detached the frames from the hot-melt glue attaching them to the hull. Hey presto the hull popped into line and now has what I view as a fair sweep and looks great. I have reinstalled frame 13 at about 75 degrees to the strongback and this seems to fit much better.

The question for those with all the answers - how do I avoid this in the future (on the assumption there is a future boat)? What is the best method of fairing the frames without knowing the real run of the boards?

Cheers
Dave

[Daddles]

Sounds like the sort of problem I run into :( Ever noticed how everything fits perfectly on the dry fit but changes size and shape as soon as you reach for the glue:confused: The nearest thing to a solution I have is a handy collection of general purpose swear words :rolleyes:

Always remember that you are building a boat. This is not a nuclear reactor or a Formula One engine. It's a wooden boat. It will aspire to following the plans and, just to keep you amused, will actually do so on occasion. The aim is to build fair and sound. How you achieve that grasshopper, is up to you.

Richard

[Boatmik]

I am probably going to horrify the purists but so what.

The question for those with all the answers - how do I avoid this in the future (on the assumption there is a future boat)? What is the best method of fairing the frames without knowing the real run of the boards?

Cheers
Dave
A good purist will always let their eyes be the guide - rather than the measurements.

As far as the your question goes, you don't really need to know the run of the planks beforehand - you can check on the set up moulds.

Traditionally boats are lofted in 2D anyway - so you only end up examining the curved shapes seen from exactly 90degrees on top and exactly from 90degrees on the the side. Any batten on the final 3D moulds is smart enough to know how to add up the two lots of bend required.

The strange shapes are more likely to be from some sort of error creeping in.

Some questions - they are based on traditional mould setups - the cosine wherry might be different - but they will indicate bits that you may have missed out

1/ are you planking from the sheer (deck edge) or from the keel? From the sheer is most common and that area is relatively easy to get fair. It also provides a stable working point for when you get to the bilge area and the strips start twisting - the stable part gives your eyes a reference to see if things are going smoothly or not..

2/ Speaking of battens - did the plans ask you to run a batten along the set up moulds to pick up mistakes - it has touch all the frames. From memory the cosine plans come full size on paper - which can move significantly with humidity (1 1/2 inches over 6ft is not uncommon) - so even "perfect" plans can end up a little different - running a batten over the surface is highly effective in making sure the planking will be happy.

3/ Did the plan ask you to bevel the edges of the chipboard moulds ie run a batten over the surface and plane or belt sand an angle on their edges so the batten sits flat (if it did and you didn't then it would give exactly the problem that you mentioned). Tip - paint the edges black before you bevel - then you will be able to make sure the controlling face remains undisturbed

4/ Did you recheck the spacing of the chipboard moulds making sure that the chipboard was on the correct side of its measurement point - with traditional boatbuilding the moulds usually end up with their back face lined up with the measurement mark toward the bow. And the opposite toward the stern. The changeover point is usually the widest point of the boat. Getting this wrong is likely to cause the opposite problem.

Best Regards

Michael Storer

[Boatmik]

I am probably going to horrify the purists but so what.

The question for those with all the answers - how do I avoid this in the future (on the assumption there is a future boat)? What is the best method of fairing the frames without knowing the real run of the boards?

Cheers
Dave

A good purist will always let their eyes be the guide - rather than the measurements.

All the answers <grin>!?!?

As far as the your question goes, you don't really need to know the run of the planks beforehand - you can check on the set up moulds.

Traditionally boats are lofted in 2D anyway - so you only end up examining the curved shapes seen from exactly 90degrees on top and exactly from 90degrees on the the side. Any batten on the final 3D moulds is smart enough to know how to add up the two lots of bend required.

The strange shapes are more likely to be from some sort of error creeping in at some point - not necessarily your doing either.

Some questions - they are based on traditional mould setups - the cosine wherry might be different - but they will indicate bits that you may have missed out

1/ are you planking from the sheer (deck edge) or from the keel?

2/ Did the plan ask you to bevel the edges of the chipboard moulds ie run a batten over the surface and plane or belt sand an angle on their edges so the batten sits flat (if it did and you didn't then it would give exactly the problem that you mentioned)

3/ Speaking of battens - did the plans ask you to run a batten along the set up moulds to pick up mistakes - it has touch all the frames. From memory the cosine plans come full size on paper - which can move significantly with humidity (1 1/2 inches over 6ft is not uncommon) - so even "perfect" plans can end up a little different - running a batten over the surface is highly effective in making sure the planking will be happy.

4/ Did you recheck the spacing of the chipboard moulds making sure that the chipboard was on the correct side of its measurement point - with traditional boatbuilding the moulds usually end up with their back face lined up with the measurement mark toward the bow. And the opposite toward the stern. The changeover point is usually the widest point of the boat. Getting this wrong is likely to cause the opposite problem.

Best Regards

Michael Storer

BTW - just after writing this I did a search on Google -
cosine wherry planking problem
and found
http://www.amazon.ca/gp/product/0917436024

[OtakiriLad]

Thanks Michael

In answer to your questions:

1. Yes I am planking from the sheer to the keel and it was well into the planking that I noticed things starting to go astray.

2. Yes it did discuss bevelling the frame edge and I have done this.

3. I did run battens along all the frames and I think this is where I went wrong. As this is my first boat I didn't properly understand the what was really meant by getting it to follow the frames in a gentle curve. I think I should have been more aware of the consequences of not being an octopus with enough arms to hold everything in place. Stapling probably didn't give a good enough result. In addition the plans were so creased I had to iron them flat and this may have led to further errors though as all adjacent frames are on the same sheet I would have thought any error would have been in all frames and not mattered too much.

4. Yes I checked and re-checked the location of the frames and their correct positioning fore and aft of the maximum beam location and all that is OK.

All in all I think my problem comes down to my fairing ability and I feel any future attempts will be all the better for this learning exercise. Besides everything is peachy after my tender application of the adjuster to the frames.

Thanks for the help.

Dave

[Boatmik]

From the link in my post above it sounds like the problem at the stern was an error in the plan.

Best Regards

Michael