Forums: Whisstock Designs: reduce length of design 055? New Topic Post Reply
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sante
Posted: May 20th, 2013 at 09:53   |   Subject: reduce length of design 055?
Hi George,

due to possibly limited building space I have another question.
Can I reduce the total length of the boat a little bit, by just leaving less space between the frames?
And if possible, what is the maximal amount of length reduction such that the influence on sailing performance remains justifiable?

I have another question concerning ply. For the planks a thickness of 9 mm is specified. Unfortunately, this thickness is not easy to find here in germany. Can I use 8 mm ply instead? Or do you suggest rather to go for 10 mm ply?

thanks and regards
Sante

debenriver
Posted: May 21st, 2013 at 18:35   |   Subject: reduce length of design 055?
Hi Sante

You can reduce the frame spacing a bit and the boat should stay fair. You could probably reduce the overall length by about 200mm at maximum. You should probably reduce the spacing between the last frame and the transom by about the same amount as the other frame spaces. I'm not quite sure about the stem – I think I would be inclined to leave that as it is – set the frames up and see how fair a batten runs to the stem blocks in the jig and then make any adjustments if they are needed.

The only thing is that it doesn't do the basic characteristics any good - specifically, for a 200mm reduction, the longitudinal centre of buoyancy (LCB) moves to about 49% of the new DWL measured from the forward perpendicular (where the dwl crosses the forefoot) and it should be nearer to 52%. A reduction in the %LCB increases resistance and can also alter the trim of the boat, so that she may tend to trim down by the stern – that will depend on how the structure weight moves with the closer spaced frames.

Having said that, a lot of the characteristics in an open boat like this are controlled by where the crew sit.

Another approach would be to make the transom more upright. You could lose up to 200mm this way relatively easily and it wouldn't affect the characteristics of the hull very much at all. This would be my favourite approach. I can draw you a new transom shape if you wish – just tell me how much you would like to upright it by. We would probably pivot the transom about the current tip position, so the aft deadwood would get a bit longer. The rudder geometry would also change, but that is about all – everything else would remain as drawn.

Either 8mm or 10mm ply will be fine. 10mm will make a slightly more robust but heavier hull (probably about 5kg heavier) but the thicker hull will displace a little more, so the LWL will probably stay about the same place. Same goes for 8mm – the boat will be a little lighter but will displace a little less so the LWL will stay about the same! I think I would probably go for 10mm, unless I was trying to keep the weight to an absolute minimum.

Hope this helps.

Cheers - George
sante
Posted: May 22nd, 2013 at 13:11   |   Subject: 073 as a first study project
Hi George,

thank you for your reply.
A reduction by 200mm is not enough to solve my building space problem. In this case I prefer to let her at the original length and try to find a bigger building place.
As this may take a little while, I am considering if I start with the smaller Design 073 as a first "study" project, which I can carry out in my normal size garage. Does this make sense? Can you give an estimation on building time for Design 073?

Regards
Sante
debenriver
Posted: May 26th, 2013 at 12:51   |   Subject: 073 as a first study project
Hi Sante

Sorry for the delay in replying.

Most builders seem to take about 6 months to build No. 073. I would estimate in absolute hours, working at the project fairly full time, with a reasonable home workshop, it would take around 300 - 350 hours.

Just to put this into context, when I started work at my family boatyard, we reckoned to build a 10' clinker dinghy in two weeks or less, with a boatbuilder and apprentice (a "man and a boy"). We had a pattern for the transom, a couple of moulds, a stem pattern and a rudimentary jig. We worked a 45 hour week at that time, so that would be about 180 hours total. One mustn't forget that the boatbuilder would have been very experienced and that everything was to hand. He would also hope to get an experienced apprentice, rather than a new one! Apprenticeships were at that time, 5 years, so an apprentice three or so years into an apprenticeship was becoming really useful.

And in the context of your previous question – we did indeed vary the size a bit from one dinghy to the next by adjusting the mould spacing!

I think that with Design 055, it would indeed be better to expand your workspace rather than reduce the length of the boat – although lengthening her a little is actually beneficial ...


Cheers -- George