Ready for design review

[viking north]

lewisboats-- Yup made a bo bo there, bloody calculator-- (now I know why I didn't make money building boats and no it wasn't me who fueled up that passinger jet for the non stop flight to Vancouver. It ran out of fuel over Manitoba and had to make an emergency landing on an abondand military air strip due to a mix up between metric and imperial fuel measuring.(I seem to recal something about NASA doing that also) Being old school I think in imperial(standard).
Ok lets see wher I went wrong--(3900mm div. by 10 = 390cm.) ( 390cm. div. by 2.54 cm. to the in. = 153.5in) (153.5 div. by 12 = 12.79in) ( 12in. x .79 =9.48in. rounding off to 10in.) So she's 12ft 10in. LOL Man don't know where the 17ft. came from. Getting old, brain is shrinking, there are days when I don't know which of my first two names is my name so what's a few feet in the sceme of things--she'll still be a better sailer with a bit of nip and tuck, not to say there's anything wrong with the design-good work all around but when one dares to ask for opinions on boat design possibilities expect responses ---- Geo.

 

[viking north]

Ok Tony whats going on, after all that math to get her LOL at 12ft. 10in. you now change it to 13ft. Geo.

 

[Tony_C]

I've updated the plan.

Pulled the transom into about 80% of the maximum beam.
While I see plenty of boats at about 65% as Tom suggests there enough precedents (only takes one )for me to think 80% might be about the sail/motor compromise I'm after.
I looked at the Dudley Dix Challenger 13 and Argie 15, John Welsford Truant and Oughtred Shearwater that have transoms at about 70-80% of beam.

Actually I think it looks heaps prettier than before which is actually the number one criteria of all

 

[tom28571]

Thanks so much for all your input.

Here's my thinking:

Firstly the metric conversions: LOA: now 3960mm = 13' Beam: 1600mm = 5' 3" WLB: 1412mm: 4' 7.5" (Is the WLB narrow compared to the beam? will that be a bit tippy? or is that what you call "reserve buoyancy"?) Displacement: 316 kg / 695 lbs

What about planing under sail - am I just dreaming? On those occasions when I'm pushing the sailing performance it will be just Mum and Dad as crew at 150kg / 330lbs so well under loaded and sitting high in the water - not sure what that does to its sail performance? Of couse it might be better to bring the kids for ballast

Now it is beginning to get a bit fat. Better before. For a good normal boat, don't stray too far from a L/B ratio of 3 and this is 4.

Haven't gotten into it, but that rig is both complicated and kind of big for this boat, especially if you are not an accomplished sailor. 10 sq m is a bit much and there is no need for a gaff and topsail complication, not to mention more cost. It is a small boat. Either a cat rig or simple sloop would make more sense in my opinion. A sail area with cat rig of 75 sq ft (~7 sq m) or sloop with 85 sq ft (~8 sq m) fits the boat better. Another and even simpler way to go might be an unstayed mast with a balanced lug of 7 sq m.

 

[Tony_C]

Tom

Thanks for the follow up.

The boat is the same beam as the first version - just looks fatter with the smaller transom.

I had to google l/b ratio :/ so I'm basing my calcs on this page:
http://dan.pfeiffer.net/boat/ratios.htm

L/B = 3960/1600 = 2.475
LWL/BWL = 3925/1412 = 2.78

Seem to me that small boats get necessarily fatter to be functional / stable. E.g. Oughtred's Auk is 7' 10" by 4' 1/2" for an l/b of 1.97!

My proportions came from many stock plans I looked at e.g Fisher, Oughtred, Welsford where 12' by 5' (l/b 2.4) seemed to be very common.

 

[PAR]

Yacht design understanding, not dramatically different design homogenization is what is necessary here.

 

[Alik]

Details of hull shape have secondary effect on performance. If You are not designing a racing yacht, look at other functional features as seakeeping, wetness, weight distribution (consider possible crew movement as well), etc. Say, for such a small boat volume of stern is essential for launching by trailer. Flare at stern is essential for wetness at quarter wave.

 

[Tony_C]

Par - thanks for the reminder - I am seeking an understanding as far as possible for my untrained brain - that's the fun part for me. But being an amateur I'm shamelessly using crowdsourcing of the the body of work out there as a starting point. Notice how I carefully avoided the word rip-off

Alik - now I understand your comments about volume. The narrowed transom above made a neglible difference to displacement on the design waterline. But trailer launching is certainly a requirement. Note that I intend the lightest possible boat - I've been reading/listening to online construction guidance from Michael Storer.

Tom - besides the question of drag you've got me worried with your mention of potential steering issues with the wide transom. I'm considering chine 2 in my plan as a 'hard chine' (particularly in the aft 2/3 of the boat) that might add some tracking ability when heeled. (E.g. Goat Island Skiff.) If that's true then the hard chine is fairly parallel with the skeg in the wide version. I think that's a good thing?

On the transom width:
Now I'm caught between the wide and narrower transom as in the pics above. Part of me wants to justify the wide transom under sail as follows:
1. With a wider transom the righting moment would be greater for a given heeling angle so it should sail flatter in the first place. Not sure how to measure the significance of this difference though.
2. If you're heeling signifcantly then I hope its because you're getting plenty of power in the sails and the extra transom drag is the least of your worries.
3. Conversely in light conditions you'll be sailing flat, avoiding heeling and the associated transom drag.
4. No sure about this one: if you're heeling under good winds the hard chine digging in a little might help control some of that nasty weather helm?

Also note that I'm planning three thwarts with side benches between the aft two. So plenty of options for moving people around to maintain balance.

Feel free to mercilessly point out the flaws in my thinking - I have no pride

Tony

 

[PAR]

If you want a sailboat, alter the lines to suit. Tuck in the aft water lines as seen on the top half. The entry is basically the same as yours, though getting that broad strake to twist 30+ degrees in the last foot of hull will test a builder's will. Personally, I'd make it a V bottom. You've gone to the trouble of making all these strakes, why not employ them to help flow. Even an arc bottom would be better then that single garboard.

A powerboat would be a different approach completely. Powerboats and sailboats settle into plane mode differently, which is why it's difficult to have both.

 

[Tony_C]

Thanks so much for the specific feedback Par. I can see how that will be close to a traditional round bilge. And that's exactly what I'll do if I can get to understand why performance wise.

Just playing devil's advocate for a fat transom a while longer I'd like to submit this design for comparison: http://www.javelins.org/
More importantly they kindly give open source lines plans here:
http://www.javelins.org/Technical/buildyourown/mcneillee_offsets.pdf

What if my aim was only to give the illusion of a traditional boat?
What would stop me from using those exact lines below the waterline and dropping a couple of lapped strakes above that for the illusion. In that case maybe all that's wrong with my boat is it needing about double the sail :O

Some other designs like the dix paper jet also seem from photos to have wide transom though I can't find a plan right now.

 

[bntii]

Your plans are reminding me of a Lawton tender:

I built one of these a couple of years back.

 

[tom28571]

Thanks so much for all your input.

Here's my thinking:

Firstly the metric conversions: LOA: now 3960mm = 13' Beam: 1600mm = 5' 3" WLB: 1412mm: 4' 7.5" (Is the WLB narrow compared to the beam? will that be a bit tippy? or is that what you call "reserve buoyancy"?) Displacement: 316 kg / 695 lbs

These are the beam numbers that I referred to for the "fat" comment. I omitted the decimal point and it should have been .3 and .4 ratios.

 

[Tony_C]

These are the beam numbers that I referred to for the "fat" comment. I omitted the decimal point and it should have been .3 and .4 ratios.

Ahh B/L
Hmmm it is fat isn't it. (Especially compared to what I consider desirable performance like the Goat Island Skiff:0.321 or Pheonix III:0.316)
Funny enough the only requirment on the hull design from my first mate is that it should look 'cute and fat' like an old dinghy. Which of course makes my desire for reasonable performance a contradiction. I guess I could change my requirement to 'having a decent size rig to look pretty' then the wide beam would make sense for stability and we'd just go slower :|

The only fudge I can think of is to have the sides bulging midships above the water line so that at least the waterline B/L ratio is better. Of course when heeled I'd have a fuller waterline on lee side whatever that does. And would that make it tippy considering that the intended side benches would partially be hanging in mid air? Or maybe that's good for putting the kids on the side benches to balance against heel.

 

[PAR]

I don't think she's especially fat, unless you want a rowboat. I do think the aft waterlines are too wide. I also think you're wasting valuable flow possibilities with the flat bottom on essentially a lapstrake hull form. You could harden up the bilge aft with a V bottom, which would help her sailing lines considerably as well as under power. The V bottom is only one more strake. The harder bilge would also help form stability for your crew.

 

[Aharon]

...passinger jet for the non stop flight to Vancouver... ran out of fuel over Manitoba and had to make an emergency landing on an abondand military air strip due to a mix up between metric and imperial fuel measuring.(I seem to recal something about NASA doing that also)

that was Apollo 13, the ill-fated 13...

Being old school I think in imperial(standard).

For me it's the other way around (been born and raised in a decimal country, Brazil), so inches, feet, yards are never automatic thinking. They are a headache.
I once read that, since the USA have a truckload of atom bombs, they don't give a shoot about what the decimal lovers think!

LOL Man don't know where the 17ft. came from. Getting old, brain is shrinking, there are days when I don't know which of my first two names is my name so what's a few feet in the sceme of things ---- Geo.

In my book, people who do not take themselves much seriously are the best to cultivate as friends. I run away from "truth owners"!
Now seriously, I joined the forum yesterday and I am grateful for the trove of knowledge you guys have and share. Thanks a lot.