Ready for design review

[tom28571]

Tony,

On the steering issue: When a boat with a very wide transom heels the hull shape in the water can change drastically. The axis of the hull changes from the centerline to a line from the leeward transom to the bow. The leeward immersed stern also lifts and causes the bow to go down which can introduce bow steering issues. This can get much more complicated and is not always bad but, on a first try, should probably be avoided.

Wide sterns are used on many boats but they are generally high performance ones with traps or racks and other means of keeping the boat flat. They may also accept some loss from the above problems in favor of high speed planing with only the aft part of the hull in the water. Not the cute little boat your wife wants or would even be caught out in.

Paul, The comment on fat refers to the second set of numbers Tony introduced which are considerably fatter than the first set.

 

[viking north]

Thanks Aharon, If ya ain't got humor ya just ain't got it, All the next generation of Canadians(my sons generation) also only know metric. The U.S. is slowely conforming to the world, most U.S. vehicles have both in their fasteners, all their medical is metric, and the state of Maine while not economically advanced is thinking advanced have the speed limits in standard and metric on I 95. -Geo.

 

[Aharon]

Viking, while I respect tradition, I believe it makes a lot more sense to have a standard that is easily divisible than the size of the thumb of a long dead king!
The French (oh, them darn despicable, double-faced, cowardly bunch! lol) got a bar of platinum treated with iridium, a combination that supposedly immune to physical changes under varying room temperatures, and made a convention: this is one meter (or one metre, which is the same size - oh, those darn high-nosed brits! hohoho!) , divided it in one hundred units, then each in 10 units, and voila. Le sisteme metrique...because we have 10 fingers.
The Apollo 13 flick (with Tom Hanks?) did not give away the blunder in terms of metric/ imperial mismatch, but rather presented it in a similar vein: how to couple a round tube to a square inlet (it had to do with the air renewal system, I guess). Obviously the rocket scientists resorted to high technology: silver tape!
[edit] Oh, and the time it takes my tired brain to add 50% plus 10% to convert miles per hour to kilometers, I have got another speed ticket already!

 

[viking north]

Ok back to the thread, metric or not there is alot of good info here in terms of hull design and dynamics and the right guys are on the job of fine tuning this hull for it's intended purpose. As usual it's a game of trade offs, and in the end the owner will be much more aware of the characteristics of his vessel. ---Geo.

 

[PAR]

Tom has hit on a point and one I've been attempting to drive home since the beginning of the thread. The transom beam, as Tom has mentioned can effect several things, but you really have to have a good feel of the hydrodynamics of it all or you could end up cutting and hacking at a freshly launched boat to fix stuff you needn't.

For example, that stern will necessitate a higher lead, then a more conventionally drawn set of waterlines. Her asymmetric heeled shaped will induce a larger turning moment and thus require a taller lead. The question is how much. Lets says she'll be a sloop of fairly conventional proportions, which places the lead in the 13% to 17% range. Well you're going to want to adopt a lead on the high side of this range, just to compensate for the aft waterline shape choices. It's these types of subtleties of yacht design that have caused all but a handful of designers to go prematurely bald and there's no substitute for the understanding of these subtleties.

 

[Tony_C]

Tom and Par

Those last posts are gold. I think the lights are going on. The fantasy physics in my head were about the exact opposite of reality.
I can see why this forum sometimes gets frustrated with amateurs like myself asking the same dumb questions and then not listening to the answers!

Goes back to Par's original comment that I didn't understand at the time:

In profile you have a sailboat, in plan you have a powerboat and the transom is clearly a powerboat. Pick one . . .

So I'm picking sailing first. The motor will be tiny just for putting around so no need for heaps of bouyancy at the transom and no point looking for planing area aft either.

Tonight I'll work on the lines again - bringing the aft in per PAR's suggestions and maybe lifting the aft lines vertically a tad too might help the exit (and look better).

Funny how looking pretty seems to me to be a good guide for have good hydrodynamics. Here's a nice rear that helps me understand this thread and won't get me in trouble with the first mate:
http://www.duckworksbbs.com/plans/jw/navigator/index.htm

 

[tom28571]

John Welsford can be depended on to design well thought out boats that will keep the crew safe and look good doing it. That example of the Navigator carries what we have been talking about in narrowing the transom a bit further than I would do it but John puts a lot of emphasis on the seaworthy factor. Maybe Navigator would not plane in other than a narrow range of conditions but that is just a quick observation on my part. I like enough aft bearing to allow planing in any boat that is otherwise capable of it. So here I am after asking for a narrower transom, defending a wider one with less tuck up aft. Balance of all design elements is everything.

 

[PAR]

You need to state your question much more precisely Shemar and more appropriately start a thread of your own, rather then change the focus of this one. Welcome to the forum.

I have to admit I'm a bit like Tom and want a preformance potential from small sailing craft, unless specifically designed as a trainer or wholesome cruiser. This means I'll desire some bearing area aft, a stiff bilge turn and a pretty flat run. The Navigator linked is the other end of the spectrum, with a well tuck in set of waterlines, the transom clear, steeper buttocks and a pretty clean exit. This is great in following seas, when sailing on her her, leaves the water relatively undisturbed, etc., but also limits it's ability to go much faster then 1.4 S/L. I'd rather commit suicide then be relegated to 5 MPH.

Tony, the key to developing what you want is knowing what you need and desire. You now know you can't have standing cabin headroom, in a shoal draft 18' sailboat and other oddities . That's life, so whittle down your needs and desires to the things you can have, in the size you want. The design you are looking for currently exists and Tom's good buddy Graham has designed (Tom did you build the first one?), which is the LapWing. This boat will offer the preformance you desire, the easy sailing your family may want and it looks good too. Tom has built a spectacular example and I'll let him try to sell you on the rest.

 

[Tony_C]

The lapwing is a beauty but a fair bit harder to build for me.
I'm hooked on the custom design process now so too late to look at stock designs I reckon. I don't mind if the convergant evolution of design means I re-invent the wheel. At least it will be my own wheel.

some bearing area aft, a stiff bilge turn and a pretty flat run.

With all those subjective adjectives around I've done my best to deliver these qualities. Happy to hear if you think I'm getting warmer.

I still didn't go to a vee bottom though. For weight and simplicity I'm intending no keel at all and no floor boards at all. So the flat bottom section will double as the floor.

 

[Tony_C]

And here's a 3D view. Upside down since I figure that's the view an experienced builder will be used to.

 

[tom28571]

Yes, the first Lapwing was designed from my brief by Graham. My wife Liz named the Lapwing as she names all our boat from birds. I guess you could ask why I did not design it? Well, Graham sometimes returns a favor this way and I was helping him build a boat at that time. I've been sailing for a long time but think he is the best small sailboat designer I know. Why would I not accept the offer and besides, my designing studies are directed more toward powerboats. I gained a very good boat and he got another design in his portfolio.

 

[tom28571]

Since we are all telling Tony how to design his boat, I have another item. I always put a keel with stainless rubstrip on the bottom of my boats other than pure racers. With the centerboard offset to one side of the keel or keelson, the 1' deep keel not only protects the hull bottom but also protects the centerboard slot and centerboard from damage or being jammed by gravel or other stuff when beaching. The small offset of the CB is no issue to performance in any way.

 

[Aharon]

Tony, the construction method will be stitch and glue?

 

[Tony_C]

I'll go with the CB offset beside the rub strip - excellent plan.
I'm planning lapstrake ply at this stage.

But first I think I need to visit the rig and balance discussion hinted at earlier:

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.

This choice is largely a personality thing I think. I'm a fast learner and easily bored so I'm looking for something that can grow with me. I don't plan to build another boat in only a year or two's time :O Also want the boat to ooze personality - I'll probably be dressing it up as a pirate ship or something just for fun.

The plan is to treat the tops'l as optional until I know what I'm doing. That gives 89 sq ft with main and jib.

Also this rig's mast fits in the boat according to me. Or I'll have to trim the plan down a bit so that it does for the intended storage in racks at the marina down the road (no where to park it at my place).

Bit worried I don't have enough lead though. Based on the hull shape, any suggestions for the lead in this specific case? Yes I've read all the thereads on how subjective an art this is - but I'm sure I have no idea where in that 12-17% range this boat should sit. Its currently sitting on about 13%.

Tony