Ready for design review

[Tony_C]

I'd love some feedback on my first ever design. (Shameless crowdsourcing here.)

It'll be lapstrake ply, used for both sailing and fishing with a little outboard.

Over here you'll find pdfs of the linesplan, hydrostatics and sailplan:
And the Delftship file for those who have it.
http://cid-5492f908e7d2782c.office.live.com/browse.aspx/Public
(Hope that works)

Disclaimer - first ever ever amateur design.
And I know I'd be better off buying stock plans, but I'm a risk taker and the design process is just too fun!

Tony

 

[Tony_C]

After adding some rake to the transom as advised there's an update:

 

[PAR]

That's a lot of transom to drag around. Tuck in the waterlines aft for a better exit and steering qualities.

 

[viking north]

Nice computer work, wish I had that talent. Agree with Par on the stern, as is she's a skimmer under power, she'll get up on a plane fairley easy, However under sail you lose that ability and she'll be a bubble generator. Good example is observe alot of dingys being rowed. ---Geo.

 

[tom28571]

As noted, she will be better under power than sail but there are other boats on the water that are worse off. I see too much beam aft for a sail also. As a start, I'd want the transom beam of the second chine to drop from what looks like about 95% to about 65% of midships beam of the same chine. Pull in the rest of the aft sections to correspond. That should make transom drag when heeled under sail more respectable and also help with steering under the same conditions. These comments are only a guess for average displacement speed winds and with a big powerful sail plan, she will likely be fast as is with enough hiking power. Good job.

 

[peterAustralia]

I had a look at your lines, and the first thing I did was go to the Dory Book by John Gardener and look at the semi- dory.

There is a 12ft model, a 14ft model and also there is a 12ft power skiff. All tend to have the bottom chine narrowing inwards as you get close to the transom.

Now you may use your boat with a good engine, get good speed and on smooth water, this way your wide stern which is optimised for planing can be used to good effect. If you use google images, and put in semi dory lines you can find some lines that may be good for inspiration

here is one that puts a lie to my idea that the stern should be narrowed, but maybe this is more a power boat (the one at the bottom)
http://www.oldwharf.com/possibilities/blog.pl

another thing to consider, as when the boat is sailing, when it heels, the stern will big in and create extra drag near the chine, this extra buoyancy will tend to push the bow down. So maybe your lines are more optimised towards a power boat than sailing-motoring boat

 

[viking north]

Tom- my comliments on suggested corrective details on your post, not only a reply but a reply with meat. Wish I had that ability also but alas i'm not a designer but a builder with the gut feeling of experience--and Toni ditto again on the work-- Geo

 

[Tony_C]

Thanks so much for your excellent feedback with principles to help me understand. I think I'll digest and update the design in the morning. Tony

 

[Alik]

Nice work; I disagree on comments about transom - sich a small boat needs enough volume.

 

[viking north]

Alik at 3900cm.(17 plus ft,) thats larger by 3 ft LoL than most of the car toppers we use here for lake and bay fishing with a 25hp. outboard, so she's really at the upper end of her class both in dingy and day sailers with a cutty cabin. Her lines are very typical to our 14 to 16 ft. aluminium runabouts being just about full beam from midships to transom although this one does have some rise and narrowing in her underwater stern section. I think the drift from the post is if one wants to improve the sailing qualities it would be benificial to tuck her in a bit. It depends on how important the sailing performance is to the owner. As a rough rule for smaller craft, I usually compare sailing to rowing, if a hull operates efficiently under oars then that will usually be the case under sail as far as dragging water behind her is concerned. Under oars this one will have some of the same although not as bad characteristics as our typical alum. runabout . As always is the case
between power and sail it's the game of trade offs---Geo.

A yacht is not defined by the vessel but by the care and love of her owner

 

[lewisboats]

I think you need to check your math...3900 cm is 3.9 meters or 12ft 8in (ish). I do agree though that the lines aft could do with a bit of surgery but it is really a good effort. My question is do you want the boat to plane under power? If so then leave it as is and bear the consequences when sailing or rowing...or if those are more important then tuck up the tail and narrow it some and your performance will increase under wind and oar but it won't plane worth a darn.

 

[PAR]

As a powerboat, she trim out with her nose in the air, likely uncomfortably so. As a row boat, she's got too much beam and as a sailboat she has more bearing are aft then she needs. Dead vertical leading edge appendages are great, until you run over stuff, as they'll tend to wrap around and stay attached, so generally some aft rake is necessary the shed sea weed and six pack rings.

Pick one Tony. By this I mean you can have an easy to row hull form, that hasn't the bearing area to stand against a press of sail, or you can have a sailboat that's a bit fat for rowing or you can have a powerboat that gets up and scoots nice, but attempting to have 2 or three from the same design, just means major concessions from one or two of it's duty demands.

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

 

[cthippo]

I think you need to check your math...3900 cm is 3.9 meters or 12ft 8in (ish).

3900 CM is 39 meters or 128 feet.

100 CM=1M
1000mm=1M

 

[tom28571]

Obviously its 3900mm = 12' 8" LOA and about 4' beam. Its not the most slender craft but it is not fat either. Many one design racers of mid teen length have similar dimensions. In fact, looking at some dimensions, the waterline beam is less than most so the wetted area is not high, except for the stern. That is why the thought of pulling in the stern beam would make for a better all round sail boat. While there is much more to designing a boat than just lines, I feel this is a far better first effort than most offer here. To me, it shows more than just ability to run a software program which is also refreshing to see.

Carry on, Tony

 

[Tony_C]

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

Its not intended for rowing at all though I see that good sailing form will help the rowing ability anyway.

I started out with a strong view that planing under power was important. But having priced outboards I think the budget only extends to a 4hp. I doubt it would be be planing anyway with that.
Having said that I'd like to retain some planing ability in case I upgrade the outboard in the future.

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