Wild Hair

Tru, 6 kts may have been a little optimistic. I DO consistantly read of small boats exceeding their 'theoretical' hull speed by 30 to 50% in winds up around 16-20mph. There is much debate (at least in my own mind) over whether the 1.34 x sq rt DWL shouldn't be bumped up a bit. Even so, DWL is about 9.2 ft...Giving about 4.1 kts 'theoretical' hull speed. If I take liberties and add 30%, I get 5.4 kts. Close enough to say "around 6 kts" I think. I'm not real worried if I never see it tho.

Steve

 

I have been tossing around the idea a lot and have pretty much decided it is too complicated to mess with. The keel would have been two flaps of 1/2" thick steel plate tho, pretty tuff stuff. You would crank it up as you coasted in then hop out and beach it. I have been considering twin leeboards and simply beaching on the hull. There isn't that much Vee in it, so it wouldn't list too much when grounded. Will probably have ballast inboard, in the form of painted blocks of steel or cast iron. That way I can chuck 'em overboard if necessary. What do you think of a keel that is an inverted Y?

Steve

 

I was refering to the 9' LOA. Small boats that reach high speeds have hulls with straight buttock lines so they can plane. Your design has rounded, tucked in lines. The longer hull, with the same type of shape, may do 1.2 x sq.rt. WL.

 

Ah...good! Now I have even more incentive to build the thing. Gotta see how fast it'll go. Gonna have to beg, borrow or (yeowch) buy a GPS. I have been procrastinating on my building, mainly because I had a bad epoxy experience glassing the joints on the side panels. The epoxy didn't have enough hardener...so...Sticky Goop. I peeled the tape off and kinda threw everything aside in disgust. I haven't touched in a month. Today is my day off, so I'll try to get to it after I get some sleep.

Steve

 

Why would it need to plane?

Regards,
Leo.

 

When you exceed theretical hull speed by more than 40% or so, the exact limit is debatable, the hull is planing. That is, it is overcoming its bow wave and has a large amount of dynamic lift.

 

At quick glance you have several things restricting this little hull from speeds over a 1.3 speed length ratio (you'll need 1.5 speed/length to plane)

As has been pointed out, the run has too much sweep, the buttock angle to steep to effectively permit planning.

The entry is quite bluff, though the forefoot may produce some dynamic lift, it will carry a large wave train along with her, further limiting her ability to overcome the theoretical hull speed.

I'm not sure how much sail area you intend for this little craft, but it would seem you have a lot more lateral area in her appendage then would be necessary for craft of similar size and/or sail area. This excess wetted surface will do no good for the speed/length ratio you need.

The hull shape looks rather burdened, not at all conducive to going fast. This shape will carry a load, but not quickly.

There is a great amount of windage to the topsides structures, again not helpful in the go fast areas.

The "butterfly" appendage is clever, but restrictive. The struts will generate much more drag then you've estimated and the slot or gap between the appendage halves will also cause turbulence, all of which will need be dragged along with her, until enough power is provided to get the boat over 1.5.

A ballasted craft with such a short LWL designed to move at 6 knots is a very tall order to fill. Unballasted it's a tough nut to crack.

Weight, shape and resistance must be reduced dramatically or you'll be well pressed to do anything close to 4 knots.

Have a look at some of the successful small planning sailboat figures and note how far off your numbers are in comparison. Then try to shape your numbers (and hull) to get close enough to have a chance at increasing the speed/length ratio. Over canvassing isn't the answer, you'll just get knocked down when the wind strength is high enough to provide the power necessary to overcome this design's short falls.

 

OK...OK!!! Short, Fat and SLOW... Sounds like ME! ... so it should suit!
I'm still building it, even if it DOES sink!

BTW, the hull is a separate entity (as opposed to having the cabin attached) and has very good load carrying capacity as a tender/playing-around-sailor.

Steve

 

If it set down on some of the slimey, mucky bottoms here, a bunch would stick to it and hinder it from closing all the way, which would seem to double the surface area subject to some sort of drag, plus our muck isn't very streamlined anyway. Sam

 

Got a good cure going today, so I'll probably take it 3D tomorrow. I'm looking to get the bottom, sides and transom connected together (Stitch and glue). Actually, from that point you can build it as a dinghy or continue to add the cabin on. In dinghy form it can be used to sail or row, perhaps as a tender. It should easily handle 650 lbs gross weight. If the boat runs 110 or so, thats 540 lbs net load with a nice waterline. 770 lbs would be max gross weight you would want to run, with a bit of the transom dragging. I am hoping the cabin will be light, using 1/8" and 1/4" ply. Looking to come in at around 75 lbs or so. Cabin furniture will run another 35lbs for about 110lbs. anticipating the hull to run 60 lbs bare, this makes 170 lbs. Add 80 lbs of ballast somewhere for a dry weight of 250 lbs. With my 200 lbs this means I can stock around 200 lbs of supplies and 'stuff' and still have a decent waterline. Add a few gallons of water and other liquid consumables that will go over the side via the kindney pump, I can approach the 770 gross wt with enough supplies for a couple weeks at least.

Steve

 

Ya think I could add hydroplanes to lift the derrier end and get it to plane?

Facetiously
Steve
PS: it would also make a 3 point landing system for beaching.

 

Got sides and bottom together. Lotsa stitches! Chipped a bit in the front bottom...not from the stitches but hit it on something.


Steve

 

hey lewisboats

Never mind the experts - go build your boat and ENJOY shape looks like it WILL work one way or another, who cares as long as you enjoy it (and don't kill yourself in the process) But keep us posted we're all interested!