Hull Shape Enquiry

This thread dried up. I appreciate the help so far. But I don't think we're there yet. To summarise the conditions I have given again, save you guys reading all the above stuff:

Boat only gets towed, no independent use.
Range of speed up to planing.
Low drag required at all speeds (i.e., not a lot of power available).
Total weight 250 lb including load, which can be moved to ootimise.
Towing boat wake will not be an issue.
Yawing can be controlled.

However I had a further idea after rereading the posts, inspired by Stumble's mention of a windsurfer. My though is, the closest approximation to the above conditions is a surfboard. It's not towed, but sliding downhill and getting a tow are not so much different. More or less flat bottom, smooth rounded entry, skeg at back. Where could I get lines for a surfboard, so I could adapt them to a larger shape with internal space? Does that sound too crazy? The only thing that worries me is the effect of wave mechanics, or rather the lack of. I'm not sure how much surfboard performance relies on what the water is doing below the surface.

 

Hi kayaker,

A surfboard or windsurfer would seem to be an interesting starting point.

The difficulty comes, I think, in getting one to carry any kind of load. If you just scale one up to the desired displacement, you'd end up with something insanely long and wide, with almost no draught- it'd pound like crazy. But maybe, if one were to take a flat-bottom double-ender and give it a windsurfer underside and skeg, keeping the hullsides? Might that work?

 

Dory

I have towed a small dory sort of boat that seemed to tow fine, although I never loaded it with more than a small motor and some fuel. Probably a 100lbs load in a 100lbs 10' boat. It was designed as a power dory, so the bottom and the run aft was almost flat. The bow wave was noisy unless you loaded it enough forward to keep the "V" bow in the water. It had a very small skeg aft and tracked well. Bruce

 

As has been previously mentioned, with the very wide range of S/L ratios expected, you're not going to find a one shape fits all hull form. A form that has quite low resistance at low S/L figures will squat badly at higher and a hull form that is efficient at high S/L numbers will be a tail dragging pig at lower numbers.

 

I'm not so sure, Par. A surfboard has to be paddled by the surfer's arms so it must be half-way efficient at low speed and it certainly planes well enough. It would make a lousy boat, it would not track. It would also have poor stability at low speed so CoG would have to kept low until speed built up, because it's so narrow. It would not be very efficient if its bow were pressed too deep in the water but I suspect surfers put their weight well back until they have enough speed to stand, and then move forward.

Some of the newer designs are rather extreme and special purpose but the older, bigger boards have a lot of buouyancy and are reputed to plane esily in smaller waves suggesting less drag at low speeds. Since I will not have to paddle it with my arms I could make it wider, and since I am not overly concerned with manouverability it could have less rocker than is usual to plane earlier. A single fin around 40 sq in should suffice to minimise yawing.

Any surfers out there?

 

There's a big difference between "half-way efficient at low speed" and your original wish for "Low drag required at all speeds (i.e., not a lot of power available)"

A surf board isn't especially efficient below S/L's of say 1.5. It's light weight permits it's paddling ability. You're correct they do plane quite well, but in reality they're actually controlling a fall, much like a drogue anchor does. As a surfer "catches" a wave, they get "over the hump", then the controlled fall begins as they literally slide down the face of the wave. The interaction between the board, gravity's desire to drag it into the trough and water pressure on the relatively flat aft portions of the board, limit the amount of fall possible. This permits the board to skid down the face of the wave. Having personally "fallen off" many waves, I can attest it's truly a fall, not propulsion from a wave shoving you and board forward.

In other words, I think a surfboard is a bad analogy, certainly not a very suitable hull form for the range of S/L you desire.

If you had a target maximum S/L or mother craft speed (so we can calculate the tow vessel max S/L), we could work around a reasonable coefficient for this figure, with the penalty for having to high a Cp, at lower S/L's being much less detrimental to the overall performance envelope.

It would seem length to be the limiting factor in regard to a towed craft. Ideally a long lean plan form with high initial stability would be called for, but I'm not sure what you're really after. If it's a dinghy for a yacht, then you'll likely have other limitations concerning practicality for that particular size and arrangement of yacht.

I still think you should look over the lines of the older tenders.



This is one of mine and patterned after them. There are several examples of this dink in operation. It tows quite well, it rows very well, can pack a large load, doesn't wander around and can be built very light. Now if you attempt to drag it behind a power cruiser running an inlet at 30 knots, you'll likely have a fancy anchor by the time you're through. As a tender, servicing a yacht that's capable of 9 knots, she's running at a S/L of 3.07, which is about as fast as you'd want to drag her. Frankly, her run should be firmed up more for sustained 3.0 S/L operation, but her lower S/L efficiency would suffer. At 3.0 S/L she's getting pretty unstable, but can be managed with trim. Much faster and she'll barrel roll eventually, with her tucked up stern quarters. Unfortunately, it's those very stern quarters that permit efficient low S/L performance.

 

Towed dinghys

Keeping to the same theme, two dinghys out of Bolger's "Boats with an open mind" - Auray Punt, and a little larger- Car Topper, both come very close to your requirements. (ignore the sail rigs). The Auray Punt is an up-dated version of a well proven towed tender. Bolger's comments, as usual, are informative and well worth reviewing. Bruce

 

Par: I see what you mean; I was thinking of a boat bow on the front of a surfboard but the stern is going to want to bury itself in the water. The slope at the face of the wave effectively predetermines the drag which is higher than I want.

For this to work I probably need to start with a planing dinghy hullshape and modify the stem to eliminate tendency to yaw. that would cause sailing performance to deteriorate somewhat.

p.s. don't you ever sleep?

 

I have a great deal of difficulty generating a "normal" sleep routine. My body seems to run on a different internal clock, based around a 32 hour day, instead of 24.

Instead of looking at a planning hull form, I'd look at a good low S/L performer that had it's aft bilge turn close to the LWL. This way as she squats with speed increases, these quarters would offer some bearing, which could also be trimmed in. Now you have, pretty much what is shown above, a craft that can manage low to 3.0 S/L with reasonably efficiency across the table. Directional stability can be handled easily enough with appendage(s).

 

After rereading this I started to think of some of the problems I have had towing in the past.

The one thing that keeps comeing to mind is the amount of water that always seems to come over the bow and winds up dragging the stern down. This problem is makeing me think that a pram bow, with its higher freeboard in the bow may help reduce this problem.

I would also look at installing a self bailer like 420's use. It may increase initial drag some, but it should also keep the boat relatively water free while underway.

 

I have sailed the East coast of England for a good few years now. Most of this time it has been with a 9ft pram dinghy dancing in whatever yacht we had at the time's wake. My father found a design in Edwin Monks book Small boatbuilding published 1946 The Design was called Stubby and is a pram with a narrow bow transom. My old man built several variations only differing in their freeboard aft which in the original design was rather too low and constuction for which we substituted plywood. The last of these dinks i have had for 20 years and she is the best tender i have ever had, she rows well, carries a good load and tows real good, planing after the yacht like a good un. She has nearly climbed into the cockpit a few times but has always missed us!