Sea Sled Madness: It's Incurable

As you say, waterline changes. So it's meaningless unless it's LOA.

 

Right, that explains the confusion. Thanks @BlueBell . It's 6", or 15cm.

 

It is indeed 'rough'...a start i agree, but very rough.
Now you really need to draw it up properly to scale with as many items on a possible, showing how it would like - as built.

You need to break your weights down into proper sub groups too:
1. Structural
2. Mechanical
3. Electrical
4 Communications/control
5. Aux system
6 Outfitting
7 pyrotechnics (i.e flares etc)
8. fluids in system
Then add a healthy margin...in this case 10%.

Until you do this, it is guessing into the wind.

The more work you do now..the less later....
Lesson from Serenity!!!

 

One of many lessons learned. So here's the conundrum: this first iteration above assumes Anne driving, me annoying her, winter gear. If it's just her LCM moves aft ~6"/15cm. If there are four of us up front, it moves forward that much. If someone is delivering a load of shingles or plywood it moves aft three times that much. With such a range of LCM on a normal day what nominal location do I select, and how much does the position of small fixed components matter when their effect is only a few percent of the effect of someone getting or out of the boat?
Serenity is certainly an object lesson here, but it's also true that every single movable weight is as far aft as practical already. Knowing typical LCM in advance would have allowed me to accurately paint a boot top. I could have built structure forward lighter than structure aft, but how much lighter before it's too light for Serenity to actual serve her role? Total weight savings would be a little more than my own weight and that's not enough to notice.

 

That's where the correct selection of hull configuration comes in. If you select the hull shape before the LCM (which is a function of the arrangement of bits and pieces) is fixed, then you go backwards in the design spiral. To do it "forwards" you fiddle with the arrangements first, then check what influence the resulting LCM will have on the required shape (ie LCB, LCWlA, LCLat).

Here comes the difficulty with the "standard" SeaSled hull shape; its transverse area is continuously increasing from stem to transom; its LCB thus is comparably far aft. That made sense with the oldtimers; they were driven by heavy engines (far heavier per hp than modern engines) placed close to the transom, while the load (a couple of persons) is comparatively light and placed close to midships.

Now with reference to your comment about the Serenity above; the issues with her trim is simply that the hull lines are not suitable for the required position of the LCB; she has far too much buoyancy aft for the actual arrangement. This is one of the reasons that I use a shape with a practically constant V all the way from midships and aft. The other reason is that it gives a higher L/D ratio at optimum trim angle.

One example here, although a traditional monohedron: The planing hull of series 62 is one of the easiest driven hull shapes available (and over a wide speed range), and it is also relatively insensitive to loading, compared to most other "constant V"-hulls. The key to this is a LCB closer to midships than what you see on newer designs. This is achieved by narrowing of the chine beam going aft, which also results in less side-wetting resistance. Why then are "modern" hulls more heavily loaded aft, you might wonder? To some extent it's a question about arrangement (big heavy engines aft, high resistance met with high powers and cheap fuel), combined with taxation laws based on hull length. Good seakeeping boats.....naaah, good workboats with forgiving loading characteristics.....oh no!

For a small boat like the one we discuss here, I would start the design spiral with the dog-house aft, leaving the forward deck open for loads, while the crew sits aft, where the acceleration levels are the lowest and the comfort (well, sort of....) is the best. With a decent hull design, the hull is stable enough to allow the house to be pushed slightly to the port side, allowing side entrance and also long object to be carried alongside.

 

Interesting indeed @baeckmo ! The only other designer I know with more than one sea sled built said that regardless of how heavily loaded forward his boats were, they always got on the plane immediately. So he did as you suggest, and put the cabin at the back, and cargo forward. Is there an industry accepted site line over the bow to the water? I'm probably phrasing that badly... From the operator's eyeline down to the bow- if that's a -4° angle, and the boat is planing trimmed at +4°, the operator can't see the water at all, only the horizon. My feeling is -14° is desirable, but the industry may have worked that out a century ago.

 

I've had enough problems to know static weights need to maintain lcg where best suited for the hull. Dynamic weights need to be loaded near to the needed lcg. You can always move loads, but not fixed structures...a few bundles of shingles here and a few bundles there on each side of the cabin is what you'll end up doing anyway in a smaller craft. On my fishing/camping/netting adventures, in a small open skiff, we still have to load manage as will you.

I realize my bias and duly apologize, but I am very nervous about that drawing as u know.

For my work on Skoota, I have a blind area in the front of the boat. It was my understanding that it is measured in boat lengths and based on vessel speed. I have a blind camera for such a large vessel to avoid running over a kayak..perhaps.

There was no such angle provided, but rather for me to decide, based on speed, how many boat lengths of blind distance. In my case, I designed for under two or 72 feet. In addition, I designed at seated(correction~I designed at standing and seated is higher), but can get up higher if required by a perch which you may find hard to accomodate. For you, take your seated height and probably one to one and a half boat lengths or two would be my approach. This, based on the deck, then establishes your head height, and probably the rooftop and then even whether you keep a sole. Another thing to consider is a tiered rooftop. This is done to allow standing aft and sitting forward. So you enter the house crouched or step down in if onto the hull and when you sit the headroom is less. We see this all the time here in canvas tops. It gives your aft deck standing headroom should you keep the angle going for aft benches as well. Anyhow, something else for you to consider. The Skoota cabin is 6'3" at the back and drops 1" every foot iirc. So about 13' up, we are at 5'2, but we have a step up 7", so really only have 4'5" or so. The step up helps the feet not dangle and gets the seats up for better views (which you wisely are realizing here).

And you may find it handy to step down between the stringers and then step up onto the helm bench sole. This gives you a bit more headroom entering and exiting, and keeps feet from dangling etc., in case that was clear as mud. I have a 7.5" step down into the cabin, but have room for a 12" landing inside..

I should add, these changes and the 6'3" head clearance inside took the designed 16" riser to a 27" riser...to get my standing height to the 2 lengths. Seated is actually a bit higher, but atm, I have unhappy dangling legs..etc etc..

I'll shut up for a bit. Ergonomics are a major challenge..

 

With a working "tool" you have to see where your'e going. Without measuring, I'd say that most of my boats allow you to see the surface about 6 to 8 m ahead of the bow at full speed / maximum trim. And don't design a deckhouse that requires double-bending to enter, you will hate yourself (and so will your crew).

 

Tiered is not the right word.

I mean something like this where the roof is higher in the back.

 

Ergonomics, the (far too often) Forgotten Factor!

 

Indeed.
And having correct weights and a very detailed, as much as possible, aids tremendously in this part.
You can never have too many weights itemised in your list...boats always grow in weight, never get lighter!!

Having a decent drawn to scale GA, will help identify correctly what can and what cannot be moved to influence where the final LCG light and loaded will be.
This then becomes essential, in selecting the correct hull form.

For boats of this size/duty/nature....1 boat length should suffice, but 1/2 a length is better.
Since if you are under speed, your boat will not stop at lest than 1 boat length.

Except to those all misty eyed and wishing to start building for the sake of it...and then it becomes painfully reactive.

 

Ergonomics is one aspect of Serenity that has turned out extremely well.

 

There is an ABYC standard which I cannot access now
I would expect a query to. ABYC HELM REQUIREMENTS. Or the Canadian equivalent TC xxxx would get you a freelook

 

@Ad Hoc : misty eyed! I'm sure there's drops for that at the local chemist

 

@Barry you're right. ABYC says 50m or 4 boat lengths, whichever is less. In Serenity it's about 1 boat length.