Sea Sled Madness: It's Incurable

That does make sense. Not necessary in a sea sled though.

 

Something else for my SOR: I dislike the exaggerated flair on the TX18 and exaggerated shear on the HS18. Maybe they are there to make them look more "boaty". But they contribute nothing to function. Function isn't the main thing with me, it's the only thing. From above my next boat should look like a brick, with slight tapering at the front end.

 

A good example of size and proportions I have in mind. This boat is extremely practical and the operator has vast local experience. And my respect.

 

Flair sheds water so to make the boat dries.

sheer is sort of arbitrary, generally

 

As suggested. A first rough sketch. Naturally I made some assumptions, since it's impossible to begin otherwise. I treated the structure as homogeneous as far as mass distribution is concerned. Any given longitudinal section of boat masses the same as any other equal length. Not entirely unreasonable: scantlings are the same end to end. The cabin is a conspicuous volume forward, but its top is the same as the sole cutout. Topsides taper forward, roughly compensating for the cabin sides. Transom is the only massive component throughout, also offsetting cabin and seats forward.
Scale is 15cm, or 6" to the square.
Component masses are placed as force couples, moments about the nominal LCM. I found that in this load condition (Anne and I, no cargo) the LCM moved forward about 16" or 39cm. Just Anne, about 6" or 15cm. Down by the head a few degrees, as is typical in sea sleds at rest.

 

Closer to 10" not 6".

 

So you're going away from the HS18 design?
This will be another custom?

 

It will have elements of the HS18.

 

What's the minimum acceptable height in the doghouse?

 

Maybe it is the hour, but if you are insisting on a top against the designer wishes; wouldn't you want it to not affect the lcg?

And, are you suggesting tripping about inside over the stringers to cut weight when you say the roof doesn't count?

And would not the front of the boat be less mass than the back per each section by nature of the vertical height? So the starting lcg is much further back?

Obviously, Bodo is the man, but I am just asking questions.

My intuition says the cabin is too far forward and a few assumptions wrong which make starting lcg further back, compounding the matter. Perhaps you understand and I am merely being patronizing?

This badly reminds me of my current problems. The hull volume forward is also less which means the lcb moves forward until it and the lcg are aligned. This means the boat tips even more than expected...corrections welcome, I am still learning..but somewhere up there is the tunnel and you don't want to sink on it...

 

Here is an article discussing static lcg of the hull at 40% per a builder calc and 40-45% per designer. I believe as measured from transom at dwl.

TX18 centre of gravity - Page 2 - Boat Builder Central - Builder Forums https://community.boatbuildercentral.com/forum/viewtopic.php?p=506418&hilit=Tx18#p506418

And here is a pic of the hull bottom which is why I worry about not considering the aft section has more volume under water..as well as the assumption on each sections mass, etc.

ps-he added a chunk of glass aft for abrasion which is why the color diff, but a fun pic

 

[removed error]

And the cabin aft edge starts at his waist. You can built it ultralight with cedar strip and the thing will only weigh a few hundred pounds; even drop down to 1/8" lexan lights. Mertens says no hard cabin because he doesn't want some idiot to raise vcg way up with 1/2" ply and end up dead..

 

@fallguy , I appreciate your enthusiasm. Slow down and read again. Scale is 6" square, not 10". LCM, or LCoG is taken as a % of loa, not waterline. Waterline is not a particularly useful number in a planing hull, particularly where gross mass can vary by 100% between fully loaded and empty, and waterline length can change by 70% between loaded at the dock, and planing at WOT. I marked nominal LCM on the sketch based on @baeckmo 's empirical observation that his own 6m sleds performed best when the LCM was at ~36% of LOA. And so I marked it there.
I have a challenging SOR. I require cabin space for 4 adults that keeps them comfortable in miserable weather, when they are already wet and tired. I also need to be sble to load up work materials for a day. So far the heaviest that's been was 500kg, or 1100#. That much variable weight needs to be centerable as closely as possible to the nominal LCM. So the flat deck, and the tall cabin with the short longitudinal footprint. Intuition is for... people not us. Engineers use math. I'm particularly fond of vector mechanics.

@baeckmo , that cabin height is ~52" or 1.3m, which is enough that I can sit upright as on a hard chair, and my head will just touch the underside of the cabin roof with my feet on the hull, no sole. I assumed such a vertical position because I wanted clear deck at the LCM, without pushing the cabin so far forward that weight and balance became unworkable or normal pitching motion was uncomfortable. Also I was checking sight lines between the operator, the leading edge of the foredeck, and the massive log floating barely awash in the early morning fog, and I thought, eyes better be able to see at least 15° downd from horizontal.
I could lower the cabin roof if I stretch the cabin fore and aft.

 

I don't think I'd risk for a second moving the static lcg forward for any loads. [removed error]

If you want to haul long lumber; that may require more creativity than moving the cabin forward.

It will be interesting to see what baeckmo says. Mertens says in the thread I linked that lcg must be centered over the center of planing lift which of course moves aft as speed increases.

Sorry for my 4am math. I see the TX18 is actually 18' loa, so 36% of LOA or 40% forward of the transom at the waterlind are nearly the same.

0.36•18 = 6.48' = 77.75" forward of top of transom
40% ahead of transom at dwl is actually
0.4•18 + 4.5" or about 91", so Mertens lcg is in your favor, unless he means 40% of lwl???

 

As I pointed out earlier 25cm is not equal to 6 ", it is closer to 10".
Your scale doesn't make sense.
Is it 6" to a square or 25 cm?
It can't be both...