Big Mac 65

Buttocks too

Forgot buttocks...
Pic3 - Side view with buttocks

 

Why not a perspective too, anorexia clearly shown.
Pic4 - Perspective with DWL

 

OK, Pics posted.

I was very pleased with what I had designed until I heeled the boat 35 degrees the first time (using trim that RhinoMarine gave me) and less pleased when I saw that I had rudder ventilation

The design was slender and what I was certain would be a well balanced yacht, I have changed a bit after that but I must say that I don’t like the results of what I did. I have gained one degree heel before rudder ventilation (now at 33 degrees), mainly by moving down the aft. I have also made it much more slender aft, to the level where I just think / feel that is should be unnecessarily slender aft of midships. The “slendering” aft made almost no difference at all indicating, I think, that I was pretty good already.

LCB is at 53% in the pics I post, down from 53.3% before the anorexia started. Don’t think that it matters very much, I can find no reference regarding LCB position for light displacement yachts. Principles of Yacht Design specify 53.4 – 53.5% but Larsson also writes that it is for medium to heavy displacement boats. I would think that it matters less (or require less?) on light displacement but I am not sure.

I still have some fairing to do the around the transom, I’m losing a bit WL length when I heel, changes will be small though so shouldn’t matter.

All input is appreciated
Mikey

 

Mikey

how would your design be affected by flaring the sides a little more?

(looks like very straight sides atm)

 

The design looks rather wide aft but that is mainly at deck level, I already have flare, moderate at midships and growing moving aft and forward, a lot more flare aft than forward.

Here is a pic of the station at 5% aft of midships (between perpendiculars). I don’t think that this looks too fat, flare is moderate though, don’t want too much.

Mikey

 

Small correction, pic of flare posted shows station 6% aft of midships, not 5 that I wrote.

From an interior volume perspective, I am OK flaring a bit more midships. The pilot house sits there anyway, but I am not so sure that it would be good for the design. I don’t think that cruisers should have too much flare, and that the flare I already have at 6% aft of midships is “appropriate for the design”, my design goal is a cruiser and not a racer (albeit a fast cruiser). Opinions?

I feel that I am flaring too much aft now, flare is increasing considerably the further aft I go (after 6% aft of midships) and it makes almost no difference at all when it comes to the rudder ventilation problem. I feel like “I’m eating interior volume for nothing”.

I don’t have any information regarding optimum placement of LCB for light displacement yachts but think that the 53% I have now already is a bit low. I would like to be at 53.2 – 53,3% or so. But I can’t get there without either fattening the aft one way or another, or moving aft perpendicular (AP) forward 8 – 9 cm. AP is 56 cm from aft extremity now, OK, large overhangs are not good but I think 65 cm should still be OK, and I don’t mind reducing the WL length a bit. It is just that I don’t think that it would help anything in reality, or would it?

Aft extremity of rudder is another 125 cm forward of AP now. Is the problem more on the rudder placement or aft design? I’d be grateful for input here. I have a whole lot of question marks in my head…

Mikey

 

The angle between the hull and the water seems to be around 13 degrees, which is OK for a slender yacht like yours.

I think there is a slight misunderstanding regarding rudder ventilation. Ventilation is air being sucked down along the rudder, which then looses its grip. What you're talking about is (I think!) that the rudder is being lifted out of the water as the yacht heels. This is more or less unavoidable and no big problem as long as your rudder is big/deep enough. I wouldn't worry about it - right now it's much more important to get the lines right!

I would make the hull a little bit deeper amidships and less deep above the rudder. And then a bit more overhang and a bit less freeboard aft... But that's just me. The famous Danish philosopher Søren Kirkegaard once said something like "Taste is not debatable", so...

 

mikey, re your rudder problem
I have noticed on some race boats a tendency to very long slim rudders, might be worth looking at

 

another thing I have noticed with some race boats is their wedge shape...

how would that big fat end influence cruising?

 

Thanks for input,

I have reduced angle from 20 as far as I could, if 13 degrees is OK, then that makes my life easier.

Ventilation
Well, I assumed that if the rudder is lifted out of the water, then flow will make air get sucked down. But I notice a difference in our thinking, you use the phrase “along the rudder” which means, I think, that you understand this phenomena as something differently from what was my understanding. OK, I understand what you mean and you are perfectly right, what is left then is the rudder being lifted out of the water and I have tried to avoid the unavoidable... Flow and run of the water around the rudder should be good (diagonals are straight and nice) so I should not experience excessive problems in this area.

I just stared at the lines for two hours yesterday, and then I went back to the version I had before I started messing around... It is not a Swede 55 that I plan! And I bet that the Swede 55 shows a bit of the rudder when heeled 33 degrees anyway. That version is less deep above the rudder and more in line with what you suggest.

But I don’t agree with making the hull deeper midships and that is for 3 reasons;
1) I would actually like to have more canoe body draft than I have, it’s just that displacement increases to more than I think I will use
2) Being a slender yacht, I want to improve heel resistance. A bit of form stability should not hurt.
3) A very round bilged hull form has less roll dampening, negligible impact? Most of that should come from the keel anyway :?:

This of course unless there is a design reason affecting characteristics of course. Is my thinking wrong?

I majored in Industrial Automation and I suppose that is shining through, in manufacturing, you like straight lines... OK, you are absolutely right about less freeboard aft. Point taken.

Talking about philosophers, I have two from Oscar Wilde, a favorite of mine that suits me right...
Experience is simply the name we give our mistakes.
Experience is one thing you can't get for nothing.

And another one that matches some who post in this forum
In America, the young are always ready to give to those who are older than themselves the full benefits of their inexperience.

Mikey

 

Forgot overhang...
I plan to increase the overhang aft with 10 cm but leave the forward one as is. I am not WL length crazy, wouldn’t mind decreasing the WL length with half a meter or more if it would make a better design, I simply think that it is “about right”. I was thinking like – the more overhang, the more effect waves will have – move the drawing board out into the Atlantic, sort of. Now when I am sitting here in 5 meter waves, what do I want? ”. Would there be a design reason for more overhang aft or forward?

I slaughtered the aft in the version that I posted pics of, that’s why it looks so fat the aftmost meter or so. That part is just touching the water when heeled and does not increase buoyancy much so should be OK, I think.

Mikey

 

How marriage ruins a man! It is as demoralising as cigarettes, and far more expensive.
Oscar Wilde

Now, I'm going to have a cigarette

 

Well, if you remove a little displacement by making the hull less deep above the rudder, amidships would be a good place to add some - it'll make it much easier to get the run nice(r) and straight(er). You might end up closer to the 10 degrees. 13 degrees is just OK - not more than that!
I don't think it'll hurt overall stability much - we are, afterall, talking about how much deeper? 1 cm? Same goes for the roll dampening.

With 10 cm more overhang aft (and a bit less freeboard), it looks just fine.

BTW: Not everybody knows how to design sweet sheerlines. It's easy to design something that looks good on paper, but paper only shows you one view. On the screen, you can rotate the hull, but that doesn't make it easier. The trick is to determine three points on the sheerline. These could be one at each end and one at BMAX. Now create a straight plane that passes through these points. The sheerline is created by the intersection of the straight plane and the hull side and will look sweet from any angle.

 

Stolen Thread

I was thinking - I have stolen this thread for my own, it's not about Big Mac 65 any longer. I will open up a new thread in a couple of days, please keep a lookout and welcome to comment there all of you.

Sorry Guys, I will open up a new thread instead of messing up others next time.

Mikey

 

No worries- I think your Mac65 questions have been answered, so this thread is probably done.