Big Mac 65

Diff early / later versions

Was just thinking, the difference in displacement between the early and the later versions of the Mac 65 is 9,000 lb and that should come up to the later versions floating something like 4 inches lower in the water. Transom would then just about hit the water if no hull modifications has been done.

Anyone knows the difference in hull form and what changes were done to the newer version? What is the transom clearance for the heavier Mac 65?

Mikey

 

Alright...alright...so I'm in the wrong century. I like a relatively large keel, maybe not a FULL keel, but a stable boat is a safe boat. Ok, numbers (generally) don't lie. Go for the Mac 65. Don't get the 26, but the 65...ok. Go for it.

 

No hull differences

I think the specs for the race version list 22,000lbs, and the pilothouse at 32,000lbs. The hull forms are exactly the same- Macgregor never changed the molds. They're still at the factory in Costa Mesa. The difference is in how the interior was fitted out, the keel (race= deep draft 10,000lbs, pilothouse= shoal draft 10,500lbs), and the shape of the deck piece attached to the hull. The race is a relatively flush deck, and the pilothouse is (duh) a pilothouse.

I don't believe that transom clearance was ever a measured spec- you would have to go around with your tape measure in the marina to compare.

If its any help, my boat had the freeboards measured for a new Americap certificate this year (for the TransPac), and apparently it is sitting a little lower in the water than it was in 1989, when it was measured for the Japan Cup. So maybe fresh from the factory there was 5" under the transom? ...

 

Thanks dcory,

Sad that the moulds are just lying there collecting dust, really.

Well, I have never heard of any hull changes but it just hit me that with 10,000 lb difference, the newer pilot house version simply must do something, float lower and/or re-trim. The Mac 65 hull form probably displaces somewhere around 900 lb per inch, that’s over 4 inches. It sounds a bit much to re-trim

There are few Mac 65s in Europe and none in Thailand. I have only seen one in my life, that was in Sweden around 1990.

Could you estimate the sheer you have – how much higher is the deck at the bow above deck at, say midships? Doesn’t have to be exact, something like “6 -8 inches or so” is enough.

Mikey

 

I hope you aren't doing too much math based on my loose estimates, but here's what I found this morning wiht the tape measure: The difference in freeboard between the bow and midships (right at the shrouds) is about 10", and further aft, at the pirmary winches in the cockpit, its another 4" lower. If you look at pictures of the boat, that's what gives it the low-slung, racy look.

As for the transom measurements, realize that the transom in rounded, so the very back tip, the part about 4" out of the water, isn't very wide. If you look at one of the first pictures I posted, follow the white waterline stripe all the way back- it ends a few feet forward of the transom, and at that point, the bottom of the white line sits about 8" out of the water.

Hope this helps...

 

Looking at one boat only is copying an existing design, comparing many and draw conclusions is investigation. I use input from a variety of boat types, even Nauticat. A hundred different boats actually :tiredsmiley (can we have a tiredsmiley, jeff? Very appropriate to describe how I feel now)

dcory, it does help
I already have more buoyancy and flare at the bow than the Mac 65, one thing coming out of this is that I have increased flare more (because of your submarining comment). I have a displacement of approx. 45,000 lb, nearly double the Mac 65 racer and 40% more than the pilot version, I won’t be as fast, I’m quite a lot fatter because I want to reach “my acceptable level for interior volume”. But hull form is apart from that actually quite similar. This was not a planned move at all, it is just that I like long slender boats. They are efficient. MacGregor hasn’t patented the long slender hull form I hope... Never mind, there isn’t a singe measurement that is the same, just the long and slender concept, nothing else.

Thanks for the sheer figures, have more than that forward already and will keep what I have, should be enough to keep the cockpit dry, besides, it is 50 foot away and I have a small sporty fixed windshield. Being 4 inches lower further aft makes a boat look nice but headroom must not be compromised too much. I will drop what I can aft, keeping 6 foot 5 inches headroom in the aft cabin. What is the headroom in the aft cabin (just stepped in to the aft cabin)? Just approximate again, - Oh, six foot three something is good.

Transom is actually quite different from Mac 65, have 6” transom clearance but don’t know if that is enough (I have almost double the displacement). I have 10-11 inches where you have 8, and my transom is also a bit wider, I must admit that I just don’t know enough about hull induced wave building to get this right. I am working on waterline lengths when heeled now, rudder starts to ventilate when heeled 34 degrees, and that is without compensating for sail forces which will give bow down trim, and I am not happy... Experience, experience, experience. At least we are learning every day.

Is there anything that you would like to improve with your boat? More buoyancy / flare forward (beat you on that one), sail configuration, center of gravity, headroom, anything...

Mikey

 

Jeff, how about this one,

 

I'm not sure I would add more flare forward, at least to this boat. We only started submarining in somewhat hairy conditions (35-45 knots wind, 15-20ft seas), and only because we continued to push for top speeds. I saw some pictures recently of a Volvo 60, I believe, that added little winglets near the bow for just that reason. It keeps the hull narrow, so performance ins't diminished, but the winglets provide lift to the bow if digs in that deep. Since you're not totally performance oriented with this design anyway, flare is probably the way to go.

My boat does not have an aft cabin, those only came with the later pilothouse versions. Headroom in the head, about even with the companionway entry, is still about 6'3".

 

The clearance between the transom and the water is not necessarily the most important thing here. The angle between the underside of the hull and the water is, I think, more important. For "normal" designs, it should be about 10 degrees.

 

Transom clearance and rudder ventilation

Thanks Søren, 10 degrees, hmmm, I have closer to 20. Then there is the question of if my design is normal. It is definitely slender, but not as slender e.g. the Mac 65 of course, except aft maybe. I have LCB at 53.3% (LCF at 54.4%) but I have not reached this by following today’s trend and making the aft wide. Hull bottom aft is still rather flat.

I have approx 10 degrees angle between the underside of the hull and the water where the rudder is attached but not further aft where the transom ends. Since the aft is “slightly deeper than normal” (or is it?), I have probably more like 20 there. I moved the rudder forward a bit a few days ago so aft extremity of the rudder is now approx. 1.5 meters from aft extremity of transom (which is 15 cm above DWL). The clearance will probably go below 10 centimeters if I go down to 10 degrees. I will check tonight but I definitely have a problem here. Have you got any suggestions?

The rudder ventilation surprised me. I was not expecting a rather slender yacht with an even more slender aft to do that. Hull waterline when heeled 32 degrees (with 0.5 degrees trim according to RhinoMarine) “misses the top of the rudder”. Maybe “slightly deeper than normal” is not correct. But it is slender all right and the buttocks are straight and nice, so are the diagonals, so the aft should not lift much because of the heel. How on earth do yachts with a wide aft avoid rudder ventilation??? ???

What do you think?

Mikey

LOA: 21.33 m
LWL: 19.65 m
Bmax: 4.27 m
Bwl: 3.53 m
Disp: 19500 kg (that’s excluding keel & rudder, plus 600 kilos or so)
Ballast: 8000 kg

 

No, not without having a look at your lines.

 

Well answered Søren

I will have to ask my partner in this before I do that. Hang on.

Mikey

 

Martin is OK, so side view and top view pics coming up.

 

well i'm definately conviced of the merits of the mac 65... wonder if changing to dual keel would help the heeling?

 

Pic1 - Top view with DWL
Pic2 - Side view with DWL