200Kg on Keel ??

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john.pompei
200Kg on Keel ??

I saw today an ad for a 26M for sale in Sweeden

(https://www.boatsandoutboards.co.uk/Trailer-Sailers-for-sale/macgregor-2...)

with the following as a feature

"Extrakeel 200 kg led mounted on the centerboard with el.winch for up or down. Gives much better
upwind performance and used together with ballastwater makes the boat very stiff."

There is a picture also of the lead bulb on the bottom of the dagger board and the motor for raising it.

Apart from the difficulty of getting it on the trailer, is the Mac built strong enough to take the extra load on the board and casing ??

I'm not thinking of changing boat or adding lead - just musing on the net.

John

ASHANTI 2008M

leigh.ross
leigh.ross's picture
Well let’s start with the

Well let’s start with the facts. The daggerboard was not designed to take the loads imposed by an added 200kg. Neither were the case or rigging. That’s not saying that they aren’t strong enough, just that they weren’t designed for those loads. So you’ll definitely be a test pilot for this. Unless of course you’re qualified to do a finite load analysis of the boats structure.

But heres the real question. Is an extra 200kg needed. I’d submit that it’s only needed if you can’t reach hull speed under sail. If you can already do that , then there is already sufficient stability to extract enough power from the rig to get the boat going as fast as it can. An extra 200kg won’t help. If you’re just hoping to reduce the angle of heeling at which this occurs then the extra weight will help. But then your back to the strength issue. Reducing the heeling angle with extra ballast implies greater loads on the board and case.

So, how much money are you willing to gamble that the structure can take the added loads?

Leigh Ross

Crieff

0777 558-4561

1990 MacGregor 26S Ptarmigan 

1992 MacGregor 26S Pelican 

rick.jones
rick.jones's picture
Insane!

Insane!

200kg is the weight of about 3 people. This extra weight will in itself slow the boat down, regardless of whether it keeps it more upright.

It would probably be just as effective to take 3 extra crew along and have them sit on the windward side!

And as you say, it must prevent the boat going on the trailer, and it would also prevent the board being removed from above. In fact to get it in and out you'd need to lift the boat on a hoist, and you'd probably need a fork-lift to take the weight of the board from below.

As I said, insane, you'd think the guy would have bought a different boat 🙄 !!!

Rick Jones (Treasurer) 1994 Mac 19 "White Lightning", Isle of Wight

mike.clarke
Adding 200 kg to a Mac keel

Adding 200 kg to a Mac keel is insane as commented above by both Rick and Leigh - it will only slow the boat. If you want to reduce the heal angle then all you have to do is reduce the sail area by reefing. The boat doesn't go any slower as you still project the same sail area to the wind. Some yachts are designed for an optimum heal angle as it increases the waterline length slightly and can reduce the wetted area but this doesnt rearly apply to the Mac hull shape. The main drag of a yacht comes from the wetted area (which is why catamarans are faster) and the maximum displacement speed is a function of the waterline length with longer being faster. If you want a Mac to go fast then take the centre board nearly all the way up when bearing away on a broad reach and get her up and planing - you cab easily get more than 6 knots this way. (don't leave all the board down when planing though as the boat can trip over it and broach - which is not good.)

The standard dagger board only weighs 12 kg so is both strong and light for what it does. Its a good symmetric aerofoil section to minimise drag and also seem to be sensibly designed to break before it does damage to the hull casing which is good.

Mike C Tarke 26M

mike.clarke
Just looked at the advert  -

Just looked at the advert - there a a few mods like the bow extension, the electric dagger board winch and the 70 HP motor. I can see why he needs a 70 HP motor to manage 15 knots with that great lump of lead on the dagger board but I think it would be an interesting conversation with his insurer when he wrecks the hull and owns up to a 200 kg (10% of boat weight) addition to the keel and a motor 20HP greater than the boat is certified for with its CE mark.!

The other thing that struck me was how does he remove the dagger board and what happens when the boat is beached?

Mike C - Tarka 26M

dave.newton
 

The issue is one of righting moment vs wetted area. Adding 200kg to the end of the dagger board is about twice as effective as adding it uniformly over the whole board. Hence the design of bulb keels. But any extra weight increases the displacement and hence the wetted area. This increases hull drag and unfortunately not in a linear fashion. Say a Mac fully kitted is around 1500kg adding 200kg is about +13%. Displacement increases 13% but wetted area will only increase approximately 4% (very approximate as it depends on the scantlings, i.e. the shape of the hull). The draft will increase a little but only about 2.5% (again very approx).

The bit that is very hard to calculate without the original design parameters is if the added performance from less healing will outweigh the loss from the increased drag. However Roger Macgregor is a very skilled yacht designer and I'm willing to bet he got the design close to right. If more ballast was going to help he'd have simply made the water ballast tank larger.

Dave Newton Sailbadthesinner

john.pompei
I knew this one would get our

I knew this one would get our Tech’s brains working !!

Happy Days

John

ASHANTI 2008M