Balancing weight fore/aft

[Paul H (admin)]

As Graham has said many times small cruisers tend to drag their heels, because the weight concentrates aft and MilliBee is a good example of the problem.

So I'm trying to move weight forward and I'd like your opinions on moving her 2 batteries forward. They weight 37kg and I can move them 6 feet forward compared to the Thames Raid:



I can still lie out comfortably and they could be boxed in, but my concern is that the C of G of the batteries will be about 4in above the theoretical waterline (marked with a green blob in the photo).

When placed mid-ships the batteries are 6in lower so the heeling stability is less with the batteries forward - is that a problem with her 6ft beam when sailing in the Solent?


There is also storage space under the floor that has never been used. I can move food/beer from its usual place in the rear lockers to a position under the forward floor..about 4-6in lower than the rear lockers which might help.

-Paul

[Jeremy]

My experience is based on the effect of moving the anchor locker on my old gaffer, but may be relevant.  My old boat used to squat down aft, especially with two or three people in the cockpit.  She was quite narrow aft, with the maximum beam just forward of amidships, so much of the buoyancy was in the forward sections, whilst much of the weight (crew, engine, etc) was aft.

To fix this I added an anchor locker right up in the bow, and switched from using ten feet of chain and a long length of rope to an all chain anchor rode of around 150ft of 1/4" chain.  Adding all this weight right up in the bow fixed the trim problem, and sailing in the loch was fine and well behaved, if anything she handled better under sail than before.

Venturing out into the North Channel of the Irish Sea revealed that this fix had an extremely unwelcome side effect, though!  With weight concentrated in the ends of the boat she would pitch and corkscrew in a very unpleasant way in any sort of a sea, with the weight acting like a big pendulum trying hard to drive the bows under on the down swing and up in the air on the up swing.  After two trips like this I switched back to using the rope rode and the problem went away.

Might be worth thinking about for a light boat like Millibee.  My guess is that she will handle better with weights concentrated closer around amidships.

[Paul H (admin)]

Jeremy,

That's interesting, lets see what the Ancient Marinated has to say. I think Chris approves of weight forward and he thinks MilliBee sails better with the crew weight as far forward as possible, including perching on a rather narrow and uncomfortable midships bulkhead! We have ideas for a bridge deck for the crew to sit on...more details later.

I could move the batteries a foot forward of midships without affecting the sleeping position, also for coastal sailing I won't need two batteries as I won't rely on the electric outboard - a petrol outboard is more powerful in a coastal emergency. One battery forward (18kg) might be ok.

-Paul

[alopenboat]

Won't that get in the way of the proposed new steering wheel? It also
may constrict the footwell in the forward steering position. Or have
you abandoned that idea? :-)

Your best bet is to try it and see. If you decide it puts too much
weight forward try one battery forward and one amidships. But
whatever else you do get the weight out of the aft locker.


On 13 Jun 2012 at 15:14, adminHBBR [via UK HBBR Forum] wrote:

>
>
> As Graham has said many times small cruisers tend to drag their heels,
> because the weight concentrates aft and /MilliBee /is a good example
> of the problem.
>
> So I'm trying to move weight forward and I'd like your opinions on
> moving her 2 batteries forward. They weight 37kg and I can move them 6
> feet forward compared to the Thames Raid:
>
> http://uk-hbbr-forum.967333.n3.nabble.com/file/n4025164/IMGP3292.jpg 
>
> I can still lie out comfortably and they could be boxed in, but my
> concern is that the C of G of the batteries will be about 4in above
> the theoretical waterline (marked with a green blob in the photo).
>
> When placed mid-ships the batteries are 6in lower so the heeling
> stability is less with the batteries forward - is that a problem with
> her 6ft beam?
>
>
> There is also storage space under the floor that has never been used.
> I can move food/beer from their usual place in the rear lockers
> underneath the forward floor..about 4in lower.
>
> -Paul
>
>
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--
Hoping for calm nights

Alastair Law,      
Yeovil, England.
<http://www.little.jim.freeuk.com>          

[Jeremy]

I looked at the reasons for the horrid pitching motion afterwards in some books on yacht design.  It seems that having a boat with a high polar moment of inertia (i.e. having the weight concentrated in the ends) makes it harder to start pitching, but when it does it pitches through a greater angle, as the ends have more momentum than if the weight is concentrated in the centre of the boat.  My case was extreme, as the anchor locker I made was right in the forepeak (I just added a small bulkhead up there, with a deck pipe to take the chain down through the deck and into the locker).

The problem only shows itself when the boat gets into waves that start it pitching, and was worse under shortened sail than full sail and was dire when motoring with the sails down.  Having all the weight in the centre of the boat makes it pitch more quickly, which can give a sharper motion, but at least it tends to stop the thing burying its bows in the oggin when it gets a bit rough.

I'd suggest that getting the weight of yourself and the batteries closer to the centre of the boat might be the best way to go, depending very much on where you intend to sail.  If you're going to find yourself in seas with a short chop a lot of the time then weight at the ends might help ease the motion, if you're more likely to be out in seas with a regular swell then weight in the centre would probably be better.

[Chris Waite]

I agree with Al that the best thing is to suck it and see

All boats will have a natural pitching frequency, which will be altered by where the weighty aspects of the boat are located aboard.  The Solent however does seem to have a malevolent Sea God that judges what your vessel's most pitchy wave length should be and then tries it out on you.

'Tit Willow' was just awful on her first trip out and only a serious load of ballast as far forward as possible sorted it out, turning her into a very well behaved young lady, though she still gets jumpy if the waves are a certain sort of short.  She has improved again as outboards and fuel tanks have been depleted aft

And is now strictly sail and oar

Ancinent and Marinated

[Chris Waite]

Ancinent?

I'd say so