how to determine the buoyancy in the bow

[Chuck Losness]

As some of you know I would like to have a cat ketch. One thing that concerns me is how do you determine if you have sufficient buoyancy forward to support the weight of the mast, boom and sail. In the size range I am thinking about the mast, boom and sail would probably weigh somewhere between 100 lbs to 150 lbs. There would also be probably 250 lbs or so of ground tackle in the bow. Any suggestions on how to calculate this?

 

[daiquiri]

Hi Chuck,
you should consider the buoyancy of the full boat, not only forward.
In case of a ketch, the masses of main and mizen masts are summed and their common baricenter and center of effort is found. After that, you make your calcs just as you would in case of a common sloop boat.
Cheers

 

[Chuck Losness]

Daiquiri
In a cat ketch the forward mast is located within 5 feet of the bow. If the bow is too fine the boat will be down by the bow because of the weight of the mast and sail. I am talking about a 28 to 30 ft light displacement boat. Total displacement will be no more that 4000 lbs and hopefully closer to 3000 lbs. So I am wanting to know how to determine if there is sufficient buoyancy in the bow to support this weight. I am not a NA and may be phrasing my question incorrectly. I know there is a fineness ratio but that seems to apply to the whole boat. Can you use this same principal and only apply it the forward half of the boat? Is there a typical range for the fineness ratio?

 

[daiquiri]

Regardless of where the main mast is located, the proceeding is exactly as by my previous reply. You don't (and actually mathematically cannot) consider just the bow and just the main mast contribution to the trim of your boat. You have to consider the boat as a whole and match the LCB of the submerged hull to the LCG of the whole boat.

Another thing is if you have an existing sloop hull with known hydrostatic properties and are trying to change it to a ketch rig. Then by adding a fwd mast (main mast) with a known mass placed at a distance X from the current CoG of the sloop you can get a new trim via the formulas of stability, case of longitudinally-moving loads.

Cheers

 

[sharpii2]

I think you will find that, though the fore mast will put more weight in the bow than the single sloop mast would, the Mizzen Mast will make up for a lot of that, as it is placed considerably further aft than the single Sloop Mast. The Mizzen will probably end up somewhere near the front of the Cockpit.

 

[PAR]

Chuck, it's not so much a matter of "if you have enough volume in the bow" as it is, the cantilever over the CG, as it imposes it's moment to trim changes. As you understand the bow will trim down, but this can be offset, so she trims where you'd prefer. This is the butt kicker to yacht design, getting the puppy to float on her lines and not trim out one way or the other. Some designs aren't well suited to having the mast in the eyes of the boat, but most can tolerate this trim change with some adjustments to other stuff.

 

[SukiSolo]

Surely the displacement is of the whole mass of the boat? Normally you include everything, however some elements such as people can be 'dynamic' ie move balance position. With a single forward mast the main force which tends to make the bow bury is the gust from astern. As PAR's comment about the cantilever. Mostly this is countered by moving weight if possible and/or having sufficiently 'full' sections which can lift the bow. These can be done afaik by having flatter lifting shapes low down or a flared out part towards topsides.

BTW this also affect the stern design as in if you can't sink it a bit, it will help bury the bow. Check the Oz skiffs which tend to do this on bear away.

 

[PAR]

Get an idea of your moment to trim, using the water plane and work from there.

 

[UNCIVILIZED]

I have no idea how to calculate it, but you also need to consider the effects on the boat's trim created by the sails. As this can & does significantly influence how much the bow is depressed or lifted on various points of sail, & in different wind strengths. It's something that commonly is tuneable through moving crew weight, but the boat needs to be fairly well balanced to start with.