Cat designs in regards to Hobby Horsing

[Cotemar]

Originally Posted by zboss

40 feet LOA or 40 LWL? Or am I splitting hairs?

40 feet LWL.

This is one of the reason that you see circumnavigating sailors buying 45 foot boats.

[jacktheflyer]

I've seen boats with large underwater bulbs on the bows as in an ocean going ship. Is this to provide buoyancy or is there any other explanation?

[Cotemar]

Originally Posted by jacktheflyer

I've seen boats with large underwater bulbs on the bows as in an ocean going ship. Is this to provide buoyancy or is there any other explanation?

A Bulbous Bow is a protruding bulb at the bow (or front) of a ship just below the waterline.

The bulb modifies the way the water flows around the hull, reducing drag and thus increasing speed, range, fuel efficiency, and stability.

Large ships with bulbous bows generally have twelve to fifteen percent better fuel efficiency than similar vessels without them.

A bulbous bow also increases the buoyancy of the forward part and hence reduces the pitching of the ship to a small degree.
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[catsketcher]

I like it when I have to be clear about the terms I use. I like the idea that all lengths can hobbyhorse - we all want boats that pitch to go over waves but dampen any pitching very quickly. Shorter boats will pitch more quickly and this can be more tiring than a longer boat.

As for bulbs, we don't see them much on cats anymore. Our Australian guru, Lock Crowther, used bulbs a lot in the 80s. They did increase prismatic co-efficient (fullness of the ends) but to be truly effective bulbs have to be designed for a specific speed, I think the goal is to be out of phase with the hull wave.

Ships have a certain speed they motor at so you can design a bulb for this particular speed range. This is pretty much impossible for a sailing boat and so bulbs aren't seen on modern cats. A bulb was even tried on a 6 metre boat in Australia but the bulbs were dropped in favour of fuller bows - buoyancy is buoyancy no matter what shape.

Of course we can do what the AC cats do and dramatically reduce hull volume above the waterline up front. This reduces (inverts?) flare and reduces the above water volume so that pitching moment is reduced. It makes for a poor cruising boat when at anchor though and is easy to fall off.

Paulinoz - I reckon the jerking from the short heavy chain would have been a pain in the Airlie fetch

cheers

Phil

[BigBeakie]

"Of course we can do what the AC cats do and dramatically reduce hull volume above the waterline up front. This reduces (inverts?) flare and reduces the above water volume so that pitching moment is reduced. It makes for a poor cruising boat when at anchor though and is easy to fall off. "

Could you elaborate on what you see as the impact at anchor?

[catsketcher]

The AC style, water shedding bows are great at shedding water, or anything else that wants to try to stay on top of them, like people trying walking around on deck. A cruising cat has nice flat decks which don't shed water well. On top of that, I don't really want to sail on a boat designed to pierce waves rather than sail over them. It would be very wet and not very comfortable. Nicer motion though.

[ZULU40]

Originally Posted by UNCIVILIZED

Less rocker in the hull's shape generally helps to cut down on hobby horsing. And a huge factor is the hull's buoyancy distribution, including where the designer locates various amounts of reserve buoyance through the use of hull flares. They play a Big factor in controlling a boat's motion. And of course bow shape, & volume is another one.

I think a more plumb bow is desirable.
For those that know the Heavenly Twins line of cats, which hobby horse quite badly. There is one modified by her owner, Grey Dove.

Grey Dove was given a plumb bows by her owner and it stopped hobby horsing. I surmise its due to straight entry to waves, a longer waterline and proportionally less rocker

Heavenly Twins


Grey Dove

[rabbi]

Originally Posted by ZULU40

I think a more plumb bow is desirable.
For those that know the Heavenly Twins line of cats, which hobby horse quite badly. There is one modified by her owner, Grey Dove.

Grey Dove was given a plumb bows by her owner and it stopped hobby horsing. I surmise its due to straight entry to waves, a longer waterline and proportionally less rocker

Heavenly Twins


Grey Dove

+1 on the HT being a bad example for hobby horsing

Last year we shared an anchorage with a french Heavenly Twins. They left an hour before us, going exactly the same direction as we, hard on the wind in ~15kn of wind.
Our boat (back then it was a FP Mahe 36) was far from immune to hobby horsing but for us it was still OK and we were doing around 7kn.
The HT literally made no progress but was just hopping around. Shortly after we passed them they just turned around to get back into the anchorage.