Replacement Anchor much larger than Factory

[Lost Horizons]

Originally Posted by skipmac

Putting weight in the ends of the boat increases the moment of inertia meaning as the bow or stern moves up or down on a wave the inertia of the mass in the ends of the boat want to continue moving up or down even after the wave passes by.

It does. Which in its turn increases the resistance to rotation. Although, it would be hard to measure the difference if you shift 200 lb of chain a few feet forwards in a 20,000 lb boat with 30-something waterline length. Seriously, the amplitude of "hobby horsing" will increase by an order of one percent while the response speed will proportionally decrease, which may have (an unnoticeable) positive effect.

[skipmac]

Originally Posted by Lost Horizons

It does. Which in its turn increases the resistance to rotation. Although, it would be hard to measure the difference if you shift 200 lb of chain a few feet forwards in a 20,000 lb boat with 30-something waterline length. Seriously, the amplitude of "hobby horsing" will increase by an order of one percent while the response speed will proportionally decrease, which may have (an unnoticeable) positive effect.

Of course the resistance to rotation is increased but the other side of that is once the rotation begins then it is harder to stop.

Result, wave comes and the bow first buries into the face of the wave due to the mass and inertia of the bow, slowing the forward motion, then the wave accelerates more due to the large increase in buoyancy of more of the bow in the water so the bow rises higher, continues rising then falls into the trough, burying again and again slowing the forward motion.

It's been a while since I did this kind of math but I can just tell you anecdotally from personal experience on a 34' boat when i moved weight out of the ends the difference was easily noticeable.

[Mike OReilly]

It's clear that some boats are more, or less, affected by added weight in the bow. My previous 34', 12,000# ketch was sensitive, so we stored the weight more mid-ships. On my current 37', 30,000# full keeler I've seen no effect when piling all my ground tackle up forward.

[Lost Horizons]

Originally Posted by skipmac

Of course the resistance to rotation is increased but the other side of that is once the rotation begins then it is harder to stop.

Result, wave comes and the bow first buries into the face of the wave due to the mass and inertia of the bow, slowing the forward motion, then the wave accelerates more due to the large increase in buoyancy of more of the bow in the water so the bow rises higher, continues rising then falls into the trough, burying again and again slowing the forward motion.

It's been a while since I did this kind of math but I can just tell you anecdotally from personal experience on a 34' boat when i moved weight out of the ends the difference was easily noticeable.

I think of the effect of the moment of inertia on pitching in a similar way the mass inertia affects linear acceleration, which generally results is increase of comfort. However, as you described, cyclical effect of multiple waves may have a cumulative negative effect at certain wave periods. Which would in its turn depend on the boat geometry, as noted by Mike. I now have 450 feet of 5/16" chain in the bow and do not notice any discomfort. Maybe one day I will get bored and decide to experiment, and move the chain closer to CG, but I doubt it. This boat's motion is much more comfortable, including going through the waves, than what I had before on a slightly shorter boat with 35 feet of chain.
It has been a long while for me too with respect to these type of calculations, and I am not even sure how to address multiple waves effect.

[skipmac]

Originally Posted by Mike OReilly

It's clear that some boats are more, or less, affected by added weight in the bow. My previous 34', 12,000# ketch was sensitive, so we stored the weight more mid-ships. On my current 37', 30,000# full keeler I've seen no effect when piling all my ground tackle up forward.

Exactly. The calculations and the effect is much more complicated than how much weight is moved what distance on what size and displacement boat. Will depend overhang, beam, how the beam extends for and aft, and much more.

[rslifkin]

Hull shape definitely has a big impact on how a boat behaves in pitch. A hull that doesn't damp out vertical movement of the bow well will tend to struggle more with hobby horsing when adding weight to the bow. A boat with more rocker will also tend to hobby horse more easily I think, compared to something with flatter aft sections (which will move the center of pitch rotation further aft).

[skipmac]

Originally Posted by Lost Horizons

I think of the effect of the moment of inertia on pitching in a similar way the mass inertia affects linear acceleration, which generally results is increase of comfort. However, as you described, cyclical effect of multiple waves may have a cumulative negative effect at certain wave periods. Which would in its turn depend on the boat geometry, as noted by Mike. I now have 450 feet of 5/16" chain in the bow and do not notice any discomfort. Maybe one day I will get bored and decide to experiment, and move the chain closer to CG, but I doubt it. This boat's motion is much more comfortable, including going through the waves, than what I had before on a slightly shorter boat with 35 feet of chain.
It has been a long while for me too with respect to these type of calculations, and I am not even sure how to address multiple waves effect.

Well as I mentioned in my reply to Mike, I think the effect will depend on a lot of factors. The boat I referred to was a moderate displacement performance cruiser built in the early eighties. Certainly much worse in just the right wave height and period (probably at some harmonic of the rate of pitch of the boat) but it was there in some degree any time I was close hauled in more or less short, steep waves.

Current boat is a lot heavier, more beam carried forward as well and have not yet noticed any difference with a 65 lb anchor, 150' of 3/8 chain in the bow locker and another 250' of chain in the forepeak.

[Lost Horizons]

I agree that for the boats designed to float over the waves there may be a more pronounced negative effect compared to the boats designed to pierce the waves.

[Lost Horizons]

By the way, has one ever considered the effect of a long keel on pitching motion in comparison to the fin keel? It changes the moment of inertia characteristics much more that the ground tackle type and location.

[wingssail]

Originally Posted by skipmac

Of course the resistance to rotation is increased but the other side of that is once the rotation begins then it is harder to stop.

Result, wave comes and the bow first buries into the face of the wave due to the mass and inertia of the bow, slowing the forward motion, then the wave accelerates more due to the large increase in buoyancy of more of the bow in the water so the bow rises higher, continues rising then falls into the trough, burying again and again slowing the forward motion.

It's been a while since I did this kind of math but I can just tell you anecdotally from personal experience on a 34' boat when i moved weight out of the ends the difference was easily noticeable.

I certainly concur with your assessment. This kind of hobby horsing becomes quite disabling if motoring into steep chop of any size or into short period swells.

If you also add weight in the stern it exacerbates the hobby horsing, and of course the weight aloft of an arch and a dingy also add to it.

I often hear of cruisers who state that they were reduced to 1 or 2 knots motoring into waves around our nearby notorious Cabo and after experiencing that one time they will do anything to avoid those conditions. They will often depart before dawn even though there is no wind for sailing until 12:00, simply to avoid that chop, through which they wind up motoring anyhow and never get to enjoy the nice several hours of great wind and flat water that started at 12:00.

Racing boats owners, however, know that keeping the ends light means much faster and more pleasant upwind passages in waves. Quick motions riding over waves is preferable to heavy motions plowing into and through them. Sailing at an angle also reduces the motions rather than directly into the wind and waves.

We have taken great pains to keep the ends light. Our normal anchor is a 44 lb Bruce, (and it has not failed us in 25 years). We use 5/16 HT chain and carry it well back, almost to the mast. Our spare anchors are stored below in a central location. We have none of our stores or equipment in the bow or transom areas.

We enjoy sailing. We sail whenever there is wind to do so, and motor rarely, even when going upwind in nasty conditions. Keeping the anchors light and off the bow helps. Every little bit helps and it all adds up.

Plus, we have confidence in our anchor and anchoring technique and we do sleep exceptionally well even without an 85lb anchor.

[wingssail]

Originally Posted by Lost Horizons

By the way, has one ever considered the effect of a long keel on pitching motion in comparison to the fin keel? It changes the moment of inertia characteristics much more that the ground tackle type and location.

The moment of inertia is increased by weight a further distance from the center of rotation. A full keel 36 ft boat with a substantial portion of the keel weight 10' from the center of rotation, both fore and aft, is affected more than a fin keel which may be down 6 feet from the center of rotation. A 10 foot fin with a bulb on the end is probably more equivalent to the full keeler.

But a major component to the hobby horsing is the shape of the bow sections. Deep forefoots slam less because they can plunge down farther until flotation stops the inertia. The shape of flat bow sections stop the hobby horsing, although we all know how that feels (and sounds).

[Lost Horizons]

Originally Posted by wingssail

A 10 foot fin with a bulb on the end is probably more equivalent to the full keeler.

And then, off course, there is the effect of the mast, which contributes significantly more to the rotational motion characteristics than the location and size of ground tackle.

[wingssail]

Originally Posted by Lost Horizons

And then, off course, there is the effect of the mast, which contributes significantly more to the rotational motion characteristics than the location and size of ground tackle.

Absolutely, And that is why boats without masts are extremely more jerky and rolly in waves.

And another reason why light weight carbon masts are faster; not only do they increase stability and reduce heeling but they reduce the moment of inertia, both rolling and pitching.

But of two boats, both with similar masts, the one with less weight in the ends will have less hobby horsing motion and will be faster as well as faster in its reaction to waves, otherwise known and feared by the "blue water cruiser" crowd as jerky motion. Deep plunging hobby horsing is much preferred, as is motoring rather than sailing.

Please excuse my cynicism.

[a64pilot]

Originally Posted by Mr B

I have a 24 KG and a 40 KG anchor on the front of my boat, Plus 100 feet of chain, Ready to go,
My plimsol line is 3 inches above the water line at the bow,
My rear is about one inch above the Plimsol line,
Im 34 feet long,


I have a bit of water in my bilges, Its dry where the pumps are,
So in order to get the pumps working, I and my girl walked down to the bow to see if it would make the bilge water flow forwards,

Our combined weight on the bows should make it sink, So Ive been told in all these anchor threads,
Nope, I didnt make the slightest bit of difference, My bilges are near flat and level,

Our combined weight would be close to 180 Kgs,

There is some thing wrong where people say that one extra anchor will make the bows sink so far that the boats bow will be near under the water, And very Unseaworthy,

I cant see how a 20 or 30 KG extra anchor would make any difference on the bow of a yacht.
Does a normal person, 70 to 100 kgs make your bows sink and its noticable, If you walk onto the Bows of your boat, ???????????????

Cheers, Brian,

I made my mind up on that one time when all four of us were right on the bow watching the Dolphins.
My extra large anchor and excessive amount of chain is apparently balanced by my dinghy, davits and Solar panels, because the water line is exactly level with where it started out, boat sits lower of course as you can’t add weight and not displace more water, but it’s only a couple of inches or so.
But remember my boat was completely devoid of any gear, didn’t really have a battery bank etc.

[wingssail]

Originally Posted by Mr B

...There is some thing wrong where people say that one extra anchor will make the bows sink so far that the boats bow will be near under the water, And very Unseaworthy...

Well Brian, that is an exaggeration. People don't say that.

What they do say, and it is true, is that the extra (additional) weight of an 85lb anchor and more chain in the bow adds to the hobby horsing, especially when combined with weight of an arch, solar panels and dingy on the stern (racers don't allow anyone to stand on the bow for long).

What I have seen, and photographed, are boats sitting several inches below their lines in the stern due to weight aft and aloft.

What I think is un-seaworthy about it is the loss of stability, the additional windage and drag, and the effect on the sailing ability, mainly due to the weight and windage of the arch and, dingy, and gear at the back. The weight in the bow only compounds it. If a boat is hindered in its ability to sail upwind (maybe not good to begin with) then it is un-seaworthy. Depending on a motor to get off of a lee shore is un-seaworthy.