How Much Weight In The Bow Is Too Much?

[Alan Mighty]

Originally Posted by Jim Cate

(actually weight times distance from pitch center)

I thought the contribution of moment of inertia from a given piece of equipment was mass times (distance from centre of gravity)^2

where ^2 = raised to the power of two or squared

Al

[a64pilot]

WAM, or weight X arm = moment
Not applicable here?

[UNCIVILIZED]

A lay explanation on Polar Moment of Inertia(ish) is this. Which is one of the things that the guys were just explaining (in different terms) above.

If you take a 2 cylindrical shafts weighing 5lbs each. One being 2' long, & the other, 5' long.
Now hang a 10lb weight off of the end of each one of them. Overall weight of the two is the same, right. 25lbs

But now attempt to spin them like cheering (cheerleading) batons. Which is easier to get up to high speed or stop in a hurry? The shorter one... even though they weigh exactly the same.

The same physics holds true in a boat. If you've got a lot of weight in the ends, it wont take much to get her to hobby horse, as compared to if you have that same weight at/near her center.

Of course, aside from being less fun to be aboard, a boat which hobby horses more is slower.
Because
- A: More of it's overall energy is spent going up & down.
- B: It has further to travel.
This is due to, instead of it going "point to point", like when you're driving in flat country.
It's going up & down, over an endless series of small mountains. In addition to it's travelling horizontal distance ("point to point").

So the hobby horsing vessel & the smooth riding one "appear" to travel the same distance, say, when viewed from overhead. But in terms of actual distance travelled, to borrow a phrase, one's got a lot more miles to put underneath of her keel to get to where they're both going, than does the other.

That's why racers go loco getting weight out of the ends of the boat, & especially up high (longer lever arm). As well as trying to sail on smoother water, and or go only "downhill" (attempting to steer a course to stay on only the downhill faces of waves when there's a sea running, as opposed to sailing up & over the whole wave).

[UNCIVILIZED]

Correction/Adjustment:

Originally Posted by UNCIVILIZED

The same physics holds true in a boat. If you've got a lot of weight in the ends, it wont take much to get her to hobby horse -> Stay in Motion <-, as compared to if you have that same weight at/near her center. .

Okay, no more explaining things a'la Physics 201, as I've obviously not had nearly enough caffeine.
Can we all just go to Shop Class now, & do something fun. Like build Honeycomb Cored, Carbon Fiber dinghys :-)

[Suijin]

In your particular case I think the experiment of sailing the boat in varying conditions with varying bodies at the bow will be highly illuminating and help you to not only understand the impact of weight distribution on your boat but also exactly how much tackle mass you want in the bow.

Everything is a compromise on a boat, and ground tackle weight and placement vs. performance is no exception. In the end you need to decide what outfitting and configuration works for your particular needs.


Sent from my iPhone using Cruisers Sailing Forum

[zeehag]

if your boat plows you have too much weight in bows.
if it pounds you have too much weight astern without compensatory midships weight....
if it hobbyhorses, too much weight at ends, need to redistribute to center...
it is all dependent on how YOUR boat performs, no one can tell you without being there.

[barnakiel]

How the displacement is distributed will play a major (but not exclusive) role: a barge will be affected less, a fine entry upwind killer will be affected more.

Any amount is bad but some amount you cannot avoid: aim for a viable minimum.

b.

[carstenb]

Before yo start worrying about too much in the bows, you might want to consider how much weight will lower you boat 1 cm in the ater:

here is the formula

Tonnes per centimetre immersion // TPC // "The TPC is the mass which must be loaded or discharged to change the ship's mean draft by 1 cm. When the ship is floating in salt water it is found by using the formula: (1.025 x A) : 100, where A is the area of the water-plane in square metres."

you like to read and especially some kind of scientific proof.

Here is a short article that will allow you to mathematically determine your boats center of buoyancy, TPC and trim moments. Reading this and understanding the equations will enable you to gain clearer picture of how much weight you can put into your bows and how it will affect the trim of the boat.

Good article and concise (I learned all this when I took my Yachtmaster - forgotten most of it, I look it up when I need it)

Trim and Stability for Ships, Boats, Yachts and Barges ? Part 2

[accomplice]

Where you wish to cruise will say a lot about the ground tackle you should carry. On my previous 10,000# boat in New England I found that 50' of chain and another 200' of rope worked great everywhere. With my new boat, now much farther from home, all-chain is a must for chafe (and the extra caternary is great too).

As far as weight, I would suggest if you carry a lot of chain that you find some way to store it farther back and lower than the typical anchor locker on a 30'-er. If you can store it in the bilge, you'll be able to get away with a lot more weight.

[CCR2580]

I have a 40' Catalina MkII and Catalina recommends no more than 300lbs in my bow locker! Just a FYI!

[Alan Mighty]

Originally Posted by carstenb

Here is a short article that will allow you to mathematically determine your boats center of buoyancy, TPC and trim moments. Reading this and understanding the equations will enable you to gain clearer picture of how much weight you can put into your bows and how it will affect the trim of the boat.

Good article and concise (I learned all this when I took my Yachtmaster - forgotten most of it, I look it up when I need it)

Trim and Stability for Ships, Boats, Yachts and Barges ? Part 2

Looks like your 'yachtmaster' course was a joke perpetrated on you by clowns, Carsten.

MTI (moment to trim one inch) and MTC (moment to trim one centimetre) are great for static trim.

More important is the dynamic situation, when a hull is responding to wind and wave. That's when resonant pitching, hobby-horsing, and similar phenomena become significant.

And the formulae involved are not simple tonnes/centimetre immersion stuff. Nor are they simple mass times lever arm.

Instead the formulae are moment of inertia or polar moment of inertia, because we're interested in oscillation or repeated cycling around the pitch point. So the formulae are similar to those for calculating moment of inertia in rotation.

The key difference between static moment and moment of inertia in dynamic situations is the difference between a64pilot's WAM (weight x length of arm = moment) and the moment of inertia I = mass x r^2 (where r = distance from the pitch point or the centre of rotation).#

Let us look at a real example - the 15 kg mass of Led Myne's Rocna anchor. If that anchor is in the anchor bow roller, 4 metres forward from the pitch point of Led Myne, it contributes a 15 x 4 = 60 kg.m negative trim moment, pushing the bow down. If I move that anchor to its bluewater cruising stowage position, only 1 metre forward from the pitch point, it only contributes 15 x 1 = 15 kg.m negative trim moment. A 400% difference in trim moment.

But the dynamic moment of inertia is 15 x 4^2 = 15 x 16 = 240 kg.m^2 when the anchor is in the bow roller and 15 x 1^2 = 15 x 1 = 15 kg.m^2 when in cruising stowage. That's a 1600% difference in moment of inertia.

So is it worth taking an anchor out of the bow roller and to stow it closer to the pitch point when sailing offshore or when likely to face chop on a long beat to windward? Is it worth stowing an outboard in a locker or to leave it on a stern rail mount? Should the outboard be left on a dinghy in stern davits? Would it be better to mount a radome at the masthead or compromise and mount it at half mast? Would you pay for a tapered mast extrusion or be happy with a mast extrusion that is a simple column?

In the dynamic situation, you consider (distance)squared. Any change in r is magnified by being raised to the power of 2.

Why was your Yachtmaster course a joke? Because the course did not emphasise the difference between static trim and dynamic moment of inertia.

You have to think about mass and (distance)^2 when considering whether to stow a dinghy in stern davits or an anchor in the bow roller.

See:
* https://en.wikipedia.org/wiki/List_o...nts_of_inertia
* http://hyperphysics.phy-astr.gsu.edu/hbase/mi.html#cmi

Al

Note # I = mr^2 is a convenient simplification for a single mass, such as an anchor or an anchor windlass that can be considered as a point mass. The contribution to moment of inertia from an anchor chain that extends unevenly over an area is slightly more complex. But the general idea is the same - consider mass x (distance from the pitch point)^2 not just mass x distance because you are in a dynamic oscillating situation, not the static situation a cargo ship is when it's tied up to a dock.

[SkiprJohn]

Originally Posted by Cheechako

If the waterline is still nearly level you are good to go!

There are marks on the hull where a waterline should be established (plimsol). If your bow goes down on these marks you have too much weight in the bow. Only stow as much ground tackle on your bow that you will need. Keep everything else as close to center and amidships as you can.

If you stand on the dock and watch what happens when one of your 200 lb friends walks forward you can get an idea of how it effects your waterline. Now imagine another friend joining him/her.

[carstenb]

Originally Posted by Alan Mighty

Looks like your 'yachtmaster' course was a joke perpetrated on you by clowns, Carsten.

MTI (moment to trim one inch) and MTC (moment to trim one centimetre) are great for static trim.

More important is the dynamic situation, when a hull is responding to wind and wave. That's when resonant pitching, hobby-horsing, and similar phenomena become significant.

And the formulae involved are not simple tonnes/centimetre immersion stuff. Nor are they simple mass times lever arm.

Instead the formulae are moment of inertia or polar moment of inertia, because we're interested in oscillation or repeated cycling around the pitch point. So the formulae are similar to those for calculating moment of inertia in rotation.

The key difference between static moment and moment of inertia in dynamic situations is the difference between a64pilot's WAM (weight x length of arm = moment) and the moment of inertia I = mass x r^2 (where r = distance from the pitch point or the centre of rotation).#

Let us look at a real example - the 15 kg mass of Led Myne's Rocna anchor. If that anchor is in the anchor bow roller, 4 metres forward from the pitch point of Led Myne, it contributes a 15 x 4 = 60 kg.m negative trim moment, pushing the bow down. If I move that anchor to its bluewater cruising stowage position, only 1 metre forward from the pitch point, it only contributes 15 x 1 = 15 kg.m negative trim moment. A 400% difference in trim moment.

But the dynamic moment of inertia is 15 x 4^2 = 15 x 16 = 240 kg.m^2 when the anchor is in the bow roller and 15 x 1^2 = 15 x 1 = 15 kg.m^2 when in cruising stowage. That's a 1600% difference in moment of inertia.

So is it worth taking an anchor out of the bow roller and to stow it closer to the pitch point when sailing offshore or when likely to face chop on a long beat to windward? Is it worth stowing an outboard in a locker or to leave it on a stern rail mount? Should the outboard be left on a dinghy in stern davits? Would it be better to mount a radome at the masthead or compromise and mount it at half mast? Would you pay for a tapered mast extrusion or be happy with a mast extrusion that is a simple column?

In the dynamic situation, you consider (distance)squared. Any change in r is magnified by being raised to the power of 2.

Why was your Yachtmaster course a joke? Because the course did not emphasise the difference between static trim and dynamic moment of inertia.

You have to think about mass and (distance)^2 when considering whether to stow a dinghy in stern davits or an anchor in the bow roller.

See:
* https://en.wikipedia.org/wiki/List_o...nts_of_inertia
* Moment of Inertia

Al

Note # I = mr^2 is a convenient simplification for a single mass, such as an anchor or an anchor windlass that can be considered as a point mass. The contribution to moment of inertia from an anchor chain that extends unevenly over an area is slightly more complex. But the general idea is the same - consider mass x (distance from the pitch point)^2 not just mass x distance because you are in a dynamic oscillating situation, not the static situation a cargo ship is when it's tied up to a dock.

Alan,

I'm aware (and my Yachtmaster course did indeed teach us the difference between static and dynamic loads) of the difference. In tryin gto keep things a bit on the simple side - the formulas I gave will give KISS a platform on whihc to start his thinking.

The hobby horse effect will obviously be a product of more than just his anchor and chain. And the solution will be more than just moving his anchor and chain.

My suggestion would be to start with a staticly correct trimmed boat - sail it in dfferent conditions to ascertain what, if any problems the loading incurs. Then make the proper corrections.

Every boat is different. Some hobby horse for a good word, others are much more resilent. Hobby horseing is also a product of the wave type you are sailing in. Here in the Baltic we have very steep waves with a short period (ideal for producing hobby horseing), whereas blue ocean waves will normall have a much longer period.

I'm not aware of any math formula that wil predict a boat's tendency towrads hoby horsing. There may be one that takes, load, heel or toe trim, boat length, wave period, wave height and hull design into account.

But I'd love to see one

[Jim Cate]

Another factor that influences hobby horsing is hull shape. Hulls with a lot of rocker and symmetrical water lines tend to do it more than those with flatter runs, no rocker to speak of and somewhat assymmetrical water lines. The contrast between our old boat and this one, good examples of both hull shapes, is profound. The Sayer design does not HH at all, but will pound a bit going to weather in a chop and at speed. A good tradeoff in my eye, but not to everyone's taste.

Jim

[Alan Mighty]

Originally Posted by carstenb

I'm not aware of any math formula that wil predict a boat's tendency towrads hoby horsing. There may be one that takes, load, heel or toe trim, boat length, wave period, wave height and hull design into account.

I don't know of any such formula. I think tthat for a newly designed hull, one designed by a nautical architect using one of the several dedicated software packages, the software could produce figures for the boat in notional lightship condition.

For a boat with gear, fuel, water, and crew on board, you could calculate some index of hobby horsing, but it would require a massive amount of data specifying the mass and 3D location of every bit of gear, fuel, water, and crew members, plus an index of rigidity for the crew members etc.

I mentioned anchors in my earlier post, because an anchor can be regarded simply as a point mass and its contribution to polar moment of inertia estimated easily knowing its distance from the pitch point. An anchor chain, extending over an area, would require integration of its mass distribution. But the point of mentioning them is that anchors and anchor chain are dense and can be moved easily.

There are easy ways to get a handle on tendency to hobby horsing and other measures of the time period of motion of a boat. And the easy ways also let you see if a different stowage scheme will reduce the tendency of hobby horsing, especially in the conditions you often experience.

The easy ways are:
(1) start with your boat in 'departure condition': fuel tanks and water tanks full, provisions loaded, crew aboard in their usual positions (e.g. one below, one or two in the cockpit - whatever is your usual situation), but still at a dock with loose docklines.

(2) start the boat pitching (by whatever means is at hand - pushing down on the bow, getting a crew member to move to the bow), and then time 10 pitches of the bow, calculate the period of a pitch cycle, and the frequency of pitching. Call that 'natural pitch period, departure condition' and 'natural pitch frequency, departure condition'.

(3) do the same with roll, resulting in 'natural roll period, departure condition' and 'natural roll frequency, departure condition'.

(4) repeat #3 and #4 when you've arrived back after coming close to exhausting provisions and stores.

(5) repeat #3 and #4 in lightship condition (if you're keen) or in different stowage schemes (especially one that follows of the rules of (a) keeping the ends of the ship as light as possible - including the mast; (b) getting the anchor off the bow and moving anchor chain as close to the pitch point as possible; (c) stowing everything that is heavier than water as low and as close to the pitch point as possible; try also breaking each of those rules to see what happens).

(6) if you have discovered a way of starting a 'standard' pitch (such as getting the same person to jump onto your bow and then jump off) you can also count the number of discernible pitches before the movement decays and the hull is at rest.

(7) in your local choppy conditions, measure wave period as experienced by the boat hitting waves when beating to windward (i.e. it's pointless measuring true wave period, you need apparent wave period in the sense of the wave period the boat experiences at its usual speed).

Then:

* avoid stowage systems that cause 'natural pitch period' in any load condition to be close to apparent wave period; That leads to amplified pitching and is no fun.

As several others on this thread have commented and as I expect you know already, usually: the lighter the ship, the better; the lighter the ends, the better; and the longer the time before pitching decays the better (a surprise to some - you want the ends of the hull to respond to the waves; if pitching decays quickly, it usually means you have a barge, not a boat, or the ends are heavy).

For Led Myne, the natural pitch period in cruising trim is 2.5 seconds; natural roll period in cruising trim is 3.8 seconds (in design light ship trim, meaning the rare occasion the waterline of Led Myne is at the design waterline, 3.9 seconds); roll acceleration (in design or lightship trim) 0.04397 g; and Ted Brewer's Comfort Factor (design lightship trim) 37.0.

As a fun thing to do, if you're at a boat show with new boats in the water (or at a marina dock), you can choose a boat or three and start them pitching by rhythmically pushing the bow down and do the same measurements. At a marina, it's even more fun to have a team of people start a number of boats pitching, so you can see which decays the fastest and the difference in pitch period and frequency. Always good fun.

Al