Rethinking Hobbyhorsing/Pitching

[terhohalme]

Yes, Rick, transversal RM has nothing to do with hobby-horsing, but longitudinal stability has. As I see it, the longitudinal inertia of boat is the generator of hobby-horsing and the longitudinal stability is the damper of it. You can decrease inertia to move masses towards LCF (longitudinal center of flotation, nearly in the middle of DWL in catalac) or remove extra masses out of the ship!

The main problem with Catalacs, Heavenly Twins, some Wharrams, old Hobby Cats and other heavy, short waterlength boat are the low longitudinal stability (or low longitudinal metacentric height). This is a function of longitudinal second moment of waterplane devided by the mass of the boat. Longitudinal stability depens on the third power of waterline length, so the influence of it is huge. Sorry, you cannot lengthen the boat...

I think, if the calculated longitudinal stability of the boat is well under 10 m, the boat is sensitive to hobby horsing. You can check it from the spreadsheet here:

Boat Design Forums

Just remove all the extra mass out and leave only essential. Or think less weighty solutions.

Terho

[ssullivan]

Originally Posted by rickm505

Oh, I'm fine with your post. I just wanted to get the discussion back to Sean's hobby horsing issue. Check your PM

And thank you for doing so, Rick.

The RM/CB/CG models at hand do have nothing at all to do with hobbyhorsing, in the configuration they are being discussed. I'm all for a good discussion... but I hope to harness all the pent up analytical skills present in this thread to talk about the same figures - RM(a different one), CB, and CG in a fore and aft, rather than athwartships line.

Take the catamaran in your examples, and rotate it in the plane of the water by 90 degress (without waves, wind or anything). Now continue the debate.

Ha ha I'm having some fun with this, not being a jerk. Hopefully, that comes through.

I have no trouble with capsizing, RM from loads and wakes coming athwartships, etc... my bows seem to shoot for the sky too often, resulting in a hard fall back to Earth (sea?) and sometimes a slam.

Today, I realized a little bit of my perception of the bows shooting skyward might have something to do with it being an aft cockpit boat, coming from a center cockpit. When a boat pivots on its central point (where CB and CG come together at rest on a cat and mono), if you are stationed above that point, you don't percieve as much of a lift in the bow. When you are stationed way aft of this point, you can see more of the hobbyhorsing, since you're sitting on the extreme other end of the see-saw.

So anyway... should I move my stuff forward?

[ssullivan]

Good post. This is what I had observed today. It's like it rocks back and forth on those rounded hulls - like a real hobbyhorse. I also felt that if I had some LWL (tacked on the bows especially), I would have less of a problem.

I mean it's nothing to worry about and nothing too uncomfortable or annoying. I just like things to be perfect on a boat.

I kept picturing adding on some long bows extending about another 10ft - plumb bows. Seems like that would slow the initial acceleration skyward somehow. I will have to give all of this some more thought when my head isn't fried from the delivery I'm on. The theories are proving a bit difficult for my 1/3 of a brain cell to work with each night.

Originally Posted by terhohalme

Yes, Rick, transversal RM has nothing to do with hobby-horsing, but longitudinal stability has. As I see it, the longitudinal inertia of boat is the generator of hobby-horsing and the longitudinal stability is the damper of it. You can decrease inertia to move masses towards LCF (longitudinal center of flotation, nearly in the middle of DWL in catalac) or remove extra masses out of the ship!

The main problem with Catalacs, Heavenly Twins, some Wharrams, old Hobby Cats and other heavy, short waterlength boat are the low longitudinal stability (or low longitudinal metacentric height). This is a function of longitudinal second moment of waterplane devided by the mass of the boat. Longitudinal stability depens on the third power of waterline length, so the influence of it is huge. Sorry, you cannot lengthen the boat...

I think, if the calculated longitudinal stability of the boat is well under 10 m, the boat is sensitive to hobby horsing. You can check it from the spreadsheet here:

Boat Design Forums

Just remove all the extra mass out and leave only essential. Or think less weighty solutions.

Terho

[Fard12]

Nothing I've said here, changes anything I said previously. If you think I've made a factual error, then please address that point directly - I would be glad to discuss it with you. We'll get more out of the forum if you avoid the cryptic pot-shots.

Kevin[/quote]

Sorry, I didn't mean it to be a pot shot towards you.
And from you post you now seem to have a reasonable understanding.

[Maren]

Originally Posted by ssullivan

I'm having some weight distribution issues with my new boat.

Now, since we are talking hobbyhorsing or pitching, (bow up, stern down, etc...), the discussion is exactly the same as for a monohull, with the exception that I don't have that extra bit of weight at the fulcrum point of the pitching.

I know there are some other issues you've had (previous owner repowering) but I think the main issue is hull form and I don't see you being able to fix that.

This article by John Shuttleworth discusses seaworthiness in multihulls. There is a section on hobbyhorsing and the description (fine v-like hulls and lot of rocker) match the photos and plans you presented.

Hope this is of use.

[Lodesman]

Originally Posted by David M

I think we are thinking along the same lines concerning a catamarans stability.

David, I agree. The 'cryptic pot-shot' was from Fard12; sorry for the confusion.

Kevin

[David M]

Originally Posted by ssullivan

Good post. This is what I had observed today. It's like it rocks back and forth on those rounded hulls - like a real hobbyhorse. I also felt that if I had some LWL (tacked on the bows especially), I would have less of a problem.

I mean it's nothing to worry about and nothing too uncomfortable or annoying. I just like things to be perfect on a boat.

I kept picturing adding on some long bows extending about another 10ft - plumb bows. Seems like that would slow the initial acceleration skyward somehow. I will have to give all of this some more thought when my head isn't fried from the delivery I'm on. The theories are proving a bit difficult for my 1/3 of a brain cell to work with each night.

Sean...that's exactly it! Put in technical terms, more buoyancy put further forward would shift the center of buoyancy further forward as the trim changes (the boat pitches). This difference between the location of the center of buoyancy and the center of gravity is your moment arm that wants to force your boat back to level. The greater the moment arm (force times distance) the more force there is to put the boats trim back to where the CB and CG are in line with one another again (back to being level).

[Tropic Cat]

Originally Posted by Maren

I know there are some other issues you've had (previous owner repowering) but I think the main issue is hull form and I don't see you being able to fix that.

This article by John Shuttleworth discusses seaworthiness in multihulls. There is a section on hobbyhorsing and the description (fine v-like hulls and lot of rocker) match the photos and plans you presented.

Hope this is of use.

Great article Maren. Mr. Shuttleworth's paper hits the nail on the head. As I recall, readng some of his papers probably gave me the idea about rocker and hull shapes. David is right, more buoyancy forward seems to be the answer.

From Shuttleworth 's article...

"Many early multihulls were prone to hobbyhorsing, and pitching. This was caused by too much rocker on the hull profile, and fine V sections both fore and aft. As hull shapes improved tending towards more U shaped underbodies particularly aft, pitching still remained a problem, because the large width of the stern sections caused the sea to lift the sterns as the boat passed over the wave, driving the bow down. However we now know that pitching can be dramatically reduced by finer sections at the stern combined with the center of buoyancy being moved forward in the immersed hull, and aft in the lifting hull (ref 2 and 3 ). This effect can be achieved in both cats and tris, giving a very comfortable and easy motion upwind. At the same time windward performance is improved, because the apparent wind direction is more stable across the sails."

Boy was I wrong..

[Lodesman]

Originally Posted by ssullivan

I kept picturing adding on some long bows extending about another 10ft - plumb bows. Seems like that would slow the initial acceleration skyward somehow.

From the keel profile of your Catalac, it looks like pitch aft would cause the CB to move aft considerably, thus giving a large RM. However when it pitches forward, the CB will not move very much at all - so the RM would be relatively short. This would make it seem very tender when pitched fwd. I wonder if the PO moved the CG aft (by moving the batteries aft) to counter this. Conceivably, you could fix this by adding buoyancy forward - perhaps bow-bulbs.
When you previously described the problem of the bows shooting skyward, what I considered to be the problem was that as the swell hit the bows and had the effect of immersing them, it moved the CB well forward. Since the CG is located aft, the large effective RM caused the bows to pitch upwards. Unfortunately, if this is the case, adding buoyancy forward will only exacerbate the problem. The only solution would be to move the CG forward.
I don't believe that removing weight is the solution, as this will tend to cause the CG to rise, which would make the rocking worse, without really helping the bow-pitch-up problem. Your best bet is, in addition to moving the CG forward, is to try to lower the CG by moving or adding weight low in the hulls, and/or removing weight from up high. Good luck.

Kevin

[bill good]

SSULLIVAN,
G'day you have not stated how the boat is sitting!! Is it on the waterline?

Also what is it like under sail? The other quest of expected speed from the motors 27' 6.5Kts not sure what mod motors but 2500RPM sounds about right for 5kts. My 1GMS are 3100 for 7Kts but the 2bladed folding props are not the best.

Regards Bill Goodward

[Nightwatch]

Good point. Trim is important. I was wondering what type of ground tackle you have and how far forward is your chain locker(s). I am thinking Maine (rocks); the islands (coral) equals a good bit of chain somewhere. You might be able to play around with moving the chain to different lockers to play with your trim. How much other stuff is aboard. Bareboat charters are usually pretty stripped out short of bare necessities. Owners have a tendency to collect stuff, even knowledgably multihullers.

[ssullivan]

I think a lot of the thread contributors are picturing my pitching motion in a way different from how it is happening.

Here's what's happening:

When I hit a wave head on, the bow goes quickly up for the sky (I see nothing but sky from the helm), then comes belly flopping back down. The bow isn't too heavy from what I can tell... it's too light relative to the long list of stuff I listed in the stern. At least... that's what it seems like to me.

I figured adding extra length to the bows would do this:

*Add a lot of weight forward way at the forward end of the boat, keeping it down when it was going to want to pitch skyward. It would do that, and at the same time would have a little bit more buoyancy to allow it to ride up any really steep waves met head on.

I wonder if hobbyhorsing was the wrong word. Maybe it would be better described as a "blast off." I see nothing buy sky most of the time, in between smashing belly flops.

In my experience (my last boat, which handled like a dream), I was standing at the center of gravity, center of buoyancy, and it basically went up and over waves like an elevator. It just went up 10 feet and back down 10 feet without the bow going skyward or stern going skyward. It was balanced as the waves went below, just rising and falling with them.

This boat seems to be driving up the wave, then launching the bow into the air, then belly flopping.

The balance is off.

Make sense?

Yes, I do have 120ft of chain and a 45lb Delta forward. That's it though. Not much weight compared to the stuff aft:

6 Trojan Batts
2 Yanmar 30 horse engines
2 Diesel tanks
RIB Dinghy
8hp Outboard
Small portable gas generator
Propane tanks

.... all in the cockpit area.

The water tanks are amidships, as is most of the food and all my tools.

I'm just not sure what I need to do to rebalance the boat, especially given I don't have a budget for it. Bow bulbs sound great, but I have about $10 to spend on this project... lol

Some answers:

My trim would be defined as "bow up" and much more so when under power. The sterns sink into the water under power enough to put the exhaust thru-hulls right down in it at times.

On the waterline, the boat is "bow up"

I am also following what Lodesman said. Makes sense, and this is what I seem to see. The bows seem light and the stern seems heavy.

[Tropic Cat]

All boats have their engines towards the stern.

It's just that monohulls have all that ballast that trims the boat out. Your catalac seems to be suffering from an extreme case of stern heavyitus (is that a word?). So how about you take your $10 to the nearest home improvement center and invest in 4 - 50 pound bags of concrete mix for your foredeck lockers? Let's load this baby up.... (OK, I'm only 1/2 serious). But , As Gord mentioned earlier, more mass = less movement.

Sean, you didn't answer the earlier question of whether or not the boat was on her lines.

Does anyone know if it's safe to store / charge T105 wet cells below the cabin, in the bilges?

[BigCat]

Cats with prop shaft drives typically (though not always,) have their engines well forward compared to cats with saildrives. In this particular case, judging by the boat's behavior in wakes or waves, it sounds like the boat is down by the stern.

[ssullivan]

Originally Posted by rickm505

All boats have their engines towards the stern.

It's just that monohulls have all that ballast that trims the boat out. Your catalac seems to be suffering from an extreme case of stern heavyitus (is that a word?). So how about you take your $10 to the nearest home improvement center and invest in 4 - 50 pound bags of concrete mix for your foredeck lockers? Let's load this baby up.... (OK, I'm only 1/2 serious). But , As Gord mentioned earlier, more mass = less movement.

Sean, you didn't answer the earlier question of whether or not the boat was on her lines.

Does anyone know if it's safe to store / charge T105 wet cells below the cabin, in the bilges?

Hey, now *that's* a fix within my means! ha ha

I was really thinking about something like that. A little weight forward to keep the boat on a more even trim as it rides up and down the waves. It should be, as I said in the other post, like an elevator, not like a roller coaster.

I had Trojans down in my bilges on my last boat. I didn't die... I do have to do something to re-distribute weight, I think. Experimenting by just tossing heavy weight in the bow might be something quick, easy and inexpensive to test out the theories.

As to trim, here is a picture of it.

PS: I can do one better than $10 for concrete. How about $0 for some rocks found on the shore??