Centre of gravity

[IanPereira]

Hi
I've added a hard top on my aft cabin cruiser. I have 2 x200 watt solar panels on it. The boat rolls a fair bit and I'm wondering whether the extra 150++kg right up top has affected the boats balance. Should I add additional weight to the bilge to compensate?.

Thanks

 

[twistedtree]

The hardtop and solar panels will definitely affect the boat's balance, center of gravity, and roll period, and a number of other things. How much, and is it too much is a whole other question that is nearly impossible to answer without a lot more info and knowledge about your boat.

Your best bet would be to ask the manufacturer if they happen to still be in business. You could also engage a naval engineer, but that would probably be costly.

What's the boat's displacement? Does it roll more now than before the additions? How heavy is the hardtop?

 

[IanPereira]

The boat is a 44 foot hiptimco aft cabin boat. Company has closed down. With the solar panels, radar and fibre blasts hard top and stainless support. I would say roughy 150kg right up top. No information available prior to the addition as I added it just after purchasing the boat and previous owner had left the boat unused for the past 10 years.

 

[Nomad Willy]

IanPereira,
The fact that you asked is an indication that you're concerned. Any unnecessary weight aloft is undesirable. But weight on the cabin top will probably dampen the roll. That is make it a slower motion and more comfortable. Sometimes ballast is installed as far outboard as possible under the decks near the cap rail. We have 4000lbs of ballast deep in the hull (put there by the builder) and we probably have more latitude than most to add weight to our cabin top but in I'd say "avoid it".

Is this necessary or a "green hobby"?

 

[FF]

Check the roll period , rock the boat from side to side , measure the reverse time .

down on port side changing to coming up, to the same condition when it repeats after a full roll.

Should be 4 to 6 seconds, or so.

 

[Tad Roberts]

I'm not really familiar with these boats, is there any designer named? If not it could well be there never was any stability data or study done. That would not be unusual. Boats built in the 70's and early 80's were pre-PC and stability studies were huge time consuming projects that were almost never done on pleasure boats. And none of that necessarily means there is or could be a problem.

But stability is not intuitive. I do stability studies for a living, these are of commercial vessels for certification by Transport Canada. Almost invariably, prior to my study, the owners will tell me there is no boat as stable as theirs is.......Yet they often will barely or not meet the requirements. Capsize of fishing vessels was epidemic in BC in the years 1995-2005. During that time something like 150 boats capsized and 160+ guys drowned. Many of these accidents were due to fishermen modifying their boats without consulting anyone qualified in stability information. So now we have rules requiring that, plus a stability education program that has changed totally the incidence of capsize.

A single hardtop and solar panels are probably not significant. The ones that scare me are the fully enclosed sundeck lounge, fully enclosed flying bridge, huge roll bar with multiple antennae, giant hydraulic dinghy crane, and a 1500 pound hard-bottom inflatable to top it off......yikes!

 

[markpierce]

I suppose there are scientific ways to measure stability.

 

[IanPereira]

The designer of the boat is Ed monk jr. Soft chine aft cabin cruiser.
yes I was a bit worried about it. But the boat didn't come with a gen set and I added one in the engine room. Would this compensate for the additional 200-300 lbs up top.

 

[IanPereira]

Ff. sorry 4-6 seconds after a full roll?. How do you measure the roll. I understand the test requires the boat to be pulled down onto one side and released. I don't have the facilities for the test. Is there any other method. This boat weights around 23000 lbs. not easy to make it roll without a wave.

 

[dhmeissner]

I suppose there are scientific ways to measure stability.

He's wearing a tie, so he must know what he's doing!

 

[FF]

Is there any other method. This boat weights around 23000 lbs. not easy to make it roll without a wave.

No problem , just move from side to side to get the boat rolling.

The depth of the roll is not important (no gunnel to gunnel roll is required) just the time period.

Just as important as time (although there is zero you can do about it) is how hard the boat checks and reverses.

For a seakindly ride a snap , quick reversal is not desired.

The weight on the top will help slow a snap.

In many old fishing schooners an anchor would be hauled up the mainmast to slow the roll as a storm came on.

 

[Tad Roberts]

You are talking about a roll period test. I think with google you can find a USCG roll-period test procedure.

The depth of the roll is very important. Every boat has a different roll period, flat-bottomed powerboats or large outside ballasted sailing yachts will have a short roll period, sometimes less than 2 seconds, and an ocean liner will have a roll period of over 20 seconds. Hopefully your boat will be some place in between.

A full roll is.....All the way down on one side (port or starboard), over to all the way down on the opposite side, and then back (to the starting point) to all the way down on the first side. Get three friends, a stop watch, and paper and pen. Tie the boat loosely alongside a dock. The dock lines must not restrict the rolling. You stand on the dock with stopwatch and paper. Your friends can stand with one foot on the dock and the other on the rail of your boat, and just shift their weight from one foot to the other to get her rolling. Or they can, at your call, get on and off the boat in unison to make her roll.

So they step on and off and get her her rolling, you yell stop when she's at the bottom of one roll (towards the dock is easiest to recognize). Start the watch when you yell, your friends stop jumping on and off and stand on the dock. You time 2-3 rolls and stop the watch. Do this at least 3 times, then average a single roll period.

The roll period should be close to the maximum beam of the boat in metres. This indicates adequate GM length. G is the center of gravity of the boat, M is a mythical point that's a mathematical function of the hull shape and weight (displacement). As long as M is above G your stability is positive. A rough formula for GM (feet) from roll period is

GM = (2R / T)^2

T = roll period (one complete roll)
R = 0.21-0.23 * Beam in feet

Your GM should be somewhere between 1 and 3 feet.

 

[rjtrane]

We've done static incline tests using a plumb bob and a measure able weight that we can move from the CL to the beam.

 

[Tad Roberts]

We've done static incline tests using a plumb bob and a measure able weight that we can move from the CL to the beam.

Right, the inclining experiment is the established method for ascertaining an accurate GM. But you need other accurate data to gain that accuracy. If you guess at displacement it will be just as inaccurate as the rolling test (which uses fudge factors and averaging). For the inclining calculations you need the hull lines drawing (or a 3D computer model), accurate flotation measurements, specific gravity of the water in which the boat is floating, and accurate info on current loading (including tank contents, inclining weights, and crew).

For many stock production boats an accurate hull lines drawing does not exist, or at least the owner's can't lay hands on them.

In either test (inclining or roll) liquid free to shift in tanks will screw up the results. It's called free surface effect. To maintain accuracy tanks must either be completely full or completely empty so that the contents shifting to the low side does not increase heel angle.

 

[RickB]

For many stock production boats an accurate hull lines drawing does not exist, or at least the owner's can't lay hands on them.

We often have a hull laser scanned to develop the hydrostatic data we need to produce an updated stability book as builder's data is either wrong, very outdated, or unobtainable.