correcting my attitude?

Grehan

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Grehan is a GRP river cruiser, 12m x 4.2m, 0.9m draft and fairly flat hull shape with a long central keel, pointy at the front and squarish at the stern. Approx. 7 tons. Her attitude is noticeably bows-down by something like 50mm, whether by design fault or change since she was built (she has a different engine, but it's in the same place - nothing else significant that I can think of). She's not loaded with anything much at the bows, or forward of her mid-point. Tanks are located one on each side, in line with the engine).
The situation means that water collects in 'downhill' places when it ought to run aft and away.
Is there any rule-of-thumb calculation for how much weight I'd need to add at the stern (it would be possible, I have some unused below-decks/waterline locker space running across the stern) to make a correction?
Is this actually feasible and if so, what's the best/cheapest way to do it?

As ever, thanks for any wisdom on the matter.
 

capsco

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The easiest way I can think of is, to have increasing numbers of people stand at the aft end until you have the desired effect on your crafts trim.

Then either guess their weights, or have them on the scales????, when you have a required ammount yo[U will have a better idea of how to proceed and with what materiels.
 

Twister_Ken

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Sounds as if she may have been re-engined with something lighter?

If so, can you find out the weight of the original? Then subtract the weight of the new and you'll have a correction figure. Make it a bit less if your ballast space is actually aft of the engine.
 

Jegs

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Trim

I believe some of the river cruisers positioned the engine off centre using hydraulic drive to prop & then needed to ballast to balance - one owner told me that his boat's engine was located in the starboard aft quarter & that concrete [sic] was then used to balance in the port quarter..... then more concrete was needed forward!

John G
 

ProDave

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Surely to get 0.9 metre draft in a (nearly) flat bottomed boat, it will have to be loaded with a lot of ballast.

I think it's a question of lift the floor boards to see what ballast is used (concrete blocks often, certainly in narrowboats) and either just remove some of the ballast from the front, or move some from the front to the rear.
 

Grehan

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ok, responding to the points.
1. The engine is located just aft of the mid-point and the Nanni replaced a Peugeot, similar spec and size (I guess) and I think the difference in weight at that position would have had to be substantial to cause the problem. But I don't know, obviously.
2. The engine's fully on the centre line. Prop shaft drive as from original.
3. There's no visible ballast - presumably other than is (hidden) in the keel. Maybe (?).
4. The people idea is a neat one . . but I'm holed up in the (gorgeous) French countryside at the mo' and other than grabbing the occasional rambler and holding them hostage one by one until numbers build up . . . We've had a bunch (half a dozen, maybe) of people on the aft deck in the past - didn't seem to make a lot of difference, but I wasn't really thinking of attitude at the time, more like alcohol . .

I had thought of using sacks of sand or finding some dead lead batteries to use as ballast, but don't want to waste my time if I need a mountain of either in order to make any correction.
 

Twister_Ken

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ok, responding to the points.
1. The engine is located just aft of the mid-point and the Nanni replaced a Peugeot, similar spec and size (I guess) and I think the difference in weight at that position would have had to be substantial to cause the problem. But I don't know, obviously.

Could well be. Modern engines are quite lightweight compared to earlier generations. Comparing a 24hp Bukh (older designer) with a 20hp Nanni gives respective weights of 210kg and 106kg. The Bukh is twice the weight.
 

Wunja

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Could well be. Modern engines are quite lightweight compared to earlier generations. Comparing a 24hp Bukh (older designer) with a 20hp Nanni gives respective weights of 210kg and 106kg. The Bukh is twice the weight.

Agreed. The difference between the Volvo MD6 and it's replacement Beta was something like 90 KG. I also have water that needs to run uphill to get out of the scuppers, but having had two hefty peeps in the cockpit, I didn't notice that much change in attitude.
 

Grehan

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Yes, if I (only a bit less than 100kg) walk from stem right to the stern it makes narry a jot of difference to her attitude; there's quite a lot of buoyant hull surface area (~40sq.m) to 'press into' the water.
Maybe it would take 10 people to make a difference, which could be ~1,000kg (a metric ton) :eek: (or about 35 100Ah batteries or bags of sand)
Does that sound right? (It doesn't sound feasible)
 

capsco

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With the hull shape of your boat I guess it does sound right, any boat that has a bow down trim is usually difficult to steer, it would be worth considerable effort to cure this problem alone.
 

neil_s

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You say the stern is 'squarish' Does this mean you have a transom (flat) stern? Something to bear in mind is that if you immerse the transom by adding ballast at the stern, it will add considerable drag and might lead to decreased speed and higher fuel consumption.

Neil
 

Grehan

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ok, thanks for all the comments - a few more notes . .
restribution of weight? - not really, she isn't carrying (that) much and very little of that forward.
transom and steering effects? - well I doubt it, she's a river boat normally making 6kph/3kn maximum on the inland waterways and 50mm (max.) isn't going to make that much difference anyway - except to the 3 or 4 pools of water that collect where they should run away.
I think I'm resigned either to forgetting a relatively minor phenomenon and swabbing, or to finding spaces for quite a lot of sand - don't think I can afford lead, but maybe steel grit?
ah well :rolleyes:
 

Downsman

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Rather than recruit human ballast or haul batteries aboard, get a couple or three of the the large blue plastic barrels that are used in industrial products transport. Drill them low down and fit bungs/taps for emptying. Put them on your stern deck empty, (they're quite lightweight) then fill them with water until the boat trims as you want it to. Water per gallon/litre has a known weight and you'll know how much you ship via a hose because the barrels are known volume.
You can then use ballast to that weight once you've got the trim as you want it.
 

Andrew_Fanner

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What about using water? as its in plentiful supply.. just need to find something to contain it in..

Find an old oil drum or too, fill them up? 1litre - 1kg...

Is the boat a "Bounty" style with forward controls similar to those you see on the Broads? I ask because a club member owned one a while back and I recall he found water below decks up forrard. All bilge rather than a leak anywhere, but might be worth a check. Spread around a bilge a tonne of water is not actually that deep.
 

rotrax

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Grehan is a GRP river cruiser, 12m x 4.2m, 0.9m draft and fairly flat hull shape with a long central keel, pointy at the front and squarish at the stern. Approx. 7 tons. Her attitude is noticeably bows-down by something like 50mm, whether by design fault or change since she was built (she has a different engine, but it's in the same place - nothing else significant that I can think of). She's not loaded with anything much at the bows, or forward of her mid-point. Tanks are located one on each side, in line with the engine).
The situation means that water collects in 'downhill' places when it ought to run aft and away.
Is there any rule-of-thumb calculation for how much weight I'd need to add at the stern (it would be possible, I have some unused below-decks/waterline locker space running across the stern) to make a correction?
Is this actually feasible and if so, what's the best/cheapest way to do it?

As ever, thanks for any wisdom on the matter.

Hi Greehan, I suspect a ballast problem as previously mentioned by other posters. Howerver, many years ago when I was a young man my wife and I plus a neibour and his wife purchased an 18 foot plywood cruiser from the Thames near Oxford. Once it was sorted and we started using it we had a similar problem. It had a particular hull shape-by design or accident-we never were sure, which meant it only had the correct attitude when under quite high power. As this was above the speed limit of that time we had a permanent bow down attitude. Some old 56lb shop weights fixed that OK. About 5 years ago at a Boat Show I picked up a leaflet advertising an "Attitude Correction Kit" I took it to work as I had a workmate who needed help in this area......... It was good to meet you last month-hope you can come to another WHCC talk. Best Regards, Rotrax.
 
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