Weight distribution / layout

lakelandterrier · 28 May 2008, 21:46

I've been considering the layout & weight distribution in my A400 & am wondering if ther's a fundamental problem with it that's affecting performance as she's very slow to step on to the plane and difficult to keep there.

She's configured as a family boat with console & side seat (helm on starboard) with bench seat / locker behind. There's an additonal seat onn the bow locker. REviewing the weight distribution we have 90Kg of Merc. 40hp EFI on th transom, then 0.5m further forward the bench seat plus fuel tank & battery - about 50kg, then me - 80Kg on a lean day, all in th last 1.4m of the hull - a total of 220Kg. Even allowing for the rear tube sponsons this weight is ALL in the aft half of the craft. Is it this that is making her so stern heavy & bow light?

This was the configuration sold to me by a main Avon dealer so I have assumed the weight ditribution would be balanced, but the more I think about it more aft heavy it appears to my non-engineering brain. Does this amount of weight in the after half sound correct or not? What is the ideal dsitribution?

If the layout is wrong how difficult / expensive would it be to replace he connsole & side seat with a single jockey, retainng the bench seat, remembering the loa is only 4m?

Alternatively am I doing something else wrong? All thoughts & opinions - especially considered ones warmly welcomed.

LT

Locozodiac · 28 May 2008, 22:13

Quote:

Originally Posted by lakelandterrier

I've been considering the layout & weight distribution in my A400 & am wondering if ther's a fundamental problem with it that's affecting performance as she's very slow to step on to the plane and difficult to keep there.

She's configured as a family boat with console & side seat (helm on starboard) with bench seat / locker behind. There's an additonal seat onn the bow locker. REviewing the weight distribution we have 90Kg of Merc. 40hp EFI on th transom, then 0.5m further forward the bench seat plus fuel tank & battery - about 50kg, then me - 80Kg on a lean day, all in th last 1.4m of the hull - a total of 220Kg. Even allowing for the rear tube sponsons this weight is ALL in the aft half of the craft. Is it this that is making her so stern heavy & bow light?

This was the configuration sold to me by a main Avon dealer so I have assumed the weight ditribution would be balanced, but the more I think about it more aft heavy it appears to my non-engineering brain. Does this amount of weight in the after half sound correct or not? What is the ideal dsitribution?

If the layout is wrong how difficult / expensive would it be to replace he connsole & side seat with a single jockey, retainng the bench seat, remembering the loa is only 4m?

Alternatively am I doing something else wrong? All thoughts & opinions - especially considered ones warmly welcomed.

LT

A photo is better than one thousand written words, send one of your boat/engine configuration for a high tech evaluation... Is your 40 HP engine achieving full throttle speed ?

Happy Boating

walruz · 28 May 2008, 22:33

You are carrying many more seats and options than I have in my 5.4m. I think you do have an over stern heavy boat which may be helped by shifting your fuel tank as far forward as possible. I would also think about losing some of those extra seats. That weight adds up fast and you are only running 40hp on her.

Ian

BumbleAbout · 29 May 2008, 11:34

I too had a stern bench seat/ locker containing fuel tank and bettery box (on a 4.2m Zodiac). In choppier water it ideally needed someone to sit right at the bow which was not very comfortable (or safe) to get on the plane.

I have now moved the fuel tank to a newly constructed bow locker which also holds anchor, chain, warp and a spare fuel can. The battery box is now under the (small) console and the rear locker is only used for lightweight storage.

My rough calculation was :
- A 6 (imperial) gallon fuel tank weighs about 20kg (1 US gallon is 6lbs from memory)
- The pivot point on my boat is approximately 0.5m from the stern when planing and the rear mounted fuel tank and battery were roughly on this pivot point
- Moving the 20kg (full) fuel tank 3.5m forward is equivalent to an additional 70kg weight at the bow (mass x distance to pivot)
- Moving the 10kg battery box 2m forward is equivalent to an additional 5.7kg at the bow

Net result 75.7kg additional weight at the bow with a full tank. In practice, anything between 50kg and 90kg of body sitting at the bow worked (I like it when theory is proved in practice).

This has made a huge difference and means that the boat is far less sensitive to who sits where. However, I still move things around when towing to achieve an appropriate tow bar weight (empty fuel tank and anchors banished to the stern locker again).

I would suggest that you can add weight at the bow (sandbags/ bodies/ water/ spare fuel etc) to work out what works for you before moving fuel tanks etc.

lakelandterrier · 29 May 2008, 21:58

Thanks for the replies.

Total seats are only 3 - console side seat plus the bench, so I can't lose those with current layout.

I could move the fuel tank into the console, but that's only moving 25Kg 1m forward. spare fuel's already in the console. Anchor locker's moulded & full of anchor, chain, anchor rope & 60lb of lead ballast added at the start of the season & has made no difference.

Is a relay the best option with a jockry helm position futher forward?

LT

spartacus · 29 May 2008, 22:50

Your boat sounds a bit like my Zodiac 7-man Pro. Bench seat and console with bow seat and anchor locker. There's a 25 litre fuel tank under the bench seat with a 25 litre spare fuel tank under the console. Powered by 2-stroke 40hp Yamaha. Only difference is 3.5hp Tohatsu auxilliary engine (not fitted here).

With 3 people on board - two are on bench seat and one on bow locker. Any more - are located up front on tubes. The engine is trimmed to central position. Again, this could be adjusted to keep the bow down.

Moving to a jockey seat configuration will be a hassle, not to mention unecessary expense given your boat length. You could always trade-in 40hp and upgrade to 50hp if transom is rated for slightly larger engine.

The Yamaha 40/50 are effectively the same engine with uprated carbs and exhaust manifold. Not sure about Mercs though?

BumbleAbout · 30 May 2008, 11:06

Quote:

Originally Posted by lakelandterrier

Anchor locker's moulded & full of anchor, chain, anchor rope & 60lb of lead ballast added at the start of the season & has made no difference.LT

If you have an 75kg person (or some equivalent weight sandbags or water containers) on the anchor locker - does that make a big difference to the balance ?

60lb of lead ballast might not be enough and you really need to establish how much the weight/ balance needs to change before you undertake more drastic measures (like moving consoles or changing seating). If the performance is badly affected by additional weight, then it might just be a case of strapping the spare fuel cans (whilst on the move) near the anchor locker and moving the fuel tank to the console - this CAN have the same effect as a 75kg person sitting on the anchor locker !

PeterM · 30 May 2008, 21:10

Same boat & similar (ish) problem from maindealer. If you search for posts by me you will see pics in 'lets see your rig' thread. I've got a 50 4/s Yam ( heavy) on the back & bench seat 55 cm from transom - then 36 cm in front of that is the console( slightlybigger than normally fitted to A400 - but not much) - in this is 25 l tank & battery. Bow locker has 5 m chain & 5kganchor.

We used to get loads of bow lift up to planning speed ( 12-15 knts ) as a big wall of water was pushed along. Looked at trim tabs & doelfins. Went for the fins & a much happier boat at all speed - now much less bow lift & planes at 9knts. Lost 2/3 knts from top end ( max now 30knts). horrible moment drilling the holes in a new outboard when I fitted the fins but well worth it & I thought what the hell- if it doesnt work new lower leg & move on to moving seats etc.

Nothing heavy goes in bench seat in mine ( apart from 2nd tank if I'm going a long way ) . I would seriously recommend try the fins - best worse case in 4 holes in plate which wont have any worthwhile effect if left .

I thought the same as you with my boat , but all fixed for £25 ! PM me if you want to go through the set up etc. I'm off to look at a bigger RIB tommorow anyway - I have the bug - mainly due to great fun being had in a 4m avon over the last18 months !

PeterM · 30 May 2008, 21:23

post 348 of the thread showspics of my 'rig'

PeterM · 30 May 2008, 21:46

Why put a bigger HP engine on it ? at low speed where the problem is you are not using the power so it wont help.........I have 50hp (max rated for hull) & its 110kg ! If I out 100hp on it @ 180 kg I would swamp the thing

BumbleAbout · 02 June 2008, 09:35

Hmmm - I'm now tempted to try the doel fins on mine ! I had a play on a 6.2m (Rib-x) with a 150hp Etec at the weekend - that certainly puts a smile on your face - unbelieveably smooth progress through the waves !

Rodi · 10 December 2023, 02:36

Hello, just looking for some advice on my new 6.8 Tornado rib, considering weight distribution, looks to me that there is to much weight affecting the bow, to much weight in the bow and forward centre of the boat. Sharing with you guys a pic so you can relate to the subject

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I’m thinking on moving the 70 kg batteries to the transom box! What are your opinions?

The middle of the boat is between console and sit box.

Console : 70 kg of batterie’s.
Sit box with 140 lt aprox. More than 140 kg
Trolling motor on the bow: 33 kg
Motor 150 hp : 220 kg