Gambian pirogue - building a keel

Hello everyone!

I'm new to this forum but have just spent the last 2 hours reading through the site - it's great to see how helpful people are! Maybe someone out there can offer some advice for my little project?

7 years ago I built a birdwatching and fishing lodge on a freshwater island 200 miles up the Gambia river in West Africa... www.bsc.gm ...and have been encouraging people to come and visit ever since!

As we're on an island (and Gambian roads have more potholes than tarmac...), boats are an important way of getting around. We have a couple of traditional Gambian pirogues... hopefully you can see my photo - if not see www.hiddengambia.com/live/discover_the_river.asp. [I'm no boat expert, but I've been told this design of boat was brought to The Gambia by the Portugese in the 15th century].

Whilst the larger of our 2 boats (the Lady Hippo) is fine, the Safari Queen (which is somewhat smaller and narrower) suffers from a balance problem - especially when too many people are on the top deck (...and all rush over to one side whenever someone spots a hippo!). We're currently planning some repairs to the wooden hull of the Safari Queen and expect we'll be concreting it with ferro-cement. Now is therefore an ideal time to also address the balance issue.

It would seem that we need a keel.....

I'd be immensely grateful if any of you wise old folk could help advise - or point me in the right direction. i.e.

- What shape should the keel be?
- How deep should it be? There are some sandbanks in the river...
- Where should it be positioned?
- Which materials should we use?
- How should it be fixed?

I guess I'm trying to learn in a day or two what others have spent a lifetime to perfect... Nonetheless if you can help us avoid some obvious (to you) blunders that would be great.

Many thanks.

 

Although I think it's not the answer you want to here, in my opinion you need ti limit the amount of people on the top deck!!! By adding a keel you'll be adding weight, and to the looks of it, you don't have that much freeboard to add a keel and carry the same amount of people.
On cruise ships, one of the stability calculations is with the passengers on one side of the vessel. If the vessel fails the criteria, the designer may add tables (fixed to the floor) in the design, as they represent a space where no person can stand and by that the weight distribution of the crowd is changed. You get the picture?
By having people seated you might solve your problem.

Good luck


BTW: Love your signature, nice saying!!

 

Vieux dicton français, ou plutot serait-ce une citation d'un auteur du 17e ou 18e siecle du genre de La Fontaine?

Let's come back to the boat Dutch Peter is totally right: you have to limit the number of people on the roof et to keep them in the middle of the roof, in the pics the boat seems narrow at the waterline with a hull in V shape. Adding a keel won't solve the problem, except to make structural problems and overweighting the boat. As you're in river, where generally a shallow draft is needed a deep keel would not be a good solution.

The boat looks like the traditionnal fishing boats of North Portugal. I make a guess; often this kind of boats are made of a monoxyle (one tree trunk dug with fire by example as a traditionnal african or south american canoe) which is the keel, ribs are added, and the top sides planked.

These boats have a cross section in V, fairly slim with a very low overall stability. If you add weight like people on a roof 2.5 m above the water line the metacenter goes so high that the boat can flip over in a few seconds.

In short words the center of gravity is to high, and the side stability of the boat insufficient. It's a design problem.

It should be more practical (and less dangerous for the passengers and the crew) to build a new boat with a flat bottom, almost vertical underwater sides and a bit wider keeping the general look of the traditionnal boat (I guess you want for your tourist clients to keep a traditionnal gambian look). The base is a traditionnal good design in north american fishing boats, easily made by your local carpenters and with little modifications it will look like the portuguese descendance gambian boat. The goal is to have a boat with greater lateral stability and thus safe.

That takes me to the ferro cement: repair a wooden hull with ferro cement is a truly very bad idea for the following reasons:

1/ wood resists (or try to) to rot because id acidic, at to put a highly alcalic material like cement in contact with the wood is to deprive the wood of defense. It will rot at a alarming rate.

2/ ferrocement is a very poor material for boatbuilding, and it won't glue to the wood.
Ferrocement works (the last country to use it for fishing boats is New Zealand, you'll find some links) if is done in a very technical way: pure silica sand, high quality hydraulic cement with puzzolane, plastifier additives, and retardant additives. It needs a lot of steel bars and chiken wire. After cementing the hull you have to make a humid curing under plastic sheets during 3 weeks and to let slowly dry after, or it will crack.
After you dissolve the surface salts with a weak acid, rince, dry and the best quality of peinture will be need to protect the ferrocement from the effects of tropical river hot water full of microbes...
It's expensive for a small boat.

3/ ferrocement is very, very, but truly very heavy for its low strenght. On big barge rivers is not a problem, on a pirogue is a major concern.

4/ ferrocement is fragile, it cracks at the least impact. If you don't believe me make this simple test: ferrocement has a density of 3 kg/liter, common wood 0.5 kg/liter. So make 2 pieces for testing 60 cm long by 15 cm wide representing a portion of the hull. The first in wood 3 cm thick, the second very well done in ferrocement 0.5 cm thick. They weight exactly the same. Place the samples on two supports espaced of 50 cm, and put weights in the middle of each sample until they break. And you'll understand immediately what I mean...the ferrocement sample will break with very little weight, while the wood will need a lot of weight before breaking.

5/ ferrocement will make a too heavy boat adding very little strenght to the hull. The wood will start to rot immediately and the repairs won't last. The two materials have none affinity.

6/ I'm almost sure that using local woods, a local carpenter will make a good d safe and durable hull (you salvage from the old one the engine and all accessories) for a competitive price compared to the thrown out money used in cementing and/or making a keel.

The best way on a power boat to get stability is a wider, rectangular cross section (or to make a catamaran). To add a keel (obligatory shallow on a river boat , I do not think you want a 1.5 m deep keel) on a round or V shape may be stable but with angles of heeling incompatible with people on a roof watching birds. Look a the old sail boats: narrow round shape going gracefully to windward with a 30 degrees inclination...

If you need more help...

 

Thanks very much indeed for your thoughts.

We do limit the number of people on the roof to 3 at a time and ask people to be conscious of the boat's balance - but that's not really ideal...

Re. keel - clearly not the answer then... I had thought of stabilisers (like on a kid's bicycle) e.g. 2 small canoes fixed to the sides, but this is probably another daft idea! It would look silly and probably cause a lot of drag?

Re. ferro cement - thanks for the warning. Unfortunately, it's too late for our other boat - we already did the Lady Hippo last year! Despite my fears about the added weight, it didn't seem to affect fuel consumption too much. More worrying are your comments about rotting... wish I'd found this forum earlier!

Our problem is compounded by lack of time... the boat needs to be back in the water in just over 5 weeks! Looks like we should scrap the ferro cement idea and just replace the worst timbers with new ones. We'll need to do a more thorough overhaul next off season, when we can do things properly...

What's the best material for plugging gaps between planks? The guys in The Gambia use putty and tuppac (a kind a of greasy horse-hair material). I don't like putty though, it takes ages to dry - especially in the rainy season! I can imagine tar (with the tuppac?) would be better? I'm planning to fly over to The Gambia within the next couple of weeks and could bring some materials with me from the UK.

Thanks again.

PS. I learned that saying from my French teacher at school. Just looked it up in the dictionary and found it should actually read: "Si jeunesse savait, si vieillesse pouvait". It's attributed to Henri Estienne, a French printer and publisher 1531-98.

 

This would solve the stability problems as you increase the beam of the vessel substantially. If you don't mind how the boat looks!? With your time schedule this is a pretty good option I think.

 

The idea of "stabilisers" is good and can be done within your schedule. It will look a bit odd, but safety first.

Do not use canoas or pirogues: your main hull won't withstand the stresses and strong beams will be needed.

A very old solution exists; the flying proas ( in the site http://www.multihull.de/proa/history/p_history.htm you'll se a lot of pics and you'll understand. Look carefully to the system of beams and connections)
which use counterweights: the outrigger is too small to have an effet by buoyancy, excepted when stopped.

What's the situation? you have a boat with low righting moment, and becoming highly unstable with 3 o 4 people watching birds on the roof. The boat has a potential of structural problems (planks are rotting, probably the remaining is not better) so it's better to not add stresses.

What do you need? For the very moment, something counteracting the effect of the bird watchers at small angles (people would feel very unconfortable on a boat reaching its equilibrium with a 20 degrees heeling), easily done and not overstressing the main hull.

Instead of adding weight in the keel, which is not the best place in your boat and a dangerous thing in a low volume hull, the weight will be added on the outriggers in the way used by the oceanian people on their proas and trimarans. As the lever arm of an outrigger is very efficient at small angles, the weight needed is relatively small.

The outriggers are simply slim tree trunks, in a medium density wood; 0.5 to 0.65 kg/liter will be fine. The simplest to make. A so slim and small in heigth and width, but fairly long, outrigger adds very little drag and is almost unsensitive to wave's effects.

How it works? When the boats begins to incline at 2 or 3 degrees, one outrigger goes underwater but it's low buoyancy will add very little to the stability, however it will dampen the rolling movement. The other outrigger will lift out of the water and its weight, multiplied by the distance from the center of buoyancy of the main hull, will stop the inclination. This leverage arm makes that a relatively small weight is needed to get the desired effect.

Let me the time to make some schematic drawings I'll post probably saturday night and all will be cristal clear for you. I'll give you also a very crude and simple method to calculate how much weight do you need.

The stresses on the main hull will be low and minimized by elastic (not too much) wood beam and almost all connections will be lashed. So the stresses are dissipated in the beam and ropes and the main hull will have to stand little stresses. If you're interested by the solution of oceanian outriggers I'l give you a drawing of the beams fixations you'll adapt to your boat.

I've made a private message for you, please read it.

 

You don't have a keel problem,you have a logistics problem.The outrigger with canoe sounds fine,put it on one side.The side most people run to when something occurs.Leave the dock and plan your travel expecting this switching of sides. Treat your boats with the best stuff you can find in your area,do not listen to people who have ten marine supply stores within twenty miles of them.Do stuff that works in your area, that others have used and know how to apply,stuff that gets you through another month.Contract to run medical supplies,save your money,buy a Carolina Skiff or comparable wide stable fiberglass hull with flotation,have it shipped to Gambia and transfer the wooden seats,cabin,etc.A friend of mine from Ghana took a Carolina Skiff back home with him from the States and now he runs a river service.

 

I agree totally with you that is a logistic problem and your counsels are good: the boat has not been designed for such use and a new boat, suitable for bord watching and safe is needed.

The polynesian outrigger provisory solution is a low tech but effective method, invented by very clever people who had only some wood and vegetal fibers. Polynesians were first class naval designers.

Buying a fiberglass hull in the USA and sending it to Gambia is a difficult task. USA boats are very expensive for 3rd world incomes, shipping to small 3rd world countries is very costly, local transportation of big items is skyrocket priced and the customs taxes may range from 40 to 300 %... Generally the imported item is more than unaffordable.

Fiberglass well made boats are unaffordable luxury in most countries. Do not forget the corruption and you may be ransomed by customs and others if you want to get your boat. Sometimes the boat is simply stolen at the customs and after you'll see any high official sailing with your boat; you won't say nothing unless you have the means of leaving the country fastly... Human rights and state of law are unknown in a lot of countries.

I have a lot of first hand information by colleagues about building fishing boats in Africa. It's generally cheaper to build locally; in many countries with maritime tradition you'll find a lot of able marine carpenters and often well priced wood (but always green).

The first thing to do is to heavily treat the wood against rot with for example octoborates (there the green state is an advantage) and dry it well, after to use better stuff during the building (hot dipped nails, good screws etc), retreat the wood with a vegetal oil which oxydizes in gums and varnish (in Africa raw soya or arachide oil for example) doped with 2 % weight metal of zinc naftenate (a cheap treatment of mold and rot) plus a tar coat in the bilges before painting. The wood won't rot.

In fact it's boat building like at the beginning of 20th century, plus some modern technics.

Importing stuff like octo, nails, screws etc do not present generally great difficulties. Sometimes it 's even found locally: small zinc hot dippers work for electricity companies and for fishing boats, a local paint fabric have the naftenate (a dryer for oil paint), octoborate has agricultural purposes also etc...

A bit of imagination and experience are useful; spiraled concrete nails are very good, common zinc plated threaded bars stand corrosion drowned in plumb minium. Examples are numerous.

A well made and treated wooden boat will last years, and can be easily fixed. Surely it will need maintenance.

The transverse planked V bottom traditionnal boats of Carolina are an excellent basis for a design... these boats were built fastly.
A double diagonal bottom planking is easy to make, strong and watertight. Nailed/screwed laminated keels are a good technic when you have not glues (clipper keels were made in that way)

There are many options and designs, technically simple and affordable for building a simple displacement boat.

 

Thanks again to everyone.

Bad news is that my guys over in The Gambia have already started with ferro cementing the hull i.e. boat is currently covered in chicken wire. I'm afraid it looks like we're already beyond the point of return on this...

Our priority is to make sure that the boat is safe until the end May 2005. To this end, it looks like we should implement the outrigger idea. The examples on the Proa web site are very interesting - a good design will be key. I look forward very much to receiving Ilan Voyager's schematic drawings on this (immensely grateful for your help).

PS. Will be travelling to The Gambia on 24th September.

 

We are waiting what trail will take the dangerous hurricane Ivan... and preparing all for the worst. Drawings will be done ASAP.

 

Phew! Looks like you might have been lucky...

http://storm1.herald.com/auto/miamiherald/tropical/tracking/at200409.html

Dimensions of the boat are:

Length - 16 m (end to end)
Width - 2.6 m (distance between top of sides)
Depth - 1.9 m (top of side to bottom centre of boat)

N.B. Measurements of what is in the water are less than this i.e. approx:

Length - 15 m
Width - 2 m
Depth - 1.35 m

Hope this helps?

 

It's ok I have enough for a very rough calculation

 

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