Converting a boat to displacement?

[ThreeSummers]

So, I hauled my other boat (see pic below) out the water yesterday after four years of cruising, and need to improve things somewhat. It's too noisy (I do birding safari cruises), it's too ineffecient - it runs a 1.9 litre Toyota diesel driving a Hamilton jet, max speed around 6 knots (I have towed it with my 15hp inflatable faster than that!!) and it's too heavy - that diesel motor again!

So my thinking is to convert it to a pure displacement hull, taking out the Toyota and jet, and installing a little 15hp 2 cylinder diesel weighing less than 100kg, by extending the hull as shown in the picture, more or less.

It's a V hull, 20 foot long, and the extension proposed takes it to around 25 feet (yay, more passengers!).

Can anyone offer thoughts on the concept, do-able or not?

Good idea/bad idea?

Regards,

Tim.

 

[TQA]

No problem to extend. Yards in the Windies do it all the time.

Why not go electric if you are looking at your audio footprint?

 

[Rigger Mortice]

Sell it and get something more appropriate.

 

[PuffTheMagicDragon]

Most things are doable but a major factor is the material of which the hull is built: Steel -> easy, GRP -> not difficult, Wood -> not easy.

The easiest way of 'jumboising' is what we do (did) on ships. In simple terms you cut the stern section off near a station where the sides of the hull are near parallel (usually just aft of amidships), build the new section that is to be added and then fit this between the separated fore and aft sections of the original hull. Add reinforcements as necessary (or as recommended by a naval architect if you cannot do the calculations yourself).

Please note the words that are in bold!

 

[Lakesailor]

Sell it and get something more appropriate.

That would be my approach. And I am an inveterate messer.

6 Knots with a jet drive sounds a bit slow. That would be a displacement speedanyway for a boat that length. Did you miss a 1 or a 2 out there?


Would displcement speeds curtail your cruises' range?
There was a lovely diesel/solar electric cruise boat for sale on Coniston last year. http://www.conistonlaunch.co.uk/index.htm

 

[ThreeSummers]

Wow, some quick responses here!

No problem to extend. Yards in the Windies do it all the time.

Why not go electric if you are looking at your audio footprint?

Not practical to charge - boat lives on the river and no power available nearby.

Sell it and get something more appropriate.

I've been thinking of having a replacement built for several months now, and it will cost me a lot less to convert - doing the conversion myself. Besides, no one in their right mind would buy this boat as is - the jet, local regulations, commercial licence attached to the boat etc..

Most things are doable but a major factor is the material of which the hull is built: Steel -> easy, GRP -> not difficult, Wood -> not easy.

GRP boat - I'm thinking of cutting the transom out, extending it with a plywood core and sheathing in epoxy/glass. My other boat I built is done this way, and 6 years after first launch is still sound.

Regards,

Tim.

 

[ThreeSummers]

6 Knots with a jet drive sounds a bit slow. That would be a displacement speedanyway for a boat that length. Did you miss a 1 or a 2 out there?

No, 6 is right. Though speed increases noticeably when my pax move the the front of the boat, lifting the square transom out the water... so I figure building a curve in there can only improve things. 10 knots would be ideal.

My understanding of jets is that the nozzle needs to be out the water for best thrust - I can't achieve that with this boat/motor/drive combination.

Range is not a prob - we only have 7.5km of navigable river - but it's very popular with tourists and birders.

Tim.

 

[PuffTheMagicDragon]

GRP boat - I'm thinking of cutting the transom out, extending it with a plywood core and sheathing in epoxy/glass. My other boat I built is done this way, and 6 years after first launch is still sound.

Regards,

Tim.

The reason for retaining the original aft section is that it is usually more complicated than a mid-ships section. In your case I would consider laminating the new section in situ and forgetting about the plywood core.

What I would do:

1) Cut the hull across at a convenient station.
2) Chamfer the cut edges using an angle-grinder with a flap-disk and extending around 8-10x the hull thickness.
3) Separate the two halves by the required distance and fix them firmly in position making sure that the alignment is correct.
4) Join the two sections with melamine-faced hardboard fastened shiny-side inwards using self-tapping screws into the existing hull. (It might be advisable to apply parting agent to the hardboard because not all are the same).
5) Laminate the new GRP alternating between CSM and woven rovings until the required thickness is reached.
6) Do the same for any stringers, bearers or other structures that were cut.
7) Remove the hardboard panels and make good the outside.
8) Fair the outside as necessary.

For that size of open boat I wouldn't bother with epoxy; regular isophtalic polyester resin is quite adequate.

Please note that the above is what I would do; it's your decision what you should do.

 

[DownWest]

Puff
he needs to get rid of the square transom, so a middle cut and plug is not suitable. But second your method to mould the extension. He just needs to bring it up gently to a level above the loaded waterline. Easy on a hardchine hull.

 

[PCUK]

Have you tried changing the jet fan, sounds far too fine if you're only getting six knots. Does it rev away without doing much? Have you got trim tabs. I'm thinking it may be cheaper to get the current set-up working better.

 

[ThreeSummers]

Puff
he needs to get rid of the square transom, so a middle cut and plug is not suitable. But second your method to mould the extension. He just needs to bring it up gently to a level above the loaded waterline. Easy on a hardchine hull.

That's exactly what I want to do. Though I still prefer glass over ply - that's what I have experience with, and the ply becomes it's own mould if I shape the frames right. Cheaper too than laying up 10mm or more of laminations, which is more or less what the rest of the boat is.

Have you tried changing the jet fan, sounds far too fine if you're only getting six knots. Does it rev away without doing much? Have you got trim tabs. I'm thinking it may be cheaper to get the current set-up working better.

There's a number of reason I want to do away with the jet, one of them being that it's damn hard work to steer - like pushing a shopping cart backwards with one finger. It needs constant correction otherwise the boat wants to go in circles.

Tim.

 

[Pete7]

I would have thought a honda 4 stroke outboard about 15-20 hp would be the best choice for power. They are so quiet at low revs and will be a lot lighter than a diesel.

Pete

 

[ThreeSummers]

I would have thought a honda 4 stroke outboard about 15-20 hp would be the best choice for power.

An outboard will get stolen.

 

[Scotty_Tradewind]

in your 'extension' in red, why a counter/upturned stern.


I'm no marine architect but......

If I were doing it I'd consider keeping the keel line to the transom, more parallel to the surface of the water. This will increase the internal volume, give more buoyancy for those extra passengers and increase the water line length.

Which would possibly make the whole thing more efficient.

Could you make up a small model and tank test the differences?
S.

 

[maxi77]

I get the impression from the OP the boat is used for commercial purposes thas as a fare paying punter I would wonder just how much stress and strength calculation was done during the design phase, and also just what proof there is to the work actually having been carried out to the design specs. One suspect that the insurers may also have some concerns as may thse who licence the boat, it has a big registration number which tends ti indicate a need to comply with certain rules etc.

 

[ThreeSummers]

in your 'extension' in red, why a counter/upturned stern.

To reduce drag - the square transom drags a huge amount of water behind it and I want to give it the right shape to have the water flow up the stern smoothly, as well as have a propellor higher than the keel - the water gets quite shallow sometimes.

I'm also thinking that an extension of the keel, in the form of a skeg, up to the prop is a good idea.

I get the impression from the OP the boat is used for commercial purposes thas as a fare paying punter I would wonder just how much stress and strength calculation was done during the design phase, and also just what proof there is to the work actually having been carried out to the design specs. One suspect that the insurers may also have some concerns as may thse who licence the boat, it has a big registration number which tends ti indicate a need to comply with certain rules etc.

Yes, it is commercial, and while I can't really have the boat tested to destruction, I can have it inspected during the process of building by the local maritime authority. They also inspect the boat annually in order for me to keep my DTC number.

I'm currently certified to carry ten pax - by extending the boat a few feet I can up that to twelve. Any more than 12 pax means I need to get a passenger vessel endorsement on my skippers ticket, and have a head on the boat, neither of which are on my wish list.

Tim.

 

[rogerthebodger]

" I'm currently certified to carry ten pax - by extending the boat a few feet I can up that to twelve. Any more than 12 pax means I need to get a passenger vessel endorsement on my skippers ticket, and have a head on the boat, neither of which are on my wish "

At least you don't have to have a ferrymans ticket like we do in Portnet controlled harbours.

Didn't know you have to have a head on a "passenger vessel " I know at least one ferry in Durban that can carry more the 12 passengers the does not have a head on board.

 

[Davegriff]

Extending/rounding the transom most likely won't increase the speed, but will be much more efficient at hull speed (6 knots or so) than the enormous drag caused by a flat transom.

10 passengers is a fair amount amount of weight to move around, so I'm not surprised she's not particularly fast as-is. More efficency means less power used so less potential noise - must be good for bird watching and general sight-seeing I'd have thought.

GRP over plywood probably cheaper and simpler to do then pure GRP. Single curve surfaces required would be fairly easy to mould in either.

 

[Tranona]

An outboard will get stolen.

Have you considered an outboard on a false inner transom, bolted to the transom? This will leave more space for passengers and keep the mucky parts at the back of the boat. Using an inboard diesel will mean you will have to put the engine in the middle of the boat which means building beds to take it, a box around it and a stern tube to take the shaft. If you are buying a new engine you are looking at £6k+ for all the bits. It will also be much more noisy than a 4 stroke petrol outboard at the back of the boat. You can either fix the engine and have a rudder or steer with the engine. Adding a long keel will make the boat more directionally stable.

 

[maxi77]

Yes, it is commercial, and while I can't really have the boat tested to destruction, I can have it inspected during the process of building by the local maritime authority. They also inspect the boat annually in order for me to keep my DTC number.

I'm currently certified to carry ten pax - by extending the boat a few feet I can up that to twelve. Any more than 12 pax means I need to get a passenger vessel endorsement on my skippers ticket, and have a head on the boat, neither of which are on my wish list.

Tim.

I would with the greatest respect suggest you discuss this project with your local inspectorate and you insurers to make sure yo do not spend money in vain on this project. I am sure it is feasible but I suspect care is needed.