Converting a boat to displacement?

[William_H]

It seems to me that jet drives really have their advantage at high speeds and are inefficient at low speeds. (this theory comes from jet versus prop aircraft). They are of course valuable for shallow water operation.
If you are operating at low speeds then yes you need to have something more akin to a sailing boat hull shape at the stern. ie the bottom rising to near water level and tapering in from the sides to some degree. As you say the easiet way to do this is with an addeed stern section. Wooden structure used as a mold for GRP would be easiest. Just remember that the wood is just a mold (which will be left in place) and the real structure is the GRP so make it plenty strong. Not like a GRP covered wooden boat.
A propeller drive would be far better. Preferably variable pitch but at least with prop you can get the correct pitch for your 6 knots operation.
I would however suggest that you simply keep the Toyota diesel unless you really hate it. Weight will not be a consideration as a displacement hull and the multiple (4) cylinders should be much quieter and smoother than a 2 cylinder diesel. Just a waste of excess horsepower. Give it a decnet sized rudder and add an external long keel of a few inches deep to add to manouvering and ease of steering and you should be a lot happier. good lcuk olewill

 

[ThreeSummers]

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.

It is a local municipal by-law as far as I can tell - a lot of our river regulations are kinda murky, much like the water is right now after heavy flooding...

Have you considered an outboard on a false inner transom, bolted to the transom?

We are in an abalone poaching hotspot here - untended outboards do not stay on boats for very long, no matter how secure.

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.

Easy to do and good advice, thanks.

It seems to me that jet drives really have their advantage at high speeds and are inefficient at low speeds. (this theory comes from jet versus prop aircraft). They are of course valuable for shallow water operation.
If you are operating at low speeds then yes you need to have something more akin to a sailing boat hull shape at the stern. ie the bottom rising to near water level and tapering in from the sides to some degree. As you say the easiet way to do this is with an addeed stern section. Wooden structure used as a mold for GRP would be easiest. Just remember that the wood is just a mold (which will be left in place) and the real structure is the GRP so make it plenty strong. Not like a GRP covered wooden boat.
A propeller drive would be far better. Preferably variable pitch but at least with prop you can get the correct pitch for your 6 knots operation.
I would however suggest that you simply keep the Toyota diesel unless you really hate it. Weight will not be a consideration as a displacement hull and the multiple (4) cylinders should be much quieter and smoother than a 2 cylinder diesel. Just a waste of excess horsepower. Give it a decnet sized rudder and add an external long keel of a few inches deep to add to manouvering and ease of steering and you should be a lot happier. good lcuk olewill

Everything you say is exactly what my train of thought is.

First bold: I figure if I keep to similar scantlings as the boat in my avatar, which is one of the boats I've built from scratch and handles far more stress than the proposed boat (i.e. 32 knots), then I should be fine. The new stern section will not be stressed by anything more than fuel tank, buoyancy, steering and me sitting in that new space. New diesel to be mounted on the same bed as existing one.

In fact, I could even leave the existing transom in place, thereby completely preserving the existing structural integrity.

Second bold: It's too big, and takes up too much space - looking at the Vetus catalogue I can get 15hp to fit in less than half the space (quarter volume?), which is where I will gain the extra two passenger seats.

Thanks for all other comments above - I feel I'm on the right track, and will start drawing a new stern. I'll post drawings here when done for those interested - going to be a fun project.

Regards,

Tim.

 

[ThreeSummers]

Okay, lets forget about issues such as legalities, insurance etc. and think about practical design for a while (I'm having this same discussion on two different forums right now)...

The concept of only bringing the bottom of the boat up to meet the new transom (original pic posted) in my mind creates too small a radius for effective water flow, so, having been on my pondering chair this afternoon, what seems like a good design came to mind...

Instead of two planes coming up (the two sides of the bottom of the hull, why not four - two creating flow to the prop/rudder from the sides, shaped to meet at a vertical sternpost where the prop is mounted, and two planes coming up to meet the new transom?

The added advantage to this is that I can have a reverse curve to the "flats" as it comes to meet the transom - which can give me added stability to compensate for the "V" of the existing hull.

Thoughts?

Pic below might be slow to load, so be patient - btw, most times I draw on screen, but sometimes on paper is easier...

Regards,

Tim.