Stretching a hull

I'm considering stretching the hull on one of my boats.



LOA is about 9,2 meters and beam (at chines) about 2,9 meter. Deadrise circa 12 degrees. Weight around 5500 kg. LWL about 8,5 meters.

The reason for me stretching her is that she runs like she's to short for her beam in combination with being under powered. And since the engine is to be replaced I might as well go all the way.

Since the aft part of the hull has the same shape (minus the keel) my idea is to stretch her by adding between 2,2 and 2,5 meters. The engine, now sitting in the "normal" position, will be relocated to the stern and the coupled to a remote v-drive via a shaft.

The current 260 hp at 2600 rpm Ford will be replaced with a 380 hp at 2000 rpm FPT. The 22x29 prop will be replaced with something in the 27-28" region. The shaft will be repositioned and the keel reworked but the general design will be the same. The only difference being that prop and rudder will be situated about 250 mm forward of the current position in relation to the transom in order to give clearance for a stern thruster.

The "new" specs would be in the area of LWL 11, meters, beam 2,9 and weight 7000 kg. Centre of gravity will be moved rearward (don't know by how much) but that will be a good thing as she's currently prone to sit on her nose in low speeds.

Today she'll do 23 knots at WOT, which is about where she starts to run fine in the water. Without anything more than a gut feeling I think the longer hull, albeit heavier and with more appendage, will be about as easy (or heavy) to push through the water. But even allowing for it to require more (say 300 hp for 23 knots) will give me a decent margin to 380 (and the engine is heavy duty rated so it can go at WOT for an unlimited time).

Are there any special considerations when doing something like this? Or common mistakes that can be avoided?

 

A usual way of stretching a hull is to cut it in the middle and add a section.

 

But on this ship, given the shapes that can be seen in the picture, that would probably be a very big mistake. Better, in my opinion, to add the new body aft.

 

Could you explain the probably very big mistake you see in stretching this hull . . ?

 

I'd too like to understand why TANSL needs to go through this process, as I see a monohedren hull form, with slab sides, which a pretty thing to stretch. As to his questions, yeah there's lots of things that can cause a bad hair day, but with a good initial plan, nothing that couldn't be overcome. The leaner (narrower) end result will make the hull slightly more efficient for it's new length, stability and other factors will also improve too.

 

Unless the gunnels and chine are perfectly parallel where you cut through the hull you will have a dislocation of the smooth lines.
Take a triangle, cut it completely through half way up from one side. Then separate the pieces say a few inches. You will see that you will not be able to make fair lines to join the previously
adjacent points.
So the bottom of the hull would have interrupted fair line and so would the sides.
While this ship is a work boat, perhaps it would not matter to have these dislocations. Certainly if you got closer toward the place where the butt weld show on the sides, it would show less. Adding to the back will increase the beam at the back which for a planing hull will help it get up on step easier.
If you want the boat to have smooth looking lines, add to the back

Regarding the stern thruster, there are quite a few models around that mount on the transom above the rudder that would do the trick without perforating the hull sides

 

Thank you for your input but in this case it would be a lot of work to stretch it in the middle. The bow starts to take shape very early so I'd need to cut her up under the cabin.

My thinking too. You can see a weld on the side of the hull, below the most forward side window. From there and aft the height of the sides (for lack of a proper word...) is the same which also goes for the deadrise and beam.

This boat doesn't know what smooth is. She was built in Finland using a L-square and a sledgehammer.

The L/B-ratio would increase from about 3,0 to about 3,8, in general a good thing if I understand it correctly.

 

Problems with current design:
1.Too small cabin.
2. Engine sits partially under the cabin.
3. The engine compartment has a hump the takes space both from the cabin and from the cargo deck.
4. Under powered.
5. Very noisy (in cabin).
6. Not very buoyant nose.

Solutions and benefits with new design:
1. The cabin can be stretched by the necessary 1100 mm.
2, 3. The engine is moved to the stern where it can sit completely outside the cabin allowing:
* Easier maintenance.
* Better use of cabin space/floor area.
* More and better insulation and totally separated engine compartment from cabin leads to less noise.
4. No explanation required.
5. The curent noise level comes to some extent from the engine but mostly, I suspect, from the prop. Today it runs very close to the hull, no more than an inch tops, which leads to an almost painful droning sound at prop speeds above 1050-1100 rpm.
6. She has a sharp nose for her boat type and cuts neatly through the seas but as a result she's not very buoyant. Moving the engine allows for removing the "hump" in the cargo deck which in turn allows for better cargo mass distribution (i.e. the heavy stuff as far back as possible.

I'm not worried about the stretch as far as hull strength goes, she's about the sturdiest I've ever seen as everything is oversized compared to every other aluminum boat I've come across (and they are a few).

 

Dear PAR, I really did not have any need to go through this process but I wanted to avoid that the OP, following the comment of Gonzo, made the mistake that Barry has described so well with his scheme.
By the way, I do not think anyone needs to go through this process, it's not a necessity, it's just about helping, if you think you can do it.

Why you need to say that. No, the most probable thing is that the initial stability diminishes but, without taking into account the new distribution of weights and submerged volume of the hull, it is unprofessional to say anything.

 

Barry, thanks for explaining the stretching issues so well . .

In this case it looks to me that most of the below decks part of the old transom could act as a bulkhead in front of the new transom, right ?

 

Don't know how it was when your boat was built (what year was that ?), but to my understanding nowadays, since about 13 years, Capo Marin and Conrad Shipyard build their Capo series in Gdansk in Poland, these are all newer models though, starting at 9.7 m (31' 10") LOA, and all have different means of propulsion...

Conrad Shipyard ---> Yachts ---> Special ---> Special Purpose ---> Capo Series

Capo Marin ---> Our Boats | left column | ---> About Capo Marin

 

I'll guess she has to be, as being a max 10 cm (4") solid ice breaking boat. Think the bow has this form to slide up the ice, and then sink through to break it, and so repeated in a continuous process till she reaches her destination, right ?

 

If a new machine is to be installed in the extension, it may be feasible.
Must of course be calculated carefully based on how the boat's lines are today.
Can toothed belt be suitable between machine and propeller shaft?

JS

 

The boat is indeed an early Capo, from 1988 (po took delivery of her in October 1988).

I've considered a belt drive but I think the driveshaft/remote V-arrangement is more common and therefor easier to work on. The boat can only be out of production for a few weeks so proven designs are safer.

10 cm is about max according to po but then I'm not making any speed at all. Also it's depending on the ice of course, rotten ice is much easier to deal with.

 

Magnus,
There is a butt weld on the side above the blue support. How far toward the bow does the existing engine and vdrive or whatever you have for the transmission etc go?
And the location of the fuel tanks? If you have pictures of the engine compartment showing a bit more detail, there may be another possibility which could reduce costs
and drydock time.