Anyone have experience with hull extensions?

We lengthened a 45' hatters double cabin hull 6'. If the aft section of the hull is straight you can build mold off the existing hull or build them with plywood and masonite. I wouldn't do it without moving the rudders and running gear back.

A rule of thumb for Naval Architects has been 10% of the waterline line for hull extensions would not require shifting rudders, shafts, etc. There have been several hull extensions that have pushed beyond that level (without changing running gear) with mixed results. I know of some 42' Uniflites/Chris Crafts that were extended to 48' without modifying the running gear and were fairly successful. Seen it done on some Hatteras and Californian Double Cabin Motoryachts as well.

It is not something overly complex for a competent boatyard, and you will usually offset the gain in bouyancy with an additional FW tank. Scarf into the existing stringers (greater than a 12:1 ratio) keep the lower existing transom as a flange to connect to, and through bolt your FRP bonding angle to it for good measure.

I think you are within reasonable limits at 3.5', although you could consider larger rudders which may require an upgrade to the steering system, autopilot and the rudder foundation.

Hi Cavalier,

If it's only a couple of feet, you might be able to extend the vessel without extending the underwater part of the hull, because doing that, your increasing the buoyancy, which will change the Centre of Buoyancy and Centre of Gravity, and may upset the performance (I doubt much though). Basically, a few more things the Nav Arch has to worry about.

I've attached a little concept I did on paint, it's just a thought, but you can extend the above water portion of the hull, material would/could be a foam cored composite to keep the weight to a minimum. Plus, you wouldn't know it's an extension when the vessel is in the water. Cheers

Far

Hard to say what a yard would charge today, but there should be some one hungry for work. I would hope you could get it done for around $60K - $75K, but that is just a swag on my part.......

If your boat currently runs better with a some Trim Tab, the increase in waterline length may even provide a benefit in a (little) more speed.

I don't think 4' would effect the handling too much, however extensions generally make the boat surf around in a following sea. They'll probably add a fuel tank to conteract the bouyancy of the extension.

The work was done at the family boatyard in Freeport, Tx. My Dad usually had some major winter project to keep the crew busy. The boat performed great. We added two 500 gallon and a 250 gallon fuel tanks midship and used the factory fuel tanks under the back deck. With a pair of 671's it would make 10-13 knots . This was in 1970-73 so I don't know what it would cost in today's dollars. I would think around 50k depending on where you get the work done. I got out of the repair business about 25 years ago. I don't know how much I believe about polyester resin and poor secondary bonding. In the 70 s we didn't know it could be a problem. As long as you grind the original glass really well and keep things clean its not coming apart. As previously stated scarf into the existing stringers and use a long scarf. We wanted the rudders moved back since every think we built got used as a towboat/workboat and it got used hard. You would be surprised how many people put their boat on the bank not paying attention. On wet salt grass they don't stop until the struts and rudders hit the bank.

With regards to rudder size, a simple but effective rule of thumb for Naval Architects is to estimate the underwater profile area, and take a percentage (1% - 3%) depending on what type of vessel it is - planing, semi-planing, or displacement.

Take a scaled drawing, estimate the average length of the underwater profile, then take the average depth of the fairbody (centerline of the hull, no keel) and you will get the underwater profile area. Now take a percentage of that area, 2% should be a good average, and divide by the number of rudders. This will give you an idea of your individual rudder area and whether or not if it is large for your size of vessel. Of course, you would have to recalculate if you are making a hull extension.

I've seen several 58 Bertrams stretched to 80 and beyond without moving any machinery or underwater gear.

As far as this particular boat is concerned, the lines look like they lend themselves to being stretched another three to five feet without disturbing the overall balance of the vessel (at least aesthetically).

Extending the hull

If you see this projest to completion, you must post pictures of this project here from start to finish so we can see them. That would also be a big help to see the process for other boat owners looking to do this kind of project on a similar boat.

The challenge for an extension on a boat of this size will be how to compensate for the additional bouyancy after the structure is added. You probably won't have an option nor would want to go to the expense of moving your existing fuel tanks, so I could see the following options:

To estimate (ballpark figure only) the added bouyancy: 3.5' length of extension x 10' average chine beam x 1.5' average depth (acounting for some deadrise) across the extension would give you about 52.5 cubic feet of volume, with saltwater at 64 lbs/cubic feet you have added approximately 3,360 lbs of displacement.

The square footage for fiberglass could be estimated as: 2 sides (port & stbd) x 3.5' long x 4' tall from chine to gunwale = 28 sf. Transom would be about 4.5' x 11' across = 49.5 sf, hull bottom (port & stbd) would be about 2 x 3.5' x x 5.5' along the deadrise = 38.5 sf, cockpit about 3.5' x 11' = 38.5 sf, and cockpit coaming about 2 x 3.5' x 1' + 11' x 1' = 18 sf. Rough Total is about 173 sf, add a margin of 10% and yo get about 190 sf of frp lay-up without stringers, bonds, overlaps, etc. At about 10 lbs/sf, you are at 1,900 lbs and need to make up about 1300 pounds in equipment, tanks or whatever you choose to neutralize the effects of the added bouyancy of the extension. A litlle more weight would even get you to be trimmed down by the stern, desirable for a planing craft.

Added Fuel Tank - not my first choice, as you probably have an aft fuel tank, and if you have to use the additional fuel tank which is further aft, your vessel will be trimmed by the bow and this condition will only increase, definetely making the boat squirelly in a following sea and increasing the pucker factor running an inlet.

Added FW Tank - better overall use for trim if the main FW tank is forward, but will have some trim issues as well. Maybe better to use it as an emergency back up FW Tank or FW Washdown. Water may get stagnant due to lack of use.

Equipment - add something heavy and usefull, such as an Inverter Bank tied to solar panels on the hardtop (if you have one). Would not affect the static or running trim and would be easier to deal with. Obviuosly, you could add other pieces of fixed equipment, within reason of physics and pocket book - not sure which comes first