Hull Extensions for Catamaran

[44'cruisingcat]

Originally Posted by Southern Star

Minaret, 44 C's methods may be primitive in comparison, but so will mine. Afterall, the question here concerned a DIY project and I am grateful for the contributions of others who have done the same.

Brad

Well OBVIOUSLY, before you start work on the boat, you need to build the multimillion dollar environmentally controlled work area....

[44'cruisingcat]

Getting back to the actual topic, a few questions: how long do you intend to make the extensions, how deeply submerged are the current transoms, and what beam are they?

It would be interesting to work out roughly how much extra bouyancy you're going to be adding, so you can determine if the work is actually going to be worth it.

For instance, I built a composite targa arch, rather than having one made from stainless steel. Probably saved around 50kg over the stern of the boat. (As well as hundreds of $)

http://www.cruisersforum.com/gallery...00&userid=3477

Maybe getting weight out of the bum would be cheaper and easier than building in the capacity to carry it?

[Southern Star]

My intention was to build extensions of between 24 and 30 inches. I am not currently at my boat in order to take measurements, but based upon line drawings the transoms narrow considerably and are only about 28 inches at the waterline. Currently the transoms are submerged by about 4 inches (in fresh water). I anticipate that I can get them to clear readily with 30 inch extensions based upon: 1. the additional buoyancy from the extensions; 2. the upward slope of the underbody (approximately 1:10, so 3 inches with 30 inch extensions, 2.4 inches with 24 inch); 3. The additional buoyancy in salt water (where she will be heading and remaining).

Yes, losing weight aft would be a very good thing; however, my davits are aluminum and not S/S and the weight of the inflatable and outboard are not going to change (in fact, they will go up if I switch from an air floor to an RIB). I have 3 x 80 watt solar panels mounted on the davits - and I have no desire to move them to another location as they are not shielded by the mainsail. Yes, I could move the radar to the mast, but prefer not to have interference with the headsails and increased weight aloft. Ditto for the wind generator.

Everything else that is mounted towards the transoms is rather light - e.g., the lifesling and the BBQ; furthermore, they are obviously more practical where they are.

I am also considering installing a rigid dodger/bimini and, of course, this is only going to add more weight aft. Fortuantely, as my cockpit is forward of a small aft deck and the aft cabin coachroof (it is essentially an aft-center cockpit), the impact will not be as great as on many boats. Since I am gaining 3 inches from the upward slope of the underbody alone with 30 inch extensions,I am now leaning towards that extensions of that lenght rather than 24 inches. Thoughts?

Brad

[catty]

Originally Posted by Southern Star

My intention was to build extensions of between 24 and 30 inches. I am not currently at my boat in order to take measurements, but based upon line drawings the transoms narrow considerably and are only about 28 inches at the waterline. Currently the transoms are submerged by about 4 inches (in fresh water). I anticipate that I can get them to clear readily with 30 inch extensions based upon: 1. the additional buoyancy from the extensions; 2. the upward slope of the underbody (approximately 1:10, so 3 inches with 30 inch extensions, 2.4 inches with 24 inch); 3. The additional buoyancy in salt water (where she will be heading and remaining).

Yes, losing weight aft would be a very good thing; however, my davits are aluminum and not S/S and the weight of the inflatable and outboard are not going to change (in fact, they will go up if I switch from an air floor to an RIB). I have 3 x 80 watt solar panels mounted on the davits - and I have no desire to move them to another location as they are not shielded by the mainsail. Yes, I could move the radar to the mast, but prefer not to have interference with the headsails and increased weight aloft. Ditto for the wind generator.

Everything else that is mounted towards the transoms is rather light - e.g., the lifesling and the BBQ; furthermore, they are obviously more practical where they are.

I am also considering installing a rigid dodger/bimini and, of course, this is only going to add more weight aft. Fortuantely, as my cockpit is forward of a small aft deck and the aft cabin coachroof (it is essentially an aft-center cockpit), the impact will not be as great as on many boats. Since I am gaining 3 inches from the upward slope of the underbody alone with 30 inch extensions,I am now leaning towards that extensions of that lenght rather than 24 inches. Thoughts?

Brad

Where does it all end?
Brad, I assume the guy who designed your vessel new what he was doing.
Why then are you altering it by adding reserve buoyancy way aft, in a location that will cause the stern to lift in a following sea and depress the bows. Geeze even seawind worked out that the real solution was to reduce weight and not just keep adding length at the stern (xl2?).

From your figures you are going to be lucky to get 15 to 20 kg of buoyancy per stern, then you have the not inconsiderable weight of the addition, giving you a net loss, so overall the crap you add will cause your vessel to sit lower in the stern with reduced wing clearance aft.

Surely as a long term advocate of multihull safety, including not overloading, its time to take some of your own advice and lighten your vessel so it floats on its designed lines, or buy a new one that can carry all your stuff safely.

Hope this is of help.
Regards

[downunder]

Originally Posted by catty

Where does it all end?
Brad, I assume the guy who designed your vessel new what he was doing.
Why then are you altering it by adding reserve buoyancy way aft, in a location that will cause the stern to lift in a following sea and depress the bows. Geeze even seawind worked out that the real solution was to reduce weight and not just keep adding length at the stern (xl2?).

From your figures you are going to be lucky to get 15 to 20 kg of buoyancy per stern, then you have the not inconsiderable weight of the addition, giving you a net loss, so overall the crap you add will cause your vessel to sit lower in the stern with reduced wing clearance aft.

Surely as a long term advocate of multihull safety, including not overloading, its time to take some of your own advice and lighten your vessel so it floats on its designed lines, or buy a new one that can carry all your stuff safely.

Hope this is of help.
Regards

Yeah ! But an inacurate post.

I thought the Seawind 1000XL was the extended version of the original 1000 and the lighter XL2 was a result of seawind starting to use resin infusion which mostly improves resin/glass ratios and produces lighter vwessels. Nothing to do with an extension I would have thought.

Simply progress and something the top monohull and cat builders have been using for a while. A good move by Seawind.

cheers

[beiland]

Originally Posted by 44'cruisingcat

For instance, I built a composite targa arch, rather than having one made from stainless steel. Probably saved around 50kg over the stern of the boat. (As well as hundreds of $)

100 6706 - Cruisers & Sailing Photo Gallery

What vessel design is she??

[Mark Johnson]

It would need to be extended using vacuum bagged foam composites, to be worth it, and the boat's designer, or A multihull designer, should crunch the numbers. Be aware... It will change other things, like the balance of the sail plan too.

You may find, as I did on contemplating this mod to my boat, that the extra buoyancy achieved is about equal to the weight added to the stern, for "0" net gain!

I have seen some "solid glass" sugar scoop hull extensions, that actually weighed MORE than the buoyancy gained! The amount of hull in the water back there is precious little, if only 6" deep.

$500 sent to a NA is money well spent here!

BY FAR, the easiest fix is to get some crap off of the boat. OR, as we did... spend that money replacing the interior flat panels, like floorboards & cubby lids, with light weight composites. We then went through EVERYTHING, & removed EVERY socket or wrench that has no matching nut or bolt on the boat, all redundant pages in books, like the chapters written in Japanese in our Yanmar service manual, all unnecessary books, too many spares, more fuel or water than necessary for your next leg, etc...

There are easier ways to raise the WL.
M.

[David M]

You will want to do a trim calculation so this is quantified so you know exactly what your change in trim will be. This way you will for sure get your desired results rather than hoping it works.

If you don't know how or you are not interested in learning then a naval architect can do this for you.

Once you have the numbers, the calculation itself is easy. It's a basic moment arm calculation. You can create your own coefficient to know what moment changes the trim by one inch by placing a known weight on the transom, noting its distance from the boats center of buoyancy and noting how much the transom submerges. Then it is a matter of knowing how much buoyant force the new transom creates minus its estimated weight and then running your calculation to see how much the trim changes in inches or whatever units you want to use. The moment arm is measured from the boats new estimated center of buoyancy to the center of buoyancy (center of lifting force) of the new transom extension.

The amount of "rocker" in the hull towards the transom (analogous to a surfboard) may prevent the extension from providing enough buoyant force to make this worthwhile. The more rocker, the less likely.

I would not go guessing at this, you may be disappointed with the results.

[Factor]

Originally Posted by catty

From your figures you are going to be lucky to get 15 to 20 kg of buoyancy per stern, then you have the not inconsiderable weight of the addition, giving you a net loss, so overall the crap you add will cause your vessel to sit lower in the stern with reduced wing clearance aft.

Wrong

[cat man do]

Originally Posted by Mark Johnson

There are easier ways to raise the WL.
M.

Dont forget the added WLL gained form an extension.

Any extension I have done I believe "added WLL for vessel weight" was what increased sailing ability as it went from a vessel that had immersed sterns to one where they floated free.

[Southern Star]

Precisely, cat man do - it is decidedly not just an issue of added bouyancy aft. Raising the transoms out of the water can only help performance and I fail to see how it can have a negative effect on safety.

Brad

[Southern Star]

Just to clarify, if the sole purpose of the extensions was to provide static buoyancy then David M and Mark J would be correct. However, if one reads my IP you would see that I said "adding a couple of feet aft will not only increase buoyancy aft, but also increase the LWL, reduce pitching, provide a swim platform and raise the transoms above the water" ( and subsequently I suggested extensions of 30 inches in length, rather than two feet). Lets consider the following (and if my math or logic are wrong, please correct me):

1. increased buoyancy aft: a.) static buoyancy - unless the extensions are grossly overbuilt (and they won't be since they are intended as essentially sacrificial extensions, leaving the existing transoms intact) they will not add considerable weight - my boatyard estimated no more (and potentially less) than 40 lbs each with reasonable scantlings. Keep in mind that since my intention is to use them as swim platforms, they will not go up to deck height, but rather extend only about 10 inches from the bottom of the existing hulls, curving up gradually to about 5 inches at the transoms for the extensions (this will leave the transoms about 1 inch above the water without the aid of any additional buoyancy). Yes, the static bouyancy will be limited due to rocker, but by my calculations the added bouyancy will nevertheless exceed their weight (for simplicity, approx. 2.5 feet long by 2 feet wide by .2 feet for the average submerged depth gives static underwater volume of about 1 cubic foot per extension, or 124.8 lbs. total). b.) reserve buoyancy - this will be more significant and will assist in reducing hobbyhorsing as, when the motion tends to submerge the extensions aft beyond the static waterline, the bouyancy will also increase. For example, a movement down of 5 inches (.4 feet) will increase total buoyancy aft by a further 249.6 lbs., for a total of 405.6 lbs, less 80 lbs. for the extensions, or rouglhy 325 lbs. Since this bouyancy will be aft of the weight of the davits, inflatable, solar panels, radar etc. that I am attempting to compensate for, it will also benefit to some degree from the lever principle. So yes, there will be some benefit from increased bouyancy aft.

2. increased LWL - this is a given and it will also marginally increase theoretical hull speed.

3. a swim platform - something I would like and which can be achieved with these additioinal benefits.

4. raising the transoms out of the water - something which will be achieved due to rocker and which will also improve performance, perhaps significantly.

Would I better off (or safer) if I forgot about the swim platforms/hull extensions and tried to reduce weight sufficiently to lift the transoms up by 5 inches? I fail to see how eliminating radar, my life sling, an inflatable with outboard, solar panels, one all-chain anchor rode, liferaft, spares etc. would make my boat safer. Indeed, even with catamarans, increased displacement increases resistance to capsize. It would no doubt be faster if I eliminated all this gear, but safer? No.

Thanks to Catty for his/her usual helpful suggestion that I should either eliminate all this gear to reduce weight (seemingly without consideration as to whether it was in fact needed for safe passages), or buy a new boat. I'd love to buy a new and bigger boat, although unlike Catty who I gather has unlimited resources, I cannot afford to. Even if I could, of course, it is not uncommon to end up with a boat that sits on its lines when equpiped for charter, but which submerges the transoms when equipped with radar, solar panels, a wind generator, life raft, spares, and stores etc. This is, of course, why many manufacturers (and many boat owners) have successfully added hull extensions aft. It is also why I intend to do so.

And Mark, yes I could replace all bulkheads/doors wtih lightweight, composite panels but that is an undertaking and expense than I have no desire (nor ability) to undertake. Kudos to you, mate, for being able to do so. Lets just say that my skill set (and my desire for speed) are much less than yours.

Cheers!

Brad

[downunder]

Definitely go for a swim platform one of the real benefits of cats.

[44'cruisingcat]

Originally Posted by beiland

What vessel design is she??

No intention to hijack the thread - the boat is a Bob Oram 44C.

[44'cruisingcat]

Originally Posted by Southern Star;85738Just t3

o clarify, if the sole purpose of the extensions was to provide static buoyancy then David M and Mark J would be correct. However, if one reads my IP you would see that I said "adding a couple of feet aft will not only increase buoyancy aft, but also increase the LWL, reduce pitching, provide a swim platform and raise the transoms above the water" ( and subsequently I suggested extensions of 30 inches in length, rather than two feet). Lets consider the following (and if my math or logic are wrong, please correct me):

1. increased buoyancy aft: a.) static buoyancy - unless the extensions are grossly overbuilt (and they won't be since they are intended as essentially sacrificial extensions, leaving the existing transoms intact) they will not add considerable weight - my boatyard estimated no more (and potentially less) than 40 lbs each with reasonable scantlings. Keep in mind that since my intention is to use them as swim platforms, they will not go up to deck height, but rather extend only about 10 inches from the bottom of the existing hulls, curving up gradually to about 5 inches at the transoms for the extensions (this will leave the transoms about 1 inch above the water without the aid of any additional buoyancy). Yes, the static bouyancy will be limited due to rocker, but by my calculations the added bouyancy will nevertheless exceed their weight (for simplicity, approx. 2.5 feet long by 2 feet wide by .2 feet for the average submerged depth gives static underwater volume of about 1 cubic foot per extension, or 124.8 lbs. total). b.) reserve buoyancy - this will be more significant and will assist in reducing hobbyhorsing as, when the motion tends to submerge the extensions aft beyond the static waterline, the bouyancy will also increase. For example, a movement down of 5 inches (.4 feet) will increase total buoyancy aft by a further 249.6 lbs., for a total of 405.6 lbs, less 80 lbs. for the extensions, or rouglhy 325 lbs. Since this bouyancy will be aft of the weight of the davits, inflatable, solar panels, radar etc. that I am attempting to compensate for, it will also benefit to some degree from the lever principle. So yes, there will be some benefit from increased bouyancy aft.

2. increased LWL - this is a given and it will also marginally increase theoretical hull speed.

3. a swim platform - something I would like and which can be achieved with these additioinal benefits.

4. raising the transoms out of the water - something which will be achieved due to rocker and which will also improve performance, perhaps significantly.

Would I better off (or safer) if I forgot about the swim platforms/hull extensions and tried to reduce weight sufficiently to lift the transoms up by 5 inches? I fail to see how eliminating radar, my life sling, an inflatable with outboard, solar panels, one all-chain anchor rode, liferaft, spares etc. would make my boat safer. Indeed, even with catamarans, increased displacement increases resistance to capsize. It would no doubt be faster if I eliminated all this gear, but safer? No.

Thanks to Catty for his/her usual helpful suggestion that I should either eliminate all this gear to reduce weight (seemingly without consideration as to whether it was in fact needed for safe passages), or buy a new boat. I'd love to buy a new and bigger boat, although unlike Catty who I gather has unlimited resources, I cannot afford to. Even if I could, of course, it is not uncommon to end up with a boat that sits on its lines when equpiped for charter, but which submerges the transoms when equipped with radar, solar panels, a wind generator, life raft, spares, and stores etc. This is, of course, why many manufacturers (and many boat owners) have successfully added hull extensions aft. It is also why I intend to do so.

And Mark, yes I could replace all bulkheads/doors wtih lightweight, composite panels but that is an undertaking and expense than I have no desire (nor ability) to undertake. Kudos to you, mate, for being able to do so. Lets just say that my skill set (and my desire for speed) are much less than yours.

Cheers!

Brad

It will also improve your LWL/BWL ratio, and thus reduce the effect of the theoretical hull speed limitation.

For me the biggest concern would be the fact your boat has 16 year old gelcoat. Matching the colour could be difficult. Getting it to STAY matching for a long time could be near impossible.