Thread: Extending the hull.

One of the known flaws with this hull, which is quite a fast one, is that it lacks stern buoyancy.

Okay, I'll bite, how did you become aware of this "flaw"?

It needs a large engine to make it go, because it weighs oer a ton, but it sits quite low at the stern at rest.

Another "know flaw" I'll presume? By the way, a ton isn't a lot for a boat of this configuration. It's a moderately deep gull wing V hull with a fairly bluff entry, fairly shoddy resin to fabric ratios, which make her heavier then necessary, but not excessively so, particularly for the era she was built.

Most power craft "seem" to sit down and the stern while at rest. This isn't bad, nor wrong, but mostly a function of the CG location on vessels of this class.

the topside fuel tank was removed and the center flotation cell cut open to install a 125l fuel tank, then decked over with a full-length hatch.

Maybe, or maybe the tank was foamed in originally, which would be a common thing for a manufacture to do.

On a boat of this vintage I'd be looking to make sure all it's "ills" have been addressed before your wish list of modifications was completed. A boat this age will likely have stringer, sole and transom issues. This would need addressing before adding additional weight and HP to the mix.

Twins look cool, but are usually more trouble then they're worth. They cost more to own, to operate, to maintain and repair. Two 75's will go slower then a single 150 and you'll have less load carrying capacity too.

Once your transom and stringers are in good health again, consider a transom bracket, rather then a home make hull extension. The good ones come with engineered buoyancy chambers.

As for positive flotation when fully swamped, well don't go crazy. Most boats will trap air in lockers, compartments and other areas, usually enough to keep a boat afloat. The amount of foam or sealed compartmentalization necessary, to insure a boat remains afloat is staggering. In small, light craft, you can do this, but in larger, heavier craft, you just sacrifice too much interior space. This is why most boats sink, unless they're small enough that a few isolated areas can be sealed or foam filled, with enough volume to keep her afloat.

lastly, if you prepare for the worst, then you'll never leave shore. taking a boat farther from shore then you can swim back to is an unnatural environment. Riding in an airplane or taking and elevator to the 50th floor are also unnatural events. Humans can't naturally place themselves in these situations, so an assumed risk is involved. We try to offer some margin of safety, but at some point we just have to have faith that the equipment will not let us down.

I have seen pod conversions on a few stern drive boats in the 21-23 foot range, where owners have scrapped the stern drive and hung 200HP+ outboards off the pod.

In these cases I suspect that there was probably not a significant weight increase in the mod per see as the automotive based stern drive would have a similar weight to the new outboard. However the fuel consumption may well increase, requiring the carriage of significantly more fuel.

The pods that I have seen appeared to fabricated from aluminium, and provide a mounting point for the outboard(s), room for them to tilt, andseal the previous transom opening for the stern drive. I am not sure how much additional bouancy they provided.

Apart from structural issues, you will also be looking for a new trailer to accomodate the extra length and weight.

Hey Exador,

I see this type of thing happening quite offen, if you do the sums, ( and Add your labour), and as Malb has said don't forget the Trailer mods, you will be miles better off, if you ditch what you have and buy what you want, something that is designed and built to the specifications that you require, without the proper training, boat mods are what keep the Volunteer Marine Rescue people busy all year round. Sorry if you are put off by my post but as I said, at the beginning I see this quite often. Sometimes we gat carried away with our expectations, I'm not trying to say don't do it, I am saying make sure that you know where you are heading and how much it's gunna cost. At the moment it is a Buyers Market and you can pick up some bargains out there.

HazzaB

Thanks for the replies, folks.

PAR, I'll take yours first, although I could have done without the "sarcasm".

The low stern freeboard I can live with, but not when it's combined with a 90s motor and inadequate flotation and any of the Brisbane bars.

I want twins for the safety of having a spare. Yes, surprisingly, I was aware that two motors are more expensive than one and that the performance won't be as good, but the reliability will be much better. I haven't settled on the final engine configuration by any means. I could be persuaded to look at a modern 150, especially 4 stroke. The 2 stroke simply sucks too much juice and I'm not confident of its reliability. It's in very good condition, but still...

The boat is currently going through a top-to-bottom, down to the glass refurb. Apart from the "ills" mentioned, it doesn't have any, hence my desire to follow through with the mod.

I've been in a boat that capsized and went down like a rock. No fun at all, so I'm making sure. I'd much rather be clinging to an upturned hull than taking my chances with the tiger sharks. At present there's at least 200kg of negative buoyancy, possibly more. That's just too much for my comfort.

The transom bracket idea may have some merit, I'll look into it.

Malb and HazzaB, the boat owes me less than $10k, plus about another $3k for this current refurb. The mod will cost me about $2k.

That means I will have a 19-20' hull with a 120 for under $15k. The rest of the boat, including electronics, internal fitout and so on is just as I want it because I've built it that way.

Even if I spend $15k on engine, I couldn't find anything similar in the size for the money, let alone with the finish of this one.

The trailer is my only regret. I fitted it with a new axle and springs only a few months ago. Still, It'll be a few months before I have to worry about it. If I do this mod it will be after the summer. Anyone want a good 17' boat trailer?

Malb, this isn't going to be a pod, as such, more a hull extension, following the below-waterline lines and continuing the chines, etc. Once finished there won't be any sign it's been done, hopefully. I've read of people adding "pods" either side of the engine to improve stern buoyancy with these hulls, but this is intended to shift the engine further sternwards as well.

Does anybody have any advice as to design considerations?

Howdy, I like pods, but they will make the problem worse not better as they move the weight of the engines back but only supply a little bit more buoyancy to float them.

The pods in some ways show that boats are not too unhappy with more weight further back.

Not that its a bad thing to look at extending the boat, but because of pods a lot of boats have weight even further back than this one.

Some older boats have the transom and the front end of the transom well (the bits that stop the water coming in) cut down below the sheerline at the back of the boat. Is that the case here?

Best wishes
Michael Storer

Originally Posted by Boatmik

Howdy, I like pods, but they will make the problem worse not better as they move the weight of the engines back but only supply a little bit more buoyancy to float them.

The pods in some ways show that boats are not too unhappy with more weight further back.

Not that its a bad thing to look at extending the boat, but because of pods a lot of boats have weight even further back than this one.

Some older boats have the transom and the front end of the transom well (the bits that stop the water coming in) cut down below the sheerline at the back of the boat. Is that the case here?

Best wishes
Michael Storer

Hi Michael, Yes, the transom is very low and the front of the engine well is also cut below the sheerline.

The Johnsom 120 weight is about 165 kg. The displacement of the proposed extension would be about 250litres, so it should float the weight of the motor on its own?

Yes, I know the CG will shift sternwards, but I have 2x45 litre fuel tanks still to install,a couple of 20 litre freshwater tanks and a bow rail, which could be made heavier if needed, all of which will compensate trim somewhat. I had planned to mount the fuel tanks under the rear seats and the water tanks close to the CG There is also a small new head in the bow end of the cabin. I have the facility to weigh each end independently if I need to, using the load guage on a crane truck. As I said, the boat has been modified already by adding a large longitudinally mounted fuel tank amidships, which makes her a trifle nose-heavy on the plane. She needs to be trimmed very high to really get going, but she rides really well once she's trimmed out.

It also occurs to me that if I add the extension the sheerline can be quite severely tapered to the waterline, as seems to be the trend in modern design, while still leaving a fair distance for any following sea to have to travel before hitting the cockpit, as well as having the extra buoyancy.

Can you offer any guidance as to where the CG should be located?

This guy suggests 40-45% of LWL measured from the transom is the ideal location for the CG. Does that sound right to everyone else? I haven't weighed her fore and aft yet, perhaps that should be my first step.

http://www.boatdesign.net/forums/boa...fts-20495.html

Howdy,

I'm very cautious about what I find on the internet, unless I know the source. There's lots of misinformation.

The article refers to Dave Gerr's book, if that figure came from Dave, I would trust it absolutely.

However, I would expect it is one percentage that would change with the normal speed the boat will be operated at - and find it a little strange that there is one range, with no method of working out where it should be in the range. It sounds sensible but I don't know if it is right.

I would suspect most production builders in the era of this boat will just mix and match and if it drives OK then it goes into production.

If you are wanting to make the boat safer, then increasing the height of the front of the splash well would be way easier and the benefit would be easier to see. Extending the hull is such a huge job, I would choose the easier of the two.

But if you are keen to do the work and to accept the risks (which reduce with the more research you do) then extending it could be an interesting project.

Best wishes
Michael Storer

Hi Michael,
I've already thought of extending that piece, but I'm also concerned about the motor getting swamped.

I'm looking into options at the moment. Replacing the S/S fuel tank with plastic would be a start, and lifting the transom and fitting aan engine with a longer leg would keep the motor a bit drier.

The idea is to make this thing a useful inshore fishing/dive boat. If I can do that without the extra work, so much the better. I'm looking at a Mercury Optimax 150 with a 25" leg.

I agree with you that the CG will vary, but I think the sea conditions affect things a great deal. It may be OK to work a specific-speed CG out ofr a competition ski boat, but not for a boat that will operate in highly variable sea condiditons at equally variable speeds.

A small update. I removed the S/S tank today and the hatch cover, to be replaced with 3 plastic 45 l tanks that will be foamed into place and decked with some lightweight ply. The hatch cover alone weighs about 25kg and the tank is very heavy - not sure how heavy at the moment because it still has fuel in it, but at least 40kg, so that's nearly the weight of a passenger between the two.

I'll try that and the higher transome/engine mount as a fiirst approximation to the solution. Thanks for the responses folks. I find the best way to think about problems is to enunciate them to others. It helps clarify and sometimes, like this one, you get useful answers from people who aren't too close to the problem to see it clearly.

Thanks again.