entension/swim platform

[jeemboNC]

I want to add a new swim platform to my 42' semi-displacement trawler that is bigger and also capable of supporting the berthing of my 600# launch (12' CC). To take it another notch, I would like to make it an extension, capable of extending my 38 WLL by 5 feet (get a little better efficiency at 8 kts). I DO NOT want to relocate the running gear, so I know I will compromise slow speed turning a bit.

What says the experts? Is this crazy? I have a reputable fiberglass shop that will make the shell for $2000 - I'll have to do the design and there will be more $$ for strength/hatch covers/bump rails. Thanks, Jim

[gonzo]

600# will not be enough to compensate for the extra floatation. You may consider moving heavy items aft. It sounds reasonable.

[alan white]

It's true, the extension, to both increase the hull's underside and also to remain dry especially at rest, will displace thousands of lbs and your boat will float high in the stern unless compensated for.
What you're gaining in efficiency due to the extension is only seen at higher speeds, but at lower speeds, the extra skin is slowing the boat.
My suggestion would be to forget about doing it unless you are very experienced in design and building. To pay someone to design and build the structure would be rather expensive, though probably workable.
If you already have a standing platform, what's wrong with davits? The dinghy will be up and out of the way when the sea gets rough instead of bashing around below. Your dinghy is very heavy however. It might be a good idea to look for something a lot lighter, A tender for your size crew, to be useful, should be a light displacement boat of no more than ten feet, and 150 lbs,
Larger boats with big crews can deal with much bigger tenders, but those are hauled aboard and if not could be hauled up quickly.

This might be worth to look at!

how thick the fiberglass?

Regards
Richard

[CarMan]

Originally Posted by jeemboNC

I want to add a new swim platform to my 42' semi-displacement trawler that is bigger and also capable of supporting the berthing of my 600# launch (12' CC). To take it another notch, I would like to make it an extension, capable of extending my 38 WLL by 5 feet (get a little better efficiency at 8 kts). I DO NOT want to relocate the running gear, so I know I will compromise slow speed turning a bit.

What says the experts? Is this crazy? I have a reputable fiberglass shop that will make the shell for $2000 - I'll have to do the design and there will be more $$ for strength/hatch covers/bump rails. Thanks, Jim

Sounds intresting !
Hawe you seen anny one doun this to your boat type (what is the boat name)

I am doing this ting to my boat righit now, I know it works fine on my boat type, it is lots of people how had don this to my boat type (bayliner 3288)

[TollyWally]

I would be delighted if Tom Lanthrop might comment on the topic of bottom loading after grafting an extension like this onto a boat.

[CDK]

Calculate the buoyancy from length, width and submerged depth, then have a crane, equipped with a scale apply that force to the transom of your boat to give you an idea of the change in waterline position. The actual situation will be even worse because the force will not be applied to the transom but at a point 2.5 ft. behind it, but the weight of the extension will work in your favor.

I increased by boats WLL with just one foot and already experienced little problems like water and dirt on the foredeck that used to run off, the kitchen sink that now has the drain on the high side of the bottom and a leaking window frame.

[waikikin]

Originally Posted by jeemboNC

What says the experts? Is this crazy? I have a reputable fiberglass shop that will make the shell for $2000 -

Sounds way too cheap, All the best from Jeff

[jeemboNC]

Thanks to all for helping me with this. To answer some questions and expand a little:
1. the boat is a 42' Present Sundeck Motoryacht (also called Ponderosa) made by Chung Hwa Boatyards (CHB).
2. I have three other dinghies/launches/tenders - an 8' Caribe RIB w/6hp Johnson (~200#), a 13' Seadoo Explorer RIB (700#), and a 12' Avon RIB w/15hp Merc (450#). I like the 12' Logic CC the best by far (fun/fast/safe/great fishing platform/indestructable). The Caribe and Avon are withing the capacity of my davit/hoist and will set on hardtop over the Sundeck (the Caribe is there now).
3. The bouyancy will be ~2600# (42 cubic ft). I guess the weight of the extension and reinforcements will be 500#. The launch is ~600#. Net bouyancy is 1400#
4. I have three potable water tanks. Two are aft, CG ~6' forward of transom, and they carry 150 gallons (~1250#). The forward one (CG ~ 10' aft of bow stem at WL) carries 75 gallons (~600#). It is difficult to discern any change in the attitude unless dead calm and I look really hard - ~1/2" delta at stem or stern from full to empty. Underway, very hard to tell. Note, my loaded craft is ~34,000#.
5. I could carry gear/fuel/water in this new compartment to offset the remaining bouyancy, yes? Will lower the static loads on the attachments, as well. Or I could waste it and flood part of the hold.
6. What I do not know is the dynamic loading at ~8 kts.
7. I will likely inset the extension ~1/2" at the transom to reduce the chance something will strike the hull underway and catch the edge.
8. I have calculated ~.5 knot increase in speed to 8 kts. Or put another way, 8 kts is the sq root of the waterline times 1.3 at 38' and 1.22 at 43'. I might run at only 7 kts to further save fuel, but I would rather be able to maintain 8 kts cruise. Barring wetted area drag (how much will that be at 60 sq ft?), I see 6% improvement in fuel efficiency.

Any new thoughts? Jim

[jeemboNC]

revision - looks like the volume is only 32 cu ft and 2000#. net bouyancy 900#.

[CDK]

Originally Posted by jeemboNC

revision - looks like the volume is only 32 cu ft and 2000#. net bouyancy 900#.

That somehow doesn't add up.
You want to add approx 13% to the WLL of a 34000 # craft. Is the stern so narrow the added volume there is only 32 cu ft?

[jeemboNC]

I was also surprised, and am now again from the math (this is why I shouldn't do these when tired and numb/dumb!). The extension top surface is/will be 10" above the static waterline. It will attach to the transom that is submerged only 8" deep at the side and 12" deep at the center. The width is 152" at the top, but it narrows at the waterline and the chine to 146". The length is 60", and at the end (trailing edge) is at the waterline (I am trying to reduce induced drag like a tailcone on an airplane or a displacement hullform).

That works out to (making the math simple) ((8+12)/2)" x ((152+146)/2))" x ((12+0)/2))" = 5.17 cu ft = 323#. Big difference! Barring major dynamic and water (poop) loading, this is NOT a demanding structural challenge until i get major stern squat.

Did I do it correctly this time?! Jim

[gonzo]

You need to measure the submerged volume when the boat is bow up. It will be more.

[CDK]

The calculation seems sound to me Jim.
Gonzo's remark about the bow up situation doesn't bother me because I was only looking at the static changes like the waterline that needs to be redrawn. Dynamic changes that will cause the steering to be a bit different, the higher hull speed and the bow not being able to lift as much as before, will certainly be there, but I do not feel qualified to calculate these.

I made my little extension under the car port, embedded aluminum L-profiles in the grp around the circumference, positioned the structure using timber, wedges etc and drilled holes through the transom and the aluminum. Then I separated the extension from the hull again after marking the errors, corrected these with a flex, liberally applied Sikaflex marine sealant to the joint and bolted it to the hull with 3/8" stainless steel bolts and nuts. In theory it should be possible to remove the bolts after the Sikaflex has cured, but I left them where they were and even tightened them a bit after a week.

[jeemboNC]

Gonzo - yes it will. I am guessing that will make the attitude flatter, yes? Allow same trim (and fuel burn) at the extra half knot? Is that bad? I might add 3" of depth to the trailing edge now, resulting in 430# of static bouyancy . With a following sea, the wave will lift this tail and could be an issue.

CDK - how did your extension work out? Have photos? Strong enough?

Thanks for helping out, guys. Jim