Hot Tub In A Boat

[rhubley]

Ok...here is a challenge for everyone. I have a 22ft fiberglass ( 8 ft beam ) cabin
cruiser hull. It has been gutted leaving only the hull and a bit of the front deck. In
this empty hull we hope to place a 6,000+ lb hot tub ( wet weight minus people ).
Under the assumption that the boat will remain stable displacing this much weight
in water ( assumption backed up by some back-of-the-napkin-calculations at this
point ) what would be the best way to distribute the weight of this tub? Obviously
it should be kept low in the boat and near the center of gravity ( stern to bow ).

Should it be supported by a set of stringers ( full length ) angled to fit against
the inside of the fiberglass hull? Should we build plywood bulkheads every
x inches and divide the weight evenly along the keel-to-sheer only under
the tub footprint? Oh..and in case anyone is curious we plan to keep this
boat moored for 99% of the time -- rarely bringing it out on a calm lake
for short little jaunts.

Any jokes, disparaging remarks, wild theories, practical tips, or brilliant
ideas welcome.

-R

[MattZ]

Depending on the quality of the lake water, you may want to consider not keeping the water in the tub between uses. I'm assuming you're planning on using the engine to heat the water. It wouldn't be too much trouble to just heat a fresh batch of lake water every use, no unpleasant chlorine that way.

As far as weight distribution goes I think you may need more than plywood stringers. Are you going to put a ready made hottub in the boat, or lay one up out of fibreglass yourself?

[Skippy]

Just buy a Mac 26, it's already installed! :D

Between the weight concentrated amidships and flotation at the ends, yes stringers should help distribute the load over the hull bottom. Also, the topsides and deck will be under fore-aft compression due to the buckling stress, and the topsides may develop a tendency to separate from the deck, which will be stretched athwartship.

[Tim B]

There is, of course the small matter of Free Surface effect reducing the stability as well as the change in CG. Free surface effect is calculable (in basic terms) but it tends to be a big problem, that will give you an unstable boat with a lot less than a hot-tub in it. I suggest that you either find a naval architect who can tell you if it will be stable, but personally, I don't think it will be if you take into account the effect of the free surface. 6000lbs of ballast (which is what it is) would come fairly close to doubling the displacement.

I would seriously advise against doing this, at the best you'll end up with a very tender boat, at the worst you'll do yourself and everyone else serious injury,

Tim B.

[FAST FRED]

Find a nice 85-125 ft boat with a combination Hot tub , boat storage well , with a sliding cover.

Install your boat in the well when not using the hot tub!

Will SAVE lots on not needing bottom paint for existing dink.

FAST FRED

[SailDesign]

I'll second Tim B's comments on Free Surfaces. Nasty things. Think of it as a dinghy with lots of water in the cockpit, and you'll get the picture :)
Steve

[Vega]

Just put the hot tub over the CG of the boat and then make an open living space around it.
After all one of the most famous American architects when he planned the perfect house for himself designed the saloon around a big central jacuzzi and that room even had a huge piano. :rolleyes:

[rhubley]

Good point about the Free Surface Effect. Nothing can be done about
that except maybe extending the hull out amidships to be more boxy like
a barge. Basically this will be a barge. It's purpose is to tie up to a house
boat and serve as a hot tub float ( on fresh water ). Unfortunately we already
have the hull so I have to find some way of working with it.

[Skippy]

Outriggers sound like an excellent idea if you have room. With the tub below the surface, the boat will be a dog as far as being a boat is concerned, but will be very stable as a barge. One way to deal with the loose water might be to seal off the deck and everything else around the tub, and leave the tub open with no deck above it, just a roof or awning. That way if it does go over, the water will spill out of the boat, which will then come back up quickly. Of course, if there are people in the tub at the time, ..... :eek: I concur with the warnings.

[steveh]

I looked at a 37' Connie that had had a hot tub installed in the foreward cabin. Fair bit of rot up there, I figured from sloshing.

I would suggest moving to the FL keys where hot tubs on boats are redundant.

[yipster]

Any jokes, disparaging remarks, wild theories, practical tips, or brilliant ideas welcome.
forgive my ignorance late seeing this tubboat thread and not having even read it all...
first thought i had was to dig a hull in as pool at a sunny backyard...

[cyclops]

Leave the boat alone. Go with a floating swim platform that completly surrounds the tub and has a 6' wide deck all around. Stabile and a place for the drinks and snacks and those who sit on the edges. Put Blue Styrofoam under the decking. We can give you deck area and foam requirements for up to 50 people if needed. Hey! Out of the tub and a dive into the lake. A Sweedish Sauna.

[marshmat]

I second that, Cyclops.
If the boat has a (semi)functional motor I might build its hull into the decking somehow- use it as a cooler/sunpad/power unit. If you want the tub in the hull, you'd probably be looking at adding some hefty amas/outriggers to keep it upright, thus getting you a trimaran barge.

[Tim B]

why not keep the boat as a boat and build a hot tub into the pontoon?

Tim B.

[rhubley]

Floating dock concept is out. So are wide outriggers. Keeping the boat
as a boat is of course an option but of no use to me. This is a gutted
hull with no engine.

In any case I looked at MaxSurf and it has the ability to calculate Free Surface
Effects. Unfortunately it is very very expensive and I only want something
to assist in this one project. Does anyone know of a free package that
can perform these calculations? Would anyone be willing to enter in
the boat design and run the calculations for a small fee -- assuming
you already own a copy of this software?

[cyclops]

If it is gutted the way you describe, I am not so sure the hull sides are now strong enough not to cave in with tub, water and 8 to 10 non swimming people in it.

[rhubley]

Hence my original question....What type of framing would be necessary to support
this in a hull?

[Skippy]

You have three tons of cargo sitting amidships. It's not really an unusual problem. The primary stress will be the same as on an upside down see-saw, downward weight in the middle and upward pressure at the ends. The stringers are longitudinal beams that support that load. The hull and deck together act even more effectively as one big beam to do the same thing, so you should replace the deck with something that will take longitudinal compression and athwartship tension, held in place vertically.

Other than that and supporting the tub within the hull, it's a standard problem of reinforcing the hull against the pressure from being immersed. Any contact between tub and hull sides will help equalize the pressure on either side of the hull. I doesn't make too much difference what shape that support is, mostly it just transmits pressure directly.

[Willallison]

www.yachtub.com
:D

[rhubley]

Will, I saw that -- nice concept if you don't mind lots of heat transfer
into the water. Also it would be nicer to have the tub up higher.

Skippy, thanks for the pointers. At first I was thinking of supporting the tub with
C shapped transverse supports but after reading a few responses I see that
both stringers and transverse supports are needed. I am thinking I will use some
large stringers under the tub with plywood C shaped transverse supports placed
in a grid pattern under the tub footprint. The stringers will continue beyond the
footprint and end in a wedge to avoid hard angle effects. Now I am wondering
if I should glass over the stringers. Are there alternative ways of anchoring the
stringers without making them enclosed ( and vulnerable to rot )?

[Skippy]

How can the C-supports form a grid if they're all transverse? :confused:
By a wedge you mean tapered ends? That's fine.

The stringers should have a glass outer shell, maybe epoxied, I'm not positive about that, with a lighter core inside. Most people seem to trust epoxy/glass sheathing these days.

You might want to also consider some supports directly between the tub sides and hull bottom fore & aft. They'll support part of the load more efficiently than stringers or the surrounding deck and hull. Of course they take up more space, and they're also stiffer and have to be kept from buckling, so it might help to consult with a professional NA.

[Inquisitor]

Sounds like an interesting problem. Since you said 99% docked...

Remember any portion of the tub below the waterline you don't really have to have structure. i.e. Put a kiddie pool in a pool and fill it up... it just sinks to the water line inside. No appreciable structure required. (You just have to account for those high heel shoes! :D) You could do a several inch thick foam core (to house your jet's plumbing, and heat insulation value) and then put several layers of glass inside and out and call it good. It would rise out of the water a couple of inches higher than the waterline inside due to the foam core buoyancy. For placement, hinge it off the stern. Easy to step down into and pull it up for your 1% outings. As someone suggested… if you’re using lake water… you could have a BIG drain in the bottom to let the water in/out while lowering/lifting and it would be self filling/draining as you lower/raise it.

[yipster]

a simple big blow-up pool at the transom heated by the engines may be the pool we all want behind our boat.
a lot of fresh water consumption each weekend though...

[safewalrus]

Two watertight bulkheads amidships some few feet apart defines the ends. stick a few holes in the sides to give a wet well, water runs in, water runs out! solves most of your free surface effects (slope bulkheads into the centre, bottom wider than top also helps,) (holes have to be fairly big), chuck a couple of logs floating in the well sorts out the splashing! Deck over the ends to give bouyancy! Shove a BIG water heater in the bottom and cover the whole bottom with a grateing for standing on. OK you heat a lot of the lake but you don't want it too hot do you? Removable outboard on the back (technical term not used this is a bath that goes out in a lake not a boat). On the odd occasion that you do go out go slowly! Enjoy!

[l_boyle]

Whew, I am glad that you all answered this..... Yes, the boat the way it designed for.. Built a floatie ring of foam, tow it, and be happy... Why, fold a boat..

[rhubley]

Hey everyone. Thanks for all the advice. I figured some of you might want to know what happened in the end. Well here is the result:

http://www.repeatmasker.org/~rhubley/hottubboat/index.html

[marshmat]

Well I'll be damned, it worked!
Looks like you did a pretty good job in the end. All that's missing now is a bar :D