How will it float?

[Plover]

Hello All,
I need a little help from you guys and of course girls. This is not a wooden boat question, however it is a boat! Apart from the on going restoration on our gaff ketch we decided to put together a hot dog boat/floating resturant. It so far came out very well I think, I knew in doing this we were going to run into a bouyancy issue, and as always the first thought is almost always right! I don't know the math or the formulas for figureing out what I need so I was hopeing some one here may be able to lend a little knowledge? So, what we have is a 28 foot Harris pontoon boat with 24 inch pontoons, it has a1988 80HP Merc on it (hefty critter), and we have had a mount put on each pontoon, one for a 60 lbs propane tank, and one for a 6000 watt inverter generator (roughly 350-375 lbs). In floating it for the first time the other day it was a bit stern heavy which I did expect. So the wife and I brought 1000 lbs. of lead ingots from our ketch to simulate people and stock on board. The people part worked great and flattened it right out figureing 5 people of 150 each. However, when we went for the stock and water the bow stayed real nice but the stern krept lower and lower! Obviously not what we had in mind! So today we are off to pull the boat from the drink and have the pontoons extended some time this week. So, the question is how long should we have them made 4,5,6 foot? (I'm thinking 5 foot at a guess)
Sitting with no lead forward she draws about 5-6 inches above the center of the pontoons and the bow is about 4-5 inches below the center of the pontoons. And slow ahead sqwats the stern just a little. If I'm not mistaken the tag for the boat says something in the area of 3200 lbs max or 25 people. Any help here would be very much welcomed! The wife did post a couple short videos on youtube titled "Rascals River Dogs" which is its name and named after our mutt Rascal. Thanks in advance to everyone and look forward to hearing from you!
Paul

[Ian McColgin]

Picture's too small so even though you may have posted this elsewhere - which ketch.

Now on: She might have gone down by the stern if the higher waterlines aft are a bit finer than those amidships and if she has a bulbous bow. Hard to tell what or why.

Likely too late to change your plan but I'd have been inclined to put in a V center hull that would not take to the water when the boat's light - leave her running about natural with that big motor but have bouyancy come into play when needed. Adding bouyancy aft may make her settle more bow down. If you can experiment with some styrofoam blocks or empty drums or something under her you can find where bouyancy is actually needed and then see if you can figure how to get it.

If you have people coming aboard with any financial at all you have passengers for hire. You're looking at enough folk that you may need to be inspected which means any modifications need to be fully and correctly engineered. It also means you need your ticket, safety gear and all that.

Anyway, figure fresh water at about 60# cubic foot (close enough) and lay in a fudge factor for the weight of the added hull material.

G'luck

[Plover]

Hi Ian,
I did speak with you about the ketch some time back, Its a Pete culler, 34 foot raised deck ketch page 31(?) in skiffs and schooners (Plover). Its drawings are at mystic # 128.73
So your saying 60 lbs of bouyancy per cubic foot of extention? So my guess of 5 foot will be about right?
Thanks Ian,
Paul

[Ian McColgin]

Now I remember. I am so forgetful - sorry.

If the pontoons are cylinders 2' diameter then you pick up a bit over 3 cubic feet bouyancy per foot extension - 5' by two hulls yeilds (all in the head here) somewhere near 30 cubes or 1800 bouyancy if totally immersed. Half that if the water line is about half way up.

But before you get carried away, double check the legal issues.

And then fix the ketch and go sailing!

G'luck

[mmd]

<Oh, crap... it's that designer guy again, sticking his nose in where it shouldn't be!>

I can't advise you on specifics because it opens up a whole can of legal liability issues unless you hire me to act on your behalf, so I'll have to just point out a few things and let you decide on your best course of action. Suffice it to say that, if you are intending to have Joe Public step on board of your boat - however briefly - you had better have your act together and understand all of the stability and regulatory issues of pontoon hulls and commercial vessels. Remember the tragedy of the little tourboat that swamped in upstate New York a couple of years ago, killing a number of senior citizens - that disaster was brought on by lack of understanding of stability and (IMHO) lack of reasonable regulation of inland commercial boats by your government. Be very aware of how easily you can kill people and you lessen the likelihood of it happening.

OK; enough of the scary generalizations part, now for some specifics:

The 3200 lbs max capacity is presumed to be an evenly-distributed load, sitting down on seats. I might even suggest that it is the max weight of the entire vessel - people, engine, boat, & picnic basket. I did a quick calculation of displacement and determined that the all-up weight of the vessel at 1" below half-depth of the pontoons (11" draft) in fresh water is 3875 lbs. More than that and you are losing reserve buoyancy very quickly.

Assume the average weight of people to be 175 lbs, not 150.

Twenty-five people is way too many to have on that type of boat, seated or not.

Pontoon boat stability is weird - not easily compared to monohull stability or even boat-shaped catamaran stability. If a pontoon is submerged beyond its halfway depth, it loses reserve buoyancy very quickly, possibly resulting in a capsize. That is why passengers must be seated - if everyone is standing and they walk to the side of the boat to watch little Junior do a belly-flop (or line up to get to the hot dog condiment counter) they can easily submerge one of the pontoons and over she goes...

I would suggest that you consider arranging the operation so that the public does not set foot on the boat - serve over the side to other boats or to a jetty.

And, of course, my usual advice is to consult a qualified designer or naval architect to make sure that all proper considerations have been made for the safe operation of the vessel in all intended conditions. It's cheap insurance in the long run.

[Brian Palmer]

<Oh, crap... it's that designer guy again, sticking his nose in where it shouldn't be!> ....

Remember the tragedy of the little tourboat that swamped in upstate New York a couple of years ago, killing a number of senior citizens - that disaster was brought on by lack of understanding of stability and (IMHO) lack of reasonable regulation of inland commercial boats by your government. Be very aware of how easily you can kill people and you lessen the likelihood of it happening.

.....

Pontoon boat stability is weird - not easily compared to monohull stability or even boat-shaped catamaran stability. If a pontoon is submerged beyond its halfway depth, it loses reserve buoyancy very quickly, possibly resulting in a capsize. That is why passengers must be seated - if everyone is standing and they walk to the side of the boat to watch little Junior do a belly-flop (or line up to get to the hot dog condiment counter) they can easily submerge one of the pontoons and over she goes...



Remember also the water taxi based on a pontoon boat that flipped in Baltimore's Inner Harbor, killing 5 passengers in March, 2004.

Brian

[Woxbox]

To emphasize the points made above -- if a full load pushes the pontoons more than half-way down, then that exact same load, if it shifts fully to one side, will totally submerge one pontoon. Once one pontoon goes under, it won't stop. The boat will just keep heeling until it rolls or everyone falls off. Also, as soon as it starts to go, loose stuff will slide down and exacerbate the problem.

The Baltimore Harbor incident was helped along by a powerful gust of wind as I recall, and the fact that the platform was curtained in on a cold day. But once the passengers all slid to the low side, there was no chance of recovery.

And Lake George aside, the US Coast Guard is very picky.

[Plover]

Gents,
As always you guys are a very valueable source of information and in truth I really don't want anyone on board at all and there is a very good chance that at best maybe one or two. I had to put seats for the liquor license (Beer Only).
I truely don't think that there is going to be a capsize issue mainly because there really isnt enough space forward to put as much weight as would be needed to swamp it unless of course they are all made of lead. However points are well made here by all! The main issue is how much to add to the length to flatten it out. All pontoons are stern heavy that is a given and some are far heavier then ours! Thank you all for your calculations and advice well issued!
Paul