Can my boat go faster?

I built my own boat. You can see the pictures of it below. It's essentially a pontoon houseboat. I over-built it with a welded steel frame, so it's pretty stout. I also built a trailer for it, so I can drag it anywhere there are paved roads that allow oversize loads. I drove it down the mississippi river twice with no issues. There's even a section of the river south of baton rouge that is all churned up with 3-4 foot waves from cargo ships and I had no issues, other than the crew not being able to sleep.

It took me a few years of tinkering, but it now floats and drives great, but the main problem is just that is slower than I think it should be. I have a few ideas to make it faster, but I figured some of the people on this forum could help point me in the right direction, or maybe save me a lot of trouble if you think it's not going to get any faster. Here are some basic details about her...

Hull type - plastic foam filled pontoons
Length - 25ft
Beam - 10ft
Total Bouyancy - about 20K lbs
Total Weight (empty) - about 8K lbs, (loaded) about 10K lbs
Draft - 18"
Freeboard - about 2 ft
Power - 2 Yamaha F115 outboards
Range - 155 gal @ 3 mpg displacement speed 6 mph = 465 miles
Top Speed - 10 mph (via gps)

Idea #1 - My first idea is to connect the 3 pontoons in the stern at the chines with flat pieces of plastic so that it can possibly plane out a bit at top speed. It doesn't add much weight to my already heavy boat, but I'm not sure it will make any difference. In case it's not obvious, the center stern pontoon is actually smaller than the others, but the chines at the bottom are lined up even with the chines on the others. It used to be the ctr bow pontoon, but I moved it to the stern when I upgraded to the larger one.

Idea #2 - Remove the center stern pontoon and build a new, longer & wider one with a flat bottom extending all the way from the bow center pontoon chines all the way back to the stern. It seems like this would give it more surface area to plane out, and it would add a few thousand pounds of buoyancy.

Idea #3 - Get rid of both center pontoons and build a center hull (maybe a modified V?). It would be a crazy amount of work, but, as you can probably see, I enjoy working on this thing.

Idea #4 - More power? Maybe upgrade to 250's?

Idea #5 - Leave it alone and concentrate on more rod holders.

Thanks in advance for the advice. I'm leaning towards option 1 since it's pretty easy to test, and then probably option 2 when that doesn't work. But I'm not confident any of these ideas will make this behemoth any faster.

 

More power. You aren't going to get 10K lbs to plane at ~8 knots no matter what the bottom looks like, which will be about as much work as dropping the pontoons and building a hull. Then you get to worry about stability when start shoving that thing thru the water faster. So maybe #5 instead.

 

Good for you to be out there enjoying boating!
We get so many theoreticals here, but you have actually done it.

The easiest way for your houseboat to go faster is to lighten it.

The next easiest is to add more power but it's not going to go appreciably faster for the added weight and cost.

What I would suggest is eliminating the centre pontoon.
But that would mean larger outer pontoons.
Long and skinny (20:1) with tapered ends.
Forget planing, it's not going to happen.
However, this would essentially be a rebuild and probably not on the wish list.

10 mph is pretty good.
My 25' x 10', 10 000 pound houseboat does 6.5 knots with a single 25HP Merc Bigfoot.

Just turn around and go downstream and you'l be flying!!!
Or get a speed boat to tow around with you...

 

Idea #6 - More beer/ tranquilizers.

I like the thing, I have a 24' trailer I was thinking of putting on a flat bottom scow, but for some reason my wife has objections.

Anything too drastic and you'd have to modify the trailer also, but you probably wouldn't mind that.

Adding length might get you a few more mph. Maybe slide the toons forward to utilize the entry shapes and then add extensions off the back of them.

 

Thanks, that helps. Going downstream in the mississippi was nice to get that extra 3-4mph. I'm not sure I could even attempt the uphill trip. For what it's worth, I added the 2nd motor a few years after the first build. It really only added maybe 1 mph. I added it for redundancy, but still pretty disappointing.

I guess this is why sailing catamarans have that skinnier shape to the hulls? My plastic pontoons are actually kind of heavy since they are filled with 2lb foam. I think they weigh in at about 2000lbs total. So if I replaced them with something like stitch and glue fiberglass, I could probably accomplish what you are suggesting without adding any extra weight. If I made them 4 ft deep by 2 ft wide by 25ft long, I think I'd float even a bit more than I am now. I'd need to move the motors to the center, but not that big of a deal. It would also help me widen the trailer. Theoretically, what kind of speed increase would you guess for the same 230hp?

 

I'm terrible at math. 20:1 would be something like 30ft x 1.5ft x 4ft. I could still do that, but more work to the trailer would be needed.

 

Stick the pontoons to the trailer, fix up the Vee Dub Bus and you could tow your "boat" to the water and then float away on your "trailer". lol

 

20' x 1' would be 20:1

30' x 1.5' as you said, would be 20:1

40' x 2' too.

I'd make them pointy at both ends or sweep up the rocker aft to a fan-tail.
And three times taller than you need.
Then you'll have lots of reserve buoyancy for snow, passengers, cargo, provisions,
fuel, water, helicopters, whatever! Fenders, rope, stuff.

Really, you're going to rebuild your Yacht?

 

That's a tough one.
So, the weight stays the same?
35' LOA x 1 1/2' pontoons (30' LWL)
230Hp
16 - 22 knots top speed but your cruising speed will be higher for the proportionate fuel consumption.
You won't have a "hull speed" to speak of. Your efficiency speed range would be broad.

 

Thanks for the input and for the math. I'm gathering with your information, as well as some research on my own the last few hours, that adding the 3rd pontoon in the middle slowed me down quite a bit. I needed the buoyancy, but it appears I would be better off just extending the 2 on the sides and losing the center pontoon as you suggested. That's actually pretty doable. I would have to reposition pretty much everything to dial in the c.o.g., but it's definitely doable with a lot of work.

But yeah, I am considering a complete redo on the hull. It sounds fun to me. My wife is pretty understanding, and I own the companies that pay me, so I guess I'm living the dream. It's a ridiculous dream, but anyway, now that I have the experience of building version 1, I am pretty interested in creating a version 2 that is faster and more sea worthy (and maybe even more road worthy). The dream is to do the great loop in this thing, but based on feedback from another post I dropped in another forum, it's not seeming like a great idea. If the weather's good, it can be done in a dinghy, but obviously you can't count on that working out too well in the gulf of mexico or the great lakes for that matter.

I can get my mind around fabricating hulls that are pointy at both ends. Not so sure I can manage the more complex designs. Would I need to reinforce them with an inner steel frame, or just bend up some 3/4" marine grade plywood? Fill them with foam?

 

How you make them is up to you.
Aluminum would be nice if dollars are less of a concern.
Stitch and glue with fiber-epoxy would be good too.
Either buy a set of good plans that fit your needs or get a professional involved...
or both.
Steel is always an option too.
And they can be round on the bottom half but an expanding square top side for easier attachment and more reserve buoyancy per inch.
You have a lot of research to do my friend.
We'll be here to bounce ideas off.

 

If you're making new hulls, drop a small diesel in each one along with some large, midship fuel and water tanks...

 

As I read it, there are two centre pontoons, one behind the other, plus the two on each side. So 4, A Quadtoon ! And they are made of what kind of "plastic" ? Do all these pontoons have the same cross section ? If they do, one solution might be to remove the two inner ones, and weld (assuming they are polyethylene) them to the outer ones, in such a way that extends the waterline length, while preserving most of the buoyancy. Naturally the weight of the structure would need to be in line with the new configuration, to keep everything on the level. But of course, the trailering length would increase, and no doubt other issues would have to be considered.

 

For more speed get rid of the steel deck and build it out of aluminum. Involve an engineer to reduce some of the size of the members if you are
trying to optimally reduce the weight
What props are you running. The 115's will have come with a pitch designed for a higher speed, ie higher inlet velocity into the leading edge of the blade. A prop designed for say 30 mph will not work properly at 8 mph pushing a barge. You need a lower numerical prop if you are using the original prop. Perhaps a larger blade area or a 4 bladed prop if you have a 3 bladed prop.
That would be the easiest way to get more speed.

 

Your Length-displacement ratio is very low. Can't do much about that, except make it much much lighter....and/or extend the tubes