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Thanks in advance Boat Designers.
Question: Can a tritoon boat design have an asymmetrical design?
Background: I am designing a Gold dredge, tritoon design, 10ft X 27 ft, 25" diameter tritoons. To be used in Ocean, max distance travel 8 miles, max speed not greater than 20 knots. In order to accommodate under deck equipment, Pontoon #3 (see photo) needs to be spaced greater than #1 and #2 (apprx 32 in, versus 12 in).
Center of Buoyancy and Center Gravity estimates are on the photo.
Assuming that I will not be over about 8-10 miles, max speed 20 kts. Are there any design problems associated with an asymmetrical pontoon layout? How best to mount the outboard: A. Center pontoon transom B. Transom mounted on geometric center line C. Along Center Gravity centerline?

 

What's the reasoning, you want enough width for the riffle box spillway ?

 

The boat will list and trim until CG is directly above the center of buoyancy (the actual center after list and heel, not the center based on a design waterline).

 

I presume that this is a 'development' of the design proposed in this thread last summer -
Fiber Glass Pontoon construction technique - Joining sections /threads/fiber-glass-pontoon-construction-technique-joining-sections.64114/

Are you still planning on fibreglass construction, or maybe aluminium?

Is 10' the maximum limit on your beam? If it is not, it might be easier to increase the beam so that you have a symmetrical plan form?

 

yes, that is correct. The sluice box "riffle box" is 28 inches X 12 ft. The spacing b/t pontoon 1 and 2 is 12in. Hence, the larger spacing b/t 2 and 3 to accommodate the sluice box.

 

Sir, thank you for your reply. Yes, I have decided on all Aluminum construction based on strength, ease of construction, and cost. I was originally going to go with 34 inch diameter 2 pontoon. But was worried about the Z center of gravity being too high ( I will be using a heavy gold dredge engine 700 lbs). 25 inch pontoons lowers my c.g. significantly. Using a 25 inch tri toon I get almost the same buoyancy as twin 34 inches (16,500 lbs vs 20,000). A few components will be fiberglassed, underside of deck, etc You noted a significant factor about going with a larger beam. Yes, Im considering increasing the beam to 11'6" to have symmetrical spacing and enough clearance for the underdeck sluice box. Added benefit is stability. But at extra cost, materials. Tough decision. Will the extra 1.5 ft beam significantly affect handling? Do you think it is better to err on the extra beam side of the equation with regard to lateral stability. I am trying to keep it light, the heaviest components are gold pump engine (700 lbs) and compressor (250). hose 200, total load under 4500 not incl pontoons at 500 each.

 

Widen two pontoons to get the displacement.
The thing is going to be a pig anyway.
Eliminate the BS.
IMO.

 

What do you think is a better design option? Two 34" pontoons 11 ft beam or tritoons with similar beam (displacement about the same) ? Yea, I agree its a pig, but distance travel is small and I dont need a lot of speed. Mostly just a work platform. My main concern is limit rocking or huge movements, which can affect gold sluice performance. And need room for a diver rest area. the pontoon supplier only has 25 or 34" options,,, my only concern with 34 inch is that the deck is getting fairly high out of the water. Just worried about capsize. I need apprx 8 inches clearance above waterline for the underdeck sluice. So going with a 25 inch pontoon and raising the deck with I cross beams to get the necessary clearance seemed a better option than sitting too high in the water using a 34 inch pontoon, But I still need the extra buoyancy, hence the tri toon...
Ideas on this line of reasoning anyone?

 

You need to calculate the submerged volume, as a percentage of the diameter to get deck height. The difference is not that significant to create a lack of stability. Tritoons have stability issues, like trimarans, on cross seas. If you need more stability, increasing the beam is the simplest solution. Also, the deck can be lowered by attaching the beams so the top of them is flush with the top of the tube.

 

thank you Gonzo. I will use your suggestions.

 

Two bigger pontoons, along with Gonzo's suggestions.
Rectangular cross-sections would give more buoyancy for the width and would not have a big effect on speed.

If you need more stability against wave action, you could put out 2 or more flopper stoppers when dredging.

 

Another argument against a lopsided triton

Wake interference between the too close pontoons.

 

I'm assume a "tri-toon" is selected because it will operate with a single engine, unlike a catamaran ?

 

To operate a cat with a central engine all you need to do is have a rear cross beam with some local structure to mount an outboard.

 

It has its problems, trying to use a central mounted single engine on a cat, I seem to recall member Alik was not enamoured of it, and he would be well placed to judge. I have seen some smaller ones that seemed to get away with it, though.