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Old 03-18-2013, 10:57 AM
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Default Floatation Pods...help 19 foot boat draft less??

I have spoken to 2 different custom aluminum boat builders and was told two totally different things regarding floatation pods....

I am considering having a center console, semi V, probably 19ft. at least 70" wide, aluminum boat built for marsh fishing. I want to hang the biggest outboard motor (probably 150hp) I can off the back and possibly fantail as well. Because the floating/trolling draft if very important to me, the first builder highly recommended adding floatation pods to the back. The thought being the extra buoyancy and surface area will keep my stern floating higher, as well as plane quicker/easier.
The next builder scoffed at the notion of the pods helping at all with the draft. He said they would only help with planing faster. If that is the case I would rather just put adjustable trim tabs on instead of "fixed" position pods.
I have also read that they may slow the boat down, neither builder mentioned that.
Any input???
thanks
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Old 03-18-2013, 11:49 AM
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Define "flotation pods"
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Old 03-18-2013, 12:07 PM
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Quote:
Originally Posted by Fiberglass1 Inc View Post
Define "flotation pods"
On each side of the transom, where trim tabs would typically go, would be 2 "roughly" squarish in shape hollow sealed "pods" welded on.
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Old 03-18-2013, 12:23 PM
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Potentially both mechanics are correct in that the description is too vague. In order for these to have a positive affect on the trim of the boat they have to provide more buoyancy ( submerged volume ) then they add in weight / displacement and their benefit will be in direct proportion to that relationship. They can only affect acceleration if they conform to the bottom of the boat, extend the planing surface.
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Old 03-18-2013, 01:16 PM
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To figure out what size pods you would need, first figure out the amount of added weight the upgraded outboard would add to the boat. This it the amount of buoyant force needed to compensate for the larger outboard. Buoyant force of each pod is the mass of water diplaced by the pod minus the weight of the pod in air. Therefore, divide the added weight of the outboard by two, and create a pod that provides that much buoyancy. This is assuming the center of buoyancy of each pod is fairly close to the center of mass of the outboard.

For example, you put an outboard that is 150lbs heavier than the boat is designed for. Each pod must provide 75lbs net buoyant force. Therefore, each pod would need to be appoximately 2200 cu-inches, or 10 gallons (assuming 5-lbs aluminum to make each pod).

This would only correct your static pitch, not dynamic lift. Therefore, trolling and sitting still your boat would be ok, but it would not help with getting on plane unless they extend the planing surface, like a trim tab would. They will only slow the boat down if they are an extension of the planing surface.
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Old 03-18-2013, 01:19 PM
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I guess they would adjust the at rest angle of the boat making it easier to get on plane by presumably countering the bow rise when accelarating. But I am no engineer and not sure of all the science involved. I guess its affect woudl be similar to a transom bracket with flotation.
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Old 03-18-2013, 01:50 PM
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Hope this helps. I have an 1874 dura craft tunnel. No pods with a 115 two stroke jet drive. My best freind has the same exact hull 1874 dura craft tunnel that has the pods on back. His boat has the same draft as mine but with a heavyer 115 four stroke jet drive. So it does make a differance. As far as planing, it does help but once on plane they are out of the water and make no differance. Only negative I see is when it gets rough he has a harder time getting his bow up. Makes his a little wetter. I would get them if I had the choice.
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Old 03-18-2013, 02:04 PM
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Commuter and Lancematic are headed in the right direction. As a point of clarification, the volume of the submerged section of the pod is the only thing that will count for buoyancy in Lancematics example. So that 10 gallons of volume would have to be below the waterline. Considering your specs (70" beam, mod V), each pod would need to be somewhere in the 18-22" long range to provide the needed volume. That is only a guess as there are several variables that aren't defined (waterline beam at transom, deadrise angle, engine clearance, etc).

Even after all that, these are just calcs to offset a given weight. To understand how the added volume would affect the overall draft, you'd have to do a bigger study. In the end, I doubt you'd get more than an inch or so of draft reduction.

As for the effect on the performance of the boat, as has been stated, if it extends the running surface, you might be able to pop up a little quicker, i.e., you'll get on plane in shallower water. But again, it's probably only fractions of an inch. Not much to crow about.

It could also be detrimental to cruise efficiency and top end performance. That extended running surface will keep the bow down and reduce trim response. As a compromise, you could cut away from the running surface. This could allow some decent trim and still give you some flotation.

It's all a compromise.
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Old 03-18-2013, 06:05 PM
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Default Pods

I built as set of pods for my 20 foot aluminum boat several years ago, they have been the single best motification I have made. (for my boat) With that being said there are some particulars and considerations.

You didnt say what the deadrise is at the transom...? mine is very flat at 6 degrees, also my hull is narrow, chine to chine it is 60".

I am running a 140 horse motor and switch back and forth from a jet to a prop, with the motor mounted directly to the transom the boat preformed well. When I decided to make the swap to a prop I decided to add a lift to make the neccescary elevation changes easier. I used a power trans lift with 7" of travel.

This added setback (~5") made a huge diffrence in how the boat sat when drifting, it sat 3-4 inches lower and went from 3 second on step to 9 (with a jet).

The pods have helped raise it back up when drifting (3-4 inches) and I get on step as quick as before, the boat will carry more weight, they are handy to stand on to clear the jet, change a prop, take a leak.
Also serve as decent beer coolers in the frigid waters up here.

If you are mounting a 150 HP motor directly to the transom on a boat w/ a 70" bottom the benefit would be minimal...IMO.........

If you decide to have some made the most important thing is to make sure they do not follow the hull, they bottom of the pod must rise as leaves the transom, if they match the bottom it could creat some bow steering by not allowing the bow to raise up.
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Old 03-21-2013, 10:36 AM
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"I used a power trans lift with 7" of travel.

This added setback (~5") made a huge diffrence in how the boat sat when drifting, it sat 3-4 inches lower and went from 3 second on step to 9 (with a jet)."

Hmmm, that has just raised a question...are you saying that by setting your motor back on a jack plate you will draft more than without a jackplate with setback?
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Old 03-21-2013, 10:48 AM
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Quote:
Originally Posted by Hooked1 View Post
"I used a power trans lift with 7" of travel.

This added setback (~5") made a huge diffrence in how the boat sat when drifting, it sat 3-4 inches lower and went from 3 second on step to 9 (with a jet)."

Hmmm, that has just raised a question...are you saying that by setting your motor back on a jack plate you will draft more than without a jackplate with setback?

Absolutely.....if I understand your question correctly, any steback will change weight and balance...that motor is a big lever hanging off the transom, my boat set 3-4" lower w/ a 5" set back, also the lift weighs some 50 pounds.

A wider bottom boat would be less senitive to this......
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Old 03-23-2014, 08:59 AM
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I built pods for my tracker 1860 that I run with an etec jet. They change the attitude of the boat, no decrease in speed but it comes on plane much quicker.

I designed them even with the bottom of the boat at the transom but they rise up 3/4 of an inch at the back. I was advised to fill my boat and gas it up and build them 1" above the water line. The outside edge follows the line of the boat inside is plumb. I put drain plugs in them and an inspection port. Mounted with 8 1/2 bolts and a backing plate with 4200 sealant.

Another big safety advantage is it is much easier to get back into the boat when you are in deep or shallow water.



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Last edited by dacheedah; 03-23-2014 at 09:38 AM.
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