Transom Pods - Flotation and Extended Planing Surface

I'm looking forward to following your new project.

 

Thank you @ondarvr . I hope all is well with you. This should be a bit easier than my last project.

 

Quick Update . . .

The boat is out of the water for the season and I took some measurements of where the Pods would be. Here are a few images of what the pod would look like (port side shown).
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The pods will be mostly submerged (at rest) with about 4" exposed above the static waterline. I calculated approximately 2400 Cubic inches of submerged volume per pod (1.4 cu ft each) or 2.8 cu. ft total for both pods. The additional buoyancy from the pods would be about 170 lbs. total.

My next steps would be to model a pod out of some foam boards to see about fit and clearances.

EDIT: corrected my buoyancy figures.

 

The additional planing surface will be about 250 sq. in per pod, or 500 sq. in. total.

 

Does the boat have a static trim problem?

It doesn't appear you can get a significant volume under the waterline based on the transom and drive configuration. And it's not clear why that is desired in the first place.

Why not replace the trim tabs with the largest you can physically fit? I have extra large ones on my rig and the impact on trim is major. This will directly address your time-to-plane and min planing speed issues. And avoid drilling more holes in your transom.

Alternatively you could fit interceptors in place of the current tabs.

Regarding steps you can read the manuals for the tabs. They generally ask for just above flush mounting, on the order of 1/4-1/8".

 

Quick Update . . .

Pre-season boat preparation is approaching, as I usually do my maintenance and modification work during April - May.

I have come up with a modified design of the transom pods that will not require re-location of the under water lights nor the trim tabs. The pods will be a much more complex shape, but since they will be sculpted from urethane foam then glassed over, it should be do-able. Basically there will be cut-out areas to accommodate the transom fixtures mentioned and the trim tabs will be flush with the running surface of the pods when retracted. Kind of hard to illustrate, but I'll post updates as I proceed.

I've seen a few boats with a similar concept of recessed trim tabs over the years, but not part of a pod.

Installation will be much easier with this 'new' design (theoretically ) I was dreading the thought of relocating the transom fixtures.

 

Another update . . .

I have the boat in the water for the season. Before doing so, I took some more measurements of the transom area and have a few templates to work from in making a couple of pods. I'm thinking I'll make the 'core' of the pods out of foam and then do some test fitting when the boat is out of the water.

My quickest approach would be to use EPS foam boards, but in doing so, I'd need to use epoxy resin when glassing the pods. A bit more pricey for the resin, but probably a wash in terms of less costly foam (Styrene foam vs. Urethane foam).

In a few associated efforts, I have done a couple of things . . .

Trim Tabs: I have 18" x 12" trim tabs, but did not have position indicators. I've added position indicators so that I can give a more precise amount of trim tab to get some stern lift. It seems to help with time to plane without much decrease in cruising speed. I need a few more 'flat water' days to do some more optimization.

Stern Weight: There are 500 lbs of batteries in the boat (Seven (7) 31M-AGM batteries). I have worked up a design to replace 5 of the AGM's with 2 LiFePO batteries, leaving 2 smaller lead-acid batteries for the engines/alternators. Also, I'm going to put the LiFePO batteries more forward in the boat. This should reduce the weight in the aft portion of the boat by about 350 lbs.

Next steps are to fabricate the pod 'cores' out of foam. If I pull the boat mid-season, I can test fit them and make any adjustments before glassing the pods.

 

FWIW - here is a picture of the OEM trim tabs. . . 18" wide x 12" deep. One of the biggest 'stock' trim tabs offered by Bennett. I could not go any wider as you can see. I could go deeper (maybe 15"), but that would be custom and might need a dual piston setup.

Anyway, using a minor amount of trim tab has helped. The time-to-plane is about half of what it was (4-5 seconds vs 8-10 seconds) and mpg at cruising speed is up about 12% (1.35 mph vs. previously 1.2 mpg). So, I'm thinking/hoping that the pods and the weight reduction will do even more and leave the trim tabs more for port/starboard balancing rather than stern lift.

 

I would be curious as to how much the static water line changes when you reduce/relocate battery weight. I would think this would be a greater change then the change you could attribute to the pods your considering.

 

It would appear that you are chamfering the outside edges, ie at the chines inwards. This will reduce the buoyancy at rest but more importantly limit the maximum width, side to side, of
any trim tabs that you can install. These two changes will reduce your ability to trim the boat. I would keep the max width and use the widest trim tab that you can install.

I have included a graph from a paper Figure 7, that provides the optimum trim angle for a particular deadrise. (optimum defined as the minimum resistance for various deadrise hulls)

If you are trying to get an "almost level" boat attitude when planing, this may not be the best fuel efficient attitude, especially if you are engaging the trim tabs at steep angles to lift the stern and adding in quite a bit of tab drag. At around 22 degrees of deadrise, you should not be surprised if the trim angle is around 6 to 7 degrees.

 

Thanks, guys.

Interesting stuff @Barry . I figured that anything over 10˚ of trim tab deployment was going to be more drag than lift. Looks like the charts support that quesswork. My boat has a 22˚ V hull, so it looks like 7˚ of trim tab is my optimum.

I do need to shape the pods with a bit of chamfer. . . . on the outside to clear the swim ladders on either side of the boat, and on the inside for outdrive/propeller clearance. My 'modified' design that I am working on will use the 18" x 12" trim tabs as they exist. (Still working on some sketches)

 

The angle that the graph refers to is the boat attitude not the amount of trim tab deployment

In order for the trim tab to provide maximum lift/minimum drag is to keep the stagnation line (highest pressure line) as wide (long chine to chine) as possible.
Of course the other alternative is to provide some of the very long tabs which increase the length of the moment arm about the center of lift but they do stick out a very long way

 

got it . . .

I don't have an inclinometer on the boat, but, I'd say the boat cruises at a lower angle than the 6-7˚.

 

Have you calculated the change to the waterline aft at the existing transom?

Does the boat sit level now at rest?

Maybe I missed it, but is there an expected speed increase?

If you only add 170 pounds disp to each corner; doesn't one person negate it?

It seems like it is all very incremental for the effort. I think you ought to at least co sider what happens if you double the size as a thought experiment...whether the drives are affected, etc.

If you limit the pods due to aesthetics; that seems foolish..

Edit: just to add, with your talents, you could probably add a step/ladder onto the pod tops and make them look like they had another purpose..on the if they were bigger

 

@fallguy . . . that's a lot of questions. How are you doing?

No good way to calculate much, but the boat sits in the water OK . . .


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I thought about adding some steps to the rear facing side of the pods, but not sure how well that would work, given the boat lives in the water seasonally and will have anti-fouling paint around/below the water line. I will keep steps/ladder in mind as I work on the cores.

As mentioned, the primary goal of the pods will be to address the planing ability of the boat. The small amount of buoyancy is a minor benefit, particularly if I take some weight out of the engine bay via the battery upgrade.