Filling in a prop pocket / tunnel?

[Prototype]

Maine winter has arrived and I have dove into my long awaited boat project. I am converting an inboard shallow draft fishing boat into an outboard Striper Seeker. It is a unique boat, custom made in Nantucket called a Grey Lady 25. Anyway, the transom seems to be plenty adequate for the conversion, however, there is a prop pocket / tunnel left from the inboard to contend with. I thought about leaving the tunnel and mounting the new 250 outboard on a Porta hydraulic bracket with a 27" or 30" setback. I figure I'd try it out and see if it created any downside like cavitation, etc. Best case it may provide a little lift in the rear.

I am re-thinking and opting for an Armstrong bracket for the floatation and swim platform aspect. Problem is, I have to attack the bottom as the Armstrong bracket follows the contour of the hull and sits only a couple inches away from the bottom at centerline.

The pocket is about 24" wide, 48" long on a tapper and 12" deep at the transom. Obviously, simply filling the pocket would be cosmetic as the structural element is already there. A friend in the business suggested filling the pocket with foam (I forget the name of the stuff), laying up a few layers of glass, and fairing. I am sure it would work, however, I hate doing things half ***. What about cutting the pocket out and re-doing properly? I would then have great useable space under the deck where the top on the pocket resides and if I ever sell the boat ("a boater's best day") there wouldn't be any question about the filled in pocket void.

It just so happens that I have access to the original mold and could have the guys lay me up a section of the bottom and transom. The problem is...then what? I don't want to get over my head here. Plus, I am on a limited budget; thus I am seeking help on the forum. Is this a viable option? or should I fix it the old fashion way (what ever that is?). How do I support it? Now it becomes a structural issue right? (by the way, the hull is solid glass with no core) Am I overthinking this or is it pretty straight forward? I could use some suggestions (or a step by step if so inclined).

Thanks in advance

 

[erikgreen]

Re: Filling in a prop pocket / tunnel?

If it was me, I'd see if the boat runs well with the pocket in place. If you think about it, that pocket was meant to guide water without turbulence past the prop. So I'd think it'd work pretty well with an outboard. You could even mount the outboard higher than you otherwise would, in line with the pocket, since it'll let water get to the prop when a flat hull wouldn't. If you're going to block the hole with your bracket, however, that won't work. Can you mount it higher?

If you really want to fill it in, I'd recommend grinding the gel off in an area big enough to glass to, then fill the pocket with pourable polyurethane foam, then cover with several layers of glass/poly/mat or epoxy/woven. Upside of filling the hole is a bit more floatation in the aft section, which you might need with that outboard.

You could also have a custom section made using the original mold, which would be gelcoated, then you could glass it into place after cutting out the original pocket. You'd want to use epoxy, and use overlapping tabs from the inside and outside of the hull. That would probably be the neatest look, but would be weaker structurally than just filling the pocket in, because you're holing your hull. Lotsa work to open up the inside of the hull to get to the pocket too, but you'd do that anyway to take advantage of the space, right?

If you think about it, you're worried about cosmetics on a part of the boat that you'd have to be underwater to see, right?

Erik

PS: I didn't see the part about being on a budget. If that's the case, I'd forget the custom molding. See if you can run the boat as-is and how well the pocket works. If you absolutely can't put up with the pocket, then grind/foam/glass it. Be prepared for the color of the new gelcoat to not match very well, plus you'll have to sand it fair with the rest of the hull, which will take a long time. Or just paint the whole hull, but that's another can of worms.

 

[Prototype]

Re: Filling in a prop pocket / tunnel?

Good points! I would rather just leave the pocket alone (aside from filling the shaft hole and rudder mount) and try it. Especially if I could justify a real advantage to having it over filling it. I agree that it may not do any harm. I would definately have to forego the Armstrong bracket and install the non floatational but very cool Porta Bracket. Knowing that all but one or two of these boats were made with outboards, I doubt that the outboard hanging 30" off the transom will upset the ride. Some had brackets and some were welled.

Here is another issue; I plan to leave the boat in the water all season. If I go with the Porta Bracket, almost 1/2 of the bracket and pump system will be sitting below the waterline at rest (according to Porta's instalation instructions). Not a huge deal; anti-foul paint?, zinks, proper grounding (of which I will need expertise).

I wish I knew for sure if there were any real concerns with keeping the pocket in place. If I go with the Porta Bracket or another type of bracket that I can mount above the pocket (open to references here) the worst case is that I have to fill it in anyway.

Any idea how much floatation a 30" wide Armstrong bracket really provides? It may prove insignificant?

Thanks for the help!!

 

[jonesg]

Re: Filling in a prop pocket / tunnel?

JUst some thought on what I would consider doing.

Leave the hull alone.

Mold up a glass and ply bracket following the identical shape of the keel, mold the same tunnel shape, this way there is no induced drag.
A stainless bracket has little floatation.
Scroll down to the pics, this guy did a good one, they cost $2-3K to buy but can be made for $300, 2 inch ply where it bolts to the transom and same for the motor mount. The mold was made up from mdf sheet, no need to make a plug.
http://www.classicseacraft.com/foru...7&page=0&view=collapsed&sb=5&o=&fpart=13&vc=1

He basically copies this design, a simple solution.
http://www.hermco.net/

 

[erikgreen]

Re: Filling in a prop pocket / tunnel?

Any idea how much floatation a 30" wide Armstrong bracket really provides? It may prove insignificant?

Thanks for the help!!

Well, if it's the sealed box type bracket, you can figure out the air volume and use that for an estimate.

If we figure 30 wide by 20 deep by 14 high, for example, that's 8400 cubic inches, or 4.87 cubic feet (8400 *0.000578703704 = 4.87). Each cubic foot of air displaces a cubic foot of water, so you get that much floatation out of it. One cubic foot of water weighs about 62 pounds, so that's 4.87 * 62, or about 300 or so pounds of floatation if it's fully submerged... which it won't be.

To figure out what it actually contributes, you have to figure out how much of the bracket is below the waterline. For example half of the above bracket submerged is about 150 pounds of floatation.

Whether it's worth it is up to you. Are you going to use a really heavy motor, or is there some other reason you want extra floatation?

Erik

PS: By the way, the floatation calculations above assume it's an empty box, which it won't be... a pump, reservoir, etc will contribute weight and take up volume. Don't be surprised if the "added" floatation of the bracket is a wash for the weight of the bracket and the stuff inside.

 

[Prototype]

Re: Filling in a prop pocket / tunnel?

So here is the latest from a couple sources:

Porta bracket says the bracket should be raised above the recommended standard of mounting the outboard 30" above the bottom of the boat. They say that the actual "bottom of boat" is now a variable because of the prop pocket. The concerns are too much drag running above the cavitation plate, not enough water or cavitation (especialy when turning), water not flowing properly through pocket ("holding water") and causing drag thus effecting performance, or porpoising.

Jeez, is this getting complicated, or is it me?

Not trained in hydrodynamics (if that is the right term), how does the water flow through the pocket? is is airated or turbulent? does it "hold"? does it flow up into the pocket and bounce back down? does it raise the "boat bottom" for a single outboard application? will it effect bow lift?

There must be some sort of science or formula to figure out if there will be true effects of leaving the pocket.

The reason I like the Porta bracket so much is that it has the lift / adjustability factor in raising or lowering the motor like a jack plate while setting the motor back 21" for tilt clearance. I do however need to get a close (within an inch or two) set of x dimensions to mount the bracket.

Apparently, industry standard is mounting the outboard 30" above boat bottom. Also for every 12" setback, raise the engine 1".

the boat is a shallow V (only 8 degree deadrise). again the pocket is a tappered 48" long 2' wide at transom, 6" deep at transom, and roughly 9" above true boat bottom at transom (following plane of bottom to centerline). What other computations can I use to see if the pocket will adversely effect performance and what height I should set the motor?