True Grit Aluminum Build

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polynimbus
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Location: Idaho

True Grit Aluminum Build

Post by polynimbus »

Hello all, longtime listener, first time caller. I am on the road to building the True Grit in aluminum, and have learned a lot from everyone here (especially Kevin's detailed posts). I have been able to answer most of my questions with research here and from traditional sources, but am coming up with the first head scratchers that I think would be solved efficiently by people who have done this before. I'm not much for introductions, so I'll jump right into it (ask me if you need any info).

I am sure I will have many questions as this progresses, but the first major hurdle I am running into is the switch to inboard propulsion. I had leaned toward a stern drive initially, but after discussions with the engineers at Volvo Penta, I don't think we would be able to arrive at a prop size that would lend itself well to efficiencies at the lower speeds we will be operating in. The plans are geared toward inboard power anyway, so I think it will be a good change. The problem is I have no experience with the hardware/structural requirements for that type of prop/shaft and we are nearing the point at which I need to commit to cutting the hole for the shaft. We have a 3"x1/2" keel (as the plans call for), but cutting a hole to install a stern tube (or shaft log, not sure what the terminology difference is) will mean cutting out that section of keel entirely. This seems like a bad approach, even if you weld in a reasonably sized stern tube since there is no continuity along the keel, and it is using the stern tube to resolve bending stress. Is there a standard convention for retaining the strength in the keel when cutting out the shaft clearance? I am envisioning sistering supports to the keel through that area, but that will create stress risers which I am also not a fan of.

I am sure this is a pretty specific answer and a bit of a long shot, but I am trying to be as much of a sponge to new information as possible so any advise or sources will help. I attached a picture of the status as of a few weeks ago, we are now pretty much welded out and checking off the last of the issues before flipping.

-Joe
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kens
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Re: True Grit Aluminum Build

Post by kens »

Hi,
Nice smooth skin on that, nice job.
I have a small lathe and milling machine, I can assist some if you need a bit of machine work.
Since you have to cut the keel at any rate, my first thought is thick wall tubing as a structural member
Let that be the member to carry structural integrity, and that accepts packing box
If the shaft sits at say, 10degrees to keel, the cuts are on same 10degree angle giving you a long scarf weld.
If the keel is 1/2" thick, then the member is same 1/2" wall thick.

here with my morning coffee, I'm thinking like this
20200826_085517.jpg
stuffing box clamps onto tube like this example
Image


this style packing box clamps onto tube
https://www.deepblueyachtsupply.com/00p ... e-assembly

Don't forget that the engine stringers add integrity to that same area, we often trim the frames on wood builds to set the engine low in the bilge, the stringers add a lot of strength.
Oak is over rated, everything about it takes extra time; then it warps, splits or checks !!! :roll:

Kevin Morin
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Location: Kenai, Alaska

Re: True Grit Aluminum Build

Post by Kevin Morin »

polynimbus, Kens is exactly right in his inboard profile sketch. A tube should be cut into the VKB (vertical keel bar- not to be confused with an external keel structure) and to plan this there are several considerations I'll note.

First, what will be the overall (LOA) of the shaft? next what will be the diameter of the shaft? Taking those two dimensions into consideration; will there need to be an intermediate cutlass bearing to support the shaft at or about half its length?

Next, if there is no intermediate bearing then one single long tube will hold the stern cutlass bearing at the prop and the stuffing box/or drip-less shaft (face) seal at the engine end.

However, the design and building sequence is complicated by the intermediate bearing if one is required? The primary issue involved is line up of the three (or two) shaft supports. As Kens has mentioned; by using heavy wall pipe or better still, mechanical tube (6061-T6 Team Tube LLC, in Sea. WA), many problems are solved and the final product works very well.

In either case, whether you have a forward stern tube and an after one or the case where one long tube will hold both shaft supports- the easiest and fastest method is to have the ends trued and machined to hold the supports- regardless of which types of bearing or shaft seal bearing arrangement you use. By machining the mech. tube- the ends of the shaft's supports are already close as they can be to aligned. By exercising careful weld sequences and some judicious preheat the mechanical tube will make the 'best' fit shaft log/stern tube in a welded boat.

Getting a pair of tubes aligned for weldout is a bit more set up work but there are a couple methods to make that work fairly accurate and reasonably quick.

I have some external keel design drawing, and some illustrations of this method of design building (somewhere?) and could get them up if they'll be of any assistance? Been years since I've discussed a keel boat, and even longer since I built an inboard so I'm not sure where those illustrations are exactly? I may have posted some of them in another discussion here? but its been a while and I don't recall exactly which boat was being discussed? The Forum member's handle, here, is North? But not sure about where the thread is? probably somewhere in this area (Metal)? You might try to see if any of those illustrations are here? Maybe they'd help your exploration of ideas of this design area?

I've seen lots of 'plate keels' in the commercial fishing boats here in the Kenai River mouth on the Cook Inlet. That lead me to design another external keel structure that is much stronger, but mostly very much less drag than the plate style. Having done those boats in the 80's this keel design has had a chance to prove itself and show any flaws; thankfully, they've been successful.

The VKB doesn't really hold all that much stiffness for the hull overall. Once the bottom panels are tacked along the keel, you'd have to tear them apart to raise or lower that V shaped (body section view) seam in profile view. And as Kens has noted, the engine beds/stringers/longerons will probably be more structural contribution to this area amidships than the VKB so cutting it to accommodate a stern tube isn't going to create a weak spot in your hull.

Always good practice to plan a hull doubler inside and outside of about 3" to 4" wide. This will keep the stress of the VKB, tube/pipe, & hull panels from focusing at the tip of the weld where the hull cutout meets the tube. This would be set of four curved, extended C shapes that butted along the keel line, and covered or doubled the hull-to-tube weld areas and extended 3-4" fore and aft this oval shaped intersection. By thickening this area of the hull, any skin or surface stress is distributed away from the intersections by the width of the doublers.

Lines of this pretty hull show nicely in your tack-up photo, I'm looking forward to seeing more build pictures? Lots of details you could provide would be very welcome here in the lonely 'metal' category of the Glen-L Forum!!

Cheers,
Kevin Morin
Kenai, AK
Kevin Morin

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kens
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Re: True Grit Aluminum Build

Post by kens »

Hi, Kevin, long time no see your post.
When I did my inboard layout, the cutlass bearing rule was 40/20 bearing support spacing.
minimum bearing spacing 20x shaft diameter
maximum bearing spacing 40x shaft diameter
The engine output shaft flange does count as a bearing support

What engine/gear / expected speed are you looking at?
Oak is over rated, everything about it takes extra time; then it warps, splits or checks !!! :roll:

polynimbus
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Joined: Fri May 29, 2020 9:08 am
Location: Idaho

Re: True Grit Aluminum Build

Post by polynimbus »

Thanks for the replies, that already helps a bunch. You guys do a tremendous service to amateurs, and I wouldn't be this far without reading all your helpful posts.

I hadn't considered how much the motor stringers would help, the plans call for 1/4" plate with a full web to the frames, so that's almost another keel's worth of stiffness from the transom to the cabin front. I like the idea of a heavy wall stern tube, and just turning the shoulder on it so I can use a standard stuffing box.

I don't quite have enough info to figure the shaft length, but should have that soon (working with VP on prop sizing so we know the angle/position of the stern tube). The current engine plan is to use the Volvo Penta D4-300 with the HS-85A bob tail. It's a fair bit more HP than the plans call for, but the weight is less than the plans call for and the fuel consumption is very good at the lower RPM range (mostly due to the turbo) which to me is the largest design constraint. I plan to operate in the 15-20kt range, depending on where we find a good tradeoff in speed vs. fuel burn. I know there is another aluminum True Grit out there with a stern drive, which I assume based on the pictures is probably 250HP at least, and that seems to be a successful pairing. Definitely open to opinions however.

A bit more info, I extended the length to 27' o'all similar to Ray Macke's build, but will probably split the difference between cabin length and rear cockpit length. I hope to use it for extended trips on the Columbia primarily, with a few near-coast excursions (mostly just to say I did).

Another question, does anyone have experience adding bow thrusters to boats this size? I like the idea of maneuverability, since the change to inboard will limit that somewhat, so I am leaning toward installing a bow thruster (95lb thrust) while we still have it upside down. I am not sure if it is necessary or if it will even be a benefit on a boat this small, but if someone has any opinions on that I would love to hear it. I added a few more pictures of the initial hull steps in case anyone is curious.



Thanks again,

Joe
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Kevin Morin
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Re: True Grit Aluminum Build

Post by Kevin Morin »

Joe, there is a website named "Boat Diesel . com" that has rental calculators- that is there's a fee to join but you get the calculators use for the min. fee. Kind of a rental of the calculators?

Reason to mention this is that you can do your own shaft hp speed calcs and even wheel and gear box ratios. These online web features allow you to run scenarios, that is several combinations of different wt.s, speeds, gear ratios and predict prop size/pitch and shaft details. Worth looking into- relying on VVpenta for these calcs isn't wise- you'd be better served to have corroborating info!

The cutlass holder machined into one end of the shaft log/stern tube and the stuffing box actually machined into the upper end is a very good way to build. Or using a soft coupler for the upper seal - still works. However, the upper seal/stuffing box machined into the end of a mechanical tube.... sure does make the line up faster, easier and setting the engine's mount plates on the timbers is very accurate too.

I do have some sketches on rudder, pintle and shoe designs too, as well as those of streamlined boxed keels to offer for your consideration if they'd help your planning? If they're of any interest? just let me know so I can dig them up and post?

From my point of view you've chosen plenty of engine to move your True Grit along! I'd recommend the bow thruster. The cabin shown is a huge sail for that shallow hull, and with a keel of some kind or another- she'll track very well but will give up a bit of agility in a breezy harbor.... so, a thruster will help make her a more enjoyable boat. IMO.

Cheers,
Kevin Morin
Kenai, AK
Kevin Morin

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kens
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Re: True Grit Aluminum Build

Post by kens »

With the thickness of the engine stringer webs, and, if you put in the doublers as Kevin suggested, I no longer think you need the tube thickness as I mentioned in the above drawing. I think you could use a thick wall that still accept the spud style packing box, without having to machine a shoulder.
From my drawing of a 1/2" wall tube, I think you could go down to 1/4" wall, delete machine work, and be just fine.
Oak is over rated, everything about it takes extra time; then it warps, splits or checks !!! :roll:

Kevin Morin
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Re: True Grit Aluminum Build

Post by Kevin Morin »

Kens, Joe,
my advocation for the extreme (5/8" to 3/4") wall mechanical tube is because I've welded in a quite a few shaft logs/stern tubes and had to deal with with weld contraction/distortion/mis-alignment and the resulting re-work or corrections of bearing placement. Not to mention excessive engine and shaft vibrations when the two or three points weren't aligned well.

So, to get a reasonably fast building method (nothing is as fast as bolting an outboard engine to the stern!) with the increased accuracy required of an inboard installation: I advocate for the heavy wall mechanical tube so the tube's resistance to weld distortion can be coupled with the lathes' bore and single axis alignment and the result is a good clean installation with long life but reasonably low building effort?

I supported a cannery fleet of welded aluminum boats on and off for years. They'd installed thin wall tubes and SEPARATE!!! bearing holders and stuffing boxes.... those shafts were so beat up in 5-10 years they were generally ALL replaced! And the work of building those boats was overly complicated by having to "shop fit" the alignment of the various bearings and shaft seals.

So, I still think Kens' original idea is the best solution- take a very heavy wall tube/extrusion and have it machined to hold the cutlass bearing aft (unless there is an intermediate?) and also machine the entire stuffing box forward, and then just make sure the fits don't leave gaps so the tube is bent after machining?

I've repaired dozens of welded aluminum boats where the three bearing points were out of alignment and its no picnic!!!

Kens, I think your intuition was right on to begin with.

Cheers,
Kevin Morin
Kevin Morin

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curtgard
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Re: True Grit Aluminum Build

Post by curtgard »

In regard to your question on bow thruster. On my true grit boat, I did add a bow thruster and glad I did. Use it all the time. My cabin is longer than most on the true grit design. Do a search on my build, "Curt's Dream"

Curt

North
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Re: True Grit Aluminum Build

Post by North »

Beautiful work! I wish my topsides would have looked as nice as yours!

I have only built one aluminum boat, so take this with a grain of salt... and I concede to whatever the experienced and very helpful Kevin and Yofish may recommend, as they build for commercial use and abuse and i built for recreational use, hours ,etc.

On my Double Eagle the internal keel portion was only for the front half of the boat.. where the stern tube exits the hull i only had to cut through the hull and not any other members. There are stringers on either side within a few inches that add rigidity to that area, and i have not seen any signs of fatigue, cracking, whatever after 5 summers of recreational use.
my design was for an inboard and before cutting the hole for the sterntube, i drew out the keel and waterline profile of the boat on the shop floor, full size and then drew out where the my shaft would need to be in order to give me enough clearance for the prop, skeg, rudder, etc. I then measured the engine and gearbox to see what angle it would sit at and how high the shaft coupler would be off the hull,. etc so that i had a full drawing of it all - a lot less scary cutting the hole after seeing it all laid out.. even then, i only drilled a small hole and mocked the engine up on wood blocks and used a couple of pieces of tig welding filler rods to see how the alignment looked (and measure prop clearance) before enlarging the hole. NOT very professional I am sure, but it allowed me to build it.

I did not follow all advice given, but i definitely took Kevin's advice and did not go with the plans which had a single plate keel. After looking at Kevin's drawings and suggestions, about 5 years ago, I built a box keel which I have been very happy with. The welding in these pics is not pretty, but has held thus far..
Where the skeg passes through the hull it extends about 8-10" into the hull and has some heavy duty framing and gussets around it and tying it to a very strong box built to support the top rudder bearing and also the rudder piston.
Darrell
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North
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Re: True Grit Aluminum Build

Post by North »

more pics of alignment..
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polynimbus
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Re: True Grit Aluminum Build

Post by polynimbus »

All,

Thanks for the input, I'm glad I'm not the only one who has had these issues to solve!

I built a gantry to help rotate the hull, I think it will be easier if I can rotate it a few times. I have the outside seams welded and the battens stitched enough to flip it. I want to do the majority of the batten welding right side up so I'm not trying to control heat crouched down and overhead. Once it is flipped I can back-gouge the seams and weld the watertight seams inside. I am waiting to install the stern tube until I have the engine pinned down and get some exact dimensions.

In planning for the topsides/carlin, I have come across another dilemma. The Stephen Pollard book recommends not doing wood decks on aluminum boats, since it doesn't save anything and won't be watertight (and it is higher maintenance). The plans call for plywood soles, but I would like to switch to 1/8 aluminum and seal weld the sole to make a watertight cockpit (This is only an issue for the back deck, the cabin doesn't require this detail). I would rather not have a full length seal weld to the side skin however (mostly for stress concentration and weld show-through). Is there a common alternative way to seal weld the sole to the side? My current idea is to extend what would be the "carlin" down to the sole in order to make an easy fillet weld all the way along (leaving a gap between that and the side skin). I would add lockers in the side wall for storage, using the frames so as to not touch the side skins with any members or shelves. I attached a simple cross section of this concept, but would like to see if anyone has better ideas. The only real downside I see is the reduction in open space, but I don't see that being a very useful space anyway. I can hug tight to the frame for toe room, so you would only be losing the width of the frame (2").
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Kevin Morin
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Re: True Grit Aluminum Build

Post by Kevin Morin »

polyn'
I think you've run into the reason why most of my illustrations come with the parts and pieces in wildly different colors? Shades of gray are about as vague as can be visually found? If you'd add some annotation and lighten the extended deck to topsides intersection it would be much more realistic as communications?

In general- if you need a water tight deck (?) plywood is out, and there well be SOME deck sides continuous weld to seal to 'something'. Perhaps you could seal to an internally located carlin instead of the topsides being used as carlin by having a well-fit fillet joint- V'd along the top of the deck where the root face of that weld is 45 deg and weld face is flush to the deck?

This type of joint is very typical, provides the least print through and if fit well, tacked often and gouged MIG tacks prior to weld out- easily provides a good fit and finish.

Not sure I've addressed your question? but this is the method I use (and so does Yofish on his builds) the only exception in my case being those boats I build from the deck to keel first and then add the topsides after the entire deck is sealed and air tested. Think I have skiff build thread here showing that somewhere? But you're past that stage of planning now, so I was trying to address your question combined with your circumstance.

Would like to see your section illustration in color to confirm I understand what you're asking and proposing?

Cheers,
Kevin Morin
Kenai, AK
Kevin Morin

polynimbus
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Re: True Grit Aluminum Build

Post by polynimbus »

Kevin Morin wrote:
Tue Oct 13, 2020 3:07 pm

Would like to see your section illustration in color to confirm I understand what you're asking and proposing?
Sorry, I went back to a 2D sketch to hopefully make it more clear. The green area is what I am proposing. The plans call for just a 4" carlin strip and open below. Let me know if that helps.
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Kevin Morin
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Re: True Grit Aluminum Build

Post by Kevin Morin »

polynimbus,
Thanks for clarifying your question so well. Yes this would work fine to stiffen the deck edge, seal it from deck wash getting into the bilge and if you put some shelves inside and cutouts to access - make for a nice set of 'lockers' for stowage of miscellaneous items. Have to do some planning to stow rods - depending on length and access to any shelves or racks.

Depending on the deck slope, camber, angles ? draining the deck of rain water with these sealed carlins will require a bit more than merely cutting an opening (just) above the deck to topsides seams; where we'd normally see a drain in this size hull.

a short run of pipe or other hollow extrusion butted to the top sides and the inner surface of the full height carlin will serve to drain the deck or, depending on the transom's construction the deck could drain aft?

Cheers,
Kevin Morin
Kenai, AK
Kevin Morin

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