Anyone ever built a hull bottom to change the shape of an existing hull?

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Sobie2
Posts: 66
Joined: Tue Apr 15, 2008 12:27 pm
Location: Juneau, AK

Anyone ever built a hull bottom to change the shape of an existing hull?

#1

Post by Sobie2 »

So I have a 1998 22' Harbercraft landing craft. The hull has had a hard life here in Alaska, but it has served its purpose. It has new power (150 Mercury fourstroke with about 160 hours), and a custom Bill Munson boats 3 window cab console. The hull as designed (and marketed) was that it is a 12 degree hull for lakes and rivers use. Well that is not how it is has been used... ever. The hull has a good structure, but I am thinking about cutting and welding a new bottom on the boat and increasing the 12 degree hull to a 16-18 degree dead rise hull. The nature of the seas around where I boat are that we have short steep seas from wind and current. The boat is stern heavy and pounds. I would also put the outboard on a small commercial offset motor bracket to account for the new hull extending down another 2"-4"

Has anyone ever heard of doing such an extensive hull modification? My thoughts are this: a new landing craft hull, even a 22'er, will set me back some $80,000 or more.

Thanks for all the comments/thoughts in advance,
Sobie2
kmorin
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Re: Anyone ever built a hull bottom to change the shape of an existing hull?

#2

Post by kmorin »

Sobie, I've changed the deadrise of a 23' skiff (not landing craft) before and it was very successful.

One thing to note is there's no need to cut off or out the existing bottom, just add the new one and perhaps relocated the tanks? Move them forward by the sounds of her pitch by the bow?

As to the bracket, moving the engine farther aft will have the effect of increasing the bow up attitude since you'll be moving a significant mass farther from her center of buoyancy (COB) thus making that moment greater. I do understand that dropping the leg to get water flow to the wheel is important but.... cutting down the existing transom is a potential design to solve that?

Another thing to consider is to put the new bottom V inside the existing chines- that is; leave a chine flat of 6-10" along the sides and just add the deeper V in the center of the hull. Not a hard project if you have space to raise her up- if you'll be stripping the hull (?) then rolling her makes this work even easier. But it can be done by sitting on back rest type mechanic's creepers on a concrete floor- that's how I did it.

By adding a vertical keel bar (w lightening holes) of a few inches in depth to the keel and planking from that keel bar's lower edges to the line along the hull (inner chine = outer bottom plate edge) you can hold up some 1/8" sheet and tack around the edges.

Framing inside this 'false' bottom can largely be done before the sheet is put up under the hull. If you will be putting another hull longitudinal on the bottom 1/2 distance from keel to chine (?) then you can bend or weld a flange to the lower edge and use overhead keyhole, plug or 'pocket' welds to weld the new bottom sheet to the new long. Of course depending on the hull's warp- that lower flange as weld back-up will have to 'roll' at the angle changes of the bottom approaching the forefoot at the bow.

When done welding; air test to 3 PSI and TIG float the leaks and put an external drain plug in the lowest after most (new lower half of transom) panel consisting of a pipe coupler so you can plug it with a SS pipe plug for seasonal draining.

A deck vent inside the hull should be added so the temp related expansion and contraction of the volume of air inside the new bottom can adjust to changing temp without stress to the hull.

I changed a nearly flat bottom net skiff to a 27 deg deadrise sport skiff, new bottom skiff runs into a head sea very nicely and without pounding and even jumps with a livable re-entry!

I realize the change added wt to the skiff but... it still runs well with the original 90hp (lowered) engine.

Cheers,
Kevin Morin
Kenai, AK
kmorin
Sobie2
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Location: Juneau, AK

Re: Anyone ever built a hull bottom to change the shape of an existing hull?

#3

Post by Sobie2 »

All good stuff to hear, thanks Kevin! There are no tanks to move, the current fuel tanks are "portable" 24 and 12 gallon above the deck tanks. I wasn't thinking of full on Armstrong brackets either. They make 8" and 6" setback motor brackets. But cutting down the existing transom is a good idea too. I would be nice to move all of the weight forward as the motors and wheelhouse being so far back already makes the boat stern heavy unless hauling a load up front. I appreciate your middle paragraphs discussion of how to go about welding it up. The bottom width is 70" and there are 2" or 3" hull chines as well.

I suppose I will remove the engine to get this done, and the boat already has 4 massive lifting eyes on it so a shop crane and cables can easily hang the boat on it's side. The Munson cab is fully welded on to the hull so flipping it over is probably not an option.

Of course this is a slippery slope because once I do something like this then there are a whole bunch of other minor issues I'd like to address on the boat.
I have a local welder who can do this job I am taking the boat to him tomorrow for some other work and I'll ask him about the idea. Thank you! I will report back!

Sobie2
Sobie2
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Location: Juneau, AK

Re: Anyone ever built a hull bottom to change the shape of an existing hull?

#4

Post by Sobie2 »

[attachment=1]40497331-528A-41D1-A937-54EB683CD148.jpeg[/attachment][attachment=0]C2894862-4C56-430D-B828-3C67FF63BA94.jpeg[/attachment]

A few pictures of what I am talking about...

Sobie2
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kmorin
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Re: Anyone ever built a hull bottom to change the shape of an existing hull?

#5

Post by kmorin »

Sobie, in regards the fuel, you can find some tank tutorials I've done here and on Glen-L to build metal tanks to stand along the topsides above decks, under the gunwales and will move your CG (not CoB) a bit forward when full. A couple of 20 gal. tanks would be appropriate and trim down by the bow if located forward of the weather helm.

The new bottom would be best located 2-3" inside of the existing reverse chine flat. This keeps new welds out of old welds, and leaves the new bottom a 'lap' weld to the edge of the new bottom sheet and the surface (inboard a few inches) of the original bottom and would be the most serviceable solution to your need for more V (deadrise).

Good luck, with your hull modifications.
Cheers,
Kevin Morin
Kenai, AK
Last edited by kmorin on Tue Nov 24, 2020 2:43 am, edited 1 time in total.
Reason: typo's as usual!
kmorin
kmorin
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Re: Anyone ever built a hull bottom to change the shape of an existing hull?

#6

Post by kmorin »

Sobie,
one other factor to consider, hope this will make sense in text?

The forefoot, curved portion where the keel curves up to the bow stem, is the topic in this post. This curve is long, slowly swept up and therefore, in the current hull's lines creates a VERY MODEST V or small deadrise in the current bow.

In order to avoid this in the future bottom designs, and allow the hull to have a sharper entry (greater deadrise at the leading edge of the planing waterline (in Plan View) ; you'll need to move the curve farther forward, making the forefoot shorter and having more of the Profile View curvature closer to the bow stem tangent point.

This will result in more V forward, and that will mean a sharper angle of entry and a smoother ride in a chop. However this will also mean the V is carried farther forward and the hull will ground on a beach farther out.

Not sure of your use for the boat, but don't your beaches fall off pretty quickly? Aren't they steep like PWS or K-Bay? The reason for asking is; adding a 'deeper forefoot' will increase the hull depth so landing on along shallow beaches is less easily done - which is why the original lines were so shallow and the forefoot curve so long and drawn out??

Just a caution about the keel-to-bow Profile View (line) transition and where it needs to be in order to reduce the hull pounding into a head sea.

Cheers,
Kevin Morin
Kenai, AK
kmorin
Tfitz
Donator '16
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Re: Anyone ever built a hull bottom to change the shape of an existing hull?

#7

Post by Tfitz »

This idea is a new revelation for me. Never occurred to me to change my 12° hull rise from the outside. I already remodled the cabin and deck maybe its time to remodel the hull. Im very interested in this post please keep us in the loop!!!
Sobie2
Posts: 66
Joined: Tue Apr 15, 2008 12:27 pm
Location: Juneau, AK

Re: Anyone ever built a hull bottom to change the shape of an existing hull?

#8

Post by Sobie2 »

I took the boat in to the local shop. We talked over bunch of things. I hope to have it back in a week... say maybe Wednesday Dec 2nd. I brought up the idea of changing the hull shape to increase the dead rise and vee. He seemed to think it is totally doable, and he will think about price. Per Kevin's thoughts yes the boat would ground out earlier... not too much of a deal for me. As you can see in the first picture I already had to extend the bow from the factory bow which ended at the shear line, the rink dink ramp was practically worthless even on a steep beach. So all good things were mentioned here. I appreciate the insights. I'll give another update with pictures of the work done when I get the boat back. Even if he I get a good rate, it may be a few months before I can spring for it.

Happy Thanksgiving everyone,
Sobie2
Sobie2
Posts: 66
Joined: Tue Apr 15, 2008 12:27 pm
Location: Juneau, AK

Re: Anyone ever built a hull bottom to change the shape of an existing hull?

#9

Post by Sobie2 »

The boat is still in the shop getting repaired, I haven't talked to the welder about price to change the hull shape yet, but his price for rewelding the chines and armoring them, fixing a few lifting streaks that rocks have crushed, and strengthening the bow door and stuff is super reasonable. He lifted the boat completely off the trailer and rotated it on each of it's sides. So... still in progress. I am now thinking I will get it back Tuesday/Wednesday Dec 8 or 9.

Sobie2
kmorin
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Re: Anyone ever built a hull bottom to change the shape of an existing hull?

#10

Post by kmorin »

Tfitz,

let's just build one from clean plate and quit fiddling someone else's starting point?

I'm in, for a full design and build, you do the work -( I may have to tune up your welding?) I'm too old to be more than the guide, but don't torture your little skiff any more! Let her go to someone who'll use her as she is?

You need a 24'- 26' (maybe a 30'er?) and I have lots of designs you're welcome too, and I'll come along as guide, team leader and tutor- no cost to you, just your work, costs of wire, argon and metal and interest in a new boat.

But of course I'll support a new bottom on the old skiff if that's what you're game to build.

However! the offer does not include Fairbanks in the winter. Sorry, neither my dog nor I have the stones to take that.

Cheers,
Kevin Morin
Kenai
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Tfitz
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Re: Anyone ever built a hull bottom to change the shape of an existing hull?

#11

Post by Tfitz »

Haha no deal my project boat works too well to let go at this time. You are right though i do wish it was at least 26 footer but I'll make do. Im up to my eyebrows in another full restore track rig project at the moment. Welding steel is SOooo much easier than working with that miracle metal.....but not near as pretty finished. Maybe if i do a outboard conversion from my existing inboard, i would add a couple feet and change the dead rise. But for right now the motor is strong and everything on the boat is Beluga stout. THANK YOU for the offer though. I do kinda miss our back and forth correspondence you beating your boat building methods into me. My ears still ring a little from you all the time YELLING at me!. But....that was fun.
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gandrfab
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Re: Anyone ever built a hull bottom to change the shape of an existing hull?

#12

Post by gandrfab »

"Welding steel is SOooo much easier than working with that miracle metal..."
I'll fight you, but alas my muscles have been weakened from years of easy work with the lightweight aluminum.
It cuts fast with wood working tools, when the hot bee bee's hit flesh they cool far faster than the evil demon steel bee bee's do.
I avoid steel work when I can :shocked:
Tfitz
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Re: Anyone ever built a hull bottom to change the shape of an existing hull?

#13

Post by Tfitz »

All true statements. But.....working with old aluminum thats seen years salt water use is not easy and i stuggle to make "Kmorin" acceptable welds. Thats why my chosen welding style is mostly the smooth, rounded and ground off look😂
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