Stossel Palm Beach Hull Riveted Rake Cap Replacement

[Slidin Gator]

We had to replace the rake caps on the Ghost after one season. Normally on a Stossel hull the rake caps are 0.06" stainless. The rake cap is a piece of flat formed into an angle shape, but at about 120 degrees vs. 90.

This picture shows 1/2 of the completed project.

This hull was built in 1986 and spent most of its life in salt water. The aircraft aluminum had corroded at the rake cap and needed replacement. The original repair involved patching in new corners with 1/8" riveted backing plates and then a new rake cap (missing in this picture).

The original plan was a salt water build, Steelflex 2000 only. To prevent corrosion, the hull was built with 0.100" Aluminum rake caps instead of SST. By the time I got the hull, plans had changed, so I knew the rest of this thread was coming eventually.

So we ran this hull hard last season. We found hands down SteelFlex2000 (SF2000) runs where Poly don't. It sucks in sand, poly is way better. Sand wears Steelflex fast. But afternoon sun, green sticky grass, SF2000 is worth 100 ft-lbs (50 Hp for the CM guys). :stirpot:


Sand is the nemesis (duh), the stuff is high maintenance. We did one patch job mid season to cover the bare spots.

The only way we were gonna keep paint on the rake caps was if we painted it on and let it cure during lunch. So one season in and we wore 0.100" aluminum down to 0.050" and the caps on both sides split.

Here is what the joint looks like with the rake caps removed.

More to come.

 

[lycomingkid]

I like seeing these kind of updates on these old hulls :thumbleft:

 

[Slidin Gator]

The new rake cap is 0.060" SST. Basically, it starts out as a 6 foot length of sheet sheared at 2" wide. On a new build it is easiest to pre drill the flat stainless on the drill press prior to forming. The hull aluminum is then match drilled. Unfortunately we are talking the opposite scenario here, we had to form and match drill the formed SST cap to the existing holes in the aluminum hull.

Here is the final goal for rake cap shape and drill prep prior to final riveting. It turns out there are a lot of steps and 3 days each between the last picture and this picture. By the Rick formula we are 36 beers in.

More to come.

 

[rick]

By the Rick formula we are 36 beers in.

That formula is +/-

Good thread as usual Slidin Gator.

Never had a Stossel, but have been around a few...they are top notch imo. Seen'em run the St. Johns switch grass better than most on our end of the river.

Rick

 

[Slidin Gator]

By the Rick formula we are 36 beers in.

That formula is +/-

Good thread as usual Slidin Gator.

Never had a Stossel, but have been around a few...they are top notch imo. Seen'em run the St. Johns switch grass better than most on our end of the river.

Rick

The 4 lbs per Hp rule is +/- too, but it is a lot easier to quote with an arbitrary but firm cut off! These boats do run the rough stuff well, even loaded beyond 8 lbs/hp. :stirpot:

A quick video showing basic shaping.

Here is the starting point. We want the side sheeting to sit outside of the hull sheeting so the sides are supported by both the Rake Cap and the rivets, otherwise the rivets will shear.

Start at the bottom transition where the angle portion needs to be flattened. After marking the initial holes and pulling it off for drilling, we used self tapping screws during forming.

Since the sheeting is only 0.100 thick, match punches won't work. So we modified a 3/16" drill bit on the bench grinder to make good marks.

Start with the hull portion as that involves essentially bending the flat section.

The side portion buckles out during forming. Ultimately forming involves cold working the metal to compress it and actually make it thicker in the bend area.

The bottom half complete after multiple on/off cycles.

Lot's of jokes about how the screws probably won't run well.

 

[lariat]

Great info! Thanks for sharing.
You beat me to the jokes about the screws not running well.

 

[Slidin Gator]

Great info! Thanks for sharing.
You beat me to the jokes about the screws not running well.

Thanks lariat, I can make a pun... I hope this helps folks understand the processes.


So forming involved some marking, some hammering and use of the rivet hammer and buck, on and off the boat. On a new build it is easiest to use an air hammer between the pre-drilled holes to form and fit, but it didn't work that way in reverse.

That's enough forming pics, you get the idea, match up a few using clamps etc., rough form, drill and add screws to pull tight. The length and bend of the cap is defined by the bottom curvature, the sides of the rake cap have to be formed in to match the bend or cut and welded. Welding tends to anneal SST, making it softer. Cold forming makes it stronger and harder, we did not weld anything here. Once all holes where drilled and we started riveting, we left the rivets loose and used the rivet gun, a hammer and back buck to continue forming. More on that in a bit.

Details on rivets and rivet tools next.

 

[Slidin Gator]

There are basically 2 different rivet styles, and several materials. Primary rivet styles are Flat Head (AN426) and Round or Button Head (AN470). This picture shows a 3/16" diameter Flat Head and a 5/32" Button Head.

The AN numbers are the most common, but you might also find them called out by Mill Spec number:

Flat Head = AN426 = MS20426
Button = AN 470 = MS20470

These numbers call out the configuration of the rivet.

There are multiple materials available for rivets, and they cover the range of useful materials, from soft to very very hard (Secs say vary vary, I guess for emphasis).

Stossel hulls normally use Medium Hard rivets specified as "AD" (AN426AD, AN470AD). "A" rivets are soft and "AD" are medium hard. "D" "DD" and "DDD" rivets are progressively harder and higher strength, but also more likely to fracture from impacts etc. So the 2117-T4 rivets are 38,000 PSI tensile strength vs. 72,000 PSI for the 7075-T6 hull material and 36,000 PSI for the stainless cap. That's correct, the hull material can handle 2X the load of the SST rake cap, but the cap is stiffer.

Note that they list shear strength values too. Shear strength is just what you think it is, the metals sliced in half because the mating parts moved. Shear strength is always less than tensile strength, but if a fastener fails in shear, the connection has already failed. The purpose of any fastener, rivet, bolt etc. is to compress two or more surfaces together. The compression results in friction between parts that resists movement, adhesives improve on this (see next post). If the mating parts move, the friction drops so all the sudden the fastener sees a shear load and will eventually fail.

The point is that the mating parts are not supposed to move, the connection tensile strength and proper tightening are the critical values. The rivets are supposed to pull two or three pieces of metal together nice and tight and keep it that way.

It is also worth noting that 7075 aluminum gets it's strength from a high copper content. Beyond making welding impossible, the copper makes this the most corrosive grade of aluminum and is the anode ("-"), which corrodes away just like the negative terminal on the start battery. The 304 SST Rake cap is the most noble ("+") material and is the cathode, which actually gains material in the corrosion loop. 2117 Aluminum rivets are a good bit less corrosive than the 7075 hull sheeting ( and a little better than 6061). Ideally the rivets would be the most noble, but we settle for less than the hull, which has way more area to corrode/consume over time vs. the small area of the rivets, which are in direct contact with the SST. If we went harder on the rivets they become more anodic ("-", corrosive) and can become the focal point of corrosion.

Rivets are a pretty basic Aviation commodity, they are not outrageously priced. Monroe in Melbourne is a local source, Aircraft Spruce is a national source.

https://monroeaerospace.com/products-rivets.php

https://www.aircraftspruce.com/menus/ha/rivets.html

The last part of the part numbering is the size. Rivets are specified in 32's of an inch on diameter and 16's of an inch on length. So these bags hold AN426AD (Flat Head, Medium Hard) rivets, 3/16" diameter (-6), 1/2" and 5/8" long (-8 & -10). Note that flat heads are measured from the top of the head as they are intended to be countersunk below the surface of the mating (SST) surface.

For the -8, 1/2" rivets, we are joining 0.100 thick hull sheet to 0.062" SST cap. So that is 0.16" grip length and 0.34" (~3/8") length to be compressed during riveting. For the -10, 5/8" rivets we have the added thickness of the 1/8" patch plates used in the last rebuild, keeping the exposed head length the same. General rule of thumb, thickness of the materials to join plus 2x rivet diameter, +/-

 

[Seven3]

Excellent posts. The topic of corrosion on these hulls is important, for the reasons you mentioned. Mainly the 7075 not being very corrosion resistant. My boat spends its nights inside a warehouse on the trailer, and I don’t own a camp, so I don’t have to worry about it sitting in water for long periods of time. Would it still be a good idea to use a sacrificial anode inside the hull? I forget what it’s called, but isn’t there one specific for aluminum boats in fresh water? Also, on my previous boats I used to completely disconnect the battery from by taking the terminals completely off when it was sitting inside the warehouse. My new boat has one of the red battery switches, so I’ve just been leaving it in the off position. Is that just as good as completely disconnecting the battery, or not?

 

[Gladesman06]

I always recommend putting anodes on airboats but especially if it is 7075. If the boat is only used in fresh water you would use a Magnesium anode. If you use the boat in fresh and salt (brackish) water then you use an Aluminum anode. If it is used in only Salt then you would use a Zinc anode. The best place to mount them is as low as you can get them on the transom. Make sure they are on the outside of the transom so they are in contact with the water most of the time.

 

[Seven3]

I always recommend putting anodes on airboats but especially if it is 7075. If the boat is only used in fresh water you would use a Magnesium anode. If you use the boat in fresh and salt (brackish) water then you use an Aluminum anode. If it is used in only Salt then you would use a Zinc anode. The best place to mount them is as low as you can get them on the transom. Make sure they are on the outside of the transom so they are in contact with the water most of the time.

This is probably a stupid question, but the magnesium anode has to be on the outside actually in the water? My boat is 30 years old and has zero corrosion, so I was more so interested just as a preventative just in case measure. But I don’t want to attach anything onto the outside.

 

[Slidin Gator]

Yes, the anode would need to be on the outside, in contact with water. DO NOT use magnesium anodes in salt water as it can make aluminum brittle due to excessive current. Zinc is correct for salt water and would be my go to for all waters for simplicity. I don't run any anodes, they would need to be all over the boat to give true protection.

This hull went 30 years extensively in salt water, that is why the corner rakes were replaced 3 years ago with new sheeting patched in.

Now the boat is stored on a trailer and I just use a shut off switch. The most likely source of stray current would be corrosion and salts built up directly on the battery. Just give it a wash down. I raise the tongue up high so the water runs out of the poly.

 

[Gladesman06]

I always recommend putting anodes on airboats but especially if it is 7075. If the boat is only used in fresh water you would use a Magnesium anode. If you use the boat in fresh and salt (brackish) water then you use an Aluminum anode. If it is used in only Salt then you would use a Zinc anode. The best place to mount them is as low as you can get them on the transom. Make sure they are on the outside of the transom so they are in contact with the water most of the time.

This is probably a stupid question, but the magnesium anode has to be on the outside actually in the water? My boat is 30 years old and has zero corrosion, so I was more so interested just as a preventative just in case measure. But I don’t want to attach anything onto the outside.

Yes it needs to be mounted outside making contact with the water

SlidingGator is 100% correct about DON'T EVER use magnesium in any kind of salt water. Putting a zinc one on doesn't really do good in fresh water so in my opinion would be mostly useless. I run double magnesium anodes on my boat but most would use Aluminum anodes. The aluminum anode would be a softer aluminum than the hull material so it would start eating at the anode first

 

[Slidin Gator]

On to the actual riveting.

Rivet hammer, 3X (4X would be better for this job, but 3X is more versatile). Round head attachments and buck bars.

Flat Head Attachment

As we went, we poured in the contact cement with the squeeze bottle.

 

[FISHSTICKER]

Damn looks like you had some 3rd graders working on your shit. Call me if you need some help.

 

[Slidin Gator]

Don't go bad mouthin my help, they work for juice boxes.

 

[lycomingkid]

It’s lookin good slidin