Thread: Running strake height.
26th Mar 2018, 11:51 AM #1
Running strake height.
I need to fit two running strakes on the bottom of a Senecadory I am building. They will be for protecting the flat bottom and also provide directional stability in turns. Thefalse keel is 90 x 19 Mt. Ash. I will use the same for the strakes but about 4mm wide. Should theybe the same height (19mm) as the keel or a bit higher?Just Do It !
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26th Mar 2018, 03:25 PM #2
Rub strakes are a good idea and they are typically about 20x40 on the flat. Round over the outboard edge, but leave the inboard edge as crisp as practical, just enough so paint will stick. The crisp inboard edge will help tracking and turning responsiveness. Additionally, since they are sacrificial in nature don't glue them, just screw them over bedding (caulk) and use small, light screws too. The small screws will permit the rub rail to take some damage, but if really bashed hard, the small screws will rip out, with the rub, instead of pulling big hunks of bottom panels with them.
26th Mar 2018, 07:04 PM #3
Thanks again for your help. 40 x 20 was my original thoughts on it. How about this - a reinforcing stringer on the inside of the hull , say about 40 x 10 and screw the strakes on from the inside ?
Alan.Just Do It !
27th Mar 2018, 02:33 AM #4
I prefer to attach strakes from the outside, mostly because the fastener penetration doesn't go all the way through the planking, which has obvious leak potential benefits.
I have a 15' power boat I just replaced strakes on. I built in in 1988 and this is her second set of strakes. She doesn't see much service any more, probably because I have way too many other boats to contend with. The hull is two layers of 1/8" (3mm) plywood diagonally molded together, making a 1/4" (6mm) thick hull. The keel is a 2x4", (40x90) set on edge and through bolted to the hull. I use two rub rails externally and these are 1x2" (20x40) and these are also through bolted, but with much smaller fasteners (#8's). Even though I've tried to keep them bedded, I've always regretted not just setting them in bedding and screwing from below, as some leaks have occurred. I've found they just aren't hefty enough to hold the fasteners well enough, to prevent movement, which lets in water.
If I had to do it again, I'd use #6x3/4" (#6x20) fasteners into the rubs, with just enough screw biting into the hull to keep them in place, but not so much to "poke through" on the inside. I'd use a tough (polyurethane) bedding, so I'd get a little extra "stickum" and would likely have less leaks.
A reinforcing stringer on the inside would help my situation and possibly yours too (my hull is pretty flexible), but I was concerned about weight, so I didn't use the extra stringers to back up the rub strips. If you do go this route, you don't need as much of a stringer as just a "backer", so maybe a 20x40 would be all I'd use, possibly less (20x30?). It's just a backer to stiffen the local areas around the rub strip contact patch. I'd still screw on the rub strip from the outside and glue the reinforcement on the inside, again to prevent leaks.
27th Mar 2018, 01:30 PM #5
The backing timber that I would use inside the hull below the strakes would not be full length as they would be interrupted by the frames (18” spacing) but I could just epoxy them in place (10mm ply strips) If the strakes are screwed on from the inside and one happened to be torn off it would probably leave the bottom panel undamaged. BUT, if I did it this way what would I use as a sealant/adhesive between the strakes and the hull? If they were epoxied on it would more than likely tear a hole in the hull ply as it said goodbye.
I have another bit of a dilemma. I have the keel pulled down and screw holes drilled ready to fasten it to the hull as you suggested, I will leave the screws in. I was going to epoxy it to the hull also but that would create a headache if it ever had to be replaced. Is there another adhesive/ bedding compound I could use in lieu of epoxy? I was going to finish it off by covering it with some 8” or 10” wide glass tape over the top.
I already have the hull sheathed with two layers of 6 oz. cloth.
Really appreciate your help. The building manual for the boat has no detail whatsoever so it is this forum and boat books or nothing.Just Do It !
28th Mar 2018, 01:48 PM #6
Covering these types of things with fabric is generally frowned on, as now you've married yourself to it being there with little hope of removing it cleanly for a repair at some later point. Rub rails are "sacrificial" in nature, meaning they're designed to be removed, so don't 'glass these over. The keel may be 'glassed over, but unless you can justify this need or the build type warrant this approuch, it's not a good idea here either.
Screw holes through the hull shell and elements attached to the hull shell should be "bonded" in an epoxy hole, to prevent moisture from getting at the wood. It's simply an over size hole, filled with epoxy, which is then drilled for the proper size shank, the fastener has. This makes a coating around the inside o the hole, preventing moisture from getting past it. The screw can be also epoxied as it's being inserted into the hole if desired too.
Sealants come in several flavors, though polyurethanes and polysulfides are the usual choices. I use polysulfides on all under water applications, but polyurethane can work too. Some have much more or less adhesive about them, which can be good helping hold things, but also can be bad making removal near impossible without more damage. 3M5200 is one of these having much more adhesive qualities about it than most others, so look around. BoatLife Caulk works pretty good, but it depends what you want.
28th Mar 2018, 07:00 PM #7
Well I have the false keel pre drilled in place and ready to fasten down. I will take your advice and not cover it or the running strakes with glass cloth. All I have to do now is find a sealant. Polysulfides seem to be hard to come by here. Sika have some good polyurethanes available but they can be a real pain to remove – almost as difficult as epoxy if it ever became necessary. I will call Sika tech support in the morning.
I used a Sikaflex 221 polyurethane last week to fit a new hatch in a mate’s caravan (trailer) roof and it went very well but will check its suitability for this application with Sika tech support. I think their Sikaflex 291 would be the correct product for this application but removing the stuff later would be a major headache
Any way I am ready to go so will see what Sika have to say.
Thanks again for your help.Just Do It !
28th Mar 2018, 07:45 PM #8
Have a look at Fixtech Fix 15. I haven't used it but as it is a MS Polymer vs Sikaflex polyurethane, it should be longer lasting and more elastic. Not sure on price.
28th Mar 2018, 09:15 PM #9
Thanks for that. I now remember seeing the product in the Whitworths Marine catalogue. I'll give them a call. Thanks again.Just Do It !
29th Mar 2018, 12:25 PM #10
I have purchased some Selleys Armour Flex to do the job. I Need something over the Easter weekend as we go north for the winter on 1st May and I need to make some progress before then. I called Selleys and the tech. rep. had no hesitation in suggesting Armourflex. It is also an SMP sealant/adhesive same as Fix 15.
Also called Fixtech but they dont have a supplier in Wagga.
Thanks for your help.Just Do It !
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