Mast Step Conversion
Tagged: mast step
29/01/2017 at 2:28 pm #10508
I’m just preparing the boat for the mast step conversion. I have a couple of questions:
The height of frame 1 above the hog is 83.5mm not 90mm as indicated in the conversion plan. Any suggestions what to do about that? Should I “build up” up to 90mm?
I understand that there is quite a bit of flexibility within the plan of the conversion but that I must preserve measurement 5.5. Anything else I should keep at the forefront of my mind?
Thanks again.29/01/2017 at 6:51 pm #10520
I hope someone who knows this conversion intimately will pick this up; I know it only from a theoretical viewpoint, and I don’t have the plans available. However a few overriding principles may be helpful.
1. Preserve the overall height of the plinth which you are building, on which the mast track rests; and of course preserve the horizontal location of the mast track, and shorten the mast by exactly the specified amount.
2. I would build up the frame to the required height, but I would also take steps to ensure that your additional filler piece is strongly bonded to either the frame below or to the plinth above (but not to both, for the reason in point 4) and is also supported against “capsizing” in the fore and aft plane. Two alternative ways to achieve the latter support are (a) fit a backing piece to the frame and filler piece, on their forward faces, bridging the joint, or alternatively (b) use a filler piece at least twice the thickness of the frame (in the fore and aft direction), bond this to the plinth above, and cut a recess out of the spline below to accommodate the (wide) filler.
3. The conversion is designed to spread the load so far as possible, so preserve this. Ensure that the plinth (the top part of the job) <span style=”text-decoration: underline;”>bears throughout its length</span> onto the parts beneath; in order, from forwards, frame 1 (if it goes that far – I am not sure on this detail), the spline, frame 2 (via the filler piece), and the recess in the centreboard case. No horizontal gaps left anywhere.
4. For an older boat which is known to have some problems, allow for the possibility that at some future date the centreboard case may have to come out for repair or re-seating. With that in mind, unless you know beyond doubt that the case is properly bonded to the hog with epoxy (i.e. modern practice, as opposed to the earlier practice of bedding it onto mastic), I would build up (and bond) <span style=”text-decoration: underline;”>everything except</span> the plinth, ensuring that you create a smooth level surface, and keep the plinth entirely separate at this stage. Apply your chosen finish, varnish or whatever, to <span style=”text-decoration: underline;”>all</span> surfaces of both parts, including what are going to become the mating surfaces. Then <span style=”text-decoration: underline;”>screw</span> the plinth into place, bedding it down onto a very thin film of sealant, but do not bond it in place. Doing it this way, if you later need to remove the centreboard case the plinth can easily be removed first, without damaging it, and it will not make it more difficult to take out the centreboard case.
Oliver29/01/2017 at 6:54 pm #10521
Sorry about the formatting gibberish. What was intended to happen was that the words between the pairs of formatting commends are underlined, for emphasis.
I hope this will enable you to make sense of it!
Oliver29/01/2017 at 7:05 pm #10522
Grovelling apologies: I have just re-read your post, and now realise that you referred to the height of frame 1, not to frame 2 …
Before operating keyboard, ensure brain is in gear … … !!
That answers the question as to whether the conversion goes that far forward; it would appear from your query that it does.
Frame 1 is so far forward that the actual load transmitted from the mast to frame 1 will be one of the smaller loads in the distributed load system (and it is of course zero prior to doing the conversion), so you will not need to worry unduly about structural strength at that point, but you will need to ensure that your plinth is reasonably level in order to get the mast track as level as is reasonably possible.
Oliver29/01/2017 at 7:12 pm #10523
Thanks Oliver, I appreciate your advice.
So far, I have the old mast step out, the bulkhead cut open and I’m making good some damage from the previous owners botched repair (fibreglass over damp wood does indeed trap the moisture in, allowing the wood to rot) to allow me to get to a starting point. I’ll post photos shortly.29/01/2017 at 8:03 pm #10524
Sorry Oliver, my mistake. I meant frame 2, immediately forward of the centreboard case- 83.5mm above the hog.
The case is actually lower than the frame so as the plans suggest, I’ll need to fill in. I think cutting the case and filling space with the plinth is a good idea.
The whole thing will be dry fitted and measured before the epoxy goes near anything.07/02/2017 at 9:49 pm #10614
Hi, I have completed the mast step conversion on three boats to date so hope I can offer some useful advice.
The important things to note are the final measurements you have to achieve to satisfy the measurement form. Setting the height of the mast step plinth is governed by the measurement from the shear lines, this is quite difficult to set up but essential if your work is to satisfy a measurer. The plinth is notched into the top of frame 2 and the front of the centreboard case, S o although Oliver says make it so the centre board case can be later removed, fitting the new mast step correctly will make a centreboard case removal very difficult or impossible. Make sure the spine fits as closely as possible to the hog so that the load from the mast is spread evenly. When finaly bonding everything in place make sure you use plenty of well filled epoxy, I use a mixture of white fibres and brown colloidal silica fillers.
I don’t know where you sourced the wood, one good source is if at the same time as fitting the mast step you take the option to remove one of the side benchs on each side. You can cut the plinth and the spine from the benches the spine is two pieces laminated to the required thickness.
hope all goes well Steve Corbet07/02/2017 at 10:11 pm #10615
Thanks for the reply Steve.
I’I just at the point where I have cut the shelf into the case/ filled to the level of frame 2 and I’m now making templates and dry fitting the components. My “measurement 5.5” is smack in the middle of the quoted tolerances.
I followed the conversion sheet as closely as I could. One thing concerns me. I cut the shelf into the case at 90 degrees to the forward face of frame 2. This means the upper face of the spine will also run at 90 degrees to the forward face of frame 2. The lower face is profiled to the curve of the hog.
This seemed like a correct interpretation to the plan for the conversion, but I’m hesitant. It seems to produce the correct measurement but, shouldn’t the mast pad be flat relative to the baseline/ waterline?07/02/2017 at 10:19 pm #10616
The wood for the spine is some 45mm thick sapelle. Once it’s epoxied, I expect it’ll far outlast the rest of the boat.
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