Minimum stem thickness

[gmeadows]

Hi all,

My offsets call for a half-breadth of 7/8" to my rabbet line at my first 3 stations (in the stem area). Given the profile lines of these stations and especially at the first station, I will only be left with a half-breadth to my middle line of 5/16", leaving me with a stem width of only 5/8".

The plans are from WB for a Dark Harbor 12 1/2 (very sleak lines with an LOA of 20'-2"). I am an otherwise experienced builder but I cannot see a stem for this boat being less than at least 1 1/2" from middle line to middle line which would leave the half-breadth rabbet distance to be around 2" in lieu of the 7/8" that the plans call for.

Thanks for your feedback.
Glenn

[S B]

Have you lofted this or just reading the offsets,because sometimes,if the plan calls for a false stem, the plank thickness will make up the needed wood. It doesn't sound right, as you have described.

[S B]

Just crossed my mind, are the plans for the moulds or the boat?

[gmeadows]

Hi all,

The plans (offsets) are for the boat and not the moulds.

Also, the plans do not call for a false stem and I personally do not feel a false stem is a good idea as there is no integrity between the forward stem and the false stem that must be screwed or glued together in lieu of just having a solid piece of wood.

If I were to increase the half-breadth distance to my rabbet, say 1 1/2" in lieu of the 7/8" the offsets call for, I would achieve my solid stem but I wood have alot more of the stem visible before the forward edge of the plank.

I guess it comes down to a judgement call or personal preference.

Thanks again for your tips/feedback/ideas.
Glenn

[Jim Ledger]

If you increase the stem rabbet sided thickness, either the beveling of the outer stem, from the rabbet to the nose, will become too steep, or you'll have to change the profile of the stem to keep the proper outer stem bevels. Either choice is not a good one, IMHO.

This matter was discussed not long ago. Maybe someone can recall the name of the thread. As I recall, the opinions ran to keeping the narrow stem siding.

[gmeadows]

Hi Jim and others,

I am not sure if I follow you Jim.

If I increase the 1/2 breadth distance to my stem rabbet, it will not change the steepness of the stem bevels. The bevel will remain the same and the amount of the stem that is visible will increase as I increase the 1/2 breadth distance to my stem rabbet.

I think the choice lies in increasing the 1/2 breadth distance to my stem rabbet, that will greatly increase the amount of stem that is visible, or doing an inner and outer stem and possibly screwing from the outer stem into the inner stem.

glenn

[Peerie Maa]

The important question is what is the sideing of the stem at the bearding line, before you start to cut the rebate.
That is what holds the fastenings and provides the strength.
Northumberland cobles were originally built with an outer stem nailed on to cover the end grain of the planking, where the inner stem had no width to speak of on its front edge.
Trust the designer.

[andrewdarius]

Hi Glenn,

I remember you asking about this a little while back. It does indeed seem like a narrow sliver of wood between the apex lines of the rabbets, but I suspect most of your strength at the bow will be in the fasteners angled through the planks back into the inner portion of the stem.

The same is the case for my 17-1/2, but there is a good amount of wood on the inner side of the rabbet.

There was mention of a few models that use a two piece stem...many designs actually have less distance between the planks before the outer stem goes on, sometimes nothing.



Andy

[gmeadows]

Hi Andy and others,

So are you saying that I should use a one-piece stem with its 7/8" rabbet from the center-line that will leave me with only 5/8" from apex to apex?

thanks,
glenn

[John Meachen]

I seem to recall a similar post not too long ago.If there is ample material behind the planking,does it actually matter how much or how little material separates the two sides?The moulded dimension of the stem is how much?

[gmeadows]

Hi John,

I am not quite sure what you mean by the "moulded dimension".

The overall dimensions of the stem before it is beveled and rabbeted is about 3 1/2 x 3 1/2.

The apex to apex distance of the monolithic stem (i.e. not an inner and outer stem) does matter. Think what would happen to the stem if the apex to apex distance was only 1/4" and the boat gets a side impact on the stem. I think that's what they call "game over" or in laymen's terms, your stem just snapped.

I'll save everyone the drama here. There is no way that the designer of this 1915 sloop, BB Crownenshield, intended for the boat to have an inner and outer stem and there is also no way that the designer would approve of only 5/8" distance from apex to apex.

So since no one has given me a definitive answer yet, I will make a command decision to increase the rabbet of 7/8" to something larger in order to get at least 1 1/2" from apex to apex and live with it. You may see more stem when looking at the boat's profile, but at least you will have a boat that is able to take the dings that all stems need to indure. Scantlings are a personal thing and I think 1 1/2" should do fine.

Still open for more feedback but I need to keep moving ahead if I want to launch her in my lifetime.

[Bill Perkins]

Glenn ;I think you've got to trust your (prominent) designer in order to proceed .You don't have to reinvent all this stuff . I’m afraid you may spoil a fine entry .

Pete Culler was an expert small boat builder and a connoisseur of the narrow stem . His rule for Minimum Stem thickness (sided dimension ): Twice the thickness of the planking , plus the thickness of the stem bolts , plus 1/8th in. He gives as an example a boat with 3/8th in. planking and 1/4 in. bolts ; so a sided stem dimension of 1 1/8th in. Just thick enough to pass the required bolts .

You could reinforce the connection of the stem area forward of the rabbet to that aft by also placing some bolts down the centerline as Culler did. As already mentioned , the wood aft of the rabbet is the part critical to hull integrity in this case .Your concern is about losing the little outer bit if it gets smacked sideways .

Adopting Culler’s rule you could use up to half inch bolts ,if you can drill that accurately . He notes that even he often nicked the bolts when cutting the rabbet ,so it would be best to use bronze ,and maybe downsize to 3/8ths bolts . You might choose smaller diam. if you want to countersink them in a narrow stem face . I once saw a published detail of a home made T bolt. Just a length of all thread with a short length of the same material brazed cross ways on one end . This can be concealed by a shallower countersink in a narrow stem than a standard bolt .

In this case you might just cut the head off a bolt and drive it as pin ,lubed with a thin epoxy . Maybe wait till you know where the plank fasteners will fall .

[Paul Scheuer]

Would there be any wisdom in having the "weak link" outside of the main structure ? I'm thinking that a "break-away" outer section would be easily replaced if necessary.

[gmeadows]

Hi everyone,

I just read my earlier post from last night and it seemed a little testy although this was not my intention so please accept an apology if you also sensed this.

Bill, using the 7/8" rabbet distance leaves me with an apex to apex dimension of 5/8". If I were to use 3/8" bolts that went through the outside tip of the stem and directly aligned with the center-line, I would have only 1/8" from the side of the bolt which should be enough given I drill accurately. The front face of the stem is a 1/2" flat face to a bronze oval and I could nip two sides of the head of the bronze carriage bolt and counter sink the head into the forward face of the stem. Now the question becomes how much should I space these bolts in the face of the stem. My guess is around 8" and given the sleek lines I would probably need just 2 bolts.

Thanks again to all for your insight on this.
Glenn

[andrewdarius]

Hi Glenn,

I began a reply yesterday, but I wanted to look at the study plans of the 12-1/2 first... I still believe it is the molded dimension (as mentioned earlier) that will give you your strength. The drawings look to show at least a couple of inches of stem behind the rabbet.

As for taking the dings of life, I hate to say it, but these knockabouts were lightly designed/built and meant to last only a few seasons before the owners commisioned a new one. That's not to say that these boats haven't lasted decades. The 17-1/2 was designed without a rabbeted keel, the garboard simply butt againt the keel, which caused all sorts of headaches, but a good number of them still exist that way.

Sometimes these details make one try to get into the head of the designer. There must have been a certain amount of value engineering going on as clubs were trying to keep a new class fleet within a certain budget...as with the 17's keel, having no rabbet meant they could bend a rabbetless plank keel over the molds and steam in frames that went from sheer to sheer...this certainly was a lot quicker than rabbeting a keel and fastening port and starboard frame heels to floor timbers. The club could then have its fleet in due course at a reasonable cost. You picked a sexy design, and there will be a certain amount of care you'll have to take. This isnt a deep ocean going vessel. Even though it has a yachty look, it's still a daysailer with the scantlings thereof.

But let's get back to your original thought. I wouldn't reloft the stem profile or rabbet. I've been in touch with a few folks who own and have built the 17-1/2 and that has never come up as a way to stiffen or strengthen these boats. You'll most likely find more friends when it comes to making changes that use plywood for the deck, or instead of searching for grown crooks to install between deck beams and frames you'll consider plywood bulkeads to hold sectional shape, or as Art Read did, you'll forego the self bailing cockpit for seats over floorboards...

There are certainly modern techniques to consider, but I think you'll be OK with that stem profile.

The Philadelphia Wooden Boat Factory received a 12-1/2 for restoration a while back and that stem didn't look too weak for the job.

Keep in touch,

Andy

[esingleman]

Hi all,

The plans (offsets) are for the boat and not the moulds.

Also, the plans do not call for a false stem and I personally do not feel a false stem is a good idea as there is no integrity between the forward stem and the false stem that must be screwed or glued together in lieu of just having a solid piece of wood.

If I were to increase the half-breadth distance to my rabbet, say 1 1/2" in lieu of the 7/8" the offsets call for, I would achieve my solid stem but I wood have alot more of the stem visible before the forward edge of the plank.

I guess it comes down to a judgement call or personal preference.

Thanks again for your tips/feedback/ideas.
Glenn

I disagree with your opinion of false stems. A properly made inner/outer false stem system is equal to if not stronger than a solid wood stem. tests on wood glued properly show the glue bonds stronger than the original wood bonds. Most false stems also have screws which and an addition level of mechanical fastening to the chemical bonding of the glue. The job has to be done correct though, no epoxy on white oak, or poor mating surfaces.

[John Meachen]

Hi John,

I am not quite sure what you mean by the "moulded dimension".

The overall dimensions of the stem before it is beveled and rabbeted is about 3 1/2 x 3 1/2.

The apex to apex distance of the monolithic stem (i.e. not an inner and outer stem) does matter. Think what would happen to the stem if the apex to apex distance was only 1/4" and the boat gets a side impact on the stem. I think that's what they call "game over" or in laymen's terms, your stem just snapped.

I'll save everyone the drama here. There is no way that the designer of this 1915 sloop, BB Crownenshield, intended for the boat to have an inner and outer stem and there is also no way that the designer would approve of only 5/8" distance from apex to apex.

So since no one has given me a definitive answer yet, I will make a command decision to increase the rabbet of 7/8" to something larger in order to get at least 1 1/2" from apex to apex and live with it. You may see more stem when looking at the boat's profile, but at least you will have a boat that is able to take the dings that all stems need to indure. Scantlings are a personal thing and I think 1 1/2" should do fine.

Still open for more feedback but I need to keep moving ahead if I want to launch her in my lifetime.

I sense a misapprehension here;if the apex to apex dimension as you describe it is 5/8" and the stem receives an almighty sideways blow you feel that the part of the stem outboard of the planking will detach and the boat will sink.Not so,The portion of the stem inboard of the hood end is what holds the planking to the structure as that is where the plank fastenings lie.The distance from apex to bearding line should be twice plank thickness,if memory serves, and this is much more important than any amount of wood ahead of the planks.If you can visualise a stem say four inches wide and five inches fore and aft,with a bevel for the planking terminating in a knife edge it will be massively strong despite the lack of material outboard of the planking.It's a shame Art Read seldom posts these days as he built a beautiful example of the boat a few years ago.

[andrewdarius]

Glenn,

In my last post, I mentioned the Philadelphia Wooden Boat Factory...if I am not mistaken, they never did restore the 12-1/2 that went into the shop. You may want to inquire about the keel and hardware. There might be a good chance to buy those items from them at a reasonable cost.

Andy

[gmeadows]

Hi all,

I suppose all of the smoke I have produced over this topic has been unwarranted purely on my part. Through the process I have gained alot of ideas and for that I am thankful that this forum exists.

What I have done is to re-visit my lofting and found that the 7/8" rabbet at the stem will do just fine. I actually end up with a 3/4" dimension from apex to apex and although this seems slim, I recognize two things. First, the integrity of the hull is secure with the inner section of the stem and secondly, that I can take a tapered bit to pre-drill the 1/2" flat face of the stem and then pop some ringed copper or silicon bronze nails to make me sleep better at night. Given the sleak lines of the bow, I might need only 2 or 3 nails. I think this might be better than drilling through the stem and placing a couple of bronze carriage bolts that have half of the head nipped off and then countersunk into the stem which I believe was a suggested solution earlier in this thread.

Either way, I am happy that the end result of my thought process has led me to be certain and comfortable on sticking to the designers offsets.

Thanks all and the WB forum
Glenn