Building a heavier transom?

[ShelbyBoats]

I am currently building a 14 skiff designed by William D. Jackson and was wondering if there was a way to alter the transom slightly for my particular use. I have attached the blueprints for the transom and its frame, and according to the initial plans this boat would be good to have a 1-5hp outboard. Now I am not a marine engineer by any means, and I know you can't just change the plans of a boat as you want without some repercussions. I want to know if there is a good way of beefing up the transom to accommodate a 10-12hp engine, I am not looking for a speed demon or anything, but for the size and weight of this boat I really feel 5hp is still way under-powered. I have a perfect vintage Montgomery Wards 12hp engine I would love to use on this boat, is there a way to make it happen? Any help as always is greatly appreciated, thank you.

Sea Skiff Transom.jpg

[Mrleft88]

Add in quarter knees and a 1/2"
pad that covers about 3/4 of the transom and give it a go. Worst that can happen is you dump your motor, and sink your boat.

[wizbang 13]

It has a 3/4 inch ply already ?
Plus one by three framing?
I’d be building it lighter for a 25 hp , not heavier for a 12 .

[Peerie Maa]

Add in quarter knees and a 1/2" pad that covers about 3/4 of the transom and give it a go.

And add a stern knee.

[MushCreek]

At a glance, it looks plenty strong as is, especially if everything is glued. I'd add a transom knee and run it. It would be good to see the rest of the design. Some hulls just can't handle a lot of power. I'm currently building an 18' rowboat, and although it will take a small motor, anything over 3 hp would be a waste, as the stern would squat and the bow would head skyward.

[wizbang 13]

Well, the “blueprints” we see are only in one view. Of course it needs knees . How old is this plan, cuz it look a bit obsolete.

[Peerie Maa]

Well, the “blueprints” we see are only in one view. Of course it needs knees . How old is this plan, cuz it look a bit obsolete.

Google is our friend.
http://boatplans-online.com/plans/BPO_Seaskiff.pdf
It has quarter kneed but no stern knee.

[nedL]

Can you post more of the plans? Given the apparent age of the plans (very 1940’s -1950’s ‘ish) and the amount of deadrise I am thinking I am thinking there may be a good bit of rocker in the bottom. If so, any horsepower more than the 5 that is recommended may be wasted. She may just squatt and not perform very well. Also, if there is a good bit of rocker in the bottom she may not sit well in the water with the extra weight.
Just some thoughts.

[Peerie Maa]

Can you post more of the plans? Given the apparent age of the plans (very 1940’s -1950’s ‘ish) and the amount of deadrise I am thinking I am thinking there may be a good bit of rocker in the bottom. If so, any horsepower more than the 5 that is recommended may be wasted. She may just squatt and not perform very well. Also, if there is a good bit of rocker in the bottom she may not sit well in the water with the extra weight.
Just some thoughts.

There is considerable rocker, as the drawings in that link show. C&P from that PDF would be a pits, so I'll leave that for you to do.

[David G]

At a glance, it looks plenty strong as is, especially if everything is glued. I'd add a transom knee and run it. It would be good to see the rest of the design. Some hulls just can't handle a lot of power. I'm currently building an 18' rowboat, and although it will take a small motor, anything over 3 hp would be a waste, as the stern would squat and the bow would head skyward.

Agree... with that much rocker, additional hp will tuck her tail and point her nose skyward.

Nothing wrong that those plans. A sturdy little fishing or utility boat. But she really is designed to accomodate a certain hp range. And don't forget that a 3 hp back then was substantially lighter than a modern 4-stroke 3 horse motor.

[Autonomous]

To oversimplify, there are 2 1/2 kinds of hulls.
1) Planing hulls which will climb up on and skim across the water given enough power.
2) Displacement hulls, yours, which will move efficiently through the water with little power but can't plane. Adding more power just uses more fuel without any useful speed increase.
2 1/2) A compromise, semi-displacement. Like most compromises it is not as efficient at displacement speeds or as fast as true planing hulls

My 17' displacement boat has a 2 1/2 hp engine which basically only makes more noise and points the bow higher over 1/3 throttle.

[Peerie Maa]

To oversimplify, there are 2 1/2 kinds of hulls.
1) Planing hulls which will climb up on and skim across the water given enough power.
2) Displacement hulls, yours, which will move efficiently through the water with little power but can't plane. Adding more power just uses more fuel without any useful speed increase.
2 1/2) A compromise, semi-displacement. Like most compromises it is not as efficient at displacement speeds or as fast as true planing hulls

My 17' displacement boat has a 2 1/2 hp engine which basically only makes more noise and points the bow higher over 1/3 throttle.

Just so, exactly correct, but

according to the initial plans this boat would be good to have a 1-5hp outboard.

I really feel 5hp is still way under-powered. I have a perfect vintage Montgomery Wards 12hp engine I would love to use on this boat, is there a way to make it happen? Any help as always is greatly appreciated, thank you.

5hp is not under powered for that sailing hull. You will have to run your 12hp at half throttle, or she will squat and dig a hole in the water.

[nedL]

There is considerable rocker, as the drawings in that link show. C&P from that PDF would be a pits, so I'll leave that for you to do.

Thank you Nick, I completely missed that link.
….Yes, that is just about the hull shape I was envisioning for that transom. She will need no more than about 3 hp.

[Autonomous]

I took another look at the plans after reading Nick's post. The hull has hard chines and a squared off transom near the waterline, it may be the fabled semi-displacement hull form. Not sure if the rocker will let her climb up on the water.
Comparing it to my hull with slack bilges and a wineglass transom was inaccurate.

[Peerie Maa]

I took another look at the plans after reading Nick's post. The hull has hard chines and a squared off transom near the waterline, it may be the fabled semi-displacement hull form. Not sure if the rocker will let her climb up on the water.
Comparing it to my hull with slack bilges and a wineglass transom was inaccurate.

No, there is too much rocker and curve to the aft buttocks for a semi planing hull form.
This is semi displacement.

Wide flat run aft, full waterlines forward, giving a full Prismatic Coefficient.

[ShelbyBoats]

I have to say I really appreciate all of the input from everyone. I had to look up the term "rocker", I honestly didn't know what it meant, but that is what I love about this forum, always learning something new. So more horsepower is not really useful for this type of boat, I never expected her to be a speed demon or anything. I know it has been stated before that this is quite the antiquated design, but that is honestly what drew me to it. I know I will have more questions as I progress and take comfort in knowing this forum exists to provide help. But just so I am clear, apart from adding a transom knee and gluing everything (which was my original plan), is there anything else I should do? Make the transom frame a little thicker or use longer screws?

[ShelbyBoats]

It has a 3/4 inch ply already ?
Plus one by three framing?
I’d be building it lighter for a 25 hp , not heavier for a 12 .

Can you elaborate? Are you saying it is already heavy enough?

[ulav8r]

Can you elaborate? Are you saying it is already heavy enough?

I think that is exactly what he meant, but wait for his confirmation. I have not had experience with large outboards, but many small outboards would not fit a transom thicker than about an inch and three quarters. Your transom is an inch and a half per plan. If you use marine ply and a hardwood frame it should be very strong. The 4 inch wide keelson and 6 inch wide frame up the center of the transom provide room for more knee that is needed. A 1 1/2" thick knee would provide plenty of room for fasteners and be hell for stout.

The above is just my uneducated opinion.

P.S. How wide is the opening of the clamp on that 12 horse motor?

[ShelbyBoats]

I am actually not sure how wide the clamp opening is, at the moment the outboard is in the back of the barn and I can not get to it to it to measure it just yet.

I do have another question regarding the transom and figured I would just pose it here. If you look at the transom frame diagram it shows 1x3 for the side board and bottom board, but the top board and middle board is indicated to be 1x6. If you read through the whole instruction set and the supplies list provided it shows using a simple 1x3x8' board for the transom frame, no mention of a 1x6 anywhere except in the transom frame diagram. My question is would it be ok to edge glue two 1x3 together to make the 1x6 sections? I have access to a lot of 1x4 lumber and would rather use it if possible. As always any help is greatly appreciated.

[Peerie Maa]

I do have another question regarding the transom and figured I would just pose it here. If you look at the transom frame diagram it shows 1x3 for the side board and bottom board, but the top board and middle board is indicated to be 1x6. If you read through the whole instruction set and the supplies list provided it shows using a simple 1x3x8' board for the transom frame, no mention of a 1x6 anywhere except in the transom frame diagram. My question is would it be ok to edge glue two 1x3 together to make the 1x6 sections? I have access to a lot of 1x4 lumber and would rather use it if possible. As always any help is greatly appreciated.

If the staves are edge glued to reach other and glued to the ply of the transom, they will be fine.

[wizbang 13]

They might even be better!

[ShelbyBoats]

That is my plan, to edge glue the pieces together, and then glue and screw the transom frame and transom together. What glue would work well in this situation, and do some people coat the wood with anything before gluing?

[ShelbyBoats]

I still can’t decide between resorcinol and epoxy, or is that overkill since everything is getting screwed anyways? I was leaning towards resorcinol, but honestly price wise it’s about the same as epoxy, thoughts?

[nedL]

98% of people would use epoxy, I use resorcinol. Coating wood with something before glueing it kind of defeats the purpose of gluing it. (In other words,… no.)

[wizbang 13]

I still can’t decide between resorcinol and epoxy, or is that overkill since everything is getting screwed anyways? I was leaning towards resorcinol, but honestly price wise it’s about the same as epoxy, thoughts?

How did you figure they are about the same price?
For an equal job, epoxy should USE two or five times the volume , otherwise yer using it wrong .
Cost should not be a factor

[pcford]

How did you figure they are about the same price?
For an equal job, epoxy should USE two or five times the volume , otherwise yer using it wrong .
Cost should not be a factor

Resorcinol requires good fits. Epoxy is a better choice for those that are not able to do them.

[ShelbyBoats]

98% of people would use epoxy, I use resorcinol. Coating wood with something before glueing it kind of defeats the purpose of gluing it. (In other words,… no.)

What is your source for resorcinol?

[ShelbyBoats]

I am curious, would I end up using more epoxy because of having to wet out every joint before using thickened epoxy to glue?

[Boatsbgood]

Epoxy likes a loose fit, if joinery is tight enough for resorcinol (ie. very tight) epoxy wont do what it does best: fill gaps and hold like crazy. And Resocinol needs lots of clamp pressure (related) . If one overclamps an epoxy joint its not a good thing. And also yes, one soaks a bit with epoxy before closing the joint. West Systems epoxy guys have a great treatise on the subject,(yes they are selling it) but really reliable info on use, you need to read it. Sorry, no link atm.

[ShelbyBoats]

So all of the joints in this boat call for a very tight fit and I personally do not like gaps or un-evenness in my woodworking projects. I am starting to think that resorcinol glue is the way to go. Now my next question is apart from gluing the transom frame to the transom itself for maximum strength, should I be gluing other joints as well? Such as the clamps, chines, and keelson to the transom frame and the stem, these are all to be screwed as well, but I am definitely a suspenders and belt kind of guy. Thoughts?

[Peerie Maa]

^ I would glue them. It will keep damp and potential rot out.

[Boatsbgood]

I may have given the wrong impression above...resorcinol needs clamping pressure, way more than screws are going to give. A good fit screwed together in a candidate for epoxy, especially with ply. And ply construction generally is more durable if end grain is all sealed with epoxy, though usually its best to also coat the entire surface...

[ShelbyBoats]

I only start to question using adhesive for all of the joints because I was watching the Tips from a Shipwright series and saw he uses polysulfide sealant for all of his joints and relies on just the screws themselves.

If I were to go the resorcinol route, would I just glue everything up with clamps first and then run in screws after it’s cured? Or would I glue it, drive the screws in, and then clamp over everything?

[ShelbyBoats]

I know that the original plans called for Kuhl's Aviation Glue and Weldwood Resin Glue, but relied more on the fasteners themselves than the glue for holding power. If you look through the plans there is heavy use of fasteners through-out the boat, so do I need to use such a permanent glue as epoxy? I guess I am worried that epoxy will make the boat too rigid and won't allow it to flex. But I definitely want to put something into every joint to keep it sealed up from water.

[Peerie Maa]

I know that the original plans called for Kuhl's Aviation Glue and Weldwood Resin Glue, but relied more on the fasteners themselves than the glue for holding power. If you look through the plans there is heavy use of fasteners through-out the boat, so do I need to use such a permanent glue as epoxy? I guess I am worried that epoxy will make the boat too rigid and won't allow it to flex. But I definitely want to put something into every joint to keep it sealed up from water.

Until the old school glues failed, they were as stiff as epoxy. Don't overthink this and just use epoxy. You do not have to be as precious with quality of fit with poxy and it is probably stronger.