1978 18' Starcraft Offshore SS I/O

[opsimath]

No Title

Picked the boat up today from being blasted with plastic media. Had just planned on doing the exterior but they did blast some of the stuck on glues etc. on the interior. The water in the bottom of the boat is from a passing thunderstorm. There are still some of the small plastic beads that will have to be vacuumed out.

All the imperfections really show up. Welded over rivets, dents, and caulked seems that were mostly hidden are now very apparent. Looks like I will need to turn this hunk of aluminum upside down, spend some time making repairs and sealing it. I have been reading other restoration threads and see that there is a large assortment of products available to seal rivets and seams.

Still undecided on the transom as it appears to be okay but as someone said; it is nearly forty year old plywood. I am always open to any opinions or advice from those that have "been there and done that".

What is the proper name for the small 1/2' thick piece of plywood where the gimbal housing bolts through the transom? Are there any other products that can be used in its place, other than the plywood?

ops

 

[jbcurt00]

Horse collar is what the ring of ply around the gimbal housing mount is typically called. Thru the keyhole, don't forget, 2.25" is the target thickness, including 2 layers of 3/4" and the horse collar, plus whatever your sealer of choice is and the exterior aluminum transom skin.

I suspect if you read every SC topic posted at iboats, the # of plywood transoms that were pulled, even though they appeared good to go, but were actually in good shape, could be counted on less then 1 full hand. Comparatively, most suspected good to go transoms that were pulled, weren't.

Wet but not yet rotten or soft areas of the transom are likely to either become rotten and soft or dry rot. Neither is a good prospect.

By comparison, replacing it now will be a simple, quick affair. Put the engine and outdrive back in and finish the boat around it, and it becomes a much more difficult process.

 

[GA_Boater]

You have her down this far, you might as well do the transom, ops. That won't add much time or ,any dollars to the project.

We call the plywood ring around the keyhole a horse collar. The purpose is to set the proper thickness of the complete transom assembly so the driveshaft meshes properly with the coupler and for alignment of the motor to the outdrive. As the drive is raised and lowered for trim and trailering, the driveshaft splines move in and of of the coupler slightly. The proper thickness of the transom is 2" to 2 1/4", which includes the outer skin and all wood - It is an important measurement and must be equal around the keyhole.

They may be other things to use for the horse collar, but plywood has been used successfully for decades.

 

[GA_Boater]

Sorry, ops. I got carried away up there.

 

[opsimath]

Hello,

Appreciate the advice on the transom. I have spent some time reading past threads on this subject and they seem to mostly favor wood. Never being one to do things the most common way, I have decided to sandwich 1 1/2 inches of PVC expanded core between the transom outer 3/32 aluminum and an added 1/8 inch inner sheet aluminum. I am considering using solid 3/8 aluminum for the horse collar. No wood at all.

I am planning on adding 1" aluminum square tubing every 16" across the width of the boat to support 1/2' MDO plywood for the floor. Flotation and sealing is yet to be decided.

Heading out of town again so it may be awhile before I work on the transom but I will post pictures of the process.

ops

 

[GA_Boater]

What is this "PVC expanded core" product. The transom supports most of the weight of the motor and outdrive. Something to consider when looking at alternative material.

 

[opsimath]

GA Boater

Expanded PVC is a plywood replacement product that will not rot. It has similar load bearing as plywood when used in a vertical or horizontal position. Lying flat it will flex more than plywood of a similar thickness and needs added support.

I am fortunate to live in an area that has a supplier of expanded core PVC. I was able to go there and inspect the properties of the product. Laying a 12mm thick piece across a couple 1x2s it would flex but not break very similar to equal size plywood. Once 2 other pieces were added to equal the 1 1/2" it became almost as rigid as the plywood. When sandwiched with 1/8" aluminum I could tell no difference. In the transom position the pvc is very strong and should perform as plywood but with no decay etc.

ops

 

[Watermann]

There should be a bracket that the transom wood rests in. The bracket will limit the transom material thickness to 1.5".

 

[GA_Boater]

I don't know, ops. Almost was used too many times in your description. I would be concerned with the transom flexing fore and aft. Maybe I'm just too traditional.

 

[jbcurt00]

I only looked at 1 supplier, and their expanded core PVC expands and contracts more then plywood. I'm assuming that's thru the thickness dimension and it isn't recommended for exterior exposure.... For either I don't think I'd want an expanding transom core, but particularly not on an I/O.

What's used to 'glue' the 2 sheets together to make a monolithic 1.5" thick transom core?

Just make sure you're familiar w/ all the specs of whatever version of it you're planning on using.

 

[opsimath]

Ga Boater,

You and Watermann likely know way more about these things than I do but I have often ventured out into unproven territory. Until retirement I was a custom home builder/designer and found myself constantly adapting to the ever changing products and codes. Most of the time they took me kicking and protesting but I worked in a field where home owners always wanted the latest technology.

I will try to make this a well documented "experiment" that will hopefully answer some of the transom questions that I encountered on this forum.

Your input is always appreciated, as stated before, this is my first time tackling a boat. :joyous:

ops

 

[opsimath]

JB,

I had some of the same questions when I visited the plastic supplier. The expanded core PVC I sampled is not designed to be exposed long term to the sun but it can be painted with most any type of paint. It is commonly used in the making of structural signs. You laminate it together with the same type glues used in bonding pvc pipe. It actually melts the pvc and welds it together for a bond no different than the pvc itself.

My only hesitation was the for/aft movement mentioned by GA. I will be counting on the "sandwich" addition of the 1/8" sheet aluminum to protect the PVC and add structural support. I do not know how much fore/aft flex is possible with the Mercruiser but I doubt there will be any more than 1/16" flex in this design.

This product can be milled and worked with just like wood and it is lighter in weight.

ops

 

[jbcurt00]

I hope its cheaper then Starboard, but wish you luck, hope that it works and works well for you.

Is it a mm spec or fractional? Might get sub 3/4" to account for the added aluminum skin. Although I guess the alumimum could start at the top edge of the transom shelf/L bracket Watermann mentioned, and only the PVC product resting in the bracket.

A solid alumimum horse collar, thats a new one too...

Be interested to see it come together

 

[GA_Boater]

What about compressibility? Have you hit a chunk with a hammer?

 

[Watermann]

Sounds to me like you're going to work with the PVC sheets and if there are any issues that arise I'm sure collectively we'll think of a way to overcome the issue. I'm glad you take all the thoughts being thrown out as they are intended to help and not as criticism of the idea. Yeah we're set in out ways to some degree but ingenuity never goes unnoticed.

I don't think fore and aft give or flex of the transom will be an issue. The reason being is the stress on an I/O vs an OB are handled a bit differently. While the OB's have knee braces, the I/O has a big chunk of Mercruiser iron bolted in front of the transom so the force is transferred to more of an area and comes out as a torque to the right..

 

[opsimath]

Hello All,

JB... This PVC product is 80.00 per 4x8 sheet. It is a mm product and is available to me in 10mm and 12mm. I am thinking I will use 2-12mm, 1-10mm and then the 1/8 inch aluminum. If I figured correctly this should just fit me inside 1 1/2 inch bottom bracket.

GA...Just tried your hammer idea on a sample piece and it leaves a slight indentation, similar to plywood.

Watermann.....No criticism taken. I like the sounding board you provide otherwise I would not be on the forum. I am reasonably sure this will not be the only idea I have. Already thinking of using my PVC transom scraps to fashion a livewell. :der:

ops

 

[GA_Boater]

ops - Please don't take my comments and questions as negative. Really playing devil's advocate because you have a new idea, that's all.

My compression question is because the keyhole doesn't have a lot of meat from the mounting bolts to the opening and the transom hardware relies on clamping to stay in position.

 

[opsimath]

GA,

Quite the contrary, I appreciate your input and your concern as you stated.... "My compression question is because the keyhole doesn't have a lot of meat from the mounting bolts to the opening and the transom hardware relies on clamping to stay in position".... and I believe you are on to something that could be a potential flaw in my design.

Because of your input I am considering enlarging the hole that extends through the PVC and putting in an aluminum tube to act as a spacer to keep those bolt areas from collapsing.

Your input is exactly the reason I joined this forum. I get to ask questions and run ideas past those such as yourself. Thanks for your question!

ops