FS18 - new build - foam&carbon fiber

[JoRoesler]

Hello,
I'm going to build a new FS18, starting some time next year. I happen to have come across lots of free dry carbon fiber fabric (recycled/production waste), and am planning to use it. I purchased a load of 5/8" recycled PET foam, and will be glueing it all together with the System Three Silvertip resin. The inside skins will be vacuum bagged onto the individual foam sheets (1st step), the outside skin will be hand layup. I will take my sweet time, as I have a well functioning PH18 at hand. Power will be a 25HP four stroke.
Questions for other builders:
What is the point of using a 15" vs. a 20" shafted outboard? Do you need the one or the other if you also plan to use a jack plate? Do you need to increase the height of the transom if you are planning to use a jack plate? Thank You.

[fallguy1000]

Short shaft or 15" motors are not readily available. The problem with them on a larger boat is backing and launching,

Backing into oncoming seas on a short shaft transom results in water coming over the back and into the boat. It happens on 20" shafts transoms as well, but lots less.

Carbon and foam are a general mismatch as elongation of foam is about 8% and carbon 1-2%. That is not to say it cannot be done, but even determining the schedule is harder.

If you build it, we'd love to see it blogged here.

[fishgitr]

I believe the fs18 calls for 20" engine. Keep it as designed. If you have a 15" shaft on hand, that's a different story. I had to run a motor well since the deck sits at around 15". May be different with a jp.

Carbon fiber is over rated. There's much more to carbon fiber. These carbon/ kevlar boats are more of a sales pitch vs reality. I am a big fan of just 10oz cloth. Much of jaqs designs call for 12oz biaxial.

I am also a big fan of basalt cloth. It has similarities to carbon/ kevlar strength and puncture resistance. It lays up better and much less cost.

[TomW1]

If you use carbon fiber on the outside, you need to vacuum bag it also to get the correct adhesion to the foam. Carbon fiber tends to float. What thickness is the carbon fiber, you may need more than one layer or a layer of fiberglass for strength. Tom

[VT_Jeff]

If possible, measure your motor before cutting your transom to final height. May not matter so much with a jackplate but my 20" transom was surprisingly short for my 20" motor.

[JoRoesler]

Thank you. So the question becomes: If I know that I’m going to use both a jack plate and a 20” shaft motor, do I need to modify the standard FS18 transom design?

[VT_Jeff]

Fair question but outside my paygrade, ill defer to those that use them.

[bamaguy0]

I’m not familiar with the FS18 design enough to say for sure, but my suggestion is to leave the center section of the transom “long” and then trim once you flip it over and can mock it up or set things in place on the mostly finished hull.

With a jackplate you can bring the cav plate slightly above the bottom of the hull so you want to make sure you have it set up where you have a little up/down play in the plate.

I made some miscalculations when I went from a short shaft with PT-35 tilt and trim bracket to a long shaft with jackplate and my jackplate is maxxed out to have the motor where it needs to be.

[Reid]

Thank you. So the question becomes: If I know that I’m going to use both a jack plate and a 20” shaft motor, do I need to modify the standard FS18 transom design?

No need to modify the transom. I am installing a jack plate on my FS18 with a 20" shaft 25 HP Suzuki. The jack plate will mount right below the level of the aft deck. I went with a Bobs Versa Jack Plate, manual but has an option to add the hydraulics. (no need for hydraulics IMO since the FS18 doesn't have a tunnel) It has a 6" set back. I also plan to run a tiller extension.
I also agree with some of the others when it comes to carbon fiber. It sounds fancy but I don't believe it will add much benefit to the FS18. As you already know, from your previous build, these boats are already lightweight by nature and build method. Carbon fiber and/or foam core will not give any significant weight savings over the coarse of this relatively small boat. It will definitely add to the complexity of the build. That being said, if you already have the materials and are doing the build this way in order to try something new then go for it! I will definitely be eager to watch this build. I would, however, have Jacques help you out with the lamination schedule. Using Foam Core will definitely change things.
Best of Luck!!!
-Reid

[fishgitr]

Good info Reid!

You will also need to layup those panels flat with some 4oz/ 6oz. If you don't lay these up prior to installing them on the frames, you will get a lot of sagging.

I would also recommend getting a lam schedule from jaq. Builds in foam core I've done, I use 4 layers of 10oz outside and 3 layers inside. But take jaqs advice over mine.