Model boat hull shape

[Thin water]

I have started making my own plugs for molds for deep V model boats for a radio controlled boat dealer. A regular problem with these hulls per the rc boat forums is a hooked hull (A concave spot just ahead of the transom, supposedly from the polyester resin shrinking).

I am sanding my plugs to a slight rocker shape to offset this. Any ideas or input on how much hook is bad (on a 44" - 54" long high speed model)? How do I keep it from happening? Will vacuum bagging with epoxy instead of polyester help? What is worse, a little rocker or the concave area.

When these are on plane nothing is in the water but the prop and rudder and maybe two inches of hull. The hook problem seems to be when the boat turns and more of the hull touches the water. They normally travel in the high 30 mph range to 60 mph. Some are much faster.

Any other things to look out for in the plug making stage?

Thanks in advance.

[Gilbert]

If you can find a boat that has this problem you could measure how much concavity there is. If you assume the plug for that boat was made straight, you could add that much rocker in your plug.
Epoxy might help in this regard. They tell me that polyester keeps curing and curing and curing; therefore the shrinkage.

[marshmat]

The extra cost of epoxy is pretty small on a model. The much improved strength and dimensional stability are not insignificant. It's certainly worth a try, at the very least. Vac bagging will help with the resin/fibre ratio as well as holding the hull firmly against the mould; the extra labour is considerable but if you're willing to give it a try it will probably result in a significant improvement in part quality.

[Thin water]

I agree, the extra strength of the epoxy is significant. I will make most of the boats with it and only vacuum bag the custom carbon and kevlar ones. It is quite a contrast, the thin precise layup of a high speed model vs the thick strength through lots of material used in my airboat hulls.

The last question remains, will a little rocker hurt the performance of a deep v style hull? Is true flat the best?

On the higher end stuff I could fair them out by hand after they are out of the mold in about an hour each.

JIM

[Greybarn]

Try using some DCPD or a DCPD/polyester resin blend. DCPD's do not shrink much at all. The trade-off is that they are more brittle (on a relative basis) as compared to a good iso, vinylester, or epoxy. Another way to go is design a small amount of rocker into the hull so it will shrink back to flat.

[SamSam]

Maybe the problem is too hot of a polyester mix. Try less hardener for a slower cure. Another possibility would be you need more internal structure such as ribs or stringers or core on the flat sides of the V on either side of the keel. I'm assuming the keel and chines are straight and it's the flat part(s) of the hull that is hooking.? BTW, what is the problem, IE, what brings on the problem and what happens when the problem occurs? Sam