Glen-L.com

1st part. I am at the stage where I am ready to lay 6 oz. fiberglass on my hull. I remember reading somewhere that fiberglass cloth does not conform well to sharp angles. My transom/bottom planking and chine/topsides edges are very crisp. How much of a radius should I take these edges down to? 1/4"r., 1/2"r. ?

2nd part. After I complete my glassing and 2 layers of epoxy on top of that, do I just rebuild these edges with thickened epoxy and sand to shape? I know my transom/bottom planking edge needs to be crisp, but can I leave my chine edges radiused or do I need to rebuild these edges as well?

I am building the flats flyer.

Depending on how severe the angle, 1/4-3/8" should be fine for 6 oz cloth. The transom edge will need to be rebuilt to crisp. The chine edge is a little more subjective...it depends...are you going to install spray rails along the chine? If so, then yes rebuild your edge. You can use thickened epoxy to transition the fiberglass edge and sand smooth. There are a few other ways to do this but your method is common.

FYI, just use enough epoxy to bury the cloth, then rebuild your edges, then add your finish coats of epoxy.

There are several ways to get the same end result...others here, I am sure, will, have other opinions.

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Bill

I told my wife we needed a three-car garage for my projects...she told me to ask her for permission next time before I buy a house.
http://www.unitybuild.net

Thanks for quick response.

Not installing sprays rails yet. I want to see how she goes at first. Good tip to rebuild edges before final layers of epoxy - thanks.

I do plan to put on two bottom fins (strakes) in the last 6'-7' of the boat, as it is very flat. I have been in other flats boats that slide that have more deadrise than I do. These fins will probably be no more than 1"-1 1/2" tall. Should I install these before or after fiberglass?

All of the advice I've seen says to install any hull appendages after fiberglassing.

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Photos of my Squirt build
El Toro I built with my Dad

Correct...after fiberglass.

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Bill

I told my wife we needed a three-car garage for my projects...she told me to ask her for permission next time before I buy a house.
http://www.unitybuild.net

Building up several layers of epoxy compatible mat or cloth will prevent the built-up edge from cracking. Several epoxy barrier coats should be applied to seal the ground down edges.

This link goes way back to the very first WebLetter. Too bad the publishing dates aren't posted in each edition !

Fiberglassing the Bottom Trailing Edge of a Power Boat (WebLetter 1)

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Paul Kane Chelsea, PQ

Building the Glen-L Hot Rod : http://www.boats.chelseacoachworks.com

Paul - Just curious. Why does a radius edge affect performance? FYI - I am restoring the square edge because sometimes I actually believe things without knowing why

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Dave

My Riviera build - the Midnight Cry Project

Video of Midnight in Action
Blog

Few things in the world measure up to the thrill and satisfaction of boating in a boat that you built.

I believe that a radius could be viewed as a very small rocker.

The following link explains the issues of having a rocker on the rear part of a planning boats bottom.

http://www.glen-l.com/weblettr/weblette ... l#notebook

Rocker in a bottom is not the same as a rounded corner. Sailboats depend on a hard corner for some added performance. But glassing a bottom and rolling the glass depends on a round over edge. You can round over the edge for glassing and then add back a measured amount of hard or sharpe edge with thickened epoxy using a structual filler after you begin your fairing and finishing process. Rocker bottoms will for sure cause a loss of planing and no amound of hard edges will correct this. Rowing skiffs are the best where rocker is actually built into the boats. Displacement hulls also are generated that way.

So Mike - If we round the corner on the transom with a 1/4 roundover bit, why does this cause a peformance issue in the high performance boats as compared to a hard edge. I can understand the sailboat issue. This seems like such a minor issue on the trailing edge of the of a planing boat. I believe I am missing something. Help me please AGAIN!!

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Dave

My Riviera build - the Midnight Cry Project

Video of Midnight in Action
Blog

Few things in the world measure up to the thrill and satisfaction of boating in a boat that you built.

THanks guys for the response and that WebLetter made it easy to visualize.
Glassing this week and painting next.

WOOT!!

Well Dave we are splitting hairs in a larger picture. What happens is that the water and hull surface squats with larger rounds since there is hull still dragging off the planing water. The more area of boat in the water, the more drag. Some folks actually use a larger round over. 1/4" is really nothing but can cause some issues with glass laying right. But most folks go back and clean up most routered edges and with transom angles in most cases a small bit will not actually round over the edge enough to lay glass across the edge without some voids either. In most cases small edges requires some nursing to make sure there are no voids along the edge.

In many cases when I only use finish cloth, I will actually glass after using a round bit over at least a 3/4" edge and then blend a really thick mix, place along the edge and hand lay the glass over the edge without using the roller that I wet out bottoms with. Pull the cloth down and wet along the upright plywood and pull gently into the resin and then wet out. The glass will actually pull some of the resin out too if done right. SO the need for a rewetting of the cloth is not really needed to acheive a hard bond in some cases.


The reason I subscribe to this method in plywood boats is to make sure that the end grains do not starve the small amount of resin thats required to wet out the thin cloth.

When using biaxall glass with a matt backing, this glass rolls over nicely and will require more resin and will hold more resin with the bulking up materials. This allows for any uneven areas to be filled and not have the issues with pinhole voids that you get either.

As a side note, large surfaces do not always allow for folks to just reach over and use yellow squeeszzzes, and for me I cannot afford to get my face over the toxic outgassing of the resins since I am so alergic to the crap.

Agreed...in SkinnySkiff's instance, 6 oz should do okay (IMHO) in most cases with 1/4-3/8" rounded edge. There are exceptions, of course, like more severe angles that have to be covered.

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Bill

I told my wife we needed a three-car garage for my projects...she told me to ask her for permission next time before I buy a house.
http://www.unitybuild.net

Sure 6 oz will round over but the corner is really too sharp which does not allow resin to "pause and stay for while" before making the turn if you only depend on the same and initial wetout, or has been my observation. Sure the transom angle helps the glass to conform better than on a right angle surface. But the amount of resin required to wet out the thin glass is just not there. My post was more to help others than for folks that have already gone down the path. Along the lines of using thin glass, there is really nothing wrong with running an additonal tab along the junction of the two parts, the bottom and the sides. Cheers.

For a complete explanation, you'll have to Google the term Planing Surface Theory and Hydrodynamics or talk to an Naval Architect ! Maybe Kevin (or Glen !) can explain it in simple terms.

My take (from very limited sources !) is that you're trying to minimize the turbulence behin the transom. With crisp edges, as speed increases, the transom flow detaches smoothly from the underside of the transom, and a depression is created on the free surface behind the transom. If the transom trailing edge is rounded over, the flow will not detach smoothly and create a great deal of turbulence.

If you're into some light reading , see Page 582, Figures 1 and 2 in the link below :

Computations of 3D Transom Stern Flows (Bill H. Cheng, David Taylor Research center, Bethesda, USA)

____________________
Paul Kane Chelsea, PQ

Building the Glen-L Hot Rod : http://www.boats.chelseacoachworks.com