Gunnels

Is there a reason for the width of gunnels?I am replacing the gunnels on an old fibreglass 1960's speedboat.The gunnels are 5"wide overall.I would like to reduce the width by 2". The upper futtocks would be jointed into blocks incorporated into the gunnels.I am replacing the upper futtocks with 3/4"grp,maybe two thicknesses laminated,the old ones were timber,bulky,and subject to rot.
The boat is 13ft long,I intend fitting rowlocks for emergency oars,is there a formula for the placement of rowlocks relative to the trim of the boat?I have yet to fit a crossways[port to starboard] seat,so the position of this will be crucial to the rowing position.
The boat has a 5ft beam at its widest point,and is about 15" above the water,is there a specific length of oar I would need?
I have converted the boat into a pleasure fishing boat,so the internals are no longer "speedboat"spec.It has a tiller controlled 18hp outboard[longshaft]
Any advice will be gratefully received

 

The side decks, if that's what you mean by gunnels, often play a structural role. It's hard to say whether the width of 5" is necessary to stiffness of the side without a study of the boat's details. Reducing the width by half with similar construction yields about a quarter the original stiffness. This may or may not matter in your case but it is probably going to weaken the structure somewhat. Obviously, you could build up a substantial enough replacement structure that's much narrower, just be aware it will be a lot beefier than the original side deck to be strong enough to replace it.

 

1/8, not 1/4

 

No, I was right.

http://www.structural101.com/Beam-Design---Basic.html

 

from the article you cited-

but I thought we were talking about stiffness- not strength. The article you cited didn't give the formula for stiffness of a rectangular beam.

see here- http://en.wikipedia.org/wiki/Second_moment_of_area.

Stiffness of a rectangular solid is proportional to h^3.

 

When advising on structural matters it is nice to have a picture. However, since you are reducing power considerably a reduction of structural strength and stiffness may be OK in this case. Wide side decks interfere with rowing so it makes sense to make them narrower.

While I’m not a rowing afficionado, a 5' beam sounds a bit much for rowing, especially with the high (by rowboat standards) freeboard. The usual formula for length is around 25/14 of the distance between rowlocks, but that might be a bit “high geared” for a heavy power boat. Also the rowing position is likely to be uncomfortably high depending on seating. Suggest you draw a simple sketch to determine how high the handles will be. A typical rowboat would have a beam of about 4' and the oarlocks would be about 6" above the seat, which in your boat may make the seat too high to allow for bracing the feet, depending on internal depth.

The fore-and-aft location of the rowing position would not be critical in a heavy boat, Close to the lowest freeboard location would make sense if nothing else gets in the way.

However a trolling motor might be a simpler alternative to all the complexities of adapting the boat to rowing, if the transom design permits. A small troller will easily exceed the thrust achievable with oars. Oars can be surprising expensive.

Phil was right actually. The web page states “With all other factors the same, a rectangular beam with depth of 12 inches has a bending strength that is 4 times that of a beam with 6-inch depth.”

Stiffness, however varies as the cube of beam depth.

 

Two fold:-

1)for stiffness to support a free edge of an open boat
or
2) To support a fixture, such as rowlocks etc

So, have you cut off the original yet..if so, how flexible is the side with it off compared to with it on?

Do you have any internal frames?

 

From what I know of your project Valvebounce, you'll have hell to pay, rowing this puppy any distance. Even if you trim her way bow down, I don't think you'll be able to "clear her butt" (which has a lovely ring to it) sufficiently, to make the row anything more than a cardiac stress test. Simply put, you can rig oars for moving around docks and anchorages, relatively short distances, but that's all you'll want to row her, trust me. All the wrong shapes to row well, so stick with a kicker if you can.

As to reducing the width of the side decks, you certainly can do this, though some "floppiness" should be expected in her flanks, if it's a lot. I wouldn't reduce the side deck width any more than the frame head's molded dimension. To do so, would require reducing the dimensions of the frame head on the upper futtock (not a good idea). You have what you have. It'll be a unique and functional piece when it's splashed again, something I'm sure you'll be proud of. Compromise is part of the game and yes, it sucks sometimes.

 

Well I learned something. Thanks.

 

Thanks

Thanks PAR,as usual you have tuned in to all the aspects of what I am looking for. There is not much floppiness in the flanks,but she has spread a little.I have the original arch pieces that supported the bow "Hood"also the fibreglass hood sheet that I am using as a former/pattern. I have fabricated a piece of timber with blocks on it which fit over the top of the outer gunnels on the bow.It is set slightly narrower than where the arch pieces fit,by tapping it towards the stern,it is closing the gap to the original distances.I have marked lines where the originals fitted.It seems to be working ok.At the moment there are no blocks or inner strips fitted on the gunnels,just the two strips either side of the fibreglass hull.I intend following the same procedure from the stern,I have a fabricated pattern of the original transom,which I made from 3/4 grp.This was actually going to be the replacement transom,until you advised me to use a longshaft engine and increase the depth of the transom.[Following sea etc.]
I have held back on fibreglassing the new stringers in,until I achieve the original boat shape,I imagine doing them first would have held the hull to the wrong shape.
Sorry if I have got a bit "Longwinded"but I really appreciate your input.V.

 

I have cut off the originals,and it was very flexible without them,I have fitted two new strips either side of the fibreglass hull,which has stiffened it up somewhat,although the boat gaped open.I steam bent the pieces to follow the hull shape,and clamped it as I proceeded,and bolted it through every 300mm with 10mm s/steel bolts.
There are no internal frames,just upper and lower futtocks,which have yet to be fitted.
If you have a look at the reply I sent to PAR,it will probably explain what proceedure I am following.
Many thanks for your input.V.

 

Thanks for the input,from what you say,it seems like the depth of the inner strip is where it gets its strength.I think a troller might be the best bet.After reading PAR's comment about cardiac arrest,I will have another think about it,and although I am fighting every click of the age ratchet,I have to admit to myself,I am no spring chicken.