round bilge for TW28

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round bilge for TW28

Post by guest » Wed Sep 23, 2009 5:09 pm

I like the TW28 design, but often woundered if it couldn't be modified to a soft chine or round bilge. The construction would have to change perhaps strip plank instead of plywood. Am I the only one who has thought of this?



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Re: round bilge for TW28

Post by TW28RJ » Thu Sep 24, 2009 7:09 am

I like the the looks of the round hull shape don't know how it would effect performance.I also like strip plank as a core though it adds a lot of work compared to large plywood sheets. I too would be interested if it were possible without it being a total redesign

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Re: round bilge for TW28

Post by jacquesmm » Thu Sep 24, 2009 8:56 am

What would be the reason for round bilges?
It can't be aesthetics because the chine is below the waterline and many classic trawlers like the Grand Banks have a hard chine.

A round chine will take some volume away at the waterline and increase the roll but it can be done.
You would replace the hard chine with a 6" radius strip planked round chine.
You need to keep extra layers of glass there as on the plans for the chine.
It's a lot of work for a less good hull.
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Re: round bilge for TW28

Post by TW28RJ » Thu Sep 24, 2009 9:44 am

I don't think that I'll ever understand the difference between the two in this context as there is so much conflicting information out there.

For instance in Dave Gerrs book "The nature of boats: insights and esoterica for the nautically obsessed " he says " In medium-speed boats particularly, the the differnce between hard-chine and round bilge hulls can merely be one of aesthetics. Both can be made seaworth and efficient. The round-bilge version will be a bit easier riding but have tendency to roll a bit more deeply, while the hard chine version will feel steadier but have a slightly snappier roll".

Okay that make sense, but the in the very next sentence he says, "Nevertheless as a general rule, hard chines are for high speed in shore work and round bildges are for low speed off shore service."

To me that seems contridictory if there is more stibility from a hard chine why wouldn't it be for off shore service. And you mention grand banks, but doesn't kady krogen have a round bilge?

Being from the northeast I see a lot of work boats with round bilges (particularly the lobster boats) and I like the aesthetics. I woundn't however want to jepordise seaworthness of the tw28 as I intend to spend most of my time on it quite a distance off shore.

I'm not being argumentitive, I agree with you, the chine is below the waterline and the adding a 6 inch radius to it will take away some of the volume below the water line. I just wonder about this sometimes.

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Re: round bilge for TW28

Post by jacquesmm » Thu Sep 24, 2009 9:58 am

At displacement speed, there is a very small difference. The round chine hull has a little bit less drag but it depends very much on the hull shape. Our chine is immersed and as such, does not increase drag very much.
At planing speeds, the hard chine is clearly superior.
In between, the round chine will be a little bit faster at semi-displacement speeds but roll more.
However, the TW28 can not be driven at semi displacement speeds.

When it comes to stability, the hard chine will roll less, the round chine will have a more pronounced but softer roll.
To get the same initial stability and displacement, the round chine hull will be wider at the waterline.
This can not be done when changing the TW28 to a round chine and therefore, a TW28 round chine hull will have a little bit less displacement at DWL and quite a lot less stability.

I worked for Krogen and for Grand Banks. Have been on each boat and there is more to them than just a difference in chine shape. I prefer the Krogen trawlers but not because of the chine.
Our TW28 hull shape is not of the Grand Banks type, it is closer to a NW salmon troller.

What Dave Gerr writes is correct but see the word "generally".
In traditional wooden boats, it is difficult to plank a watertight hard chine. That is one of the reasons why they preferred round chines.
And a boat can be just as seaworthy, if not more, with less righting arm and a softer roll.

In our case, the material dictates the shape of the chine but even if the TW28 was designed for another material, I would have given her the same hard chine.
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Re: round bilge for TW28

Post by TW28RJ » Thu Sep 24, 2009 3:09 pm

Finally something that makes sense (at least to me). It can be frustring to those of us who haven't been trained or have worked in this field. For whatever reason everyone in boating becomes a self proclaimed expert -

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