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Sailboat conversion to Trawler

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Sailboat conversion to Trawler

Postby jbo_c » Tue Apr 22, 2008 7:52 am

Read an interesting piece of an article on the SCA website about converting a sailboat to a trawler. Something I've considered before(but I never would have called it a trawler), but thought it was just one of my wacky ideas that would never work.

What 'engineering' type things would you have to take into account? First thought says just cut off the keel, mount a motor in the center of the "transom", leave the mast in the yard and go.

But after what I've learned here in the past year, I've got more questions to answer. How much keel can you cut without affecting the tracking? If you remove the keel and it's a large portion of the ballast as well in a small boat, then what effect does that have on the CG? Does that matter if you won't need to offset the wind in a sail? With less ballast(from the keel), the waterline will change - maybe a lot. Will that be enough to cause significant balance issues?

Have I learned enough that I'm overanalyzing now?

If I didn't need shallow draft and could leave the keel in place, 99% of the questions would go away, I think. I'm wondering if you could use a drop or swing keel boat and just not drop, or remove the swing keel.

Just for fun discussion. Any opinions? Ideas? Done it? Seen it done? Know where to get more info on doing it?


[/img]http://www.smallcraftadvisor.com/content/compac16/traw1.jpg[img]


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Postby Daddy » Tue Apr 22, 2008 8:07 am

jbo, been there done that. Worked out pretty well but not sure I would do it again. I bought a 22 foot sailboat, removed the keel, cut it down by half and reinstalled it, raised the roof of the cabin to give it standing headroom. It waa a fun boat but would roll terribly. The reason was you dont have the steadying effect of the wind in the sails to hold the boat steady. I mounted a 9.9 yamaha on a home built bracket and she would go at hull speed at just above idle. That was the good news, go all day on a couple of gallons.
I wouldnt say dont do it, just be prepared to rock and roll. Oh yes good point about less keel raising the boats lines I would add ballast if I did it again.
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Postby jacquesmm » Tue Apr 22, 2008 9:21 am

Daddy is correct. The sailboat is designed to get his stability from the sails, a trawler hull gets more stability from it's hull shape.
There are some motorsailer hulls that are close to a good trawler shape but most small sailboat hulls will roll like crazy.
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Postby jbo_c » Tue Apr 22, 2008 12:34 pm

Hmm. I would have thought the leverage of the sails and mast would have increased the roll and leaving some of the keel would further steady it without the mast.

Thanks for the comments.

Daddy, what did you do about steering?

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Postby jacquesmm » Tue Apr 22, 2008 12:45 pm

Ballast doesn't give much righting moment at the low angles at which power boats run.
One could leave the mast and have a large steadying sail.

The best way to improve stability for those hulls is paravanes:
http://en.wikipedia.org/wiki/Paravane_(weapon)
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Postby Daddy » Tue Apr 22, 2008 2:08 pm

jbo, the steadying effect from the sails comes from the fact that the wind holds the boat at a pretty much constant angle as it cuts through the water. The boat may be heeled over but at least does not roll. With out that effect it rolls like crazy. the hull has low initial stability but great reserve stability as the weight of the keel keeps the boat from rolling too far as in capsize. A flat bottom boat has great initial stability but poor reserve. I guess somewhere in between is where we want to be. But do the sailboat thing if you want to. We had a ball with ours, great lines too.
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Postby Daddy » Tue Apr 22, 2008 2:15 pm

jbo, sorry, I did not see the part in your post about steering. I used a tleleflex rotary. The motor was designed for remote steering and operation. I put the helm in the cockpit which was covered with a hardtop to keep the sun off.
Here is the quote from Jacques notation:
"In general boats need to be designed with paravanes in mind because they need strong mount points. Sailboats do not need paravanes since the wind pushes them on their side and keeps them from rolling. Most powerboats do not need paravanes because they skim on top of the water (planing hull) at such high speeds the water pressure forces keep the boat from rolling. Boats that efficiently plow through the water (displacement hulls) usually roll uncomfortably from side to side in rougher conditions and they can benefit from paravanes."
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Postby jbo_c » Tue Apr 22, 2008 3:29 pm

Good explanation on the paravanes. Makes perfect sense.

If I did this, it would be for small water only, mostly rivers and small lakes. Lakes are more chop than waves usually IMHO. Do you think the rolling would still be a problem or would that primarily be in a sea coast type situation? This boat would most likely never see salt water.

Got any pics, Daddy?

Thanks again.

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Postby Cracker Larry » Tue Apr 22, 2008 4:26 pm

No offense to anyone, but I never understand why people want to take perfectly good boats and try to turn them into something they aren't. There is little likelyhood of a happy ending. And you will have a product that handles poorly, possibly unsafe, and you could never sell it.

The effort and money would be better spent fixing up a boat that is initially designed to do what you want to do, IMO. :lol:
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Postby jbo_c » Tue Apr 22, 2008 4:49 pm

An excellent point, CL. In fact, my first thought is to not change anything, but to just use the "kicker" to putter around. If it suits, then I make permanent changes, if it doesn't, sell it and break even or give up a little for the experience. If it works, it has a new "intended purpose".

- and, I've never seen a boat in this size range that actually does do what I would be hoping to accomplish - short of building my own. Southwest Georgia hasn't yet been run over with small, economical motor cruisers yet.

However, in a pinch, you can drive a nail with a rock if you're patient. :lol: Especially if you don't mind throwing the rock away when you're done.

No offense taken. It's a discussion. If we all agree, at least one of us is superfluous.

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