How to properly trim Pocketship

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How to properly trim Pocketship

Postby craig on Wed Jul 30, 2014 9:30 am

I know very little about boat design, so I'm just thinking out loud here... anybody who has built a Pocketship please chime in.

I'm worried about properly trimming Pocketship. Specifically, whether I have put too much lead in the aft keel compartment.

I've noticed in pictures online that almost every boat ends up "heavy" in the stern. Here is Pocketship #1 with one passenger and no motor (so, the "ideal" condition). It is almost perfectly trimmed, and the transom is not dragging in the water. Even here, though, you can see the waterline is slightly higher out of the water at the bow than at the stern.
https://www.facebook.com/chesapeakeligh ... =3&theater

But then we add a motor, and more people, and gear, and it seems like all of that added weight concentrates in the stern and the stern falls faster than the bow. Here are just two examples I could find quickly (apologies here, but they were side-on shots and every boat seems to display this tendency).
http://www.clcboats.com/images/photos/b ... LowRes.jpg
https://www.flickr.com/photos/mtsailor9 ... 470828808/

I will flip my boat soon to fiberglass the bottom, and it wouldn't be challenging to cut open the aft lead compartment and remove some, and later add more removable ballast towards the bow. I'd rather make that change before finishing the boat than afterwards. Should I? Or am I making a mountain out of a molehill?
Titania, launched January 2015
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Re: How to properly trim Pocketship

Postby Bflat on Wed Jul 30, 2014 10:53 am

No way would I remove lead from the Keel! Moving weight upward affects heeling moment greatly (just like weight aloft does). It's much better to affect fore and aft trim with inside ballast. I noticed that Tattoo even went so far as to put lead under the battery in compartment 2. Not a bad idea, I think.

This kind of relates to my query about the floor board lift out sections. Pocket ship 1 appears to have the lift outs farther aft than the plans indicate. I'm wondering if that means that it has its inside ballast farther aft as well (or at least not as far forward as planned).
Last edited by Bflat on Thu Jul 31, 2014 10:02 am, edited 2 times in total.
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Re: How to properly trim Pocketship

Postby Shudoman on Wed Jul 30, 2014 1:22 pm

I concur with not altering the ballast in the keel. Pocketship seems to need varying amounts of extra ballast depending on who you talk to. I've seen anywhere from 120 - 250 lbs. I've got about 170 lbs of lead total on each side of the floorboards towards the FRONT of the keel. With that the boat was close to trim but still a little bow high. When I added a 58 lb house battery in compartment 2 it trimmed out perfectly and became significantly less "tender". My total internal ballast now is approx 228 lbs.

The trim IS subject to change based on your motor and where you sit. When sailing alone I sit as far forward as possible and my Torqueedo motor weighs about 30 lbs. In that configuration it is in trim. If I add passengers I tend to make them sit as far forward as possible else the transom drags. Pocketship is a small boat and IS sensitive to weight distribution in the cockpit as are all small boats.

You can see the out of trim initial sail on my blog. Again this situation was fixed with the additional weight of a battery and moving everybody forward in the boat.
http://pocketship.blogspot.com/
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Re: How to properly trim Pocketship

Postby craig on Wed Jul 30, 2014 2:02 pm

Thanks guys for your input. I remembered from the plans that I needed something like 150 pounds internal ballast, but didn't realize I could/should increase up over 200 pounds for handling and trim performance. Of course it's all hypothetical for me right now since launch day is just a dream at this point, but I think I'll leave the keel AS-IS and focus on just increasing internal ballast. Bill - good to know you got everything straightened out.

I am considering installing a water ballast tank under the floorboards, or forward between bulkhead 1 and 2. Now I need to revisit those plans to make sure I have capacity for 225 pounds of water (27 gallons or 3.6 cubic feet). That is a vast area and I'm not sure I can find the space for it all. Only about 8 gallons can fit between floor 3 and 4, for example.
Titania, launched January 2015
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Re: How to properly trim Pocketship

Postby Bflat on Wed Jul 30, 2014 2:17 pm

By water ballast I hope you mean bottles of water and not some flooding tank like water ballast usually entails. That said, lead is a lot denser than water and bags of bird shot are pretty handy to move about. I'd go with those before I'd consider bottles of water and I'd never consider a flooding tank in a boat not designed with that in mind.
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Re: How to properly trim Pocketship

Postby craig on Wed Jul 30, 2014 2:53 pm

I currently have water "pouches" that will fit under the floorboards. I also have a drum that will go forward of bulkhead 2. But, I don't see a problem with a water ballast tank, and am considering installing one under the floorboards. The disadvantage compared to lead is that it takes up more volume and so for a given weight, the center of gravity is higher. And it could leak and flood your boat (so build it well!). But you can shed the ballast weight at the boat ramp to make life easier for your car. I have seen many designs incorporate a water ballast including "pocket change" and the Faering cruiser, so I don't think it's very radical. My problem is finding enough room to fit over 20 gallons: the floor supports divide up the area and prohibit one large tank.
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Re: How to properly trim Pocketship

Postby DanaDCole on Wed Jul 30, 2014 3:45 pm

I was considering (kind of) water ballast for at least some of the ballast. The reason I was thinking about it is that I have a Toyota Tacoma with two-wheel drive. Although John designed the PocketShip so that it can be hauled out of the water with a Honda Civic, that is a front-wheel-drive car and as you know unloaded pickups don't have a lot of traction in back.

But I have decided to stick with the plans and also go a little heavy on the ballast since I will be doing a lot of single-handing. If I can't get it out of the water I'll have to ask for help. Some other thoughts are: Take some of the ballast bags out of the boat and put them in the truck bed (a lot of hard work). Get a long tow rope and slowly pull the boat out with the truck up on flatter ground (hard on the trailer jack wheel). Get a bigger truck (don't want to). Any other ideas would be appreciated.

Anyway, we don't have a lot of slime on the ramps on inland lakes so maybe I'll be OK. I'll just have to wait and see.
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Re: How to properly trim Pocketship

Postby mtsailor on Wed Jul 30, 2014 5:16 pm

I'm happy with the way Carlyn J is trimmed. I weigh 220 and with just me aft in the cockpit there is a bit of down trim on the stern, however, the boat handles very well. With another body sitting forward in the cockpit the trim is perfect and matches the painted water line, the boat still handles well. In addition to the lead poured in he keel and the piece in the center board, I have about 180 lbs (total) of lead sheets in the bilge compartments on either side of the center board well (according to the instructions in the manual) and a 55 lb battery, on the keel line, the bow side of the forward cabin bulkhead. Seems to me, in a 15 ft boat, if you agonize over exact trim, you'll never get it just right for the given situation, i.e., additional crew, gear, etc. I'd suggest a few good sails and then make ballast adjustments if needed. Good luck. Build & sail good.

Jer aka mtsailor (s/v Carlyn J) https://www.flickr.com/photos/mtsailor9 ... 470828808/
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