New daggerboard for Potter 19

West Wight Potter 19s are not self-righting.

They have a steel daggerboard that weighs about 300 pounds.

If the board were to be changed to aluminum and a lead bulb or even wing added, how much would the stability improve?

Anyone have any thoughts?

 

Chubby

Hi David,

These photos might give you some ideas:

http://www.msogphotosite.com/P19Chubby.html

The photos have information on who did the keel changes:


Cree Partridge, Terry Tucker, Bill Teplow (The original owner of Chubby)
350# bulb, NACA 64 Foil

They state it doubles the righting moment.

 

Maybe, but it will also stress the rig and hull more. It may cause structural damage.

 

David, your proposal wouldn't change the stability much, unless you placed the lead weight low in the new aluminum centerboard. Of course this would place huge new loads on the pivot bolt, case supports, case sides, rigging and mast attachments, etc., so it would be best if engineered properly.

The West Wight Potter is difficult to capsize and usually requires considerable skipper mistakes, to cause this to happen. The boat has high initial stability and sails flat, so if you've got a Potter on the "edge" you've screwed up long before and should have been moored, berthed, looking for a lee shore or reefed down. This isn't the fault of the boat design.

Is there a reason that you feel the need to alter a Potter? If the reason is more stability, then maybe you need more sailing experience or a different boat. The Potter is quite stable.

 

The point about hull and rig stress is valid. The boat in the link I posted has a considerably built up center board case that was used to mount the new keel. The rig was also later customized.

I think the big advantage to a deeper fixed keel would be improvements in pointing. The stock Potter doesn't point much higher than 50 degrees.

 

. . . and defeating the whole point of a shoal draft vessel. Putting a fixed fin of a Potter isn't a very bright idea as the result is a craft that has too much reliance on beam and wetted surface then a low CG as a result of good appendage design and ballast distribution.

In short, a Potter if designed as a low CG, fin keeler project, it would also be less beamy as well as several other physical attributes. You can attempt to make a silk purse from a sow's ear, but the usual result is heifer clutch.

A stock Potter can tack through 90 and pinch down tighter then this.

 

If the bulb keel could be lifted just as the daggerboard is lifted on the regular Potter I don't see how it would matter. I would guess that the OP would want to preserve this aspect of the boat's design. The boat in the photos above can have the keel raised for trailering. I also know the rig was sized up. But even on a Potter you always sail with the board completely down. So it is a bit of a misnomer to call it a draft sailing vessel.

But really we agree, the Potter is a great boat and very competent in the right hands. Among the Potter Yachters though there is a lot of experimentation. I just thought I would point out an example of one similar to what the OP was desiring to do.

 

Agreed, though as a rule these types of modifications are really way out there from a practicality point of view. Making retractable bulbed appendages isn't back yard, shade tree repair stuff, if you expect it to be reliable and durable.

 

Thanks for all your replies.

For mast monkey:
Thanks for the link. There's no text on that page, so it would be nearly impossible to find by searching. Also strange that this would be on a site for Montgomery sailboats. Never saw this on a Potter site.

I later found the following from http://macgregorsailors.com/phpBB/viewtopic.php?t=3805&highlight=weighted+keel ("Bill" is Bill Teplow):

Bill sails a West Wight Potter 19 named Chubbys. His home port is Berkeley California. This intrepid sailor has sailed his potter from Berkeley to Hawaii. Port Angeles, Washington up the inside passage to Glacier Bay Alaska then back around the outside to Craig Alaska and then South from Seattle to Berkeley California. After his Hawaii trip he decided to modify his stock keel by adding a led bulb to the keel. His goal was to improve sailing closer to wind, increasing the righting moment and stiffen the boat. He attached a lead bulb that resembles a squashed football to the bottom of his new thick stainless steel, glass encapsulated keel. The potter does not carry any other ballast other then her keel. The original keel is 270 lbs. Bill added 150 lbs which puts the new keel at ≈ 417 lbs, putting the majority of it well below the hull bottom. The righting moment for the stock keel at a 90-degree knockdown is ≈ 550 ft-lbs. The new keel for the same heel angle is ≈ 1649 ft-lbs. That is a 3-fold increase in the righting moment. He goes on to state that during his Hawaii trip he slept next to the trunk and the he could feel considerable flex in that stock truck. So he did beef up the area around the base of the trunk. He did his first sea trails off the Marin coast (North of the Golden Gate). The wind was at 25 Knots with 6-8 foot seas. His shakedown cruise of 2600 miles was the trip north to Alaska and then South down the west coast to Berkeley as stated above. Bill reports that she now sails much stiffer, points higher and does not round up when overpowered as well as the increased righting angle.​

Later in that same thread:

On the Potter 19 that I mentioned earlier in this post, Bill did beef up his trunk. The new keel trunk was glassed into the hull by laminating both the inside and outside to the hull. To prevent flexing the trunk had buttress installed on both sides extending from the top and angled down to the hull. He actually cut the old trunk out completely leaving a hole in the bottom of the boat.​

The site you quote says the modifications increased the righting moment by two; this site says it increased the righting moment by three.

Not fully understanding the intricacies of self-righting and righting moments, can someone explain to me how doubling (or tripling) the righting moment affects a vessel's self-righting ability? Is there enough information here and about the Potter to determine if this change actually makes the Potter 19 self-righting--and to what degree?

David

 

I haven't the numbers to crunch for the Potter 19, but I have serious doubts as to the validity of these assertions of 2 or 3 times improvements in righting leverage (the math used in the one paragraph is flawed). For one, if they (the modifiers of this boat) did crunch the numbers they'd know specifically if they doubled or tripled the righting arm and leaving it as an ambiguous "2 - 3 times" is just an absurd statement.

David, a dissertation on this subject isn't best absorbed in this venue. In short, it's a complex subject (stability) and not easily boiled down into manageable snippets.

The Potter in question on the MacGregor site, is a highly modified design, composite rig, composite sails, composite deck cap and hatches, highly modified case, ballast casting well, new board, etc. A huge set of engineering feats and sailed by a real white knuckles guy. Sometimes these sort of things can happen, but for the vast majority of folks, it's just wishful thinking, not to mention the budget damage it can do.

You can do all sorts of things to any boat, but the reasons should be justified, which you've yet offer substance to. Is there a specific set of rational, that you can apply to these types of changes? Do you have specific concerns about the Potter's physical attributes? Which are bothering you and why do you think these need to be altered?

My point is the Potter is a fine little boat, considering what it is. The size of this boat is the most popular of all boats in the world, so there are many different designs to select from, particularly if some of the design decisions and compromises about a Potter have you concerned. Again you can attempt to make a silk purse from a sow's ear, but the usual result is heifer clutch. The Potter, prepared for the LA to Hawaii trip was quadruple (at least) the cost of a regular Potter and sailed by a mad man. So, if you have a similar interest and budget, then go for it.

I'm not trying to be coy or tart. Let me take a guess at what's really going on here. You have limited sailing experience and limited boat building experience. You are enamored with the idea of micro cruising, but have limited to no experience with this as well. Your understanding of hydrodynamics in minimal and you think you can do this micro cruising thing fairly cheaply. Am I close? Yea, how close?

My recommendation is to get out there and find your sea legs. Beg, borrow and trade your way onto small sailboats. Before you do anything other then study about building and yacht design, you need to get out on the water and sail. As you build a sailing skill set, you'll also develop likes and dislikes about each little sailing adventure you have. With these adventures your ideas of the perfect little cruiser will change (repeatedly). This "base" of information about sailing and boat stuff will be your foundation for the things you'll want or not want on your little custom micro cruiser.

 

I really don't understand why two people now are questioning my experience when neither of you know anything about me. My questions are legit. If you don't want to answer them, then don't.

The only thing you are correct about is that I have little understanding of hydrodynamics. That is why I am asking a question on the Stability section of a site called boatdesign.net.

Thanks anyway, but I will go elsewhere when I have a question.

 

I don't think anyone means you any ill will; people only know the input information you've provided, If time is money, consult and pay a naval architect for the most expedient solution; otherwise, if people miss the mark when giving free advice because they don't know your background and are curious about your rationale, either don't let it bother you or give some more info to satisfy their concern and inspire them to consider your question more. I'm quite sure they only meant to help someone (maybe not you) be sure they weren't going down the wrong path.

 

Jeff, this is the same poster that wanted a sharpie micro cruiser a few years ago. He had a design, a 14' sharpie, but wanted it enlarged to 24', which most of us had considerable issue with and a slight contention arose. He then abandoned the idea for a few years to return asking for explanation of the above described Potter butchering. I suspect he has limited sailing and cruising experience and is overly anticipating things that will not occur. This is a common trait among inexperienced sailors. They think they need uncapsizeable when they just need sailing skills or stability curve all in the positive, etc. He'd realize his concerns are moot if he did some cruising, but he also appears to be the "instant gratification" type as well, which suggests a building project will also be fruitless. Maybe the SA guys can be nice to him for a while . . .

 

This thread and the points made here have been extremely informative and enlightening. I would like to start by saying that I think you're spot on, PAR.

Yeah, pretty damn close in my case. That was me, up until a few weeks ago anyway when I really started punching numbers, listing items for overhauling a small DSII, modifying the CB to carry more weight, bowsprit addition for a cutter rig, etc. Recalling my experiences in racing cars, all of a sudden this was appearing to be more work than was necessary. Then it all came together when I read this thread. It was PAR's comment that hit me.

(The following questions were directed to myself, not anyone else) Most of my experience has been with aerodynamics, not hydrodynamics. They're somewhat similar, but there is a reason they're called two seperate things... because they are. My sailing experience has been severely limited to only a few experiences locally. When pondering the ideas of shoal draft, trailerable boats I often came to the conclusion that I would need to modify the CG in order to maintain a proper balance and more efficient heeling moment. But wait? Why do all of that when YOU CAN JUST BUY A BOAT THAT HAS THAT? Sort of a self-reflective "duhh, you idiot" moment. On top of that, experience will be the only true measure of a boat's performance. How sure are you that the boat is unbalanced? Have you been out on the water in one? Do you own one? Have you even seen one in front of you that's not on a computer screen? Experience is key, education and research only gets you so far. Sailing is an art and a science, wrapped up into a dense little section of floating wood and fiberglass in the middle of a body of water. Like quantum physics, nothing is for certain and you cannot place hard facts on any facet.

A great quote that I've learned from a fellow sailor is, "Never settle for an inherently flawed design." In other words, if you know you're going to have issues with something, a boat, some piece of equipment or whatever, why try to fix it when you can replace it before you even buy it? Easier on the pocket book and stress levels as well. Just some revelations from another new guy!

 

Welcome aboard Matt. I'd forgotten about this thread and re-read it when I noticed the original poster was bothered by my comments. I didn't at the time, nor do I now feel I stepped on his toes, though apparently he did. I'm glad the thread found you Matt. So, the sow's ear thing got 'ya?