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-   -   The ratios and other numbers - can someone enlighten me or point me to a resource? (http://www.sailnet.com/forums/sailboat-design-construction/44791-ratios-other-numbers-can-someone-enlighten-me-point-me-resource.html)

arbarnhart 07-08-2008 08:33 AM

The ratios and other numbers - can someone enlighten me or point me to a resource?
 
Preferably an online resource written for the lay person. I don't want to become a boat designer, I just want to understand what numbers/ratios are important, *maybe* how they are caluculated and what the good/bad ranges are. If you want to use my boat as an example (no, I'm not lazy, honest :rolleyes: :D ) the info is below the text.

Most of what I know about my boat is anecdotal. It's mostly positive - I have never found a capsize reported, but have found reports of someone trying to on purpose in high winds and failing. I have communicated with someone who has taken one across the Gulf stream (I am NOT planning to do that). It almost seems too perfect - light weight enough to trailer easily, low draft (under a foot) with the keel up, possibly self righting (here's where I need some numerical help; I have had someone tell me that the first two points make the third one nearly impossible in a cabin/cockpit style boat) with positive flotation. Fairly stable and seems pretty seaworty (but the lack of lifelines or stanchions tells me that the designer (Harry Sindle) really wasn't thinking about ocean sailing. I am about to start (actually I have started but not way into it yet) a fairly major amount of work to fit it out. I have pretty much decided that this is the boat for me for a while. I need some cons to go with my pros and I am guessing some numbers might give me a clearer picture.


http://i43.photobucket.com/albums/e3...wportspecs.jpg

http://i43.photobucket.com/albums/e3...7_sailplan.jpg

http://i43.photobucket.com/albums/e3...17brochure.jpg

jrd22 07-08-2008 09:48 AM

Ted Brewer has a good site with a lot of info. Here's a link:

Ted Brewer Yacht Design

John

T37Chef 07-08-2008 09:52 AM

try this...

Sail Calculator Pro v3.52 - 2000+ boats

arbarnhart 07-08-2008 10:26 AM

Quote:

Originally Posted by T37Chef (Post 338357)

Thanks - I did (pasted in below).

I am going to be uncomfortable in motion, probably hard to sail, capsize easily and never go 6 knots. I get the impression that the calculations aren't really made for small boats.


boatName=Newport 17
LOA=17.75
LWL=16.1666
beam=6.3333
displacement=1000
sailArea=147

displacementToLWL=106
Displacement to LWL: A medium value would be 200. 300 would be high (Heavy Cruising Boat) and 100 would be low (Ultra Light Displacement-ULDB). Boats with low numbers are probably uncomfortable and difficult to sail.

speed=5.39
Calculated hull speed.

sailAreaToDisplacement=23.52
Sail Area to Displacement: The sail area is the total of the main sail and the area of the front triangle. I cannot be sure that this datum was entered correctly for each listed boat. A racing boat typically has large sail area and low displacement. A number less than 13 probably indicates that the boat is a motorsailer. High performance boats would be around 18 or higher.

LWLToBeam=2.55
LWL to Beam: A medium value would be 2.7. 3.0 would be high and 2.3 would be low

motionComfort=7.89
Motion Comfort: Range will be from 5 to 60+ with a Whitby 42 at the mid 30's. The higher the number the more comfort in a sea. This figure of merit was developed by the Yacht designer Ted Brewer and is meant to compare the motion comfort of boats of similar size and types.

capsizeRatio=2.53
Capsize Ratio: A value less than 2 is considered to be relatively good; the boat should be relatively safe in bad conditions. The higher the number above 2 the more vulnerable the boat. This is just a rough figure of merit and controversial as to its use.

category=racer
Sailing Category: The four categories are racer, racer/cruiser, cruiser/racer, and cruiser in order of descending performance

ppi=366
Pounds per Inch Immersion: The weight required to sink the yacht one inch. If the boat is in fresh water multiply the result by 0.975. If you know the beam at the waterline (BWL) multipy the result by BWL/Beam.

arbarnhart 07-08-2008 10:48 AM

One last follow up - I used the comparison chart feature on the site and compared it to a Monty 17, WWP 19 and Seaward Fox and they fared pretty similarly (actually capsize ratios were slightly higher for some reason).

I am still not sure about the self righting question. It seems to my simple mind that it is a case by case design question. I can duct tape a nickle on a clorox bottle and make it self righting.

The biggest things I plan to do are the outside run from Beaufort to Lookout (very near shore about 7 miles between the inlets) and possibly an Ocracoke trip (crossing part of Pamlico Sound).

http://www.ncdot.org/it/img/graphics/boatguide/10.gif

http://www.ncdot.org/it/img/graphics/boatguide/8.gif

sailaway21 07-08-2008 11:22 AM

arbarnhart,
I think you're being much too hard on your Newport 17.

I disagree completely with the hard to sail conclusion. In fact, I think the boat is easily single-handed as well as being blissfully uncomplicated for any inexperienced crew you may have.

The low displacement to length ratio, and the resultant conclusion of a bad ride and difficult to sail is a generalization that may not apply either to your sailing area or your boat. If you're out in the heavy stuff she'll not ride as well as some other boats of differeing hull forms, displacement, or length. On the other hand, she's easily sailed solo and you need only the lightest of air to get her moving. While other "more seaworthy" boats are essentially becalmed you'll be sailing right along. While your hull speed is lower, you'll achieve it much more easily and in much lighter air. In fact, you'll be reefing in conditions that heavier boats just get moving in! You'll also find that your boat is far more nimble than many others; she'll respond quicker and accelerate quicker.

As to your capsize number and heavier seas. You'll want to make sure that you can secure your keel in the lowered position in the event of a severe knockdown and potential roll over. You really don't want it slamming back up into the keel trunk and you certainly want to preserve your righting moment in such a circumstance.

You can do something about your ability to avoid capsize as well. If you do not have a reefing system, install one. Can your companionway hatch be closed and secured? What do you have for washboards? consideration might be given to increasing the size of your cockpit scuppers and drain lines. And when venturing out you can reduce the volume of your cockpit, which will decrease that capsize number, by filling it with stowed and secured gear. That big ice chest you may want to carry will take up a significant volume of cockpit space and that is space that water cannot fill when in a capsize situation.

And you may well see six knots. Chances are it will be downwind and surfing!

arbarnhart 07-08-2008 01:36 PM

Sway,

I am disappointed in the calculations, not the boat. I probably should have placed a rolleyes up there, but I didn't (and don't) want to imply that they are totally useless; they just don't seem to be geared toward evaluating this type of boat. I was hoping they might give me some insight but some of them grossly conflict what I know about the boat from experience. It will plane as well as surf so I knew that number was not a big deal. Some N17 owners beef up the motor bracket and go with a light outboard in the 10HP range to make them somewhat like a smaller version of a boat that gets a rather MiXed reception around here. :) The PO told me he was just getting it on plane with a Honda 5 (which I did not buy as he wanted as much for it as the boat) and I have not had a chance to try my old-but-new-to-me Sea King 5 yet, which is (hopefully) one of my improvements.

I have roller reefing.

I am planning to add one or two cockpit drains out the transom. One of the few design decisions I don't like is that the cockpit only drains through one hole near the companionway into the keel trunk. Besides the possibility of slow draining if pooped, it is a pain at the car wash - it is not the low point when it is on the trailer attached to the car. Another N17 owner put an inspection hatch in the cockpit floor at the transom which he opens at the car wash and lets it drain out the bilge and some have added transom drains. I am planning to add at least one drain back there.

I also plan to add some D rings so I can lash things down. Floating things don't displace water very well. :)

My hatch can be closed and secured, though the current arrangement can only be well sealed from outside and not opened from inside when sealed. I am planning to make a door for it that slips in as a one piece hatch board. The current one is a two piece.

I would really like to come up with something for a keel lock. I have heard that mentioned before. Ideally, it should be quickly/easily engaged/disengaged so as not to lose the ability to haul it up quickly if I get in too shallow.

Thanks for the encouraging words. For a lot of reasons, a bigger boat just isn't in the picture in the near future. I am so darn good at justification I have to run around asking people to poke holes in my plans for me though. :)

johnshasteen 07-08-2008 02:57 PM

Those capsize numbers have a bit more meaning to boats that are actually capable of (or contemplating, or dreaming of or wondering if they are capable of) venturing afar offshore and realistically anticipating heavy weather that they cannot outrun - a category into which your boat does not fit. What it is, is a great trailer/sailer built for the nearest lake or bay - just enjoy it for what the manufacturer says it is "...makes it a good boat for novices at sailing". Could you take it across the Gulf Stream, sure, you could go in a sailing dinghy - with a lot of sailing skills, a modicum of luck and no bad weather.
When you're ready for offshore sailing there are a number of small boats that can serve you well - however, many of them are not trailerable.

arbarnhart 07-08-2008 03:54 PM

I crewed on a friend's QuickStep 21 last October for a Beaufort to Lookout weekend trip that I am contemplating doing in mine this year. It would have been no problem. Could have done it in a Sunfish; the weather was perfect! But I have read trip reports from other outings on that same route that started that way and turned nasty. If weather like that was likely I wouldn't go at all. If it was reasonably possible but not highly likely, I might make the drive down and decide at the ramp between the outside route, the back way (a narrow channel with a lot of traffic) or not going. I would not purposefully decide to go out into high winds and big waves, but I want to be prepared for the possibility of it occassionally happening.

sailaway21 07-08-2008 04:16 PM

barney,
My Cal has a hole located near the pivot point for the keel for the insertion of a locking pin. I assume there are no unexplained and unused mystery holes in the forward end of your keel trunk?


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