h18 mast prebend concept

In the 90s Rick whites book(cat sailing for the 90s) he describes the advantages of pre bending the mast with the diamond wires and the spreaders. Have any 18 sailors tried this and what were the results as far as boat performance. Could this damage the mast?

this can be done, but, i would bet that there a lot of other factors to be considered. mast rake, batten tension, the general condition of the sails themselves, and above all general sailing tactics. pre-bending the mast is not a holy grail to speed on an average condition boat. (IMHO).

I asked that question myself after reading Catamaran Sailing for the 90's, and the answer that I got is that nobody on the forum has been able to do it, or spent much time trying to. I believe you would need to modify the spreaders to pull it off. The H18 spreaders don't rake back far enough to induce pre-bend, and the amount of tension required on the diamond wires would likely not be achievable by hand with turnbuckles.

In the book, Rick White implies that it's been done on an H18 before, but it doesn't seem to be something that anyone has any experience with, so I'd love to find out what/who he was referring to, but As far as I've asked the question, nobody seems to recall pre-bend being pulled off on an H18.

_________________
Mike

'79 H18 standard 'Rocketman II' sail #14921 RIP
'78 H18 (unnamed) sail #14921
'08 H16 sail #114312
'97 H21SC sail #238

Here are two past threads on the topic.

viewtopic.php?f=13&t=55787&hilit=mast+pre+bend

viewtopic.php?f=13&t=52126&hilit=mast+pre+bend

Jim

Short Answer do not worry about pre bend, this is a Hobie 18. Jim posted up some nice previous discussions on this topic. After reading through them I got a head ache. I think they all ended up agreeing. Pre bend as it is typically thought of, does not really apply to the Hobie 18 due mainly to the sails (not cut or of a material that can handle mast bend fore and aft) and next is the mast itself, very thick section with bendy top section.

As mentioned above and in other threads, the spreaders do not allow for much in the way of mast pre-bend. The spreaders are adjustable if one wants to get carried away, however I think this is a great distraction. Pre-bend is usually referring to bend in the major (meaning fore and aft direction of the mast.) The rotating mast and particularly the Hobie 18 gets its mast bend mostly on the minor (meaning sideways bend). This is accomplished by mast rotation and then down haul and main sheet tension, which will bend the mast side ways and flatten sail. That is not "pre bend". The diamond wires on the mast are used to allow or limit the amount this sideways bend.

Now for the practical part, mast bend should be the least of the concerns for most (99%) of all Hobie 18 sailors. (I just sailed in a Hobie regatta this past weekend in the 18 class) and I will share my observations. First the Hobie 18 is a heavy catamaran and as mentioned in the threads under powered by today's standards. For class racing this means nothing, but I point it out to make a bigger picture point. We occasionally sail against a H18SX, bigger main, bigger jib with very little observable speed differences. So even "a powered up boat" results in not much difference being made in the speed of the boat. The boat's weight and hull design have some what dialed in speed potential. If one looks at the Portsmouth rating comparison between an SX and a stock Hobie 18 they are essentially the same even with the SX having a spinnaker. So all the mast shape worry is to miss the boat, so to speak. Now to my observations on the race course. First, I have no idea what my mast diamond tension is at, it is how it was when I bought the boat 2 years ago, (so I obviously do not think it is important, but after reading all of this I may check) On the race course I notice a couple of things, one was my assumption that the wind direction and strength on different parts of the race course is and was key to boat speed. Not my mast bend or batten tension (don't even know what that was either). When we were right next to other 18 (like at the start) etc. we didn't point higher or go faster. However after we tacked and sailed off by ourselves to re-cross again with other 18's we had put 10 to 20 boat lengths on them. Some of this was by going the right way wind shifts and all, but what I think was the largest factor was they did not have a sense of their own boat speed. Meaning, they did not know if they were sailing fast when left by themselves. They would only go fast when next to another H18 so they could imitate their speed. My strategy quickly became, get away from any other boats and I would be faster. It worked flawlessly. This tells me the other boats needed more sailing time recognizing when they had the boat going at full speed, and when it slowed down when they are by themselves.

So back to the short answer. Mast pre-bend is a distraction. Read what you need to know about depowering the sail with mast rotation and down haul so you can focus on sailing. Steering the boat fast is everything, learn how to focus on that. Then look at where you have most wind, and which direction get you there fastest.

My disclaimer is, I am not trying to insult anyone. Just want folks to focus on the sailing part more as all the boat tuning stuff will distract folks way too much. The Hobie 18 does not lend itself to tuning changes making that much difference. If you do happen to be racing at the Nationals, it makes great dinner conversation, but again those guys are not winning because of their diamond tension.

Cheers,

PS I raced 18's hard in the early 80's, getting back into it and really love the 18 for its simple fun. I no longer need to worry about mast bend and batten tension

Truer words were never said. You want to sail slow? Sail in a group of boats. You want to go faster than everyone else? Get away from them.

While I agree with you Kaos that fretting over the small stuff like how much batten tension and mast rotation/bend to apply is not going to win or lose you the race, I do think that it has a noticeable affect on how well your boat will sail. I've noticed a huge difference between tight and loose diamond wires in particular, and I've gone from "couldn't keep the boat down" to "why won't this hull lift up?" just from making adjustments to the diamond wires under the same wind conditions. Mast rotation & rake is a similar gross adjustment, and spreader rake or batten tension... why even bother?

To your point, it won't win you or lose you the race, but it can be an improvement or a detriment to your sailing performance. It's all about getting the right sail shape & angle of attack given your weight, winds and wave conditions.

As you hinted at, the concept of "building leverage" on the rest of the fleet by sailing away from the other boats is really quite interesting, and something that was only recently put into perspective for me. If you are well in the lead, and/or it's a shifty day, you will likely want to stay close to the competition (same side of the course), so that they don't overtake you on a lucky wind shift or what have you. However, if you're a little behind and are counting on a lucky windshift, sailing to the other side of the course can either save you or sink you (metaphorically speaking), if one tack isn't initially too heavily favored over the other.

sabresforthecup, Your comment" ...loose vs tight diamond wires and hull lift" please explain. Thanks, dj

jim-doty, Thanks for the references to forum discussions shown back in 2014. The discussion was very thorough on mast bend. Sorry to bring it up again. dj

loose vs tight diamond wires and hull lift - He is referring to how much sideways bend you allow in your mast. Tighten your diamond wires and they will not allow your mast to bend much (which is what flattens your sails or depowers them). Loose diamond wires allow the mast to bend a lot and depower sails. If you sail with heavy crew weight say 350 lbs. or more you may want tighter diamond wires to keep the boat powered up which will be noticed by how much hull lift you get from your windward hull (all things being equal).

What Kaos said. I sail with pretty light crew, right around 300lbs, and so if we're sailing in heavy winds (15+ mph) and keep our diamond wires tight, even rotating the mast off to 90 degrees leaves a lot of shape to our mainsail and a lot of power in the sail, and so we end up flying a hull very easily and have a hard time keeping it lower to the water. (when your hull lifts high out of the water, you lose a lot of your forward thrust and slow down because of the extreme/unusual angle between your sail and the wind... the optimum angle is to have the windward hull just barely out of the water)

If the wind really picks up on us suddenly, I'll loosen my diamond wires while on the water to allow more mast bend when the mast is rotated, which allows for a flatter mainsail... less power but more speed. However, if I loosen up the diamond wires too much, my sail ends up flatter than I intended, and I no longer have enough power to lift the windward hull out of the water. The diamond wires are supposed to be a "gross" adjustment set on the beach, and then mast rotation and downhaul are your finer adjustments while out on the water. They all work together to make your mainsail flatter or fuller.

The "power vs. speed" dynamic can be confusing, but basically look at it this way: a wing, rudder or sail is an "airfoil" or "hydrofoil." A thicker foil, or a sail with more shape to it generates more lift at a given airspeed than does a thinner foil/sail with less shape, but it does so at the expense of creating more drag. However, the thinner foil can go faster with less drag, and in fact it has to go faster in order to generate the same amount of lift. It's why an airplane uses flaps on takeoff and landing. The flaps create more shape and a "thicker" foil, generating more lift at slower airspeeds, but also generating more drag and slowing the plane down. When the flaps are retracted, the wings generate less lift and less drag, so the airplane can/has to go faster to generate the same amount of lift to stay airborne.

Ahhhhhhhh, the old mast bend issue.

I tried pretty hard a few years ago as reflected on the previous posts Jim listed above to bend the H18 mast.. Because I sail both 20s and 18s, I tried to sail the 18 with the same theory as the 20. Prebend as much as you can (or at least very tight wires) and then use the down-haul to bend more while sailing for finite control and to DEPOWER the rig by twisting off the top. Worked "kind of" and kept me in the lead of the NACs in 2014 until Jim Sohn took it from me on the final day. But...I destroyed the main in the process as the dacron main couldn't handle the down-haul. (lesson learned)

Bend on the major on the H20 is huge and I just spent the better part of one evening this week tuning my H20 mast again and increasing the bend. The boat sails significantly different depending on mast tuning. We are sailing with as much as 3" of bend off the cord. It makes a huge difference in the 20s but, as some reflected above, the 18 is a heaver boat with a mast that doesn't bend well.

With that being said. I am not done messing with the 18 mast. I'm gunna take another shot next month for my fall H18 campaign. The H18 NACs are in Mexico in Oct and the winds are going to be on the lighter side. Depowering won't be an issue but more initial sail shape would be nice.



I couldn't disagree more. Small stuff matters. Tuning matters. Body position on the boat matters. Your ability to read the water matters. In the beginning when we were all leaning to sail/race we were told that "don't sweat the small stuff" because "one bad tack" could erase any benefit of whatever improvement we were thinking of.......
Well, my tacks aren't always perfect but they are pretty darn good. After that, the only difference between me and the next guy is my attention to details. Rudders, sail shape, daggers, mast rotation, mast bend (major vs minor), starts, current, combined body weight vs how the boat is tuned....etc. etc.. It is the accumulation of all of the above that makes you the winner or loser. If you don't over think it, you won't understand it. You have 9 foils on the boat that interact with each other constantly. Understand or studying the relationship(s) means you are a learning evolving sailor.

Winning the race against the best sailors means you've studied the best sailors, studied the boat(s), adapted your own techniques based on your experiences and stay true to your discipline.

_________________
H18 '85
H18 '89 "Knotty Passion"
H20 '96 "20/20 Vision"
Fleet 259 Central Coast California

it's a lot easier if you just cheat

Concur completely. Skippers who consistently win are the ones who pay attention to all of the details.

BTW, if you want to "check" your prebend; take a string and tighten it from the mast sheave to the mast or diamond wire base then measure the distance between the string and the mast track at the maximum. Note the position from the mast sheave down to that measurement (it should be around the spreaders). While you are futzing around anyway, sight up the mast track and ensure it is straight. Now lay out your mainsail and tension it on the ground along the luff. Take the same string and tighten it. Measure the distance from the string to the luff tape as well as the distance from the sail head. The distances and measurements should check reasonably closely. The distance "should" be greater on the sail by a little bit (it only takes 2.65% more cloth along the horizontal to create 10% camber in the sail at that location). You guessed it, your sail was made to fit your mast! As others have noted, too much mast prebend depowers the mainsail.

_________________
R/Thom
SeaRail 19
Triak
BMW C600
Formerly Getaway with Custom Spinnakers
Formerly raced F24 Mk II

I don't mean to say that tuning doesn't matter! I was just agreeing with what I think that Kaos was trying to express, that for your initial foray into improving your sailing ability, focus on technique over tuning, as it has the far greater effect on your performance. At the same time, half of the thrill of sailing (at least for me, as an engineer) is trying to figure out what to tweak, how to improve your tuning or adjust your sail or wind angle to squeeze out that extra half knot or two. Why else do we race, if not for a way to push our boats to the limit and try to get every last bit of performance we can? But certainly the biggest improvements to my sailing skill after reading that book were to my technique... learning how to roll tack, how to avoid getting stuck in irons, how to shift weight to trim the boat properly and keep the sterns from dragging, how to find the "groove" downwind, etc. Then probably a few tweaks from the "how to shift gears" section was the next largest improvement... which includes the interaction between diamond wire tension, mast rotation, downhaul and mainsheet tension. After that, maybe mast rake? Though that is a more subtle difference.

The point, I guess, is that everything has an effect on your performance on the water, it's just a question of how much. Technique certainly has the largest effect, then proper sail trim (of which shifting gears is part), then all of the other little tuning aspects. Spreader rake has functionally no effect (unless you modify them to rake further aft), rig tension and jib halyard tension have a subtle effect, rudder rake has a subtle effect on maneuvering (mast rake is a bigger factor in helm) but can have a large effect on tiller feedback. Batten tension & shape has a pretty subtle effect, unless they're very inconsistent from one to the next. Jib sheet fairlead position has a slightly more pronounced effect. I bought the Hobie 18 tuning & performance guide from murray's, but the most useful thing I found in there was how to measure/set mast rake. Beyond that, most of the tweaks are too subtle to make a significant difference for all but the most serious of racers. I think it's important to be aware of them and make sure your boat is close, but to spend an entire day setting up the boat, measuring, shaping & remeasuring batten shape would be really going overboard. After a year of use, your sail stretches and you'd have to do it all over again anyways.

Mast bend on the major axis would certainly be an improvement, and have a noticeable effect if you're really good at it, but on a stock Hobie 18 it isn't really a possibility. and as wscotterwin said, it's likely going to take considerably more downhaul tension to bend your mast on the major axis than the minor, which can quickly destroy a dacron sail. Mylar might take the strain a little better, but it also delaminates in just a few years in the sun, so it's not expected to last as long as a dacron sail typically does either.