Trimaran - Especially Searunner - Owners

[Cavalier MK2]

I think the thing to ponder is the increase in various stresses. If all the hulls bound across the surface they tend to move in tandem without additional wracking. It might be better to go with aft ama foils like a Catri or go for it with all 3 hulls. I'd suggest leaving well enough alone on a searunner and build something designed to work with foils if you want them. After all a SR is seaworthy already!

[owly]

Originally Posted by Redreuben

Hobby-horsing: Without going into serious structural remodelling another remedy could be the T foil rudder, I have no experience personally and wether it's a good fit on this boat or that is up to the individual but if the proponents are to be believed it's a possibility, there is even a kick up design around whereby the foil is hinged to the front of the rudder so it can stay horizontal as the rudder kicks up.

The main transom is the obvious location for your device due to leverage, but the fact that your vertical motion is greatest at this point, it aggravates the stall issue. That means to me that the engineering is not quite so simple. Hydrodynamic stall will make the foil let loose rather suddenly when the angle of attack exceeds it's stall angle. Angle of attack is not just physical angle, but the combined vectors of physical angle and the vertical motion. It's the actual angle of water flow over the foil. A 4412 hydrofoil begins to stall at 16 degrees actual angle of attack, which depending on the rate of pitch and forward speed could be just a few degrees (at slower speeds). It's all about vectors. Ideally one would want a symmetrical and extremely stall resistant foil, and even then it needs to be mounted "creatively" to retard or prevent stall. The last thing you want is a slow pitch followed by a "launch" as the foil stalls and lets go. The lift of a foil increases as the stall angle is approached, but the drag increases far faster. The trick would be to use this rapidly increasing drag to reduce the foil angle of attack. You want to create upward lift when the transom is moving downward, and downward lift when the transom is moving upward, hence the symmetrical hydrofoil. The rudder is NOT the place to mount this. It would introduce a great deal of force into the rudder, the shaft, and gudgeons. We are trying to dampen the inertial of the pitching motion of a boat weighing perhaps 4 tons fully loaded. End plates on rudders are for preventing water flow from the high pressure side to the low pressure side around the tip, thereby increasing rudder authority. These foils need to be on their own struts, perhaps on either side of the transom.

The idea has some merit, but it needs a healthy dose of engineering to make it work properly. On some boats people have gone so far as to install heavy gyroscopic stabilizers to reduce pitching and rolling. Here is an example of one: https://www.seakeeper.com/ Impractical obviously for our use, but it's interesting to see the lengths people do to to get a good ride.

I think it's worth developing this concept, but trial and error on full size boat is expensive and time consuming. A simple model in a simple home built flow tank would be a great way to do this. just a piece of flat steel on edge with a foil on the downstream end. The flat steel because we are measuring the effects of the foil, and we don't want the effects of a hull interacting with the water to confuse the results. Measuring the actual lift both static (at stationary angles), and dynamic (during pitching) would tell the story, but might be challenging. Mounting the foil with the center of lift aft of the strut it's attached to using an elastomeric mounting, would allow the rapidly increasing lift as it approaches stall to reduce the angle of attack. It's not a simple problem to address on an existing boat.

H.W.

[Buzzman]

I may be completely off beam here, but I understood - I think fomr somehting I read on boatdesign.net - that the main issue causing hobby-horsing in multis is too much rocker, especially too much rocker aft, and that more modern designs have a much flatter run aft below the waterline which tends to even this out.

Is that right, or am I recollecting incorrectly? Or was the idea just plain wrong?

Doesn't help anyone witha built boat, of course, as rectifying too much rocker would mean a redesign of the entire hull(s), but....

[Boatguy30]

Re: rocker...

My cat has a ton in aft 2 meters and does not hobby horse any more than an average 35' mono. At speed in 10+ knots water seems to break free from lowest part of hull and sterns lift up

[Boatguy30]

Schrimshaw got pretty banged up ashore during Irma. Theory is a few small tornadoes spun thru LaBelle during the eye passage.

The boat was blocked quite high and the ama fell down on a tractor tire parked nearby. Interestly, all stringers were scarfed in the same area and most of them all broke open. Apparently weldwood loses some strength over 44 years! Had the stringers held, perhaps damage would have been more modest?

All original ply is in very good shape and is a straightforward, but complex repair.

Will post more here or on my Facebook page as time allows.

[owly]

Originally Posted by Boatguy30

Schrimshaw got pretty banged up ashore during Irma. Theory is a few small tornadoes spun thru LaBelle during the eye passage.

The boat was blocked quite high and the ama fell down on a tractor tire parked nearby. Interestly, all stringers were scarfed in the same area and most of them all broke open. Apparently weldwood loses some strength over 44 years! Had the stringers held, perhaps damage would have been more modest?

All original ply is in very good shape and is a straightforward, but complex repair.

Will post more here or on my Facebook page as time allows.

Jim Brown just did a nice piece on the construction and strength and weight of the Searunner 34 versus the Marples 35. A Searunner (Scrimshaw I think) and a Marples 35 were blocked up in the same boatyard, and sustained similar damage when tipped off the blocks........ Why one would have either boat elevated on blocks for a hurricane is behind me. Both it would seem to me should have been on the ground. The CC35 weathered it better, and really the hull construction using CC results in my opinion in a superior boat structurally, and I suspect the work and cost of building would be about the same. What I don't like is what John did with the forward coach roof, eliminating head room, and completely eliminating forward visibility. While is looks sleek and fast, I'd rather have the utility of the SR34 than the beauty of the Marples 35.... but I'd rather have the construction of the Marples. One can download Jim's podcasts from the searunner website, but I presume everybody here knows that. H.W.

[Cadence]

Originally Posted by owly

Jim Brown just did a nice piece on the construction and strength and weight of the Searunner 34 versus the Marples 35. A Searunner (Scrimshaw I think) and a Marples 35 were blocked up in the same boatyard, and sustained similar damage when tipped off the blocks........ Why one would have either boat elevated on blocks for a hurricane is behind me. Both it would seem to me should have been on the ground. The CC35 weathered it better, and really the hull construction using CC results in my opinion in a superior boat structurally, and I suspect the work and cost of building would be about the same. What I don't like is what John did with the forward coach roof, eliminating head room, and completely eliminating forward visibility. While is looks sleek and fast, I'd rather have the utility of the SR34 than the beauty of the Marples 35.... but I'd rather have the construction of the Marples. One can download Jim's podcasts from the searunner website, but I presume everybody here knows that. H.W.

Since most at armature built a comparison would be difficult.

[owly]

Originally Posted by Cadence

Since most at armature built a comparison would be difficult.

The comparisons have nothing to do with amateur versus professional build, etc. The Marples has a thicker hull, and compound curves, but less internal structure.... it's not necessary where there are compound curves, which are inherently more rigid. The interior space is less convenient in the Marples due to his swoopy front coach roof / deck, which eliminates standing head room in the dressing room area, and completely eliminates the forward facing windows found on all Searunners. This loss of standing head room, and visibility, and lighting is an unmitigated negative, unless styling may be considered a mitigating factor....... streamlining is hardly an issue on a craft that travels at 5-10 kts. There are fundamental design comparisons that are relevant regardless of the builder. Presumably however builders of the Marples probably have a higher commitment, as the construction method is without a doubt more complex. You don't just build up a bunch of frames and lay sheet plywood over them. The multi diagonally biased layers of veneer or thin plywood is at least on the face of it, a more intimidating prospect, and as a result the workmanship is arguably less likely to be poor. H.W.

[Jimske]

I heard she was was down in Florida some place. Did you buy her? Are you fixing her up?

[Cavalier MK2]

Originally Posted by owly

The comparisons have nothing to do with amateur versus professional build, etc. The Marples has a thicker hull, and compound curves, but less internal structure.... it's not necessary where there are compound curves, which are inherently more rigid. The interior space is less convenient in the Marples due to his swoopy front coach roof / deck, which eliminates standing head room in the dressing room area, and completely eliminates the forward facing windows found on all Searunners. This loss of standing head room, and visibility, and lighting is an unmitigated negative, unless styling may be considered a mitigating factor....... streamlining is hardly an issue on a craft that travels at 5-10 kts. There are fundamental design comparisons that are relevant regardless of the builder. Presumably however builders of the Marples probably have a higher commitment, as the construction method is without a doubt more complex. You don't just build up a bunch of frames and lay sheet plywood over them. The multi diagonally biased layers of veneer or thin plywood is at least on the face of it, a more intimidating prospect, and as a result the workmanship is arguably less likely to be poor. H.W.

There is no reason you couldn't put a Searunner style cabin on a CC. The streamlining does help with apparent winds however. If you are going 10 knots into 30 knots of wind, very doable in a tri, you have 40 knots over the deck.

The streamlining pays when the wavetops and spray start brushing the deck too.

In fairness after having a good view out the front I wouldn't want to go back as it lets you keep a better lookout while cooking, off watch time etc.... a second set of eyes never hurts.

[Boatguy30]

I am repairing Schrimshaw for the present owner, Bruce Matlack. Made some good progress in 5 days work. Old age (most weldwood joints failed) certainly contributed to the more extensive damage as did some rot in the bottom of frame 4 and stringers.

[Cavalier MK2]

Good for you Jeff!

[Cavalier MK2]

So.... The Gougeon bible tells is that even a very small fillet or a coating can increase glue joint strength. I think Scrimshaw has a lot of bare wood in the amas etc? It might be worth it ti the owner to consider coating the interiors with epoxy or adding small fillets ti compensate for the old weldwood.

[Jimske]

Re: bare wood. I haven't been able to detect any significant rot on my 31er. (AMA hatches need work as do forward wet locker). But the interior okume of the amas look kind of bare and worn in spots. I'm wondering if I should just get in there and spray up a coat of polyurethane on the interior? Thoughts?

[Boatguy30]

I created a Facebook album. On to the rudder.
https://m.facebook.com/MoJoMarine1/p...=page_internal