Power trimaran/stabilized monohull, 6M LOA, 60HP outboard

Hmm, I am looking at 3 point hydro but I don't think this vessel could ever be the same because I have a V bottom, so no air cushion. I will continue to look at those, though.

I've been corresponding with a guy in Texas (a dive buddy's little brother) who is making a fiberglass flats boat, using his family's fiberglass manufacturing business (but up until now no boat hulls). He suggested I look into 'Power Poles' which they use down there as anchors in the flats. I'd never heard of but the concept but it could allow a little CF pod to drop down from the radar tower as training wheels. Dear god, it could even allow an ama to be swiveled, what a rabbit hole. They aren't cheap, though but at this point nothing is... Hinged amas had been suggested by a few people but a little pod that lifts in and out vertically seems less grief and risk...

 

@Barry

Any willingness to comment?

 

Ah, this?
Trim Tab Orientation Discussion https://www.boatdesign.net/threads/trim-tab-orientation-discussion.57395/

My airboxes indeed might be creating more turbulence than they should because they are parallel to the chines instead of the deadrise... As I mentioned though, this angle I think makes it very difficult for the stern to dip below a certain point when running, because if it does, the angle of attack increases and lift forces become truly massive.



If the amas--which are of course extraordinarily long in comparison to standard trim tabs--have a mild angle of attack, then they could provide a lot of lift with not a lot of drag, indeed lifting the stern and reducing the drag there, possibly increasing speed. We then get into the following seas/corkscrew thing but the amas can be relatively fine and make a lot of lift at higher speeds... I also think by having them so tight against such a narrow hull any vertical heave or corkscrew is gonna be minimized.

 

Another frustrating thread…

It has been argued in the past by others, not me, the only purpose transom pods serve is to increase the lever arm for trim tabs. Obviously, they also move the CoB aft as well. But at a price..

What purpose is served by moving amas aft of the already aft transom pods? I think it only creates drag when you want lift. The amas are no way lifting the hull unless massive. They will just increase waterplane area, I believe. No chance do they lift and reduce it. I’ll wait for an NA to explain how adding ama surface aft of pods reduces drag.

One must consider the extremes when making modifications. What effect of very long amas? Boat will not plane, imo, just drag slower, less side to side movement.

Now, if you argue the amas are less drag than sponsons, that would also be something to see. At speed, sponsons designed like a chine flat basically lift out; the amas do not..they drag

Anyhow, we can’t seem to get any sme to comment on the thread, so I’m gonna be quiet for now in hopes wiser men offer some insight.

But the amas will slow the boat for the offered stability. And I think worse if further back than the pods.

 

These transom pods give the vessel reserve buoyancy in the stern as she runs with open scuppers, has very low freeboard/deck height, so there aren't a lot of places to put that reserve. They also, like mentioned previously, insure that when running, no amount of green water will submerge the stern (threatening the outboard and downflooding into bilge) since the hydrostatic forces against them are massive; if the stern tries to sink, angle of attack on them increases and lift forces are even more massive. They basically lock the vessel into this attitude, for better or worse, when she runs. Because their angle of attack is 0 degrees in relation to the hull bottom they provide no appreciable lift so they're just outboard + bad weather/green water insurance at the expense of some drag. This vessel also has a relatively low bow, albeit with a lot of designed in reserve buoyancy, and so running perfectly flat is not ideal for bad weather/waves.

I'm not married to having the ama's aft of the transom pods, I just kinda sketched them at way. Structurally the bridge will be strongest if it extends aft far enough to be attached to the transom. One reason was I just wanted more static lift in the stern... there might have been a reason related to dynamic performance though.... I need to find it again but there was a study I read on power tri's where they moved the amas around and measured how various placements affected speed/drag/seakeeping. As one might expect everything was a compromise of one kind or another. No matter what, drag/lift will be affected by the depth of the amas and their shape/angle of attack. I'm thinking a very conservative angle of attack plus shallower amas than sketched in the first post. A better design for what I'm doing would be no transom pods--but removing them dear god would be pretty major operation on this vessel--and just narrow hull and training wheels... Ama's too far aft could also be undesirable during fast turns, causing the bow to bury. These are overall very small and hydrostatic forces against the turned in bow are gonna be really strong at those speeds, but it'd be a thing to consider.
[EDIT: Here's the article where they modeled different longitudinal amas...
Optimizing Trimaran Yacht Hull Configuration Based on Resistance and Seakeeping Criteria - ScienceDirect https://www.sciencedirect.com/science/article/pii/S1877705817332642/ ]

Re: planing...
https://www.boatdesign.net/attachments/planing_boat_theory1-pdf.60407
This is very useful to me. My experience with very narrow hulled power vessels is that they just 'rise', there is no defined transition between on plane/off plane; Lanthrop mentions that for narrow hulls, 'hull speed' can be high enough (compared to a beamier boat) that the transition from max hull speed to on plane is hidden or mostly seamless. From a seakeeping perspective I like this a lot.

 

Really good stuff in here...
Short hull trimaran https://www.boatdesign.net/threads/short-hull-trimaran.62283/

These are referenced in the above thread but incredibly useful as he's comparing Adastra to Earthrace. Earthrace has her amas extending past the stern of the main hull. I think part of why they did that was to get the ama bows clear of the main hull bow wave, something I've been thinking about. Adastra also was engineered as an efficient luxury vessel, unlike Earthrace.
https://www.shuttleworthdesign.com/...eakindly-Fuel-Efficient-Vessel-Web_Part_1.pdf
https://www.shuttleworthdesign.com/...eakindly-Fuel-Efficient-Vessel_Web_Part_2.pdf

 

Another reason for modeling is bow wake interference is hard to predict..

 

True, but since I have a running hull in the water I don't really need to try and model the shape of the bow wake because it can be observed at various speeds via drone. I mean, I'd love to run fluid sims in 3D of all this stuff but I'm basically running a vessel that's tank dummy size for a jillion dollar yacht; their design spiral is do a bunch of fluid sims on a bunch of designs and then make tank dummies to see how it really works.

This is just nomenclature, but I think you're referring to a block sponson. The amas I'm talking about would be winged sponsons. When I looked at a PJ 48 ( Motor yachts: super and megayachts - Arcon Yachts https://arconyachts.com/sale/pj-48-supersport ) it's very similar in hull shape to what I'm proposing--a few of the other narrow hull yachts have this feature if they aren't so narrow that they absolutely need amas to keep from falling over. I mean, I don't want to knock the block sponsons but if a block sponson was a truly better design I think we'd see more super yachts (where construction cost and complexity is a secondary concern) with block sponsons instead of winged sponsons. If I have a kind of jacked up fishing boat, block sponsons are really easy to build and very predictable, and construction cost and complexity will be the primary concern for that user. Winged sponsons are lightyears more to design, fabricate, and attach, so no small boat in owner in his right mind wants to mess with them.

The super yachts though are optimized to cruise at a relatively high speed for a giant boat, but I think they're still largely operating at the limits of hull speed. I am going to study the shuttleworth articles more.

 

My gut opinion is the amas will provide more stability, but the build effort is much harder. The connections need to be engineered. They will likely be smaller than the hull and provide little lift, but lotsa stability. Where divers return to the main vessel also seems to be problematic, and the connections look like they will be where I saw swimmers reboarding.

I also think amas will add more drag than sponsons. So, for me, a sponson makes more sense. Your stability issue is largely related to the tower that raises vcg and shifts lcg some as well. The problem with sponsons is they need to be pretty wide to help, but they would be well clear of bow wake.

Gary Dierking’s amas are a good candidate for this type of thing. But the akas are not suitable. You need to size the ama and then decide the forces on it as a cantilever and determine the aka. I’d say a rough estimate is 4” round aluminum tube 1/4” wall, but this is a real dart aimed to the sky. Better perhaps two 3” tubes if they can be used, but the ideal is for them to go all the way across or well into the boat. So that will ruin the boat. A wider structural wing also changes boarding.

How will divers board if you have a 2’ wide structural wing ahead of the tower?

 

Yes, the wings can provide much more stability than block sponsons and yes, they'll be so, so much more work to make. Lift and drag is quite delicious... I could actually make a cross member, put some 'skis' on it that would be something like the waterline footprint of the amas and play with different angles of attack. The cross member could be clamped to the inwales for testing.

The winged sponsons as currently sketched stick out just 12" from the hull, so divers would still board the same way; this vessel is so petite it's easy to think in 'regular' boat terms when you look at the model and assume everything is larger. Currently, boarding is too easy since she heels so her gunwale reaches the waterline when someone boards, but this is the behavior that drives anyone already in the boat crazy (the righting arm gets enormous at that point so she doesn't heel further). It doesn't need to be that easy; the original SOR intention was about a 10" lift, so going over the ama bridge will still be very easy to board, as that's about what it is on my inflatable. Possibly, someone could also step onto the rear of the ama and grip the radar tower, but I'd discourage passengers from boarding that way.

As for fabricating the amas/sponsons, I understand where you're coming from, but I'd never use aluminum tubes for this vessel. Cold-molded wood composites are stronger and lighter. I'd just make them the shape they oughta be in Fusion 360, then slice them into sections and build a one piece component with internal trusses that gets attached the hull. The bridge can be massively strong because it's a trussed box with so much more surface area than two pipes, and the current inwale is also a trussed box, so properly attached together it's two trussed boxes jointed together with thru hull fittings and a very large surface area touching for adhesion and joinery. And, like everything else on the boat, the inwale and that cold molded knee/storage compartment isn't just tabbed in, the wood portions of those things are lapped and rabetted into the hull, then glassed over, so it's so absurdly strong and stiff. My whole vessel is mostly a series of different trussed boxes. The aft edge of the bridge should attach into the transom, which can be incredibly sound from a structural standpoint and a good thing since a lot of the forces on these sponsons will be on the aft end of the sponson/ama. The Gary Dierking amas look more or less right to me (very long and slender, fine V) but for aesthetics I'll try to work off my main hull shape to do the same instead of introducing a new shape.



This is how I made the console, same way as described above but the section guides get pulled out since we want a hollow console... with the CAD it's easy to slice the solid modeled shape of the console into sections, go get them printed to life size paper patterns, 3M spray them to MDF, jig saw them out, fit them together, mold around them, then break them out and final the console. For this application I'd probably make the trusses out of sitka spruce or WRC laminates, maybe add some stringers, foam the structure, carve the foam, then wrap and vacuum process fiber around that. It takes some time but with a plywood CNC the process would be stupid easy and fast--jig sawing the sections takes forever... I probably can't justify the expense just yet but a 4' x 8' plywood CNC (no real Z axis, just for cutting sheets) has come down to around $2k. I'm planning to make any subsequent boats with one of those... workflow from CAD to CNC is so quick, and for multihulls stringer frame construction makes a ton of sense.



Can make any curved shape that you'd see on a super yacht!

 

So, I've had her in the for the second round sea trials. This time was a good deal more intense as the outboard is fully broke in and I'd fixed some little mechanical/electrical things that I discovered in the first round of sea trials... and we had we some proper stormy weather.

I also had a simple cushion that my friend Whitney (Even Keel Canvas) made me for the boat, so passengers would have something to sit on. First day was just about a 50 mile run, me and a buddy in average afternoon seas. Verdict is she'll do a consistent 20kts - 25kts with good passenger comfort in any seas I'd remotely consider going diving in. On day one I rode about 25 miles as a passenger. I'm jazzed on that, and running those speeds we're not actually going wide open.

In the next few days I did a two solo runs in bigger seas--small craft type advisory conditions (very sporty!) and the tail end of a gale with proper 10' seas and blustery wind (not a good idea!). I didn't get any usable footage as running a drone in these winds is a project and my gopros on the vessel would not stay properly mounted under the G's.

Around here, we get our deep energy swell often independent of wind speed and direction, and if one or both is big it gets interesting and we usually get a small craft advisory. This doesn't mean there isn't calm water and good diving out there somewhere if you understand the geography of the area and sea conditions. So, on day one 'weather trial' I set out for Catalina island, about a 25kt run from Redondo Beach to Two Harbors Catalina. This was running into a quartering head sea, 4' - 5' swells + up to 25kts wind before getting into glassy seas in the lee of the island. I wore the killswitch lanyard and went as fast as possible, a good deal faster than I'd go with passengers. In those conditions I could still make 20kts - 25kts, and about 18kts - 20kts would be fine for experienced passengers in wetsuits. What I discovered was that if I'm going fast enough and the bow is dropping down into a trough at just the right angle, she'll heel onto her lee side freeboard edge and decelerate. This is a butt puckering angle of heel for a powerboat, but she just won't roll further (in those seas) because her COG is so low, and she quickly corrects. It's easily avoided by slowing down--to induce this behavior I have to be already running faster than feels safe in the sea state, but by exceeding certain speeds there's no escape, even with very careful and active steering and throttling. Coming back in the same seas (so quartering following) she was superb and I was able to make a fairly comfortable 25kts with nearly zero perceived wind on the boat. Overall she's better/faster in the following but exceeding certain speeds will again induce the abrupt heel. Trying to run a small RIB in these same seas and speeds doesn't induce the heel behavior but the slamming will give you a concussion if it doesn't throw you out of the boat first.

Over the next few days, we had a proper winter gale, so I waited until the tail end when the seas would still be pretty large in the bay, but the sun would be shining and visibility would be ideal. When I left the harbor wind was about 20kts, but seas were still pumping to about 10' and (for us) a fairly quick period. Nobody in their right mind goes out in these kind of seas, there's no good diving anywhere, and I'd be a proper ******* if I had to bother harbor patrol with a mayday so this was just a brief max weather test. I didn't feel safe going above 10kts - 12kts as I would not want to heel the vessel in these seas, although it was big enough that getting dropped was enough G's to unhook the kill switch lanyard. In these seas I did a few passes with a direct beam sea which I had always expected would be the hardest for this vessel design but she really didn't seem bothered by that and just floated along the wave crest without heeling much. Quartering head sea felt the most challenging still. Direct head sea was butt puckering but she carved surprisingly well. In a following sea there a few times I throttled back a good bit on the crest of a large wave (like, peering down into a deep trough just in front of me) but overall she was pretty good like that.

So, from a power/drag standpoint, I think little winged sponsons wouldn't really affect her usable speed. I think she'd still make 20kts - 25kts in most anything I'd be running in, possibly at the expense of a little more gas. I think I also understand now how they could induce the corkscrew/vomit comet movements that power tri's can be known for. In the right kind of wave angle at the right kind of speed (quartering seas), instead of heeling so hard, the aft buoyancy wound induce yaw instead of heel. The more buoyancy the amas provide, the more they'd do this, and their longitudinal position would affect whether yaw is more pronounced in head or following seas. So... I think very modest sponsons with a fine entry/bow section (very gradual increase in buoyancy as you move aft) would be the way to go. The other consideration is splashing. Currently she's surprisingly dry when it comes to green water, meaning it's a little spray mist and trickles but no buckets dumping in. When the main hull throws water it'd be nice not to have it deflected by the outriggers right back into the boat, so the bridge shaping and above water forms will need some careful consideration.